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Post Info TOPIC: Canon de 75 mle 1897/ US Model 1897 recoil system function and manufacturing.


Corporal

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Canon de 75 mle 1897/ US Model 1897 recoil system function and manufacturing.
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Since I found a 1920 article on the manufacturing process of the French 75's recoil system by Singer, I decided to clean it up a bit (removing typos and converting units to metric) and post it, which will be on this thread.  But in order to understand the significance of this, it is important to know how the recoil system works and why it was so hard to make.

There are a few pictures of the French 75's recoil system floating around the internet:

https://www.historynet.com/canon-de-75-modele-1897-frances-recoilless-soixante-quinze.htm

http://roadstothegreatwar-ww1.blogspot.com/2014/12/weapons-of-war-french-75mm-field-gun.html

http://www.passioncompassion1418.com/decouvertes/english_fusees_artillerie.html

However, these do not accurately show how the system operates, because they are based on pre-1918 public diagrams with all the details censored out (the diagram in the second link is an actual pre-WWI image).  This can be seen on page 273 of Henry Arthur Bethell's Modern Guns and Gunnery, published in 1910, which shows the same general layout and states that "The exact details of the recoil gear are kept secret, but it is understood that the arrangement is in principle as shown in the above diagram."

The full mechanism is seen in more recent technical documents, such as the US Army's Recoil Systems Metric, from 1988 (It's a download, so don't click unless downloading it is intended).  Pages 3-9 and 3-10 show and explain this diagram:

RecoilSystemsMetricPuteauxdiagram

Though it does not state that this was used in the French 75, it is labeled the "Puteaux mechanism" as it was invented there for the French 75.  The book "Elements of Ordnance" by Thomas J. Hayes, from 1938, shows the same mechanism on page 259 and clearly states that it is used on the French 75.  

Using these 2 sources, the functioning of the recoil system can be explained, using a series of simplified diagrams:

 

Puteaux Mechanism diagram 1

In the top diagram, the recoil system is at rest.  Dark blue is the hydraulic fluid, and light blue is air.  The orange piston is attached to the gun barrel.  In the bottom diagram, the first part of recoil is underway.  The gun recoil (black arrow) pulls the piston (orange) backwards.  This pushes the oil down into the lower cylinder where it then pushes the floating piston/control rod (green) forwards.  The oil flow is represented by the red arrows.  It freely flows into the lower cylinder, and then flows between the orifice (purple) and the control rod (green) as it pushes the floating piston forwards.  A little bit of oil flows right around the control rod (the 2 very small arrows) instead of through the 2 big ports on either side, but it's not much by comparison.

 Puteaux Mechanism diagram 2

In the top diagram, the recoil stroke has progressed further.  The piston (orange) is still being pulled backwards, and the oil still is flowing into the lower cylinder (red arrows), but it can't move as fast anymore.  As the oil pushes the floating piston/control rod (green) further forwards, the fat end of the control rod fills up more and more of the orifice (purple).  This means that the oil can't flow into it as fast, and it has to slow down- which means the piston (and gun) has to slow down.  This absorbs the recoil energy.  In the bottom diagram, the gun is at full recoil.  The piston (orange) has been pulled all the way back, and the oil has pushed the floating piston/control rod (green) so far forwards that the control rod completely blocks the orifice (purple).  This means that the oil can't flow forwards at all, and the piston can't move backwards at all.  At this point, the gun has been brought to a complete and gentle stop (because the control rod slowly and smoothly blocks the area).

 

 

Puteaux Mechanism diagram 3

In the top diagram, the recoil system is returning the gun back to position (counter-recoil).  The compressed gas pushes the floating piston/control rod (green) backwards (black arrows), which in turn pushes the floating oil back into the upper cylinder (red arrows), and which pushes the piston (orange) and gun back forwards.  If this happened the same way as the recoil in reverse, the oil would flow faster and faster as the control rod blocked less and less of the orifice (purple).  This would cause the gun to slam into the front of the recoil system at high speed, and jump forwards.  So to stop this, a set of one-way valves (dark blue) block the main passages, and force the oil only to flow through the small space around the control rod (the very small red arrows).  This forces the oil to flow slowly, and push the rod (orange) and the gun forwards slowly so it won't jump forwards.

The bottom diagram shows the actual nature of the floating piston in the Puteaux mechanism.  The control rod is attached to a diaphragm, which is attached to the actual floating piston (green) by a spring (light green).  This is so that the control rod is always pressed against the orifice in the right spot when the recoil starts.  If there is an extra amount of oil in the system, the floating piston gets pushed further forward and the space between the piston and the diaphragm gets bigger, where the extra oil is stored.  This way the floating piston won't pull the control rod too far forward when there's too much oil in the system.  When the mechanism leaks oil, the compressed gas pushes the piston further back, that extra space just gets smaller to account for the lost oil, and the recoil system functions normally.  This is why the oil in that space is called the oil reserve- it's to compensate for oil leaking out of the mechanism.

 

What's notable about this system is that it was exceptionally hard to make in WW1, even though plenty of hydrospring recoil systems were mass-produced just fine at the same time.  It required extreme precision, as several sources show:

One paper, "Evolution of the American modern light field gun" from 1978, states on pages 49-50: Upon close examination, the secret of the Puteaux recuperator was revealed. Each one was hand made to an indescribably close tolerance with precision nearing perfection. These extremely close-fitting parts and highly machined surfaces could not be adapted to assembly line production needed to quickly produce guns. This is the reason the British elected not to use hydropneumatic recuperators until 1918 when they developed proper manufacturing techniques. The Germans never attempted to produce the recuperator during the war after examination of captured French guns.  The exacting construction of the recuperator also posed a significant problem to the Ordnance Department after the decision was made to produce the French gun.

Another comment on a forum quotes from Ian V. Hogg's book "The guns 1917-1918," stating that the secret to their success was "Fine tolerances, and an exceptionally close-fitting piston-head sealed with german-silver rings. Nothing more."

Since hydrospring mechanisms are only different in having a spring rather than compressed gas, logic shows why this mechanisms requires such high precision.  A hydropneumatic recoil system has 3 types of seals that might require high tolerances: those separating oil from oil (like the one-way valves), those separating oil from outside air (like the gun piston seals), and those separating oil from compressed gas (like the floating piston).  A hydrospring system only has the first 2 types of seals.  If we look at the consequences of each seal leaking, the following happens:

  • If a seal separating oil from oil leaks, a negligible amount of oil will move around the mechanism- this is negligible compared to the normal flow of oil through the mechanism anyway.
  • If a seal separating oil from outside air leaks, the oil is under much higher pressure than the air, so oil will leak out, but air won't leak in.  There are oil reserves in the mechanism to compensate for the oil that leaks out, so this isn't a problem.
  • If a seal separating oil from compressed gas leaks, the oil and gas will slowly mix.  This means that not only will the compressed gas become less compressible because of the oil (and be a less effective spring), but the oil might become more compressible because of the gas in it (and compress when pushed instead of flowing to slow the gun recoil).  This is a serious problem.

So what makes the Puteaux mechanism so hard to make is likely that floating piston separating oil and compressed gas.  It can't be allowed to leak under any circumstances, which means it would need those high tolerances to form a perfect seal.  (This would also explain why Schneider guns were easily mass-produced in WWI despite also being hydropneumatic- their recoil mechanisms didn't have floating pistons separating the oil from the compressed gas).

