drill size

[polecat]

just bought an american benjamin347 air rifle i need to fix the valve which as it is  american it's got funny thread sizes i've bought the tap to make the tool so i'm asking what size hole do i need to drill thread size is 9/32 x 32 tpi
thanks polecat

[vtsteam]

If you use rule of thumb that drill size is diameter minus pitch, that works out to 9/32 minus 1/32, or a 1/4" drill.

[mattinker]

Hi,

I did a web search for "drill size for tapping chart pdf" which led me to amongst other things:-

http://www.libertyfastener.com/Guide%20to%20information/Tap%20Drill%20Size%20Chart%20for%20Standard%20Threads.pdf

I think that the WWW is a good place to start with any kind of question like this.

Regards, Matthew.

[polecat]

thanks again lads
polecat

[vtsteam]

9/32"-32 is an odd size and doesn't appear on the usual online US drill and tap charts. Hence the rule of thumb. 

[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".

[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!

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]

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!

Example: 9/32 by 32 tpi:

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

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