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drill size

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mattinker:
Typical shop drill bits come in lettered, numbered and fractional sizes, typical imperial based! the Metric stuff is very use full!

You may well be right about Lew being right, I am advocating methods for people like me who suffer from 'Math anxiety", I don't have a "Math" reflex, I'm always looking for other ways first!!

Regards, Matthew.

vtsteam:
6.3mm then, or 1/4", whichever.

Back to post #2......if you don't have a computer nearby , and/or can't find a drill chart that covers the need for an odd sized imperial tapped hole, and you don't need to know what the minor diameter to the nearest thousandth of an inch is, but just want to get on with it....

Subtract the pitch from the diameter and Bob's yer uncle! :beer:

Example: 9/32 by 32 tpi:

9/32" - 1/32" = 1/4"

Lew_Merrick_PE:
The internationally recognized standard for tap drill sizing (known more technically as pilot hole sizing) is 75% of full thread depth.  My previously posted equation, Pilot Dia = Maj Dia - .975/TPI (which may also be "cast" as: Pilot Dia = Maj Dia - 0.975*Pitch -- for metric usage), is designed to provide exactly 75% of full thread depth.  A 75% of full thread depth pilot hole will create an internal thread that is nearly 88% as strong as a 100% full thread depth tapped hole (and the taps last a lot longer).  Thus, if the screw and the mating material have the same strength characteristics, the tapped hole's threads should start failing very shortly before the screw breaks!

There are many other considerations when doing a formal analysis of a screw/bolt and tapped hole design.  Many of these considerations must be taken into account in flight/space duty, medical, and other extreme application usages.  Few people ever run into this level of detailed analysis.  I get into it every couple of years doing load separation analysis.

Manxmodder:

--- Quote from: vtsteam on January 02, 2015, 09:28:01 AM ---6.3mm then, or 1/4", whichever.

Back to post #2......if you don't have a computer nearby , and/or can't find a drill chart that covers the need for an odd sized imperial tapped hole, and you don't need to know what the minor diameter to the nearest thousandth of an inch is, but just want to get on with it....

Subtract the pitch from the diameter and Bob's yer uncle! :beer:

Example: 9/32 by 32 tpi:

9/32" - 1/32" = 1/4"

--- End quote ---

Or 9/32 + 1/32= pilot dia and the tap will last forever  :lol: :lol: ....OZ

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