[cam53]

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

[neilw20]

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

[Geof]

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.

[rfwyzard]

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

[cam53]

yes tap is oversize

[Geof]

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.

[neilw20]

Sounds like an allowance for galvanized bolts.:withstupi

[Geof]

Sounds like an allowance for galvanized bolts.:withstupi

Some allowance, 0.015" to 0.02" on the diameter?

[neilw20]

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

[SumDumPhuckr]

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"?

[cam53]

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.

[KLUCAS4]

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

[rfwyzard]

Drilling oversize by the amout the tap is oversize is the solution. What material are you using? If it's soft like aluminum it's not as critical as if it's stainless steel or something even tougher. Also there's something called "First piece inspection". Self explanatory. With as many pieces as you are making you always allow for scrap.