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John Lawson
06-11-2005, 09:08 PM
Using a 55 degree toolbit with sharp point, to cut a full depth thread of 12 tpi. what would the compound infeed be?
If it were a 29.5 degree infeed for a 60 degree thread form with a sharp pointed toolbit, it would be .072 compound infeed. What would it be at 27 degrees for a 55 degree thread form?

Paul Alciatore
06-11-2005, 09:53 PM
The 27 degrees sounds about right. I get 0.0918" in feed for a 100 %, sharp thread. Depending on the exact thread form you are cutting it will be somewhat less than this. I would start with a 0.069 depth and measure or gauge from there.

Paul A.

John Lawson
06-11-2005, 11:01 PM
Thank you, Paul.
Unfortunately, I do not have the luxury of measuring or gauging; the thread is actually an obsolete metric pitch somewhat close to 12 tpi. (It is for the thread tenon on a 98 Mauser barrel. and it must be depitched to screw it in, so I can't use the receiver as a gauge without feeling the resistance of depitching.)It can't be screwed in by hand while still in the lathe, but must be transfered to a barrel vise and a receiver wrench used. Therefore, I must cut to the indicated figure.
Perhaps I could use the three wire system, miking the tenon of the removed barrel and cutting to a similar reading as a sort of comparator.
But, then, I would possibly overcut for the amount of depitching that will take place.
I wouldn't worry this much for a hunting rifle, but this one is a bull barrel target configuration that must shoot to sub moa over very long ranges.

[This message has been edited by John Lawson (edited 06-11-2005).]

precisionworks
06-11-2005, 11:12 PM
You might want to cast the receiver threads with low-temperature casting metal (CERROSAFE CHAMBER CASTING ALLOY from Brownells http://www.brownells.com/aspx/NS/store/ProductDetail.aspx?p=384&title=CERROSAFE%7e+CHAMBER+CASTING+ALLOY )


------------------
Barry Milton

Norman Atkinson
06-12-2005, 02:10 AM
I've been prattling about some British books.

Asquith has been doing the same.

This is all in Geo Thomas

In the Model Engineers Workshop Manual there are two designs for retracting screwcutting tools. Again, there is a rather nice handle to "mangle" the mandrel round when cutting funny threads.

For my part, the book saves on what hair remains on top.

Norman

DR
06-12-2005, 07:06 AM
Been in the business for close to 30 years....don't recall ever hearing the term "depitching".

What does this mean?

mcassill
06-12-2005, 08:33 AM
I don't recall seeing anything about it in the Kuhnhausen Mauser manual either. The people I talk to who build match rifles for a living haven't mentioned it either. What they do tell me is that on makes sure the two torque shoulders are parallel and perpindicular to the receiver ring threads, cut the barrel shank to match that dimension (or a couple thou short, or for a couple thou of "crush", depending on who you talk to) and then torque barrel in place. No mention of "depitching" anywhere in this. For another matter, every set of Mauser blueprints I've ever looked has specified receiver ring and barrel thread pitch as either 1.100"x12 tpi or .980"x12 tpi, depending on whether its a large or small ring action (or the large ring Turk I have with small ring threads cut for whatever reason).
Mark

nheng
06-12-2005, 09:27 AM
DR, looked it up in my modern dictionary and here's "depitching" used in a sentence; "Went to the game last night and depitching was bad".