Some other articles and sources bear this out, stating the floating piston was the main secret of the Canon de 75's recoil mechanism.  This article states: The floating piston was of particular interest to those wanting to copy the gun’s design because of the way it was sealed to prevent the fluid and gas from mixing. This was such an important detail that French artillery officers were forbidden to have any knowledge of it—in fact, they were not allowed to see the piston itself when it was disassembled from the gun. Various regulations were put into place to assure the secrecy of the 75’s internal mechanism.

More importantly, the US Army M1897 manual from 1942 has no repairs for the floating piston.  Most problems apparently have some fixes that can be attempted in the field, but only one problem involves the floating piston, on page 54: If the presence of air in the recoil oil is due to the escape of nitrogen past the floating piston, the recoil mechanism must be returned to an arsenal for repair.  Apparently the floating piston was so sensitive that it could only be properly maintained in an arsenal.

So, with that background, the importance of the manufacturing process for this gun should be clear.  I will end this post with one more quote, from another article about the Canon de 75 (by Robert L. O'Connell):  A break finally came when, under some­ what murky circumstances, the artillery officer training program received four worn-out 75s as training tools. The guns soon found their way to the Ordnance Department, where they were torn apart. After disassembling the guns, the army’s chief of artillery was telephoned and told that the secret of the 75’s recuperator was nothing more than incredibly close tolerances–no tolerances, actually. Each and every example had been handmade by French craftsmen, working with the precision of jewelers.  

Characteristically, the Americans chose mass production even after the French warned them it was impossible. It took time (in fact, the war was over), but the Singer Manufacturing Company, with ex­hausting attention to detail, finally man­aged to fabricate acceptable recuperators based on machine-made interchangeable parts. According to artillery historian Frank Comparato, the project “was one of the most complicated engineering tasks ever attempted…one of the marvels of modern industry.” Singer’s employees had every reason to celebrate. It is not known whether the French, when they heard of the feat, shared their joy.

 

It is this manufacturing process Singer developed that will be described in the next posts.

 



-- Edited by AN5843 on Saturday 7th of December 2019 12:12:02 PM



-- Edited by AN5843 on Saturday 7th of December 2019 12:12:42 PM



-- Edited by AN5843 on Saturday 7th of December 2019 12:13:32 PM

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Legend

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Secrets don't last forever. After WW1 the French Army published an "Atlas of Lithographs of the Canon de 75 Mle 1897".

The details of the floating piston and recoil system are shown in great detail.

http://gallica.bnf.fr/ark:/12148/bpt6k6555554m

Regards,

Charlie

 



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Corporal

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CharlieC, that is indeed an excellent source, and shows the mechanism well.  As mentioned below, that's actually one of the sources I used to confirm whether the article was actually about the Canon de 75's recoil system.

Below is the 1920 article about the manufacturing of the Canon de 75's recoil system by Singer.  It is from Volume 52 of American Machinist, and titled "Unusual Methods of Securing Extreme Accuracy," by A. L. De Leeuw.  It is a 4-part article which can be found at these sources:

Google Books source 1

Google Books Source 2

Internet Archive Source

Part 1 is on page 595, Part 2 is on page 937, Part 3 is on page 1049, and Part 4 is on page 1094 on any of those sources.  You can use those sources to read the whole article right away if you want.  

I have copied the figures from the Internet Archive source, as it has a better quality scan.  I corrected typos and errors, added metric values in parentheses in red text, and added blue letters and arrows over the original arrows and letters in figures (which made them easier to read).  The figures were moved to more suitable locations in the article (they are normally far from where they are mentioned in the writing), and I added some writing to the captions, in parentheses in red text, to make the figures easier to understand.

One interesting thing about the article (and why it was so hard to find) is that it never refers to the 75 mm gun- it only states that it is a recoil mechanism.  However, we can determine what gun the recoil system is for by 3 methods:

  1. The author, A.L. De Leeuw, is cited in several other sources (such as this one) as being in charge of Singer's efforts to manufacture the Canon de 75's recoil mechanism, and in some of his other writings he mentions first using certain milling operations (which can be seen in the article) on the 75 mm French gun.
  2. The article mentions the work was done at Singer, and the previously mentioned sources state that De Leeuw was the chief engineer there.  Upon checking contracts with the US Army during WW1 (page 60), it turns out that the only recoil system Singer made was the Canon de 75's.
  3. Just looking at the figures in the article and comparing them with actual diagrams/pictures of the Canon de 75's recoil system (mainly that on page X of CharlieC's link above) shows it to be identical.
  4. The tolerances mentioned in the article (0.0008 inches- 0.00025 inches achieved in practice) fit the reputation of the Canon de 75's recoil system for needing high precision.

So with those methods, it is clear that this article describes the Canon de 75's recoil mechanism.

 

And with that, I can start posting the article:

Unusual Methods of Securing Extreme Accuracy—I

BY A. L. DE LEEUW, M. E.
Consulting Engineer 

Accurate drilling and boring of long holes is one of the greatest of shop problems. When two holes must be bored parallel, the problem increases in difficulty. This article tells how the preliminary machine work necessary to the locating and boring of such holes was per formed. The illustrations show the problems clearly. 

Fig_0

 

(This image has no designation; I just refer to it as Fig. 0.  It shows the final shape overlaid on a rough forging, like Fig. 3 below.)

The problem of making an accurate recoil mechanism for gun carriages is a particularly difficult one, both on account of the extreme accuracy required in the component parts as well as owing to the very large amount of metal to be removed from the main, or cradle, forging. This main forging was supposed to weigh 950 lb. (430.913 kg) in the rough and 215 lb. (97.5224 kg) after machining; but, due to the lack of satisfactory heavy forging machinery, it weighed from 1,300 to 1,350 lb. (589.6701 to 612.3497 kg) and it did not seem possible to reduce this weight and still make a forging which could be completely turned up. Fig. 1 and some of the other views show the irregularities in the forging. The waves or irregularities were sometimes as much as 1 1/4 in. (31.75 mm) in depth.

The unfinished appearance of the trunnion lugs, and the curved outlines of the forgings can readily be seen. Fig. 2 shows the large amount of metal allowed, but it sometimes happens that even with this it was barely possible to true up the trunnion. Fig.3 shows the cross-section of the forging to which has been fastened a steel templet showing the cross-section of the finished part; the two holes shown are the result of the first operation.

Fig_1

 Fig. 1.  The irregularities of the forgings

  

Fig_2

 Fig. 2.  Surplus metal on trunnion

 

Fig_3

 Fig. 3.  Metal removed at various operations

Fear was expressed that, due to the removal of such a large amount of metal and to the delicate shape of the product, there would be great danger of twisting and warping of the forging after machining. It was found, however, that with the sequence of operations adopted, there was no trouble from this source. As a matter of precaution it was decided to leave a small amount of metal on the inside of the slide part to be removed after finish-boring; and, though this was regularly done, it was more than doubtful whether this operation was really necessary.

A LAYING-OUT MACHINE 

As the forgings came so rough and of such uncertain dimensions, it was necessary to lay out every forging very carefully before machining. This at first required the services of a skilled man and consumed a great deal of time. As the plant was originally laid out for a capacity of 25 finished recoil mechanisms, and required 30 of these finished forgings per day to allow for waste, a machine was designed to do this laying out quickly, accurately and with unskilled help.

This machine is shown complete in Fig. 4 with details in Figs. 5 and 6. It consists of a bed, and means of shifting the forging as desired and for finally clamping it in the proper position. To obtain this position, brackets were used which could slide in grooves in the bed, these brackets carrying feeler rods which could be placed by hand, in vertical or horizontal directions, against the forging. If these rods touched the forging in any position of the brackets on the bed, then the forging would have sufficient metal for finishing at all points. If one of the feeler rods, when placed over or along the forging, failed to touch it, then the forging was shifted horizontally or vertically, or turned around its axis, until the rod struck.

