correct thread form

[hammers-n-nails]

ive been doing some reading on thread geometry this evening and realized that i have never considered a potentally important detail.

ill use a 1/4-20 UNC external thread as an example

is the 60° face of the thread supposed to be measured in a plane that is in line with the axis of the bolt, or perpendicular to the helix of the thread? in this case 11.31° if i calculate correctly

so as a practical example when single pointing a thread on the lathe should the cutting face of the tool which the 60° cutting edges are measured from be held level or at a 11.31° angle (downwards side left for right-hand thread)

i have always ground my tools so the cutting face is co-planer to the axis of the shaft with a 0° rake and have never had any obvious problems.

im pretty sure im doing it right but i thought it would be worth asking you more knowledgeable machinists about.

thanks in advance
Zach

 

[Ken I]

It's measured along the axis plane - as if in cross section.

The helix angle has nothing to do with it.

Having no top rake on a tool helps to preserve the geometry - if you grind the correct thread angle and then add top rake the actual cut angle will vary slightly (very slightly).

Refer to a machinery handbook for the required corrections for form tools. (I can't remember the formulae but can work it out form first principles when required).

For most practical purposed I shouldn't worry (someone's sure to shoot me down in flames for such a glib comment).

Regards,
Ken