# Tap drill size for metric threads?

My old Machinery's Handbook doesn't seem to provide a tap drill size for M14-1.25 hand tap (modern spark plug size). Maybe I'm losing my marbles, but
I can't find it. Is there a simple formula for determining tap drill size, given the metric size? TIA, Dave
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diameter - pitch = the size to drill M14 * 1.25 -> drill 12.75 table says 12.8
Nick
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The general formula for finding metric tap drill sizes is: Nominal diameter - pitch
In the stated case for a M14-1.25 thread that would come out to a metric drill size of 12.75 mm.
12.75mm is 0.502 in
I assume the general formula gives the tap drill size for nominal 0.75% thread which is considered practical for a strong thread in average machineable material.
To test this I converted 14mm and 1.25mm to their inch equivalents and calculated the inch size of drill for 0.75% thread. It came out to 0.503 in.
Bob Swinney

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No! M14 * 1.25 is a fine pitch. So you would be way off for standard pitch. Your (and other rules of thump using a simple factor) are simply useless. It is always NominalSize - Pitch. No matter wether fine or standard.
Nick
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No. Not over 0.001 inch. No use arguing about a hair.
But what's the right drill (according to your formula) for: (std) means this is the standard pitch, (fine) is one of the fine pitches.
M3 * 0.5 (std) M3 * 0.35 (fine) M6 * 1 (std) M6 * 0.5 (fine) M6 * 0.75 (fine) M10 * 1.5 (std) M10 * 0.5 (std)
Thanks, Nick
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Sorry Nick. I guess we have a problem with translation here. Pitch is distance between the crests of a thread. Pitch is also the reciprocal of the # of threads per unit measure; whatever that unit may be; inches, millimeters, etc. AFAIK, there is no specific definition of (std) and (fine). For the terms to be meaningful in any calclulation they must have a specific identity re. to the quantity to be calculated. That seems to be the case in the 7 examples you cited.
There is no "your formula". I merely changed mm to inches in order to use an "inch" formula I am familiar with and to illustrate the fact it comes out with only a tiny difference compared to the "metric' formula.
I believe the "metric" formula for tap drill size is: Basic major diameter minus thread pitch
The "inch" formula I used is: Tap drill diameter = nominal diameter - [ (1.299 x %) / pitch ]
where: Pitch is # of threads per inch and % is expressed as a decimal
I'll leave proof up to you.
Bob Swinney
wrote:

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If you leave out the pitch, it is standard. I just quoted the pitch for (your) convenience. Every (standarized) nominal diameter has a standard pitch.
Pitch (in metric) is the distance between two (neighboring) crests.

It is.

Ah, in the imperial way messuring the pitch. Too complicated for me. >8-/
Sorry for any confusion I may have caused.
Nick
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http://catalog.starrett.com/catalog/images/objects/2900/2880.pdf

but
size,
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Better yet
http://www.engineershandbook.com/Tables/metrictapdrill.htm
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Dan

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D Murphy wrote: ...

Which gives the drill size as diam - pitch (for those subtractively challenged).
Except for these: Thread Table Calc M4.5 x .75 3.70 3.75 M8 x 1.25 6.80 6.75 M9 x 1.25 7.80 7.85 M12 x 1.75 10.20 10.25
M4 x .35 3.60 3.65 M10 x 1.25 8.8 8.75 M14 x 1.25 12.8 12.75
I suppose that the calculated value is not a readily available drill size, hence it is rounded (some up, some down).
These must be typo's M8 x .5 7.00 7.50 M8 x 1 7.50 7.00 M16 x 1.5 15.0 14.5 M36 x 3 36.0 33.0 !! Pity the poor guy who blindly follows the table and uses a 36mm drill for a M36 thread.
Bob (who wonders why he doesn't go to bed instead of verifying tables that he'll *never* use)
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Simply:
OD minus pitch
Like:
14 - 1.25 = 12.75mm tap drill (12.8 is more likely though)
This works for inch and metric sizes, although you'll have to convert the inch TPI into pitch (1/TPI).
Regards,
Robin

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