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What causes hourglass shaped threads?

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jwmay

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#1
That’s about all I’ve got. I singlepointed several threads recently, and finished them all with a die. After the die, all of them were full size at both ends, and skinny in the middle. Any idea? I went ahead and used them,because they are of absolutely no importance. But at some point, I’ll have something I care about, and I want to avoid this moving forward.
 

Bob Korves

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#2
If the shaft is long enough and does not have support from a follow rest, the shaft will flex while you are cutting it from the tool pressure. It will move the material over, and the portion that gets moved most is the one with the least cut. Now, that is completely backwards from what you are describing, skinnier in the middle. Could it be from the ways being worn most in the middle of the cut? No, that would also make the middle fatter. OK, I'm stumped...
 

kd4gij

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#3
If you can post a pic it would help us help you.
How long was the threads and what size?
 

kd4gij

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#4
It sounds like the lathe cut a slight different pitch than the die.
 

jwmay

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#5
It’s 3/4” of thread, 1/2-13. I think it’s the die, but was just hoping someone else has experienced it, and could verify it from personal experience.

@kd4gij , would not having the the toolbit directly on center cause the pitch to be incorrect? I’ll admit I was a bit lackadaisical in setting up for screw cutting this time.
 

jwmay

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#6
I’ll have to take a picture tomorrow. I’m not nearby to the threads in question.
 

MarkM

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#7
Hard to say without seeing the operation so here are are a few thoughts regarding your hour glass thread. Cheap die and leading taper out of proportion and possibly the tailstock not used to keep die straight. Why not single point the whole thread?
 

kd4gij

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#8
It’s 3/4” of thread, 1/2-13. I think it’s the die, but was just hoping someone else has experienced it, and could verify it from personal experience.

@kd4gij , would not having the the toolbit directly on center cause the pitch to be incorrect? I’ll admit I was a bit lackadaisical in setting up for screw cutting this time.

No off center would not change pitch.
What lathe, What material, and what brand die?
 

jwmay

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#9
Grizzly G4002 12x24, ledloy steel, Mibro carbon steel die.

Why not do the whole thread? I don’t know. I assumed the die was more accurate than I was willing to be. In a hurry I guess.
 

higgite

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#10
The back end of the thread will be thicker because the die is tapered on the leading side, unless you turned the die around backwards to finish the threads to a shoulder. But, that doesn't explain the front end threads. You would think that everything that passed all the way through the die would come out at the same diameter. Unless.... did you measure the single point thread before hitting it with the die?

Tom
 

jwmay

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#11
I didn’t measure it. The only measurement I made was to ensure my major diameter was correct before beginning the threading operation. I crept up on a sharp V, and then tried the nut. The nut started, but only went about a thread and a half on. Then I used the die.
 

higgite

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#12
Throwing spit wads here to see if anything sticks. Did you make finishing spring passes when turning the pieces to major diameter and again when single point threading? Like Bob said in post #2, if the overhang is too long and not supported, you could be getting some flex on the end of the piece. If you aren't doing spring passes that might explain the front end being thicker than the middle. That is, if the outer end is turned to major diameter with no spring passes, the middle will be less than major. The die's taper could explain the back end being thicker.

Tom
 

Cadillac

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#13
I’ve had this happen awhile ago. When I was threading a solid 3/8 aluminum rod. I didn’t single point I had just ran a die over the rod about 3”. I had-chalked it up to the die not being center and square when starting??
I can see if you were using a center with part and a lot deflection on the cut. Close to tailstock cuts full depth of hen as you get to center you may be getting deflection and it clears up getting closer to chuck. What did the thread look like prior to the die? Was the die full of chips and maybe pulled some material in causing a deeper cut?
 

