On Tue, 26 Apr 2005 19:43:10 GMT, (Marvin
W. Klotz) wrote:

On 26 Apr 2005 17:18:27 GMT, Norman Yarvin wrote:

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

Making eccentrics by shimming on a three jaw is at best a bodge. If one wants
to do it correctly, a four jaw is the tool of choice. The program is aimed at
newbies who still haven't equipped their lathes properly.

If they must use a three jaw, the technique of using a slotted tube is far more
accurate and safer. A program for this technique is included in the archive
that contains the program described above.

Regards, Marv
Marv,
I must disagree about the bodge. If a small offset, say .030", is
needed then shimming a three jaw is much faster. In fact I'd say up
about .125" offste is best accomplished with a three jaw. Of course,
I'm talking about adjustable three jaw chucks, where it is possible to
indicate the part. Sort of a 4 jaw, but much faster.
ERS