At each end of the bed a drill head was mounted, the head containing two drill spindles, each having an independent belt drive. The feed was by hand. The holes drilled in this position corresponded with the center of the holes which finally would have to be bored through the forging and were small enough so that it was not necessary to drill them with extreme accuracy. These holes were the starting point of all the subsequent operations. Fig. 5 shows the construction of the feeler rods and brackets. The method of locating the center for the trunnion parts by means of center punch is shown also. Fig. 6 shows the drill heads and jig brackets.

 

Fig_4

 Fig. 4.  Machine for laying out forgings.

 

Fig_5

 

Fig. 5.  How the machine is used.

 

Fig_6

 

Fig. 6.  Drilling the two pilot holes.  (Figs. 4, 5, and 6 show the same machine)



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I'm probably going to upload Part 1 in 3 posts- with the above post being the first.  Figure 7 in particular will probably make up most of the second post, as it's a huge image.



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On to the second piece of Part I:

THE LOCATING SURFACES 

The sequence of operations is shown in Fig. 7. The milling of the two guide strips at the bottom of the forging, operation 2, is shown in Fig. 8, where the piece is set up for operation. This operation was performed on a 24-in. (609.6 mm) milling machine with an inclined rail. The illustration clearly shows the jig and holding devices, and especially shows how the holes drilled in operation 1 were used for locating the forging for this operation. The cutting speed for this operation was 57 ft. per minute (17.3736 meters per minute), the feed 0.78 in. (19.812 mm) and the time for finishing a piece about 2 hr. Before the milling machines were completely installed, this process was done in a planing machine, requiring about 8 hr. per piece.

After this operation the lips thus milled and the surfaces thus obtained were used as gages or control points for further operations.

 

Fig_7

 

Fig_8

 Fig. 8. Milling the locating surfaces.

Fig. 9 shows operation 2A—rough-milling the sides and the stock around the end of one trunnion. This operation was not contemplated in the original lay out but the additional amount of metal of the forging made it necessary to remove some of the metal before the finishing of the sides. The method of holding the piece is clearly shown, including the hardened-steel plates against which the lips of the forging are located before milling. This operation was done on a No. 5 milling machine and required two settings, as the trunnion lug, which is practically in the center of the forging, would not allow of completing one side in a single setting. The milling machines were consequently arranged in pairs, one running right hand and the other left hand, so that one pair of milling machines could take care of a complete side. The depth of cuts varied widely with the forgings, being some times as much as 1 7/16 in (36.5125 mm). The feed was 1 1/4 in. per minute (31.75 mm per minute). The style of cutter used was the 8 1/4-in. (209.55 mm) high-power face-mill with an extra amount of projection of the blades beyond the body. Operation 2B is the same as the previous one but on the other side of the forging.

 

Fig_9

Fig. 9. Removing material from side. (The same is done on both sides)

Operation 3 is shown in Fig. 10 and consists of shaping the top of the forgings. This was done on a 24-in. (609.6 mm) horizontal machine with inclined rail and interlocking cutters. The cutters were kept as small as possible but were nevertheless 8 in. (203.3 mm) in diameter. This was caused by the fact that the arbor was 3 in. (76.2 mm) in diameter and further, that a deep section of metal had to be cut. In these cutters and in all gangs of cutters for heavy work, a cylindrical key, as shown in Fig. 11, was used. With this key no trouble was experienced in getting the cutters on or off the arbor.

 

Fig_10

Fig. 10. Milling the back of forgings.

 

Fig_11

Fig. 11. Key used in milling cutters



-- Edited by AN5843 on Wednesday 11th of December 2019 05:35:37 AM

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The third and final post of Part I:

WHERE A LIGHT CUT BREAKS CUTTERS 

Fig. 12 shows operation 4 which was done on a double spindle 24-in. (609.6 mm) machine with vertical heads, using a cutter which was 5 in. (127 mm) in diameter and 4 in. (101.6 mm) high. The illustration will show how extremely heavy this cut was. It also shows the peculiar style of stub arbor used, as it could not be expected that any ordinary kind of arbor would stand up under this heavy duty.

Fig_12_modified

 Fig. 12. A heavy milling cut.

 

By a misunderstanding of the instructions, the men in the shop divided this operation into two cuts, and invariably broke the cutter or arbor, or both, on the second cut. When, however, the entire amount of metal was removed in one cut there were no breakages. This was due to the fact that the metal to be removed lying immediately back of the body of the cutter prevented the cutter from pulling itself into the cut. It will be noticed that stops were provided on the cross-rail to keep the heads from moving under the cut. Two of these are shown at A and B. The illustration shows clearly how the pieces were clamped and how two pieces were done at one setting. It also shows the steel water guard around the table.

Fig. 13 shows operations 5 and 6 which are different only in the shape of the section of metal removed. Operation 5 was done with a relieved cutter held by means of a stub arbor and used as an end-mill. Operation 6 was done in a similar way except that the cutter was made in two parts and interlocking. The reason for this difference was that the surface made by operation 6 had to be corrected within narrow limits, whereas the surface made by operation 5 did not require such accuracy and the variation of thickness of the cutter due to sharpening would not cause trouble. It was found necessary to introduce an extra operation before operations 5 and 6. This operation is shown in Fig. 7 as 4A and consisted of beveling the end of the piece where the cutter is to enter. Without this precaution the cutter was apt to pull itself into the cut, though there was no danger of such a mishap after the cutter had once entered. 

Fig_13

 Fig. 13. Undercutting the sides.

 

A HEAVY MILLING OPERATION 

Operation 7 consists of removing practically all of the metal from the inside of the forging. This was done on a 24-in. (609.6 mm) horizontal milling machine, using interlocking cutters. As it proved to be extremely difficult to get sufficient cutters for this operation, and as the first cutters obtained were not correctly made, it was decided to split this Operation, for the time being at least, into operations 6A and 7.

It will be noticed that operation 6A can be done with ordinary milling cutters, which of course, could be made in a relatively short time, and without the use of a large backing-off machine. This left much less metal to be removed by the special cutters, and made it possible to do the finishing operation, No. 7, at higher speed and feed and with less wear of cutters than if the entire amount had been removed in one cut. However, if cutters had been available, it would have been more economical to take one single cut with cutters as in Fig. 14.

Fig_14

 Fig. 14. Milling out the channel.

 

Operations 8 and 9 are the rough-finish-milling operations along the sides of the piece and around the trunnion, Fig. 15. These operations are similar to operation 2A except that less metal must be removed and consequently a smaller cutter can be used. This operation was done on a No. 5 machine and as the table travel was only 50 in. (1270 mm) and the length of the piece to be milled from 69 to 70 in. (1752.6 to 1778 mm) it was necessary to mount the piece in a sliding fixture. Fig.16 shows the cutting of the lip of the forging by means of a relieved end-mill. This lip is made of very delicate shape and to men not versed in the mysteries of ordnance design, it would seem that the gun would shoot equally far and straight with a square lip. However, as the design had to be followed, the special milling cutter was used on a No. 5 vertical milling machine and the piece was mounted on a sliding fixture which is quite clearly shown. 

