What causes hourglass shaped threads?

[P. Waller]

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]

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. Happens to the best of us! Hopefully just once. Always check tpi with your gauges after first cut. Saves time in long run.

 

[darkzero]

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.

 

[kd4gij]

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

 

[cg285]

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

 

[RWanke]

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]

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

 

[kd4gij]

@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.

 

[Silverbullet]

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]

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