29 or 29 1/2 degrees When Threading Barrels - Which is Best?

hepburnman · Mar 31, 2014

Hi- I have heard different recommendations for the angle to set the compond at, 29 degrees or 29 1/2 degrees, for threading of barrels and I would like to know which is best and also why (if possible). My first attempt at threading used 29 1/2 degrees, and the action fit real nice, but I am not sure if I might be affecting the strength of the threads some how by using 29 1/2 degrees (as measured with the compound rotated CCCW when initially being perpedicular to the work). I understand that others like to thread with the compound set at zero degrees (perpendicular to the work piece - or they advance the cross-slide only when deepening the threads) but I have settled on threading using the compound so please DO NOT comment here whether compound or cross-slide is best to use (this could be a separate post if you'd like).

Thanks much!

manualmachinist · Mar 31, 2014

In feed however you want to! But the last few passes need to be straight in to generate the full profile of whatever tool you are using.

Willie-Grad96 · Mar 31, 2014

Using the 29 1/2 degrees on the compound lets you keep all of the cutting on the leading edge of the tool. The last few passes taken on the cross slide will let the profile of the tool cut on both edges, giving you a true 60 degrees, as manualmachinist said. Its not really a matter of which is better.

Krutch · Mar 31, 2014

My understanding of 29 or 29 1/2 is that the leading edge of the cutter takes the most material and the trailing edge skims the thread to produce a better finish. How well that works depends on rigidity and tool sharpness. I have always tried for 29 1/2 but "eyeballing" is the only way I've set it.
If feeding by cross slide (i.e.. plunging in) it doesn't matter. Just that the cutter is square with the work.
It helps to take the last couple of passes at .001" and then one last pass with no infeed. No matter which infeed you use.
Some CNC machines have canned programs that have the cutter cut in steps that uses both edges by staggering the feed so both edges get even wear.

moonlight machine · Mar 31, 2014

I feed straight in, works fine, Who can set their compound accurately to the 1/2 degree with the scale on the pivot anyway.

Artv · Mar 31, 2014

hepburnman said:
Hi- I have heard different recommendations for the angle to set the compond at, 29 degrees or 29 1/2 degrees, for threading of barrels and I would like to know which is best and also why (if possible). My first attempt at threading used 29 1/2 degrees, and the action fit real nice, but I am not sure if I might be affecting the strength of the threads some how by using 29 1/2 degrees (as measured with the compound rotated CCCW when initially being perpedicular to the work). I understand that others like to thread with the compound set at zero degrees (perpendicular to the work piece - or they advance the cross-slide only when deepening the threads) but I have settled on threading using the compound so please DO NOT comment here whether compound or cross-slide is best to use (this could be a separate post if you'd like).

Thanks much!

If you have a graduated compound, set it to 29 1/2, if you are using a protractor, set it to 30. Blunt the tip of your tool just a little to keep from breaking the tip off.

Art

hepburnman · Mar 31, 2014

Artv said:
If you have a graduated compound, set it to 29 1/2, if you are using a protractor, set it to 30. Blunt the tip of your tool just a little to keep from breaking the tip off.

Art

I have blunted my threading tool's tip now in prep for my next practice attempt. I think that somewhat square or radiused thread-valleys may have some small benefit as it might reduce stress-risers here and also keep additional material on the threads to help strengthen their root. Just like it may be better to have flat-top thread points, maybe there is some benefit to have flat thread valleys. I am not saying there may be any real benefit other than maybe niceties but sharp inside corner, etc., are always a good rule to prevent. Thanks everyone too about the cross-slide infeed and small infeed towards the end of threading. That will be my next refinement!

manualmachinist · Mar 31, 2014

You should decide which thread form you are using and grind your too to conform to a standard. American national uses a flat, the unified form uses a radius. Which form is in the reciever?

hepburnman · Mar 31, 2014

manualmachinist said:
You should decide which thread form you are using and grind your too to conform to a standard. American national uses a flat, the unified form uses a radius. Which form is in the reciever?

Good question! I don't actually know. This is an 1885 Winchester Highwall reproduction action by Wyoming Armory. I believe they've tried to keep it as close to the original as possible so that parts interchange, so I am not sure if this would determine which thread-form they used, or if they even tried to keep this small aspect per the original? Might anyone know for sure which? When I cut my barrel threads I only cut them as deep as was needed to thread onto the action. This therefore left flat-top barrel threads. Not sure if mentioning this helps in anyway other than if this is a preferred method, I'll keep doing it!

Nick Mueller · Mar 31, 2014

29 5/17° work best.
Why? Two prime numbers like with 1/2, but 5 times more precise.

Nick

hepburnman · Mar 31, 2014

I guess I should have added that my Highwall threads are 15/16 - 16 TPI 60 degree threads. I just baically assumed it was American National Standard Unified thread. I found a spec that said the threading tool tip should be blunted .007" for this size thread. It is currently flat-faced but I see rounded is optional for the thread root.

Conrad Hoffman · Mar 31, 2014

When advancing with the compound, any angle up to 30 insures that the entire right wall of the thread is cut/shaved. Zero, for example, is the same as feeding with the cross slide. There are always tolerances in setting angles and grinding tools, so 29.5 or 29.0 is commonly used. It has nothing to do with the final accuracy of the thread. The tool is always mounted square to the part. You could use 27 if you chose to. In any case, choose based on finish and what the chip looks like. The logic is that the left side of the thread gets cut with positive rake and should cut well. The right side of the thread, unless you tilt the tool, gets cut with negative rake and is harder to cut well. Thus, feeding with the compound, you do most of the work on the positive rake side.

hepburnman · Mar 31, 2014

Nick Mueller said:
29 5/17° work best.
Why? Two prime numbers like with 1/2, but 5 times more precise.

