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Tap drill sizes

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Brian G

02-25-2001 15:27:38




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Does anybody else use this "quick and dirty" method of determining drill size for taps? Find a drill that will just fit thru a nut of the same size and thread of the tap you're using. Unless I'm doing some "fine" work where it might really matter, this system works just fine.




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Alberta Mike

02-26-2001 13:47:26




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 Re: tap drill sizes in reply to Brian G, 02-25-2001 15:27:38  
Some interesting stuff here (although I must admit that some of the math seems a bit involved). Meanwhile, I've noticed that the charts that have been mentioned refer to the fractional, numbered, or lettered drills, BUT when I buy a tap (say one in a blister pack) it always refers only to the fractional inch size and never to the more exact one which is usually lettered or numbered. Last week I bought a 6-32 tap for a carb application and it referred to a fractional inch size slightly bigger than the chart. I guess it's close enough right?

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wayne

02-26-2001 06:56:03




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 Re: tap drill sizes in reply to Brian G, 02-25-2001 15:27:38  
hey brian
I`m wayne as a machinist i usually use a chart because they are readily available
but if you don`t have one and you have a calculater you can use this
major dia. - (0.6495 * pitch)


pitch = 1/number of threads eg 5/8 - 11 - unc

11 is the number of threads
1 / 11 = pitch
=0.0909
so then 5/8 - 0.0909 = 0.534
or 17/32
hope this helps



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Paul J. Decker

02-26-2001 02:55:36




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 Re: tap drill sizes in reply to Brian G, 02-25-2001 15:27:38  
There may be times when the thread you're tapping is a special and is not on a chart. The following formula works for all sizes and pitches of 60 degree thread forms that are straight and will give the 75% thread that most charts list(no pipe threads, acme or oddballs like Whitworth)

ND = Nominal outside Diameter
TPI = Threads Per Inch
TD = Tap Drill diameter

Simplified formula (you can do it in your head with some pitches)
TD = ND - (1/TPI)

Accurate formula (try it with known sizes-it works on all of them)
TD = ND - (.98/TPI)

For example, 1/4"-20 the tap drill is #7 (.201)
TD = 1/4 - (.98/20)
TD = .250 - .049
TD = .201

Simplified method works real easy with 16 threads per inch because you can just subtract 1/16" from the nominal diameter.

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Ray,IN

02-25-2001 20:03:02




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 Re: tap drill sizes in reply to Brian G, 02-25-2001 15:27:38  
I prefer to use my pocket chart, because my memory isn't what it was. I can pull it out and select the drill I need from the same drawer. Sure helps this old retired toolmakers tired eyes too.



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Brian G

02-26-2001 08:15:52




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 Re: Re: tap drill sizes in reply to Ray,IN, 02-25-2001 20:03:02  
Ray, I like your idea. Maybe I can carry it a step further. This is a good time of year for me to do this. I have the charts, so I can just sit down, "mike" up my drills to be sure I have the correct size for each tap and set up a tap/drill drawer. Then, when I want to tap a hole, the tap and the drill will be right there together. I'm sure I've got enuf extra drill bits, so I shouldn't have to use my tap drills for other purposes and vice/versa. It seems lately, the most obvious ideas have eluded me for years until someone else says something that jogs the cobwebs loose in the old gray matter. Thanks

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Al English

02-25-2001 18:08:04




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 Re: tap drill sizes in reply to Brian G, 02-25-2001 15:27:38  
Hi Brian, That method starts you out with a hole that smaller than should be used, and with most taps, will result in a weaker thread. But that's better than guessing too big, since doing that can give you threads that won't hold at all...Al English



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