Making Round Threading Dies

I used my bench mic and saw about 0.00175 difference. Machinery's Handbook does mention radial relief on dies.

I know that sharpening a tap, for example, by simply widening the gullet with a dressed form wheel, after a few grinds although it still will cut nicely, it will not gage. I surmise this is because of the loss OD/PD post grind.

Dick, I would think that the amount of relief on a small tap like that would be difficult to see without a laser mic or comparator. It doesn't require much to minimize friction in the tapped threads and very much would allow easy wandering of the lead portion of the tap.
 
In the 23rd MH, page 864 there is an illustration of what is referred to as the relief I am talking about. There are 3 drawings; No relief, Eccentric Relief, and Con-Eccentric relief. So I believe we are all correct, it just depends on the tap. I imagine due the the same principles at play, dies would follow a similar course.
 
I just checked some large new taps I have. I checked a 1-1/4" -7 and 1-1/4"-12 and the O.D. on each tap is round and not relieved at all, but, low and behold, the P.D. is relieved behind the cutting edge and it is definitely visible under 20x magnification. The flats at the crest get narrower at the trailing edge. Being that the O.D. is not relieved, it is the P.D. that is relieved. It could be that all taps are relieved at the P.D. it is just that it is hard to measure on small taps. I visibly checked a 1" 11-1/2 NPT tap and saw the same under magnification. I never checked with Machinery's Handbook on this but it would be nice to know the "definitive facts".
I think I just learned something new in my old age-"old dogs can be taught new tricks".

Dick
 
Today, I checked a couple of taps for relief, and here are the results:
I measured the PD on the 3/4"-10 tap, and even though it is not backed off on the OD, it measured .004 backoff on the pitch diameter.
I also measured a Chinese special tap, 1-1/8" - 8 , it measured .001 backoff on the OD and .004 on the PD.
Having measured and said that, I think for a homemade special tap, backoff is not necessary, it would drag some, but for limited use it would suffice, and filing a flat on the tops of the flutes as I read about would reduce the drag a bit. Also, cutting the flutes so as to reduce the land width would also reduce drag; I have some B&S tap and reamer flute cutters, they are not simple radii like one sees on modern commercial taps, and could be set to make a narrower thread land. In the 50 years that I have had them, I only ever used one twice, once to make a piloted reamer to remove condenser tubes, and to make a spiral fluted taper reamer for tie rod ends.
Peace and best wishes, Tony!
 
Your mention of filing flats on the crests reminds me of some of worst tapping memories I have. The material is BeCu. I would almost swear that the material is closing up on the tap behind it. Nearly impossible to tap any real depth in one shot, and sometimes even tougher just to get the tap out. Doesn't seem to matter whether is is by hand, CNC, or dedicated tapping machine. I hate that stuff. I learned from an old timer about modifying the tap though, and all the guys would come ask me to do it for them back when I had good eyes and steady hands. I would truncate the tap to about 50% thread depth, and use that for a roughing tap, then go with a full form tap right behind it. Helped a ton. As you may know, BeCu is a pretty expensive material, and broken taps are especially bad news when it is in a high-dollar part. Never had any success with roll-form taps in it. Not malleable enough in the heat treat we commonly used. Spiral taps seemed to do a little better in their factory profile, and I always thought, but never really proved, that it was because the torque required to drive them "unwound" the spiral just a bit and they grew on the OD just enough so that when you backed out they relaxed and went back to nominal size, and they didn't drag so much. It was just harder to tell when to replace them as they dulled. They are more difficult to remove if broken in a hole. And usually the engineers want threads about 2 turns past the bottom of the hole. I don't believe they make a tap for that. So a lot of hand tap finishing happened.
 
Been through the safety training Bob, and have called industrial hygentists from the state do some process studies. I'm well versed in dealing with it. In fact one of my best friends left NGK years ago and has his own distributorship and owns a couple of European mills where it is produced. So I have excellent sources for it and all the dangers that are known. Truth be known, it's not as difficult to deal with as most people believe. Now handling pure Be with bare hands is a no-no( and there are "windows" for Gamma tools that are pure Be because is is Gamma transparent), but unless you are welding it, the size of the airborn particles isn't much of a hazard. I rather like machining it in general......except for the tapping.
 
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