Threads on Threaded Milling Cutters

[jackrae]

Out of idle curiosity, can anyone advise if the 20TPI threads on metric end mills is meant to be very slightly larger in diameter than the equivalent size on imperial mills.

I've found the thread o/d on a recently purchased 10mm endmill larger in diameter by around 20thou than that on an imperial endmill.

I am of course using metric collets on metric endmills and imperial collects on imperial endmills (both for an autoloc chuck) and the endmills screw into their respective collets correctly.

[jasonb]

I'm 99% sure they are all 20tpi x nominal shank diameter so your cutter will be 10 x 20tpi and as 10mm is 0.47mm bigger than 3/8" that accounts for your about 20thou bigger.

[jackrae]

Yeh, that makes sense.
I'd incorrectly assumed that the thread details with respect to diameter were identical for both metric and imperial, but obviously not so.
This all came about when I obtained a set of imperial 20TPI taps to clean imperial collect threads and wondered why they didn't suit metric 20TPI collet threads.

[jonkett]

Presumably we talking about Clarkson type threaded shank milling cutters. The threads are 20tpi whitworth , but metric cutters have metric shanks and imperial have imperial shanks.