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#1
13th April 2010, 09:10 PM
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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
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Albert Haynes Roadster FAQ | Haynes Builder Locations Gallery, build thread in Lithuanian / via Google Translate. 
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#2
13th April 2010, 09:50 PM
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OH MY GOD
That diagram / drawing is awesome if not a little mind bending. 6.8mm then!!
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Sent from my iPad, from who knows where in the world I was building but I am not now!  Build Pictures Build Thread
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#3
13th April 2010, 11:16 PM
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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!
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#4
14th April 2010, 08:08 AM
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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
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