Making Round Threading Dies

Wouldn't it be easer to single point the thread on the screw?
 
benmychree said:
To my knowledge, the only button type dies that are relieved on the PD are tapered pipe dies, dies for straight threads are not relived on the PD; they would "shave" threads if forced forward or also if dragged back. Button dies are a very poor choice for cutting long threads from scratch; they tend to "camming", that is they start to wobble as the cut progresses, getting worse as the cutting continues. Cutting the thread by single pointing is the best way for accuracy, cleaning them up with a die for considerations of taper, well maybe, button dies are not known to give a particularly nice finish on a thread. The method of using a follow rest is a good one, but as the thread nears completion (in the case of a vee thread), there is very little bearing on the tops of the thread crest; with square or Acme threads this is not so much of a problem.
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John, I believe they must be to prevent excess "drag" while cutting and precisely to prevent cutting while being removed from the work. Unlike adjustable ring gages which are ground to 100% form through. I guess an experiment would give evidence. If someone were to grind the cutting edge using the holes to the point that the contact surface was only say, 1/3 the original radial or chordal length, we would be able to measure any change in the PD as cut on a sample part. Long ago I did some plaster impressions of a fairly large die to see how they could best be sharpened and that's how I remember the geometry. Of course it may have been a particular mfg or something due to the size. I have not looked closely at smaller dies. Die head cutters should exhibit the same geometry if cutting to depth with as little friction as possible is the concern since they are adjustable as well. I have some in the shop I can look at. I remember using a pipe threading machine with straight dies to make double ended B-7 studs, and you could see that they were relieved, especially on the larger ones. The last few threads are imperfect not only due to the lead in taper of the die. Of course on a machine or with a die head you get to open the action to remove them, but I believe that is so you can simply roll the workhead off the work without reversing, saving time and wear on the cutting edges. As always, I may be mistaken, but that's what I remember from some research 30-35 years ago.

Definitely agree with your comments re follower rest. Necessary evil guess. Same with cutting long sections of thread with a die. I have seen the effect of the camming you refer to. It can be quite severe, rendering a useless part.
 
Tony Wells said:
John, I believe they must be to prevent excess "drag" while cutting and precisely to prevent cutting while being removed from the work. Unlike adjustable ring gages which are ground to 100% form through. I guess an experiment would give evidence. If someone were to grind the cutting edge using the holes to the point that the contact surface was only say, 1/3 the original radial or chordal length, we would be able to measure any change in the PD as cut on a sample part. Long ago I did some plaster impressions of a fairly large die to see how they could best be sharpened and that's how I remember the geometry. Of course it may have been a particular mfg or something due to the size. I have not looked closely at smaller dies. Die head cutters should exhibit the same geometry if cutting to depth with as little friction as possible is the concern since they are adjustable as well. I have some in the shop I can look at. I remember using a pipe threading machine with straight dies to make double ended B-7 studs, and you could see that they were relieved, especially on the larger ones. The last few threads are imperfect not only due to the lead in taper of the die. Of course on a machine or with a die head you get to open the action to remove them, but I believe that is so you can simply roll the workhead off the work without reversing, saving time and wear on the cutting edges. As always, I may be mistaken, but that's what I remember from some research 30-35 years ago.

Definitely agree with your comments re follower rest. Necessary evil guess. Same with cutting long sections of thread with a die. I have seen the effect of the camming you refer to. It can be quite severe, rendering a useless part.
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Can you imagine how difficult it would be to relieve a button die on the threads, especially a small diameter coarse threaded die? Most die heads have their chaser threads at a clearance angle and a lead angle, such as the Geometric and J&L tangent and Hartness For straight threads none of them have the chaser cutting edge radial on center, always above center; only the chamfer is on center and does all the cutting, the un chamfered threads do the guiding and provide the accurate lead; the same goes for taps; mike the tops of the threads; there is no relief, other than pipe taps that take a jamb cut, same as pipe dies. If taps had relief, they might easily pick up chips and mar the (just cut) threads while backing them out, and the same would apply to solid dies. I have all three of those die heads, and have sharpened chasers many times; I know whereof I speak; you can "dust off" the cutting face id there is damage, but if you take too much off they will shave off the threads entirely; the bottom line is that threading tools that guide themselves cannot have clearance behind the chamfer or they will shave and make inaccurate threads. Again, I think the most telling test is to measure the tops of the threads on a (preferably) ground tap; there is no relief. I suppose measuring the PD with a thread mike would also tell the same story.
 
John,

A good point you make here! I have always believed that taps and round button dies were not relieved on the P.D. (I have been in the tool & die business all my life but I am not infallible and therefore I could be totally wrong). I have learned to never trust myself completely. I am going to put a thread mike to a four flute tap and see what I come up with.

Dick
 
I just checked a brand new 3/4-10 4 flute and confirmed that the OD is not "round". There are 4 distinct lobes with the peak at the cutting edge.

I'm certainly not saying it would be easy, in the home shop especially, but commercial tap grinding machines, and I believe die grinders do have the capability to grind such geometry.
 
old toolmaker said:
John,

A good point you make here! I have always believed that taps and round button dies were not relieved on the P.D. (I have been in the tool & die business all my life but I am not infallible and therefore I could be totally wrong). I have learned to never trust myself completely. I am going to put a thread mike to a four flute tap and see what I come up with.

Dick
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I look forward to your results!
 
Tony Wells said:
I just checked a brand new 3/4-10 4 flute and confirmed that the OD is not "round". There are 4 distinct lobes with the peak at the cutting edge.

I'm certainly not saying it would be easy, in the home shop especially, but commercial tap grinding machines, and I believe die grinders do have the capability to grind such geometry.
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I just did the same thing as you did, and detected no difference in feel on the OD of a new ground thread tap, the same size as you measured. Some taps need relief, like pipe taps and acme taps. I do not think taps for parallel threads need any relief. What I remember seeing in an old book was that one could file a flat on the tops of the threads, leaving a small margin at each end where the tap is fluted to reduce drag, however I have not seen one personally
 
Hi John,

I chose to check a 1/4-20 tap because having only two flutes there is more thread width to try to measure a difference if any from leading edge to trailing edge.. There was no discernible difference in pitch diameter from leading edge to trailing edge to within .0001". I also measured a 5/16-18 tap with identical findings. I don't have the measuring equipment to check a die but would assume the same to be true.

Dick
 
Tony Wells said:
I just checked a brand new 3/4-10 4 flute and confirmed that the OD is not "round". There are 4 distinct lobes with the peak at the cutting edge.

I'm certainly not saying it would be easy, in the home shop especially, but commercial tap grinding machines, and I believe die grinders do have the capability to grind such geometry.
Click to expand...
How much difference in diameter do you think you saw?
 
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