What causes hourglass shaped threads?

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.
 
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
 
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?
 
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.
 
DEC6CF62-2F82-4693-B492-393D0228CA9F.jpegHere 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.

D591830A-5F4A-4429-9163-F34751CBBDB4.jpeg

7EB6312E-131A-4060-ACB8-43FC4C53BC6D.jpeg
 
The die was clean and possibly unused. The threads looked fine prior to using the die.

@RandyM I will doublecheck my change gear setting.
 
@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.
 
jwmay said:
@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.
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Just chalk it up to a learning experience and go from there. We all make these mistakes once in awhile.
 
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.
 
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