Fig_15

Fig. 15. Finishing the sides

 

Fig_16

 Fig. 16. Undercutting the lip

 

Operation 11 requires the removal of a considerable amount of metal; only 1/32 in. (0.79375 mm) was left on the inside of the forging for finish-planing. This can be seen in Fig. 7. The central guide running lengthwise of the forging limited the size of the cutter, as it was further necessary to make some little allowance for the sharpening of the cutter and the consequent reduction in diameter. This, on account of the forging, limited the stem of the cutter to not more than 1 1/8 in. (28.575 mm). As it was evident that an arbor of 1 1/8 in. (28.575 mm) in diameter could not possibly resist such a heavy cut, the arbor was made 1 1/8 in. (28.575 mm) only at the point where it passed the lip of the forging; while immediately after this point was passed, the arbor widened out and followed closely the outline of the forging so that the portion which was 1 1/8 in. (28.575 mm) in diameter was only about 3/8 in. (9.525 mm) long. This proved so successful that no breakages occurred.

This cut was taken in a No. 5 vertical milling machine using a sliding fixture. Operations 12 and 13 are shown in a general way by Fig. 17. These operations consist in milling off the end of the forging and then milling a gap through the center part. This gap was 2 in. (50.8 mm) deep at one end of the forging and 4 in. (101.6 mm) at the other. After these operations, the forgings were 69 in. (1752.6 mm) long while the length between the gaps was 63 in. (1600.2 mm). This operation was done on a No. 5 machine with an end-mill. 

Fig_17

 Fig. 17. Slotting the end

 

AN END KEY BEST HERE 

In doing such extremely heavy work it was necessary to drive the cutter with a key at the end of the mill rather than with a regular key on a stub arbor. The regular key has a tendency to split the mill. In all these operations the fixtures were provided with setting pieces which were 0.010 in. (0.254 mm) below the desired surface, and feelers with a thickness of 0.010 in. (0.254 mm) were used for setting the cut. In cases where it was necessary or advisable to run the machine while setting the cutter, a feeler of copper was used. However, this was avoided as much as possible for safety's sake. In all milling operations large amounts of cutting compound were flowed over the cutter.



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Here's the first post of Part II:

Unusual Methods of Securing Extreme Accuracy—II

BY A. L. DE LEEUW, M. E.

Consulting Engineer 

In this installment the very important operations of drilling and boring the two main holes in the cradle forging are taken up in detail. The methods used, the types of tools and the working limits are all discussed.
(Part I appeared in our March 18 issue.) 

The machining operations which are the most important in the schedule of operations on the cradle forging are the drilling and boring of the two main holes. The entire success of the mechanism depends on the accuracy with which these holes are bored. These holes are 63 in. (1600.2 mm) long, one is 1 9/16 in. (39.6875 mm) in diameter, and the other about 2 1/2 in. (63.5 mm). The variation permissible in any one hole is 0.0008 in. (0.02032 mm), though the permissible variation between the holes of various pieces is as much as 0.002 in. (0.0508 mm). However, this allowance was of no assistance as the pieces were supposed to be interchangeable. There was nothing in the mechanism which required the two holes to be parallel, but, as very close limits were set on the walls surrounding both holes and as the metal between the two holes was rather thin and had to be submitted to a hydraulic pressure test of 5,700 lb. to the square inch (39300.12 kPa), it was thought best to make the holes parallel whether they were required to be so or not.

In addition, the holes were to have a very smooth and mirror-like finish and were to be entirely free from tool marks and even the smallest surface scratches. The result of the following operations was that the variation in individual holes did not exceed 0.00025 in. (0.00635 mm), that the roundness of the holes did not in any place show a measurable variation and that the surface was sufficiently good to withstand the very severe tests to which the pieces were subjected, of which there were more later on.

The boring operations took place in the following sequence: Drill small hole; drill large hole; first bore small hole; second bore small hole; first ream small hole; second ream small hole; first bore large hole; second bore large hole; first ream large hole; second ream large hole.

It was the original intention to drill the holes on special 30-in. (762 mm) boring lathes. These lathes had a 10-hp. (7.457 kW) motor mounted on the carriage for turning the drill and a revolving steadyrest for the work or fixture to revolve in. Delayed delivery of these machines made it necessary to rig up other lathes, originally designed for boring, to do this operation.

The 30-in. (762 mm) lathes, however, were very well designed for this class of drilling and would have been especially superior for drilling the large hole. With both the work and the drill revolving, proper drill speeds could be secured and the time of operation shortened. In drilling the large hole only the work was revolved.

DRILLING THE 1 9/16-IN. (39.6875 mm) HOLE

A general view of the 24-in. (609.6 mm) heavy boring lathe with revolving steadyrest, with the fixture and recoil body in place and the drill in working position, is shown in the headpiece (Fig. 18). The spindle is driven by a 3-hp. (2.2371 kW) d.c. motor with pushbutton control and dynamic brake for stopping and starting and a field rheostat for speed regulation. The switch and the starting rheostat for the oil pump are shown mounted at the right.Fig_18

(no caption for this, but it is the headpiece and thus Fig. 18)

 

Fig. 19 shows the recoil held in the revolving fixture A which is clamped to the faceplate by the clamp B, while the other end revolves in the steadyrest C. The fixture is turned to the same diameter at both ends. The hole is-first drilled half way through from one end, then the fixture is turned end for end in the lathe without unclamping the recoil body and the other half of the hole drilled from the opposite end. The tools met within 1/64 in. (0.396875 mm) and often within a few thousandths.

Fig_19_modified

Fig. 19. Details of holding fixture (Cyan overlay of letters and arrows added by me for clarity.  Also the "Cradle forging" lettering was added to show where the forging was in the machine- but that is the only other addition I made to any of the photos in this article)

 

The fixture is revolved at 70 rpm. in a counter-clockwise direction, viewed from the carriage. The recoil body is held by the clamps shown in Fig. 19. The revolving steadyrest C and the end of the fixture A are seen in detail in Fig. 20. The bushing F is held tightly against the end of the recoil body by the two screws shown. The oil-tube drill G is fastened to the hollow drill tube H and revolved in a clockwise direction viewed from the carriage. The drill is guided in the guide bushing E and in the stuffing-box J.

Fig_20_modified

Fig. 20. How drill is driven and lubricated

 

OIL OUTSIDE THE DRILL

Oil under pressure enters the stuffing-box J as shown and passes through the drill tube in the guide bushing E. The drill tube is smaller than the cutting diameter of the drill. This allows the oil to pass into the drilled hole and around the tube and thence along the oil clearance and over the end of the drill. As the chips are produced they are carried back through the drill tube by this oil.

The drills are shown in Fig. 21 and were made of Rex AA high-speed steel. The shank section was left soft and the land above hard. If the drill is not hard at the land, it will seize in the hole and twist off in use. It is necessary to give the oil clearance L the shape shown in order to prevent the wedging of fine chips along the edges.

Fig_21_modified

Fig. 21. Some of the drills used

 

Another important point is the shape of the grooves for chip breaking shown at N. These grooves must have square sharp corners to properly split the chip into three parts. The shape and finish of the throat of the drill where the chips enter the hollow interior must be as shown and must be polished to prevent any clogging of the chips. The short drill shown at A is ready to be scrapped; it has drilled 102 holes. Figs. 22 and 23 show the method of revolving the drill tube H. The unit O is connected to the regular carriage by a forced bar and is fed forward along the bed by a feed screw inside the shears. This drags the carriage on which is mounted the 3-hp. (2.2371 kW) motor, controller and gearing as shown. The gearing is 10 to 1; the motor speed, 1,750, giving a drill speed of 175 r.p.m. This, added to the speed of the work, gives a total speed of 225 r.p.m. The maximum cutting speed of the drill is 80 ft. per minute (24.384 meters per minute). This drive was installed as a temporary expedient but proved entirely satisfactory. A cast-iron shearing pin, 5/16 in. (7.9375 mm) in diameter by 1 1/2 in. (38.1 mm) long, drives the drill tube. This pin is reduced in diameter to 1/4 in. (6.35 mm) by a neck 1 in. (25.4 mm) from the end and is easily sheared off in case the drill sticks.

