3/4-32 minor diameter? And other fine thread questions.

OldMachinist said:
I'm trying visualize what you're saying so let me see if this is what you're doing.
You have some aluminum tubing with a .720 inside diameter and 1.25 outside diameter
You need to thread the ID and turn down the OD to ?
I'm not sure what you mean about spreading when tapping.
Maybe a sketch of the finished part would help.
Click to expand...

You got it. I want to turn it down as close to 1" as possible without making it a weak part. By spreading I mean some distortion caused by tapping a thin walled tube. What I am making are called "mods" in the e-cig world, basically a battery holder, sometimes contains some basic circuitry too. These are prototypes that I am getting paid a little to make so it's a damn fine excuse to buy tools. :))

The picture is of a typical mod. On the ones I am making I want a little coarser thread with the hope that it might be less likely to cross thread.
modthreads 001.JPG

modthreads 001.JPG
 
Put a Higbee on the threads to prevent/minimize crossthreading.
 
Higbee, I learned a new word. I suppose that is another reason to go with a coarser thread, putting a higbee on 50 TPI seems like a daunting task for a newbie.
 
I was just half kidding. It would be pretty tough with manual equipment and unaided vision, at least for me. It is similar to a blunt start, except there is a specified female radius between the minor and major diameters. Used most often on Acme and Stub Acme connections where heavy parts are involved with power make up and little chance to actually mate up the threads.

I knew I wouldn't throw everybody lol!
 
As a former firefighter, my immediate vision of a Higbee was the notch you find on one of the wrench tabs on a fire hose connection. When you line up the two connectors with the notched tabs together, the threads will engage as soon as you start to turn the connectors. Probably the same engineer worked out both to make threads easier to engage.
 
Yep, the way they work is that you jam the two connections together, turn them counter-lockwise while pressing them together. When the blunted ends pass, the joint slips deeper together, and you know that the threads are aligned. Then you tighten it.

In oilfield, where things are heavy, same principle applies.....drop the pin in the box, or box over pin with light pressure and rotate ccw until they "clunk" into place, then run CC to tighten. Not all API connections use this feature, but a few do, and most straight threads of Acme or Stub will take advantage of it.
 
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