How to determine a drill size for a thread.

Brento

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I need to tap a 1.035-40 thread in a square piece of aluminum. Is there a formula to figure out the prebore size
 
The simple method.
Major Diameter - the lead. This will get you in the ball park.
In this case 1.035 - .025 = 1.010
 
Guhrings calculator does not calculate the thread dimensions just brings up a list of standard ANSI threads.
The 1.039-40 thread is not listed in the standards and appears to be proprietary to an optics manufacturer, it is likely that this company is not keen on publishing the thread data.
 
Thats what i used as well to get a size. Funny thing is the thor labs is what the part is for lol
 
Exactly, I used the easy method and it yielded a minor diameter of 1.010 for a 75% thread fit, well within Thor's recommended sizes.
I have not used a tap drill chart in years, I just subtract the lead from the major diameter, this works for all 60° V threads.
 
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