# Thread: formula for drill size

1. ## formula for drill size

I have a 1/2-13 tap ground special with a .4715/.465 pitch dia.,need help figuring drill size?

2. ## 10.8mm

Or in medieval terms 27/64". for standard 1/2" UNC 13tpi.
BUT oops..
It may be smaller. What is with the regrind?

3. Here is my bushman's method for calculating tap drill sizes.

Find the decimal equivalent of 1/lead(pitch) in this case 1/13 = 0.0769"

Subtract this from the OD; 0.5" - 0.0769" = 0.4231"

Take the next fractional drill size up from this which is 7/16" = 0.4375" and you will get somewhere between a 65% and 75% thread; okay for most purposes.

0.4715" - 0.0769" = 0.3946" so you could choose between 25/64" or 13/32".

But all this is not necessary; the tap was ground down by about 1/32" so simply go down 1/32" from the standard tap drill size.

4. Pitch diameter on a standard 1/2-13 tap is .450, so your tap is oversize. Correct?

5. yes tap is oversize

6. Originally Posted by cam53
yes tap is oversize
Oops, misinterpreted by fast reading, I thought you meant 1/2" ground down.

Anyway my bushmans method will work for 13tpi no matter what the diameter.

7. ## Hmmm...

Sounds like an allowance for galvanized bolts.

8. Originally Posted by neilw20
Sounds like an allowance for galvanized bolts.
Some allowance, 0.015" to 0.02" on the diameter?

9. ## Rattle proofed.

At least it should be a nice fit in a 'clearance' hole.

10. 1/2"-13 2B Pitch Diameter is pitch diameter of 0.450 to 0.4565, or an average of 0.45325 with a tap drill size of 0.4219

So if your pitch diameter is .4715 to .465, or an average of 0.46825, wouldn't it make sense to have a tap drill size of +.015" larger @ 0.4369"?

11. yes ,thats what i thougth ,drilling it .015 to .020 larger . I have 1200 parts to tap wanted to be sure since i don't have go or no go gage.

12. Hey Pop, For that many parts why dont you just take the time to make you a go and no go gauge.

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