Tap-drill size (and why this needed to be done)

[rbh]

can anyone let me know what I need to drill a hole out to, to use a 1/2" course tap??

7/16???

So the reason this needed to be done in the first place was because I was impatient!!
I snapped 3 of the six heads off the blade holding devise on the tractor (blade tilt) and I needed to easy out them.
This bolt was the first one and I drilled a little sideways and by the time I got to the point to use some heat and the easy out I had made a hell of a mess.
So I attempted to square up the hole with a file, drilled it out to the 27/64th and tapped the hole.

I hope I got most of the new threads were the old ones were. :facepalm:

thanks

rob

 

[fat fanny]

Re: Tap-drill size

Re: Tap-drill size

1/2 -13 is usually 27/64

 

[Wills03]

Re: Tap-drill size

Re: Tap-drill size

http://www.imperialinc.com/pdf/I_DrillSizeDecimalEquivalent&TapDrillChart.pdf

 

[Pascal]

Re: Tap-drill size

Re: Tap-drill size

It's handy to have one of those charts that Wills03 posted the link. Sometimes tool stores give away cards or wall charts with the list on them.

In the absense of the chart, you can calculate the size.

for metric you use the size in mm and subtract the pitch of the thread........ A 6mmx1 screw needs a tap drill size of 5mm diameter.

For the SAE thread sizes it is similar. Tap drill size is claculated by using the decimal size equivilent and subtracting the amount of 1 divided by the threads per inch.


A 1/2UNC-16 bolt is calculated as 0.500" minus 0.062" or 0.438" which should be close to 27/64".

These calculations are meant to allow for 77% thread engagement.

If you have the 4 complete drill indexes of fractional, number, letter and metric, there shouldn't be many sizes of holes that you can't drill to a specified size.

 

[puddle jumper]

Re: Tap-drill size

Re: Tap-drill size

Check the side of the tap it will usually have drill size on it right below the tap size.

 

[Limited-Time]

Re: Tap-drill size

Re: Tap-drill size

It's handy to have one of those charts that Wills03 posted the link. Sometimes tool stores give away cards or wall charts with the list on them.

In the absence of the chart, you can calculate the size.

for metric you use the size in mm and subtract the pitch of the thread........ A 6mmx1 screw needs a tap drill size of 5mm diameter.

For the SAE thread sizes it is similar. Tap drill size is calculated by using the decimal size equivilent and subtracting the amount of 1 divided by the threads per inch.


A 1/2UNC-16 bolt is calculated as 0.500" minus 0.062" or 0.438" which should be close to 27/64".

These calculations are meant to allow for 77% thread engagement.

If you have the 4 complete drill indexes of fractional, number, letter and metric, there shouldn't be many sizes of holes that you can't drill to a specified size.

.438 is actually a lot closer to 7/16. For the record .4375=7/16 .4218=27/64 there's only .0157 difference in diameter, either should work for your tap drill size.

 

[Tim Frank]

Re: Tap-drill size

Re: Tap-drill size

.438 is actually a lot closer to 7/16. For the record .4375=7/16 .4218=27/64 there's only .0157 difference in diameter, either should work for your tap drill size.

That is because he divided by 16...as in 1/2-16UNC....should have been 13....then his formula works.