In , on 07/05/05
at 05:46 PM, Dave Hinz said:

OK, I now know several ways not to do this. Sounds simple - 4 holes,
evenly spaced, around a circle 0.600" in diameter.

The center of the circle, I can find easily (lathe & center drill). The
edges are concentric to the center, by definition, because I turned it on
that lathe.

I marked the 0.600" circle on the face of the disk by lightly touching it
with a cutting tool on the lathe & turning the work around, scribing a
circle at the right diameter.

Now, how do I get the 4 points layed out properly from there? I can use
a center finder to get two of 'em, but how do I get the other two evenly
spaced from those points? Or, worse yet, what if I wanted 3, or 5 holes
on that circle, rather than 4?

Dave Hinz

Use a 4 jaw chuck to hold the work, then when you've scribed the circle,
use a vertical prop ( piece of brass bar/rod is ideal) between the
underside of the 9 -o-clock jaw & the front shears of the lathe bed. Turn
the chuck anti clockwise( is that counter clockwise over there?) to trap
the prop. Now use the cross slide to move the tool ( at centre height) to
scribe a radial mark intersecting with your circle at 9 o-clock & 3
o-clock. Rotate the chuck to the next jaw & repeat using the same prop.
Result 4 marks evenly spaced on the PCD. Use a 3 jaw chuck for three
holes, marking once per jaw. 5 holes - use a 5 jaw chu.... now you are
back to trig GG
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