Nick

I can do that! If you dont check it...

ewlsey · Mar 31, 2014

In the CNC world, we use 27.5 degrees. The carbide manufacturers tell us that it will greatly improve tool life over 30 degrees. The tool should cut a little bit on both sides.

hepburnman · Mar 31, 2014

Conrad Hoffman said:
When advancing with the compound, any angle up to 30 insures that the entire right wall of the thread is cut/shaved. Zero, for example, is the same as feeding with the cross slide. There are always tolerances in setting angles and grinding tools, so 29.5 or 29.0 is commonly used. It has nothing to do with the final accuracy of the thread. The tool is always mounted square to the part. You could use 27 if you chose to. In any case, choose based on finish and what the chip looks like. The logic is that the left side of the thread gets cut with positive rake and should cut well. The right side of the thread, unless you tilt the tool, gets cut with negative rake and is harder to cut well. Thus, feeding with the compound, you do most of the work on the positive rake side.

I have been using the Aloris AXA #8 threading tool holder with a 60 degree HS threading blade. This tool allows you to set the Helix angle to your thread which I did.

Not sure what everyone thinks of this tool but I was pretty saticfied with the quality of threads it cut on 416 S.S. This was on a short piece and not actual barrel 416 S.S. so I dont know if there might be a treatment difference between the two and my results might defer when I actually try threading my barrel. From the design of this tool it looks to be pretty rigid and the blade as well. I like the idea of a commercially cut blade as I do not have to worry about grinding the tool accurately myself. This tool can be sharpened by just grinding on the top. The quality of the side-flats do not change as you grind the top off. Because the top of the blade comes flat from Aloris I guess it could be ground for a particular rake I did not do this and swiveling the tool to the thread's Helix will produce top rake but I do not know if this is positive or negative rake or what's prefer. Like I said it cut well but I did not push it and kept the work well oiled. I am certainly open to recommended improvements to the use of this tool. I would also consider switching to a better tool but so far my resulst have been quite good.

Nick Mueller · Mar 31, 2014

If you do grind a different rake, the profile will be distorted (less than 60°). 8° already are detectable, up to 3° are OK.

Nick

Finegrain · Mar 31, 2014

manualmachinist said:
In feed however you want to! But the last few passes need to be straight in to generate the full profile of whatever tool you are using.

I don't think that's quite right. The cutter is a form tool, and will produce the same form regardless of how you infeed, as long as the infeed angle is less than 1/2 the included angle. The only difference is which flank gets more removed than the other. At 0* (straight in), each flank cuts the same amount. At exactly 1/2 the included angle, one flank gets the full cut and the other gets 0. Anything in-between varies the ratio, but not the form of the cut.

In practice, there are differences in surface finish between the flanks when infeeding at close to 30* (for a 60* thread) -- the flank getting the heavy cut may tear a bit. In this sense, infeeding straight in for the final couple passes can even out the surface finish, but the form is unchanged -- form is entirely determined by the cutter, as long as the infeed is less than 1/2 the included angle.

Regards.

Mike

WAS Jr · Mar 31, 2014

Hepburnman, how do you set the helix angle? I've got one of these but I have not used it yet. I don't get the impression from Aloris that the cutter should be anything but straight up and down. Thanks, Bill S

manualmachinist · Mar 31, 2014

Finegrain said:
I don't think that's quite right. The cutter is a form tool, and will produce the same form regardless of how you infeed, as long as the infeed angle is less than 1/2 the included angle. The only difference is which flank gets more removed than the other. At 0* (straight in), each flank cuts the same amount. At exactly 1/2 the included angle, one flank gets the full cut and the other gets 0. Anything in-between varies the ratio, but not the form of the cut.

In practice, there are differences in surface finish between the flanks when infeeding at close to 30* (for a 60* thread) -- the flank getting the heavy cut may tear a bit. In this sense, infeeding straight in for the final couple passes can even out the surface finish, but the form is unchanged -- form is entirely determined by the cutter, as long as the infeed is less than 1/2 the included angle.

Regards.

Mike

Not sure I understand what your saying about my statement being wrong. As you stated the angular infeed effects chip load on either side of the tool. For the OP's situation (especially since he is using the Aloris form relieved tooling) I don't think equalizing tool wear should matter, since touching up tool wear before finishing is relatively easy.
Regardless of how you get there, the correct geometry of the finished thread has the utmost significance, and a straight infeed or a spring pass that creates full form profile is needed.
I find it easiest to accurately control pitch diameter by leaving .001" or .002" on diameter , measuring and dial the finish amount straight in. This creates full tool form profile while controlling actual pitch diameter at the same time.

hepburnman · Mar 31, 2014

This tool has the ability to swivel the blade to the left or right depending on right or left hand threading and to set it to the particular Helix angle of your thread (I think thread-insert type tools might not have to worry about this because the insert is not very thick (deep)). This tool's blade is very deep (long) and you rotate it so the long-sides do not rub excessively against the sides of your threads that you are cutting. I hope this is the correct thinking. Once set, this tool has set-screws that will hold the blade at the angle you set. I set the angle by putting the tool with blade on its side in my milling machine vise and indicating along the side of the blade until I got the right deflection amount as determined by Trig calcs.

29 or 29 1/2 degrees When Threading Barrels - Which is Best?

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