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drill size

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mattinker:
Steve your absolutely right! It is rare, I don't use imperial very often but I did find it!

http://www.tap-die.com/contents/en-uk/d297_Tap_Die_Company_Tapping_Drill_Sizes.html

Regards, Matthew

Manxmodder:
According to Matt's chart the tapping drill for a 9/32-32tpi is 6.1mm.
Using the rule of thumb diameter minus pitch 1/4" = 6.35mm which could be a bit on the slack side for a small diameter thread.......OZ.

mattinker:
OZ,

a difference of 25 hundredths of a millimetre, ie. 12/100mm per side, which would only show up at the root of the thread, isn't much! My point was more there is this extraordinary source of information which is the WWW, the reflex of asking questions through a search engine is so useful.

Regards, Matthew.

Lew_Merrick_PE:
I believe there is a document posted here somewhere titled Pilot Hole Considerations.pdf that I posted quite a while ago.  The traditional formula for UN 60° threads is: Pilot Dia = Major Dia - .975/TPI -- in your case, that would be: Pilot Dia = .28175 - .975/32 = .2508 (just to be obnoxious about it).  The paper I referenced gives you the ability to choose your Pilot Hold diameter based on the percentage of full thread desired (something that only rarely comes into play).

vtsteam:
Typical shop drill bits come in lettered, numbered and fractional sizes, with odd gaps between, so no matter if the calculation is worked out to a thou, you usually have to move to the nearest lower common size to drill a pilot for a fastener. Usually the tap cuts the difference.

Looks like the chart Matt shows is off, since I trust Lew on this one, and his .2508 is larger than 1/4".

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