Fig_22_modified

 Fig. 22. Driving the drill

 

The three-cylinder pump, shown in Fig. 23, driven by a 3-hp. (2.2371 kW) motor, supplies the oil. The tank connections to the pump contain fine meshed screens to filter out the fine chips. This shows the arrangement of troughs and the way in which the outlet A must always discharge into B, no matter where the carriage may be. The oil used was Houghtons' refrigerant base, 7 gal. (26.4979 L); paraffin oil, 50 gallons (189.271 L). 

Fig_23_modified

Fig. 23. Handling the drilling lubricant

 

The drilling data may be summed up as follows:

 

Imperial

Metric

Drill size

1.3594 in.

34.52876 mm

Drilling speed (total)

225 rpm

Cutting speed

80 rpm

Feed per revolution of spindle

0.0035 in.

0.0889 mm

Feed per minute

0.787 in.

19.9898 mm

Idle time, loading, turning around, etc.

1 hr. 10 min.

Cutting time

2 hr. 50 min.

Total time

4 hr.

Number of operators required per machine

1

Holes per drill average

48.3



 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

In drilling 726 holes, six drills were broken. Four of these breakages were before the adoption of the aforementioned cast-iron safety pin and the other two were due to the drills being worn so short that they had insufficient taper clearance at the shoulder end.

The large hole in the recoil body is drilled from both ends and the operation is very much the same as with the 1 9/16-in. (39.6875 mm) hole, the main difference being that the work only revolves. Fig. 24 shows the drill and the work. The same type and size of machine is used as for the small hole. The fixture is the same type and the function and design of the drill, stuffing-box and bushings, size and type of pump and the oil used, are the same in both cases.

Fig_24

Fig. 24. Drilling the large hole

 

The same points must be observed in making this drill as in making the small one. The details follow:

 

Imperial

Metric

Drill size

2.3906 in.

60.72124 mm

Drilling speed

80 rpm

Cutting speed

50 rpm

Feed per revolution

0.0014 in.

0.03556 mm

Feed per minute

0.112 in.

2.8448 mm

Idle time, loading, turning, etc.

1 hr. 10 min.

Cutting time

9 hr. 10 min.

Total time

10 hr. 20 min.

Operators per machine

1



-- Edited by AN5843 on Friday 13th of December 2019 11:08:40 AM

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The second and final post of part II:

BORING AND REAMING THE TWO HOLES IN THE RECOIL BODY

Both the boring and reaming of each hole are accomplished in one setting of the piece. The boring brings the holes parallel and in the correct position, and the reaming sizes give the holes the proper surface for lapping.

A modified French type of tool was used for boring, while for finish-reaming, a wood-packed reamer was developed by the Singer engineers for this purpose. Both these tools have the same bar and are pulled through the holes, one after the other. Two boring tools are first pulled through, each removing part of the stock left from drilling. The last boring tool drawn through leaves the hole straight and in the position required. The finish-reamer is then pulled through and the hole sized and left smooth. About 0.002 in. (0.0508 mm) is left for lapping and this amount is all that is required to remove the reamer marks.

Fig. 25 shows the small-hole boring and reaming fixture mounted on the stripped carriage of a 24-in. (609.6 mm) heavy-duty lathe. The recoil body is clamped in position by four brackets. The spindle nose driving the bar is screwed to the spindle and the tapered end of the bar is drawn in by two tapered keys.

Fig_25

Fig. 25. The drilling fixture used

BORING THE SMALL HOLE

Fig. 26 shows the modified French boring tool for pull-boring. The body R is made of Rex AA high-speed tool steel. The chip is produced at the cutting edge and washed by the oil flow through the slot in front of the cutting edge and out through the interior of the hollow body. The fiber washers S are fitted to prevent the chips reaching the bronze bushing T. The bushing T is ground about 0.0005 to 0.0007 in. (0.0127 to 0.01778 mm) smaller than the cutting diameter of the reamer and is free to turn on the body R. When the tool is revolved and pulled through by the bar, the body R revolves within the bushing. The fiber bushings S are ground the same diameter as the cutting tool.

The bushing T is made of forged bronze. The tool is fastened to the solid boring bar by the keys shown at K. This tool differs from the French cutter which is fastened to the boring bar by a female instead of a male taper. This was expensive to grind inside and not easily removed from the bar. The French tool is also without relief on the outside, back of the cutting edges, and also in the space which in the Singer reamer is occupied by the fiber collar. This caused the reamer to seize in the hole and break off. The Singer tool is backed off to a line as is any reamer and breakage entirely ceased after this type was introduced. The life of this reamer averaged 32 holes. Two of these tools are pulled through to straighten and position the hole. (The French tool is likely the boring tool used by the French to make their own Canon de 75s using the original manual method.  Apparently it was badly designed if this article is anything to go by.)

Fig_26_modified

Fig. 26. Some of the drilling tools

Fig. 27 shows the small reamer entered in the starting bushing at the beginning of its travel through the recoil body. The steel starting bushing is 0.0005 in. (0.0127 mm) above the size of the bronze bushing in the reamer and gives it a straight start. While working, the oil and chips come out of this bushing. In case the reamer is reduced in diameter by grinding, smaller bushings are furnished below the maximum size, in steps of 0.002 in. (0.0508 mm). Fig. 28 shows the coming-out end of the fixture. Bushing V is bronze. The oil is pumped in at this bushing as shown and the bushing is held tightly against the end of the recoil body. 

Fig_27

Fig. 27. Pulling the boring bar into the work

 

Fig_28_modified

Fig. 28. Boring bar just leaving the work

Details of the reaming operations are:

 

Imperial

Metric

First tool size

1.457 in.

37.0078 mm

Second tool size

1.552 in.

39.4208 mm

Revolutions per minute

90

Feed per revolution

0.014 in.

0.3556 mm

Reaming time

1 hr. 50 min.

Loading, changing, etc.

25 min.

Total time

2 hr. 15 min.

The-fixture-reaming is done in the same fixture as the boring, without removing the recoil body. The last boring tool is removed from the bar and the bar pushed through the hole until it protrudes from the rear. The wood finishing reamer is then keyed to the end of the bar and pulled through. No starting bushing is used, as the wood follows the bored hole, this wood being turned 0.007 in. (0.1778 mm) larger than the hole is bored.

The finishing-reamer is shown in Fig. 29. The body is made of chrome-nickel steel and the blades of a fine carbon finishing steel. Colonial No. 7 and Bohlers Gold Label proved well adapted for this purpose. After being machined and hardened the blades are lipped as shown in Fig. 30. The blades are then fastened to the bodies and packed out to allow for grinding to the correct diameter.

Fig_29

Fig. 29. The boring tool

 

Fig_30

Fig. 30. Grinding the cutting lip

Fig. 31 shows the blades in position, being circular-ground to correct diameter and tapered. The angle of this taper is 3 deg., followed by a short taper of 1 1/2 deg., 1/8 in. (3.175 mm) long. As the reamer is pulled shank first through the hole, the taper occurs on the end of the blade toward the shank. Fig. 32 shows a reamer being backed off on a Cincinnati cutter grinding machine No. 1 1/2. The edge is backed up to the circular grinding line. Fig. 33 shows the reamer being stoned. The soft copper bar in the toolpost is used as an indicator and the two blades stoned until they both register exactly. The point where the taper meets the straight part of the blade is very carefully stoned.

