What causes hourglass shaped threads?

On Tuesday I have to produce a 1"-10 RH internal Acme thread for a lead screw nut.
We do not have a gauge on hand nor a 10 TPI Acme external tool. I will have to make a gauge using the boring tool upside down on the outside of the part and measure over wires, this is not a fast way to do it.

Someone here will easily find the upper and lower PD dimensions I suspect.
 
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.
Click to expand...
I always make a scratch pass and double check the thread with my pitch gauge before proceeding. Sort of a bozo check if you will, we learn these things from mistakes. The key is to learn from it and not repeat the same error
 
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