In article ,
Marvin W. Klotz wrote:

It's a lovely exercise in trigonometry...

For the benefit of folks who want the actual formula:

w = width of chuck jaws
d = diameter of workpiece
e = required eccentric offset
r = d/2
root3 = sqrt(3)
p = required packing thickness

if (w root3*e)

{p=1.5*e}

else

{p=1.5*e-r+0.5*sqrt(4*r*r-3*e*e+2*e*w*root3-w*w)}

The second case seems to involve the edges of two of the chuck jaws
digging into the workpiece. That would render the formula inaccurate:
if the edges are blunted they won't be where calculated, and if they're
sharp they'll dig in, so the workpiece won't be where calculated. It
might also damage the workpiece, although in some contexts that would be
completely acceptable.


--
Norman Yarvin http://yarchive.net