Tap, thread and hole diameter question

[226bhp]

I have a 40mm diameter hole which I want to put a thread in, I have sourced a 42 x 1.5 tap.

Is the hole diameter ok for that size of tap?

[DVandrews]

Usually the hole is made to size required less thread pitch, so it is marginally too small, 1.5 is a very fine pitch for a 40mm thread.

Dave

[226bhp]

DVandrews said:

Usually the hole is made to size required less thread pitch, so it is marginally too small, 1.5 is a very fine pitch for a 40mm thread.

Dave

But thread pitch isn't thread depth, if it was then a 1.5 pitch would have a 1.5 thread depth and I would need a 43mm tap for a 40mm hole.
But I don't think it is, I found this on Wiki:



"Thread depth:

Screw threads are almost never made perfectly sharp (no truncation at the crest or root), but instead are truncated, yielding a final thread depth that can be expressed as a fraction of the pitch value. The UTS and ISO standards codify the amount of truncation, including tolerance ranges.

A perfectly sharp 60° V-thread will have a depth of thread ("height" from root to crest) equal to .866 of the pitch. This fact is intrinsic to the geometry of an equilateral triangle—a direct result of the basic trigonometric functions. It is independent of measurement units (inch vs mm). However, UTS and ISO threads are not sharp threads. The major and minor diameters delimit truncations on either side of the sharp V, typically about one eighth of the pitch (expressed with the notation 1/8p or .125p), although the actual geometry definition has more variables than that. This means that a full (100%) UTS or ISO thread has a height of around .65p.

Threads can be (and often are) truncated a bit more, yielding thread depths of 60 percent to 75 percent of the .65p value. For example, a 75 percent thread sacrifices only a small amount of strength in exchange for a significant reduction in the force required to cut the thread. The result is that tap and die wear is reduced, the likelihood of breakage is lessened and higher cutting speeds can often be employed."


I think I may just do it, but will do some maths, or phone a friend....

Edited by 226bhp on Tuesday 22 April 14:06

[Snake the Sniper]

According to the tapping charts I can find, a M42 x 1.5 requires a 40.50mm hole, and M42 x 2.0 a 40.00mm hole. How accurate is the 40mm hole you have? You could risk it, as reaming the hole out to 40.50mm will be a pain to do assuming you can find a bit that big.

[226bhp]

Snake the Sniper said:

According to the tapping charts I can find, a M42 x 1.5 requires a 40.50mm hole, and M42 x 2.0 a 40.00mm hole. How accurate is the 40mm hole you have? You could risk it, as reaming the hole out to 40.50mm will be a pain to do assuming you can find a bit that big.

It's a very accurate hole and in cast iron so i think I will get away with it.

My charts don't go that big, so thanks very much.

[jefword]

You don't really need charts that go that high. Just look at any metric thread that has the same pitch and angle and use the same o/d to tapping size difference.

A standard M10 x 1.5mm pitch shows a tapping drill size of 8.5mm

A difference of 1.5mm

Just subtract this from any o/d size you wish to tap.

In your case, as stated by others above it will be 40.5mm