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• # M23 thread - what Ã˜ pilot hole?

Discussion in 'Machining' started by GarethW, Sep 21, 2011.

1. ### GarethWChief ClickerStaff Member

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What diameter pilot hole should be used prior to tapping an M23 thread with a pitch of 1mm?

Material being machined is aluminium.

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3. ### PWASSWell-Known Member

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Gareth

The quick way to calculate the tapping size of a metric thread is to subtract the pitch from the diameter, so in your case it would be 22mm.

4. ### GarethWChief ClickerStaff Member

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That's great - thanks very much

5. ### kevinsmithukNew Member

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A 1mm pitch thread has a depth of 0.6134mm therefore the drilling size would be 2 x 0.6134 = 1.2268
This size should then be subtracted from the thread diameter, giving a tapping size of 21.7732mm ... which is 55/64" as near as makes no odds.
Drilling at 22mm would cause truncation of the thread although this may not be a problem for what you are making.

6. ### PWASSWell-Known Member

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All the engineering charts state the tapping size is 19.0mm for a M20 x 1 thread.

7. ### kevinsmithukNew Member

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The charts you use are actually incorrect and have been rounded up to the nearest size.
For the majority of design or engineering applications, this is considered to be close enough.

Having completed an engineering apprenticeship, manufacturing components for the aerospace industry and having spent over 30 years in that industry from shop floor to senior management prior to entering Further Education, I was always taught that near enough isn't good enough and if you're going to make something, make it right.

The drill size should be 18.77mm (18.75mm or 47/64" which is slightly smaller. Drills never cut smaller than their stated size and most often cut slightly larger due to inaccurate grinding)
Drilling with a larger diameter drill will cause slight truncation of the thread, which was never acceptable to my inspection department, however would probably be acceptable for most applications.

8. ### GarethWChief ClickerStaff Member

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Guys - thanks but it's M23 I'm looking for.

9. ### kevinsmithukNew Member

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See response #4

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