 # Metric tap drill

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#### HenryAZ

Here's something I never knew, probably all you folks already knew this but when I read it today I wanted to share with anyone who didn't already know.

To determine the tap drill size for a metric screw, subtract the pitch from the diameter.

For example, M10 x 1.5, tap drill = 8.5

I didn't believe at first until looking at a tap drill chart.

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that's because the threads have a fixed 60deg angle IIRC, so the difference between the 2 diameters is always proportional to the pitch and it's just about 1 pitch's worth.

If it's always 60 degrees, then here's the reason it should be (exactly) equal to the thread pitch:

The cosine of 60 degrees (or sine of 30 deg.) is exactly 0.5; multiply that by the two sides of the bolt, and you have (0.5 + 0.5) x thread pitch, which just equals thread pitch.

If it's always 60 degrees, then here's the reason it should be (exactly) equal to the thread pitch:

The cosine of 60 degrees (or sine of 30 deg.) is exactly 0.5; multiply that by the two sides of the bolt, and you have (0.5 + 0.5) x thread pitch, which just equals thread pitch.

I don't think that's quite correct based on the geometry. And if you neglect the "rounded tips and dips" as you seem to want to do you'd end up with quite a bit more than 1 pitch as difference between the 2 diams IIRC. But no matter, this is splitting hairs. There is indeed close to one pitch difference between the 2 effective diams IIANM, and the error is on the safe side if you do one exactly, so good enough.

That doesn't take into account the different thread classes. The smartest thing to do is look up in the Machinery's Handbook the min/max the tap drill can be for each thread fit.

If you guys want worthless trivia,

The difference between the even numbered screw sizes is .026"

So a #4 is .112 #6 is .138, etc.