Fig_31

Fig. 31. Grinding outside of boring tool

 

Fig_32

Fig. 32. Grinding clearance on boring tool

 

Fig_33

Fig. 33. Honing and testing boring tool

In Fig. 34 is shown the wood packing being turned. The wood is hard maple, soaked in hot cutting oil until the oil has permeated the wood thoroughly. It is left in the oil until shortly before the reamer is to be used and the turning is done just before using. That part of the wood which extends back along the blades is turned 0.007 in. (0.1778 mm) larger than the blade and is tapered at the shank to facilitate entry. This supports the blades as the wood leaves the hole on finishing. The oil flows back around the bar and washes the chips away as they are produced, as with the boring tools. The wood is shimmed out several times with veneer and returned. The blades are set out and re-ground for every cradle.

Fig_34

Fig. 34. Turning the wood packing

After some experimenting it was found advisable to pull two finishing reamers through the holes. This was mainly to counteract a tendency to taper the hole as the reamer finished cutting and the wood got away from the supporting walls of the hole in the cradle. Care must be taken to break the comers of the entering end of the hole to prevent shearing of the wood. When the wood is about right, a squealing noise is often heard as it revolves.

The details of this reaming are shown in the accompanying table.

DETAILS OF REAMING (for the small hole)

 

Imperial

Metric

First reamer

Size of first finish-reamer

1.565 in.

39.751 mm

Revolutions per minute

55

Feed per revolution

0.0685 in.

1.7399 mm

Second reamer

Size of second reamer

1.572 in.

39.9288 mm

Revolutions per minute

55

Feed per revolution

0.050 in.

1.27 mm

Time changing from French tool to first finish-reamer, including gear changing

20 min.

Reaming time

17 min.

Change to finish-reamer

12 min.

Reaming time

45 min.

Removing work

15 min.

Total time two reamers

109 min.

(Final hole size after lapping is thus 1.574 in. (39.9796 mm).)



-- Edited by AN5843 on Friday 20th of December 2019 07:23:31 AM



-- Edited by AN5843 on Friday 20th of December 2019 07:31:55 AM

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Thanks for posting this extremely detailed information about the French 75mm mle/97 recoil

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You're welcome, I hope it is useful to anyone who really wants to know about that gun and the mass production achievement the recoil system represents.

And with that, on to the first post of Part III:

Unusual Methods of Securing Extreme Accuracy—III

BY A. L. DE LEEUW, M. E. 

Consulting Engineer 

The third installment continues the boring and reaming operations on the two main holes in the cradle forging and then takes up the machining of the trunnions with a description of the unique fixtures and special machines used. 

(Part II appeared in our April 29 issue.) 

 

The boring of the large hole differs from the small hole only in detail. Fig. 35 shows the boring tools. The body X is soft steel, Y is Rex AA high-speed steel, and fiber ring R is used to stop the chips, instead of having a full-diameter section below the chip openings as the French tools have. These tools are very satisfactory. Care should be taken to see that the slots in the cutting tool line up with the slots in the body to give a clear passage for the chips.

Fig_35_modified 

Fig. 35. Tools for large hole

Fig. 36 shows the circular-grinding of the tool and bronze bushing. The bushing is ground 0.0005 in. (0.0127 mm) below the cutter size. After circular-grinding the cutting edge is backed off to a line. The small-hole boring tools are ground in the same manner. A fixture similar to the one used for drilling is used for boring and reaming the large hole. One end is clamped in the revolving steadyrest on a 24-in. (609.6 mm) heavy-duty lathe as shown in Fig. 28, and the other end in a special bracket fastened on the stripped carriage and bored in position, as in Fig. 37. Fixtures of the type used on the small hole were found satisfactory in this case. Fig. 38 shows the boring tool leaving the work. Fig. 39 shows the method of fastening the large bar to the spindle of the lathe. The bar screws into the collar A which has a center fitting the lathe spindle. This collar is then held in the spindle by the capscrews shown. The large finish-reamer is like the small one. 

Fig_36

Fig. 36. Grinding the boring tool

 

Fig_37

Fig. 37. Boring tool entering work

 

Fig_38

Fig. 38. Boring bar and oil connection

 

Fig_39_modified

Fig. 39. How the bar is driven

The large-hole operation details, including removing the finished work from the fixture, are shown in the accompanying table.

THE LARGE-HOLE OPERATION DETAILS

 1st Bore2nd Bore1st Ream2nd Ream
 ImperialMetricImperialMetricImperialMetricImperialMetric
Size2.476 in.62.8904 mm2.572 in.65.3288 mm2.586 in.65.6844 mm2.596 1/2 in.65.9511 mm
Revolutions per minute909055 to 6030
Feed per revolution0.014 in.0.3556 mm0.014 in.0.3556 mm0.0685 in.1.7399 mm0.050 in.1.27 mm
Time of cutting55 min55 min.17 min.45 min.
Setting up, etc.20 min.20 min.25 min.45 min.
Total time70 min.70 min.42 min.74 min.

(Final hole size after lapping is thus 2.598 ½ in. (66.0019 mm).)

This method of reaming is very satisfactory for this class of work. The holes come very straight and parallel, true to size and remarkably smooth; 0.002 in. (0.0508 mm) is left for lapping and polishing. After this method of drilling and boring was started, not a single cradle out of the 800 was spoiled in this operation. As little as 0.002 in. (0.0508 mm) has been removed from a hole with the wood reamer. The three cylinder oil pumps used deliver plenty of oil at 80-lb. pressure (551.581 kPa). Care should be taken to screen or otherwise remove the fine chips. The boring operations are covered by operations 16, 17, 18 and 19. The lapping was not done immediately after boring but was made one of the final operations. This was due to the fact that it is of extreme importance to preserve the final finish of the holes and consequently reduce the handling of the piece after lapping to an absolute minimum.

Operation 22 shows the milling of the grooves and surfaces for attaching the cover plates. These grooves were on both sides of the piece, but on the side shown in operation 22, the grooves extend the entire length of the piece, whereas on the other side these grooves were interrupted. Operation 22 was done on a horizontal milling machine using two gangs of cutters, the pieces being set at the proper angle as clearly shown in Fig. 40.

Fig_40

Fig. 40. Gang milling for side plates.

And that's it for the first post of Part III, this part has a lot of photos, so it will probably be split into 3 posts, maybe 4.  The second post will cover the milling of the trunnions, and the third will probably cover everything else.



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The second post of Part III:

The next operation was the rough-turning and drilling of the trunnions. At this stage the trunnions were square blocks or bosses and it was deemed advisable to remove the large amount of metal still left after milling so as not to have to remove much metal at the finish-turning. This roughing operation was done on a high-duty drilling machine, the table having been removed and a special fixture substituted. This was really a hollow-milling operation, the hollow-mill being clearly indicated in Fig. 41. It will be noticed that the fixture can rotate on its axis so as to present both trunnions to the hollow mill. A number of other tools such as drills, reamers, taper reamers, etc., completed the equipment for this roughing operation and are shown in Fig. 42.

Fig_41

Fig. 41. Hollow-milling the trunnion

 

Fig_42

Fig. 42. Some of the trunnion tools

Skipping a few of the operations, we now come to the finish-turning of the trunnions. It was considered to be of extreme importance to have the trunnions exactly in line with each other. The peculiar shape of the piece and its great length make it very difficult to swing the piece around the center line of the trunnion on a lathe or vertical boring-mill. Besides, such a procedure would require two settings of the piece for tuning the two trunnions. Then, too, the method of hollow-milling both trunnions on a two-spindle machine would not insure perfect alignment of the two trunnions. For these reasons it was decided to build a special machine which consisted principally of a large pulley about 74 in. (1879.6 mm) in diameter as in Fig. 43. The pulley was made in halves, with the journals for bearings of the pulley entirely in one of the halves, the other half being merely a cover. Fig. 43 shows the cover as the upper part of the pulley.

