Metric tap drill

[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.

 

[Tapage]

well probably one of few that didn't know that too ..

 

[e9999]

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.

 

[GeologistFelix]

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.

 

[e9999]

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.

 

[nat]

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.