Spud, it's not that simple. Both ends of the triangular profile are beveled, and the nominal diameter of the thread is not measured at the tips of the triangles (Dmaj+H/8), but the actual height of the external thread (Dmaj), so the math is a bit more complex. The drill diameter is Dmin here.



More info on wikipedia, as always: http://en.wikipedia.org/wiki/ISO_metric_screw_thread

OH MY GOD

That diagram / drawing is awesome if not a little mind bending.

6.8mm then!!

Get your self a copy of the 'Zeus' book. A brilliant pocket book full of all sorts of useful engineering info. Your overalls shouldn't be without a copy!

I know Alga but that calculation has worked fine for me so far, for bigger threads i just use diameter less Pitch and its worked okay, except for M18 fine thread which needs a starter hole of 16.5mm then 16mm.

Wikipedia is a really good page, equation you refer to is:






Too Early....AndyH

Just hammer a nail in there Ginge, that'll hold it...:D

Ginge, I have an MOT first thing in the morning, I need to spend a few hours in the office and expect to be home at about 3pm. I will PM you my phone number so you can call and see if I am free before coming over.
Dave.

sounds perfect to me