In this position, the upper half can be removed by unclamping the swinging bolts which are clearly shown. The lower half is arranged as a receiving jig for the cradle as in Fig. 44. There are hardened bearing spots, adjusting screws, clamps, etc. A piece of work is placed on this lever half of the pulley and fastened, after which the upper half is put in place and the driving belt is thrown over the pulley. The machine is driven by a 7 1/2-hp. (5.59275 kW) motor with push-button control and powerful dynamic brake. There is, in addition, a foot brake of which the treadle is shown in the illustration. The motor is larger than is required for the running of the machine, but this amount of power is required for the starting up of the machine.

The trunnions of the pulley are hollow. On each side of the pulley there is a cast-iron stand on which is mounted a compound rest of an 18-in. (457.2 mm) engine lathe with a four-position tool block as seen in Fig. 45. After the piece is put in place, a smooth brass cover is put over the trunnion of the piece and inside of the journal of the pulley. This is for the sake of safety so that no projecting parts of the mechanism shall touch the operator or the tools and also to provide a non-changing field for the eye of the operator. An idler puts the proper tension on the belt and permits the belt being thrown off when lifting out a piece.

Fig_43

Fig. 43. Special machine for turning and boring trunnion.

 

Fig_44

Fig. 44. How the cradle fits in the machine. (This is the same machine as in the previous figure, seen from the front with the upper half removed; those bolts sticking up out of the sides hold the 2 halves together.)

 

Fig_45

Fig. 45. Boring the trunnion. (Again, this is the exact same machine as in the previous 2 figures, but from the right side instead of the left, and focused on the open milling section at the center of it.)

TWO OPERATORS CAN WORK SIMULTANEOUSLY

As there is a stand and compound rest at each of the two trunnions of the pulley, it is possible to have two operators work simultaneously. It is plain, of course, that there was no possibility of getting the trunnions of the piece out of line with each other. The only thing requiring attention was to have the axis of the piece at right angles to the axis of the pulley and this, being once established, would so remain during the life of the machine.

The rest of Part III will be split into 2 parts, just to keep the number of images per post down- most of these are just miscellaneous operations that don't fit into any specific group.



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The third post of Part III:

Operation 24 and 24A are the milling of the interrupted slots. These required several settings and the use of sliding fixtures, one being shown in Fig. 46. The fixture shifts so that the hole B comes under the index pin A. Operation 25 was the fitting of the cover plates to the sides of the cradle. Operations 26 and 27 were the drilling, counterboring, tapping, and the fitting of screws for these cover plates.

Fig_46_modified

Fig. 46. One of the small milling operations.

A FEW PLANING OPERATIONS

Operations 29 and 29 A were the planing out of the insides of the undercut portion of the cradle, 1/32- in. (0.79375 mm) being allowed there for finish. Fig. 47 shows the piece in position on the planing machine and one of the tools used. Operation 30 is the final finish-milling of the inside of the forging and confines itself to the lips and what is called the nerve guide. This finish-milling of the lips would not have been necessary if the original cutters had been entirely correct.

Fig_47

Fig. 47. Finish-planing the inside



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The fourth and final post of Part III:

Operation 31 is the finish-turning of the trunnions. Operations 33, 34, 35, 35A, 36 and 37 are all minor milling operations shown in Figs. 48, 49 and 50. These give a good idea of the fixtures, the cutters and the gages used for determining the different cuts. Many of these cuts were at various angles, some were in the nature of an arc, others required peculiarly shaped covers, but none of them presented any unusual difficulties. The drilling, counterboring, etc., of the various holes were also ordinary operations, but some of the jigs and tools used were of interest. Fig. 51 shows the fixtures used for holding the piece vertically. Fig. 52 shows a simple drilling fixture which utilizes the trunnion as a locating point. Fig. 53 is a tapping fixture which was found very convenient.

Fig_48

Fig. 48. Milling end, and showing gages used

 

Fig_49

Fig. 49. Milling fixture for end, showing gages used for setting cutters

 

Fig_50

Fig. 50. Another end-milling job

 

Fig_51

Fig. 51. Drilling small holes in the end

 

Fig_52

Fig. 52. Locating side holes from trunnion

 

Fig_53

Fig. 53. A convenient tapping fixture

Parallel to the axis of the piece, and on each side of the center guide, there is a taper track with a very small taper per inch, which runs into a short piece of straight track. This straight part is the lower part of the track, very much like a street running level for a certain distance and then suddenly going up hill. The changing from the straight to the taper part was sudden, making a straight line across where they met. This required a certain amount of hand-finishing at the junction of these two parts and, in order to reduce this hand-finishing to a minimum, the following procedure was adopted:

The operation was done on a No. 5 milling machine with vertical attachments and lengthened table so as to give 60-in. (1524 mm) table travel. First, the straight part of the track was milled out with an end-mill to within 1/64 in. (0.396875 mm) of its final depth. Then the jig was tilted to the proper angle and the bevel part of the track was finished. As the straight part was not made to its full depth, it was possible to mill the straight part further than it should go and yet not touch the plane of the finished bevel part, so that, when this bevel part was finished, there was only a little corner left, 1/64 in. (0.396875 mm) in height. This had to be removed by hand. Operations 40, 40A, 40B, and 41, 41A, 41B, and 41C were simple, presenting no difficulty.

And that's the end of Part III.  There are surprisingly few dimensions mentioned in this section, but many figures.  The final (but most important) section has only 9 figures, so it should fit into 2 posts.



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And moving on to the first post of Part IV, the final and most important part of the Canon de 75's recoil system:

Unusual Methods of Securing Extreme Accuracy—IV

BY A. L. DE LEEUW, M. E. 

Consulting Engineer 

The concluding installment describes the most important and difficult operation on the cradle forging, the lapping to finish size. The article ends with a brief account of the high-pressure hydraulic test to which each finished part is subjected. 

(Part III appeared in our May 18 issue.) 


The real trouble began at operation 42 which was the lapping of the small cylinder. Two thousandths of an inch had been allowed for this lap ping and proved to be sufficient to remove the tool marks of the finish-reaming. It was originally thought that it would not be possible to do this lapping entirely in a mechanical way and that a considerable time would be required to instruct and train men in the art of lapping. However, it was not thought practical to attempt to train men to a point where, by the mere skill of hands and eye, they would be able to produce both the extreme accuracy required and the beautiful finish which was said to be necessary.

The following method was adopted and proved to be entirely satisfactory for producing holes of the proper finish and of much higher accuracy than required in the specifications. It is believed that the success with the recoil mechanism as manufactured by the Singer Manufacturing Co. was largely, if not entirely, due to this method of lapping.

(Because of these statements and the previous other sources, this is likely the aspect of production that required the critical hand-polishing to incredibly high tolerances, with the precision of jewelers.  The lapping method is, therefore, the operation that was believed to be impossible, and the critical innovation that made the mass production of the recoil system possible.)

The lapping was divided into two processes, lapping and polishing. The lapping was for the purpose of producing a hole of proper size and roundness, the polishing for the producing of the mirror-like finish. The amount of metal removed in the operation of polishing was not measurable and was probably more in the nature of a burnishing operation without much abrasive action.

