Geometric die head threading problem

We have been running a used geometric die head in the tailstock of our lathe for a while now, several thousand parts at least. We're cutting 3/8 -24 threads on 12L14 material at 220 rpm. We're only cutting 1-3/8" of thread length that does not thread to a shoulder. The threads were turning out perfect, until we had a part that had what i believe are called tapered, shaved, or stretched(I read another post with the same issue that called them by this name) threads. Upon inspection, we found we had a chipped chaser. So we ordered 2 more sets of identical chasers and sent in the damaged set for repair. When we got all the chasers back and setup to run more parts, we began getting these "shaved" threads. After trying all 3 sets of chasers and sweeping the circumference of the die head with a test indicator mounted in the Chuck to check for proper alignment, we refered to the geometric die head manual and concluded that our used die head might be worn.

We then purchased a brand new Rubometric die head, which gave us the same results. The style of both heads is style 3/4 D. So neither of them are adjustable for alignment. We spoke to our chaser supplier to confirm that our 45 degree chamfers on the work are compatible with the 22 degree chamfers on the chasers as well.

So my question is, what else could be the problem? The thousands of parts we successfully ran on the original set of chasers, we didn't help feed the tailstock. During our troubleshooting we did however, with no noticeable results.

Please describe what you mean by "shaved" threads, or attach a photo of the problem. Or, point to the "another post", please. Thread form too narrow/spaces too wide? Crest-to-crest pitch too small? Crest-to-crest pitch too large? Major diameter too small?

You've identified the chamfer angle on the chasers. What is the hook/rake angle?

What sort of lathe do you have? More specifically, how heavy is the tailstock and how much force is required to pull it toward the headstock? Is it under positive motion control (CNC) or just a passive tailstock?

With answers to those questions, we might suggest some solid solutions. I will take a wild-assed guess, under some assumptions about your actual problem, and suggest that you make something similar to an extending tap holder for non-rigid tapping, and mount your die head on that holder rather than directly on the tailstock. Let the die head only have to pull the extending holder open, rather than drag your entire tailstock forward.

Chasers can be swapped into different blocks,moving the chasers around in the rotation. Just keep the chasers in numerical order. Starting at the top, 1,2,3,4, or 2,3,4,1 or 3,4,1,2, or 4,1,2,3. Hope this translated right.Worked in a screw machine shop and now and then all was not "plug and play."

Don't forget the $#!T Outta Luck setting...it's actually for smaller or larger threads. If you change the chasers you might have to reset that again. Looks like SOL on the DSA heads.

Looks like you've gone over the chasers to make sure they're the same.

What about your lathe speed ? Has that changed at all ?

Short of the dies being assembled wrong, my money's on dragging the tailstock, ........on a capstan (turret to you) I was taught to ''follow'' the die head, not let it drag or push it, or you will get bad - especially shaved flank threads.

One other possibility, though is very slim indeed, is a bad batch of material

Thanks for the responses folks, here's some more info. We were simply dragging the tailstock along. I'm starting to think we were lucky and need to help it along as suggested in one of the previous posts. Tailstock weighs 36.5 pounds, we use vactra No. 2 way oil. It slides very nice and smooth.
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Perhaps adding a turret to the lathe is the way to go or the extending holder to reduce the mass of the moving tool. Also suggested in earlier posts.
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10 degree hook, 22 chamfer on the chasers, 12L14 23/64” diameter round bar stock.
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We have checked and double checked the order of chasers.
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Speed range from 70-220 rpm.

Here is a pic of some of the results we've been getting.



Thanks for the help.

That is kinda a small thread to pull 36 lbs. A turret would make it easy peasy to keep up.Try to keep the pressure off the die head to let it do it's thing.

DrDan, Sir
Hey dragging the tail stock on a manual lathe is a duffers trick. The die head was designed for a turret lathe. Find a old turret lathe that meets your size criteria..
With that rebuke.. I'll try to help you.. I run a geometric die head almost daily. So assuming the problem is not with dragging the tail stock.. Here's some things to look for: dirt/chip clean the diehead by disassembly. Don't not use air to "blow it out" all that will do is embed chips in all the wrong places. Look for chasers that need lapped. There is a detailed procedure in the manual that describes the process. Long and short of that is look for one chaser that is "frosted" and lap it to the proceeding chaser. What you are fixing is a minor timing issue. Your springs may be worn-stretched and in need of replacement. One thing I've found is the pin that you unscrew to change chaser can raise a burr over time from snapping back when the head dis-engages.. Also what type of cutting oil are you using as that can have a major effect..
Hope this helps
Stay safe
Calvin B

As above, that thread is too small to pull a tailstock that heavy. What cutting oil are you using? (as above )
And why so low on the rpm? With any decent cutting oil, should be able to go faster.
I used the Geo die heads a lot long ago, if something goes wrong, it's usually operator error.

Thanks again everyone for the continued input. We use molly dee cutting oil just by brushing it onto the part before cutting. The low rpm speeds are due to the troubleshooting process. We typically run at 270 rpm, it's just a comfortable speed for us. We're also in the process of making a tool holder similar to the above mentioned suggestion about the extendable tap holder, just made to accommodate the shank on the die head instead. If that doesn't work, we will go with a turret attachment or even look into a turret lathe. We've already explored those options, but decided to try the homemade tool first just to help save on costs. Thanks again for all the help folks.

I wonder if you could "tie" the tailstock to the carriage and let that drag it with the half-nut? If your setup will allow you to use a tie bar that you can unhook easily it might help solve your problem and let the normal carriage work be done.

You don't need a turret, just mount the die head on a tool holder and feed your carriage by hand to a stop.

Dr.Dan, sir
Just for the record.. This is how threading with a die head has been done on turret lathes for the last 100 years or so.. ( maybe you can glean some insight from this or maybe it can help someone else down the road ) First turn your part slightly under sized ( pic is for 5/8-11 and is turned to .620 )and then chamfer the end where the die will start. You can even match the chamfer to the same angle as your die. These two operations have a 'centering' effect on the die. Slather part with dark (sulpherized sp?)cutting oil. I use ridged 'dark' as it is easily sourced at a local supply house. Run die head on part using two fingers on the turret wheel. ( or that is how the old geezer taught me in my wildly misspent youth )
After bottoming out on the stop let the die head continue on til it opens automatically while keeping a slight forward pressure on the turret wheel.. Back head off.. repeat til your arms fall off or the job is finished.
As for the economy issue.. turret lathes are cheap if your not in a hurry.. They don't have to be in perfect shape to thread.. The W&S #3 M-1200 in the pics is a 1941 machine that I swapped 1000 rds of .22mag ammo for. So unless you don't have the floor space you really can't afford not to have one. BUT I digress.
Stay safe and I hope this is helpful
Calvin B

Switch to a coventry die head and you can just thread std bright bar stock to its nominal thread, no need to turn your 5 thou off!!

I only have - use coventry heads, but the alignment needs to be near and they need to be fed, for a thread that size, i use about 2Kg of pull think about double what it takes to over come my turrets drag.

If you want to experiment, get some sutable nylon rod and try threading that, it will soon rip - strip if the feed force is not about right.

Not sure how it applies to your head, but the comment about chasers kept in the sets they come in is key, equally the heads slots - mountings all need to be clean, 9x out of 10 if im getting a shaved thread its crap in the chasers mounting area pushing it out of alignment in the head.