What causes hourglass shaped threads?

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