Acme Thread MInor Diameters - Tap Drills

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I have never turned or tapped Acme threads and I won't be for the project at hand. I have a customer that request me to build some trunions for a screw jack. Said trunions are to be made from 2" x 2" bar 4" in length with a .750" x 1.5000" boss turned on each end. It gets interesting in that a hole is required in the center to accommodate an Acme tap for .750" 8 TPI Acme screw; unspecified G; unspecified hole size. UGH!

So, I go to my trusty Machinery's Handbook. Not much help there either. Next stop Google. There are as many recommended tap drill hole sizes as there were search results ranging from .5033" to .6375". That's a pretty healthy range.

So, what is it? I know my ignorance is missing something important here... (of course that is the basic definition of ignorance)

AS an aside, one chart indicated for a .625" 15 TPI the tap drill to be .0025"! Really? Two and a half thousandths?!
 
Acme taps are sure a strange breed. I have sharpened around 6 this week alone. Same dia. & pitch taps from different mfg's have different shank sizes. Looks as if your gonna need a spec.

Add in different chasers(acme) and threading inserts and confusion sets in quick. Here is a drawing from this morning.Acme Drawing 2.png

Acme Drawing 2.png
 
The thread bore should be .625 to .6313" diameter for a .750-8 ACME-2G thread.


In the download section, I have place a excel file that will allow you to calculate the different thread parts for any 2G fit Acme or Stub Acme thread.
 
The Tap Bore in the link I posted is .5917 for .750-6 - 2G. That is a lot less that .625. Which is correct?

"Billy G"
 
Bill,

You're correct for .750-6 Acme. He was asking for .750-8 Acme.

Easy Mistake.
 
Thank you for the clarification Ken.

"Billy G"
 
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