John Lawson
06-12-2005, 11:10 AM
To depitch" a thread is to forcefully cross thread it. The more common term is cross threading, but depitching goes farther than simple cross threading, which usually stops when some person yells out, "Hey, there's something wrong with this screw!"
During the 1890's, Germany used a now obsolete metric system. The Mauser was designed with all metric dimensions.
When thousands of Mausers were brought back as war trophies, American gunsmiths discovered that the barrel tenon threads were very close (but not identical with) 12 tpi pitch. So, for generations, gunsmiths have been depitching Mauser threads to seat the barrels. This usually does not cause problems, since the majority are used as hunting rifles, and if you have ever been on a public range during sighting in days and heard the great cry arise after a box and a half of ammo has been expended "Close enough!"
If you find a drawing of a Mauser barrel with 12 tpi, it is technically incorrect, but is considered by most to be "good enough." When it is reefed in against the shoulders it will usually be just fine for the intended use.
I'm trying to get a thread that, when depitched, will not leave gaps that could interfere with the proper harmonics of the bull barrel I'm installing. This could be over or under the indicated infeed for 12tpi.
Since nobody even knows what I'm talking about, obviously, I will have to muddle through by guess and estimate.
The Kuhnhausen books omit a great deal of vital information. Matter of fact, you could fill another volume with what he leaves out. This can be maddening when you order a book to find the answer to a specific question and find that he has totally ignored the problem because he cannot provide an answer.
There are American made castings available for a very well designed retracting tool holder. On a coarse thread, in back gear, I don't have a need for a retracting device...I do it with a flip of the wrist. It works.
Number of years ago a draftsman made a set of very authentic looking blueprints for the 1911 pistol. Bought a set. Found the hard way that he had AVERAGED the dimensions from several specemins of commercial and G.I. pistols. (I ended up with a very average pistol) Another case of "About yea long..."
I've been a gunsmith since 1946. I've been on the ragged edge of total distraction ever since.
A few months (or years) ago, an author of an article in either the HSM or MW mentioned the mauser thread and gave the exact metric pitch and sited the depitching necessity. I cannot find the article. It may not have even been a gun related article. Would the author please step forward and provide more information?

[This message has been edited by John Lawson (edited 06-12-2005).]

mklotz
06-12-2005, 11:18 AM
Draw a side view of the thread triangle and it's obvious that:

D = (P/2)/tan(27.5)

D = thread depth (sharp crest to sharp root)

P = thread pitch (1/12" in this case)

Doing the arithmetic, D = 0.080".

D would be the amount to infeed directly, i.e., as measured on the cross-slide
feed. To obtain the compound feed, divide D by the cosine of the compound
angle. For example, if the compound is set to 27 degrees,

D' = D/cos(27) = 0.08/0.8910 = 0.0898"

Regards, Marv

Home Shop Freeware - Tools for People Who Build Things
http://www.geocities.com/mklotz.geo

John Lawson
06-12-2005, 11:30 AM
Thank you, Marv. Thank you gentlemen for responding; I think that some of us have learned something we didn't know previously.

Norman Atkinson
06-12-2005, 12:24 PM
I was ploughing my way through my Machinery- which is the 11th Edition!!!!!!!

Initially, I would agree that there are Legions of info.
However, this ancient tome contains the following

a German Metric Thread and a Metric Fine Thread- German Standard.

These differ from the French ones!

Could I have the Major Diameter
Minor or Core

From these, it may be possible to suggest
the possible figures.

What is given is that it is similar to the International Standard but the depth =
0.6945Xpitch and the clearance and root =

0.045Xpitch

The radius of fillet is 0.0633X pitch.

I have book marked the pages- to avoid an abnormally large intake of pain killers

Norman

DR
06-12-2005, 12:39 PM
So, if I understand what's been posted here, "depitching" refers to approximating a thread such that it's very tight fit. I do that occasionally for special applications where there's significant likelyhood of the fastener coming loose due to vibrations. Variable pitch threads and other methods are used in high vibration areas.

John L, you used the term cross threading. When I think of cross threading I think of threads that are totally screwed (pun intended) up for any practical use.

I guess I don't understand enough of what you're doing to understand why you don't cut the exact pitch needed. Yes, it might take some fooling with the gear train on the lathe, but it must be possible. (easy for me to say, since I cut threads on CNC lathes).

[This message has been edited by DR (edited 06-12-2005).]

Norman Atkinson
06-12-2005, 01:12 PM
DR gives a clue.

This is the 120-127 transposing gears to cut metric- or imperial.

