Metric Thread Forms

[Charles Lamont]

That's expected as these thread profiles specify radii on crests and roots. They are not optional. Aren't these threads obsolete?

Virtually all standard 60° thread forms allow sharp roots and taps and dies are typically supplied this way.

Well, I have examined some of my metric and UNC taps too. They all have
radiused or, in the case of the German made ones, flat crests and mostly radiused roots. They are arranged to give clearance at the roots of the threads they cut. An M10 tap measured 10.10mm over the flat crests and a 1/2" UNC one was 0.507" over the radii.

No way do any of these taps have sharp crests. For one thing, if they did, they would have no strength and the sharp tips would soon start to crumble in use.

Equally, the longer his grinding wheel lasts between being dressed, the cheaper it is for the tap manufacturer is to grind the tap, so it is easier to grind a radius than a (sharp cornered) flat at the root. A sharp pointed root would be virtually impossible to grind.

 

[goldstar31]

Equally, the longer his grinding wheel lasts between being dressed, the cheaper it is for the tap manufacturer is to grind the tap, so it is easier to grind a radius than a (sharp cornered) flat at the root. A sharp pointed root would be virtually impossible to grind.

Quite right! I use a worn hand grinder disk to 'do' the female curvature on the 'top' of my rear parting lathe tool. Of course, the top is at the bottom- as the proper Thomas's- rather than the doubting ones will appreciate.

oops!:wall:

Norman

 

[mu38&Bg#]

Ah well, I admit defeat. I did find some that were sharp, but 98% or taps had flats on the OD. Only a few had true radii. The root is irrelevant on the tap as it does not cut.

For major diameter of external threads, I always shoot for the high end. If pitch diameter is important, I measure it. Otherwise figure infeed from actual OD to nominal pitch diameter.