Two styles of machines were used for these operations, though the style which was used for the lapping would have been suitable also for polishing. However, the fact that some machines of the second style were immediately available, and that the first style was not absolutely required for polishing, was the deciding factor in making this division. The main difference between the two machines consisted in the fact that the piece to be lapped was held in the first style of machine in an indexing fixture, and in the second style of machine was held stationary. The first style machine is shown in Fig. 54 which shows the entire machine, and in Fig. 55 which shows the feed mechanism and the holding fixture. The feed mechanism was disconnected for short-stroke lapping.

Fig_54_modified

Fig. 54. The machine for lapping holes

 

Fig_55_modified

Fig. 55. Details of feed mechanism (In the actual article there is an error; the captions for Figs. 54 and 55 are switched.)

The machine consisted of a bed A on which was mounted the fixture B, in which the holding fixture C could revolve on trunnions D. At the outer end of the machine a motor E was mounted. This was a variable-speed motor of 10 hp. (7.457 kW) capacity, running from 600 to 1,800 r.p.m. but of which only the lower speed was used. The motor was reversing and the reversing mechanism F operated by the dogs H and the tappet G, Fig. 54, would reverse the motor in a small fraction of a second, so that it was even possible to use the machine on strokes as short as 18 in (457.2 mm). This was an extremely exacting duty of motor and controlling mechanism, but this part of the apparatus, furnished by the General Electric Co., stood up well and did not give any serious trouble. The motor shaft was directly connected to a screw I which had 3 1/2-in. (88.9 mm) lead, double thread, and would give a lapping speed of 175 ft. per minute (53.34 meters per minute). The lapping was not done, as is usually the case, by rotating but by reciprocating the lap in such a way that about half the length of the lap would project beyond the end of the piece at the end of each stroke. After each stroke the piece would be rotated through a small angle, the rotating mechanism being shown in Fig. 55.

AN INTERESTING FEED MECHANISM

The feed was pneumatic, a feed dog tripping the inlet valve so that the air can push the piston and rack to the right. At the extreme right the piston closes the inlet and opens the exhaust. A constant air pressure on the right side of the piston returns it to its position at the left, but before the end of the stroke the rack closes the exhaust valve to form a cushion. The movement is transmitted through a one-way ratchet and wormwheel with gears, with teeth so calculated as to avoid uniform positioning of the work at each stroke.

Fig. 56 shows the tools used for the lapping operation. A is the shell of the lapping head quite worn out; B and C are new shells for the large and small holes respectively. A is mounted on a head with taper wedges for expanding the shell by means of the nut D; E is the hollow bar to which the head is attached.

Fig_56_modified

Fig. 56. The kind of laps used

The lapping compound consisting of No. 4 F carborundum and vaseline was forced through the hollow bar by means of a little force pump. It was squeezed through the small holes visible in the shell and carried by the right- and left-hand spiral grooves of the shell to the work. As the piece and not the bar revolved, or rather indexed, every part of the hole was subjected to exactly the same action. The result was that the hole became round. The fact that all of the outside surface of the shell carried lapping compound made the action relatively easy.

After lapping, the cradle was washed in soda and hot water as shown in Fig. 57. It was then put on the second type of machine as shown in Fig. 58. In this machine the piece was held stationary, and the lapping bar reciprocated and was also indexed. The reciprocating motion was obtained by a planing-machine drive and the feed by means of ratchet pawl and dogs. In this machine the same style of lapping bar was used but with an entirely different lapping head. In this case the lapping head was made of aluminum and of the shape as shown in F and G respectively in Fig. 56. The stones H and I were placed in the recesses of the aluminum head, spread apart by the springs J and kept in position by a piece of twine before they were inserted in the hole. The stones used were made of the so-called "water-of-Ayre" stone. This stone is exceedingly soft and being absolutely free from grit, does not produce any scratching. Great care had to be taken, however, to wash all of the abrasive material out of the hole before going over to the polishing operation; a grain of carborundum left from the lapping might imbed itself in the water-of-Ayre stone and scratch the surface. The reason why it was not so important to have the work indexed during the polishing lies in the fact that this polishing did not remove a measurable amount of metal.

Fig_57

Fig. 57. Washing holes after lapping

 

Fig_58

Fig. 58. Machine for polishing the holes

In the original method of lapping, but which was not followed by the Singer company, a lapping head was used similar to the one used for polishing. Carborundum stones were used as abrasive material with thin oil as lubricant and the lapping bar was indexed while the work was held stationary. This was satisfactory so long as the two stones were in a vertical position, but when the stones were horizontal the weight of the head and bar would cause the stones to rub in the hole with a wedging action and gradually cause the center of the hole to drop. In other words they had a tendency to produce an oval instead of a round hole. In the machine which was originally designed for this purpose, the lapping bar was supported close to the work. This in the opinion of the writer may be classed among many other good intentions, and led in the same direction as good intentions are generally said to lead. This support caused the lapping bar to overhang very little at one end of the stroke and very much more at the other end, so that the tendency to wedge, and to cause an oval hole, was not the same during the entire length of the hole. The result was that the hole was not only out of round, but that it did not have the same section throughout.

The pieces which have to move into these holes have to be packed with extreme care and this packing must conform to the section of the hole. If the hole was oval but of uniform section, the packing might adjust itself gradually to this oval form and thereafter work satisfactorily. But if the hole is of non-uniform section, then the packing must assume a different shape every time it moves along the inside of the hole, and this would certainly lead to failure in the function of the mechanism. It was for this reason that it was thought absolutely essential to have the work rotate and to have the bar entirely free and without support.

And that is the most critical part of the whole operation.  The second post of Part IV, and the last post of the entire article, will cover some final minor operations, but this is what I found to be the most interesting and valuable part to take away from this article.



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Corporal

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And the final post, the second post of Part IV:

In addition to turning trunnions on the outside, they are also bored and threaded with the aid of special attachments and devices. One of these is shown in Fig. 59, where the trunnion is being reamed with the taper reamer shown. The fixture for holding the work by the outside of the trunnion is plainly shown, as well as the long bushing which guides the reamer into its correct position. Fig. 60 shows the taper reamers, as well as the tools for counterboring for the thread.

Fig_59

Fig. 59. Reaming the trunnion

 

Fig_60

Fig. 60. Tools for reaming trunnion

In Fig. 61 is another special machine built to mill the threads in the end of the trunnion, and also to line-ream the brackets which carry the sighting mechanism after it has been assembled in place. The thread-milling is done by the nearest spindle, while the one in the background reams the two holes to insure their correct alignment. After the cradle is completed, the holes, which are threaded at the ends, as shown in Fig. 62, must be plugged and tested to be sure the steel is of the proper strength. The kind of connection used is shown in Fig. 62, where the small hole is being tested. This is tested to a pressure of 5,100 lb. per square inch (35163.26 kPa).

Fig_61

Fig. 61. Milling trunnion thread and drilling

 

Fig_62

Fig. 62. Testing the forgings at high pressure

 

And that's the article on manufacturing the Canon de 75's recoil system in the US using mass production techniques.



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Brigadier

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I really only know of one person that has recently rebuilt a French 75mm mle/97 recoil system for live fire.  That is Matt Switlik.  This is a link to a YouTube interview with him about the French 75mm:  https://www.youtube.com/watch?v=JDz0uW2jex0&t=602s

This YouTube link shows the 75mm mle/97 in Matt's collection live fire with projectile and recoil:  https://www.youtube.com/watch?v=rl-dUgLIo0U

By the way, I have a French 75mm mle/97 but without a functioning recoil system. 

R/

Ralph Lovett

 



-- Edited by Ralph Lovett on Wednesday 25th of December 2019 04:04:28 PM

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