I've been b******** about changing from a Metric Lathe and onto an old Imperial one.

Nothing to do with the question but after all, we are only doing simple arithmetic.
In my old Imperial- one set of transposing gears is based on 21- which multiplies to 123- and not 127 which is the PI approximation. Then, I have a 63 which gives
126- but not 127. Of course 127 is a Prime but still only approximates Pi. And then, if we all want to go further out on the branch- and fall off- our leadscrews are probably not 1/8th or 0.1250000000!

Sorry, John, my last incursion with firearms cost a dollar. It would fire damned near anything!!

Norm

quasi
06-12-2005, 02:02 PM
I have rebarreled a few dozen mauser 98's, always used 12 tpi 55 degree whitworth thread form.

Paul Alciatore
06-12-2005, 06:51 PM
John,

If I understand correctly, the 12 TPI thread is a slight mismatch for the original and you are using that mismatch to give it a very strong locking action to prevent the barrel from becomming unscrewed/detached from the receiver. Yet, you do not have the old barrel from which to take proper measurements of the thread. And you hope to be able to cut a thread that will fit properly on the first try because the receiver can not be used to test the thread while the barrel is still on the lathe. And you do not know the exact pitch or any of the diameters (OD, MD, or PD) of the original thread.

If you ask me, that is a tall order. I don't see how you are going to do it.

First, not knowing the exact diameter will severely limit your ability to even determine the OD of the barrel to start from.

Then, not knowing the exact pitch of the receiver threads will make it difficult to get a controlled amount of interference.

Finally, using a sharp Vee thread, which is a stress multiplier, on a high stress item like a rifle barrel sounds like a distinctly bad idea to me. And when you add the additional stresses from the mismatched threads...

If the 12 TPI is a slightly shorter pitch then it may work for you but if it is a slightly longer pitch then the threads will become looser as the stress is increased.

At the very least, I would consider putting a small radius on the cutting tool to relieve some of the stress there.

And I would definitely take some measuremente of the receiver threads before starting. Perhaps a variant of the three wire methoud could be used, with three small ball bearings and an inside mike. The measurement between three balls and the diameter of the balls should allow you to do a very good drawing of the thread in a CAD program. A casting, as Barry suggests, would allow a good estimation of the actual pitch.

Just some throughs. I'm not a gunsmith so take it for what it's worth.

Paul A.

Carl
06-12-2005, 09:16 PM
I think I would make a plug gage between centers (so you can take it in and out of the lathe) and perfect the fit on it (maybe using some blueing). When it's right measure it with thread wires and use that measurement to cut the barrel threads.

[This message has been edited by Carl (edited 06-12-2005).]

Spin Doctor
06-12-2005, 09:29 PM
This is my $.02. In any matters relating to fire arm perts that are threaded when making new new parts kind of close doesn't cut it. The screw leads should be the same (granted there may some discripicies between lathes) and the fits should also be the same. This calls for thread wires or micrometers used to check the original part

John going back and reading all of your posts (I think) as I should of do start with there is a way to come up with the exact gearing needed to cut the threads you need in Machineries handbook if you know the lead of the thread you want to cut versus the ratio of threads per inch of the lead screw on the lathe. In the past I have used it to cut threads of Hardinge HLV-Hs not in the Hardinge users manual

[This message has been edited by Spin Doctor (edited 06-12-2005).]

JCHannum
06-12-2005, 09:42 PM
There are two methods for accomplishing this, the simplest is to purchase a purpose made tap and chase the threads. This will produce a knowm thread.

http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=528686&utm_source=nextag&utm_medium=cpc&utm_campaign=649

The second method is commonly used in blueprinting an action, and is singlepointing the thread. This is the better choice, as it is done to assure the thread is concentric with the axis of the action. This will also produce a known thread.

Use of a surplus Mauser action may not be the best route to follow, as the money saved over purchase of a good modern action such as a Winchester or Remingtom will give a much better base to build on.

http://www.bryantcustom.com/faq.htm#9