Lotus Elan

Tapping Drill Sizes Rule of Thumb for UN threads

PostPost by: gerrym » Wed Mar 09, 2011 4:22 pm

Apologies to all of you who know this one...

For normal fitted threads, metric tapping drill size is just nominal size minus pitch (both in mm). Eg for M10 x 1.5mm, use 8.5mm tapping drill. Known this for years, no need to consult tables etc just quick mental calc.

What I didn't know until recently (I can hear some old timers chuckle) is that for UNC and UNF threads there is an equally simple rule of thumb formula: nominal size in inch - 1/TPI (fractions of an inch). Works a treat for normal materials, class of thread etc etc. Eg 1/2" UNF would need 1/2"- 1/20" or in 64ths 32-3 = 29/64" tapping drill.

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PostPost by: Higs » Wed Mar 09, 2011 4:37 pm

Gerry

Have I missed something here? Surely 1/TPI is just another definition of Pitch? Therefore the formula is the same for metric and UN threads?

Richard
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PostPost by: gerrym » Wed Mar 09, 2011 4:46 pm

Richard, yes and no.
YES :Both are 60 degree threads so yes the formula should be the same
NO: Its not normal to describe inch threads explicitly via pitch so sorry, wasn't obvious to me.. and in any case are your drills metric or imperial or do you mix and match?

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PostPost by: Higs » Wed Mar 09, 2011 5:15 pm

Gerry

Sorry to have added any confusion. All I was saying is that the rule of thumb for both is:

Drill size = nominal diameter - pitch

However, for UN threads, you have to calculate the pitch by using the inverse of the TPI.

Nothing desperately clever but merely math(s).

I tap holes so rarely that I would look it up on t'internet anyway!

No need to reply!

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PostPost by: alexblack13 » Wed Mar 09, 2011 6:31 pm

LOL.... I've a chart on the workshop wall! Faster than thinking right now..Didn't use to be though... LOL....

Cheers for that though Gerry...Many thanks..From all of us!! :wink:

Alex... 8)
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PostPost by: CBUEB1771 » Wed Mar 09, 2011 7:21 pm

alexblack13 wrote:LOL.... I've a chart on the workshop wall!


Exactly, I don't think I've ever been in a machine shop that didn't have a tap drill table posted on every wall. I've got mine up in the garage.
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PostPost by: bcmc33 » Wed Mar 09, 2011 9:36 pm

Since my apprentice days (over 500 years ago :)), I've always taken my ZEUS book as gospel. Having said that, I also have the metric revision at hand.
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PostPost by: alexblack13 » Wed Mar 09, 2011 10:39 pm

Hi Brian, Guys...

The zeus guide is the standard top pocket book of words. Always in the top coat pocket. Ahemm! Me being me I put it down now n then and cant find it. Hence the Myford givaway chart on the wall. :wink:

Great little bible in its own right..Never without it. Except when I cant find it!! :lol: :lol:

Have fun guys,

What stage you at Brian?

Al' ............
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