MozamPete

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#14
Did you initially turn the bar down to size at high speed?
If it is long and thin the center can be 'thrown out' with respect to the ends held in the chuck and the tailstock center, so that the major diamter would be narrower in the middle before you even started threading.
 

jwmay

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#15
DEC6CF62-2F82-4693-B492-393D0228CA9F.jpeg Here are the threads in question. More accurately, the middle threads are gone. Anyhow the bottom picture is the threads in action. Axles for new pneumatic tires for my little air compressor.
 

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RandyM

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#16
In my opinion, that is what a thread looks like when it has been cross-threaded. Was the die the same pitch as the lathe?
 

jwmay

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#17
The die was clean and possibly unused. The threads looked fine prior to using the die.

@RandyM I will doublecheck my change gear setting.
 

jwmay

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#18
@RandyM

I was going to post a picture of the change gear setting, but no reason to now. Apparently, I was single pointing 11 threads per inch. Boy do I feel stupid. Thank you! Better double check that from now on.
 

RandyM

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#19
@RandyM

I was going to post a picture of the change gear setting, but no reason to now. Apparently, I was single pointing 11 threads per inch. Boy do I feel stupid. Thank you! Better double check that from now on.
Just chalk it up to a learning experience and go from there. We all make these mistakes once in awhile.
 

ttabbal

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#20
Been there, done that.

I try to always test the pitch setting using a piece of PVC or similar to verify with a pitch gauge when setting up.
 

P. Waller

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#21
Torn thread often caused by chips in the die. Did you use a threading head or threading die or a "Thread Repair Die" as found in hardware stores?
If you spent the time to set up and single point a thread why would you then use a die afterwards rather then finish it?
 

Cadillac

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#22
Ive heard some will clean the thread up after to make sure it’s correct minor major. As you see in OP you cannot rethread or change a existing. :rolleyes: Happens to the best of us! Hopefully just once. Always check tpi with your gauges after first cut. Saves time in long run.
 

darkzero

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#23
Only time I got hour glass shaped threads is when someone with an hour glass shaped body walked by when I was threading. Luckily (or maybe not) that does not happen very often where my lathe is. :big grin:
 

kd4gij

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#24
I always, always make a very light pass and then check with a pitch gauge before cutting threads. Did I say ALWAYS?
 

cg285

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#25
i once did something wrong with threads - actually twice but the second time was metric so that don't count
 

RWanke

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#26
Reminds me of the engineer at the place I used to work. He sent out 6 rudder shafts to a shop because we where to busy to do it done in-house along with a print to build them. The end of these shafts are always drilled and tapped to put a lifting eye in for installation and on the print he specified to drill and tap 3/4-11. Well that's exactly what we got. Fortunately I was the one trying to install the finished rudders and could not get the lifting eye to thread into the shaft. I grabbed my thread gauges and checked and discovered the mistake, then I checked his print. If it had been one of the less "conscientious" employees it would have been cross threaded with the possibility of a heavy rudder shearing the threads out of the hole if it got hung up on the install endangering everyone around. I asked what we do about it and he was really flustered. I told him the quickest solution was to single point a special lift eye to go with the boat and make sure the engineer on the boat was made aware of it. I made the lift eye, installed the rudders, then painted and fancy labelled the lift eye and put a mount for it in the rudder room and made the engineer aware of the situation when they crewed the boat.
 

jwmay

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#27
Thanks to all for helping me find my mistake. In the future, I’ll be sure and be more methodical.
 

kd4gij

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#28
@RandyM

I was going to post a picture of the change gear setting, but no reason to now. Apparently, I was single pointing 11 threads per inch. Boy do I feel stupid. Thank you! Better double check that from now on.

I thought by your description that is what happened and your pic conformed it. You are not the first one to setup the wrong thread. I have done that on a quick change lathe, but caught it with the light pass.
 
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#29
Try to Ck a scratch line with your thread gage it'll tell if your off the size you want per inch anyway.
 

rock_breaker

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#30
You don't need another comment on 2 different threas on the same shaft. Don't ask me how I know.
Ray
 
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