Dave Hinz wrote in
:

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?

You have to use trigonometry, or a protractor. Then there are a lot of
ways to skin this cat.

If you are laying this out by hand to drill in a drill press where you
don't have an X-Y table, then scribe a line across the face of the bar
using the center attachment on a combination square, or use a height
gage on a surface plate. Set your protractor to 90 degrees and scribe a
line 90 degrees off of the first line. Where those lines intersect the
..600" circle you inscribed is where you center punch and drill. Or use a
height gage, or surface gage set to .300" to scribe a line across the
face of the bar. Rotate the bar 90 degrees, by lining up the first line
to your machinist square, and scribe again.

To lay out more or fewer holes spaced equally, simply divide 360 degrees
by the number of holes, the resulting number is the number of degrees
each hole is spaced from the last.

If you have a mill with an X-Y table, find the center of the bar, set
your dials to zero, move X-.300" and drill, then move X+.300" and drill.
bring X back to 0.00" and move Y-.300" drill then Y to +.300 and drill.

To trig it out draw a quick sketch of the bolt hole pattern. Draw right
triangles from the center of the bar to the center of each bolt hole, by
drawing a line from the center of each hole to the center of the bar.
Now draw lines from the center of each bolt hole to the center of the
adjacent bolt hole. Bisect each of those lines by drawing a
perpendicular line to the center of the bar. The distance from center of
the bar to the center of the bolt hole will be the Hypoteneuse (side
"c")of the triangle, 360 degrees divided by (2x the number of holes)
will be Angle "A". Side a=Sin"A"* side"c". Side "b" will equal the
square root of (c^2-a^2). From this you can derive all of your table
positions, or the dimensions to scribe to using either a protractor or
surface gage.


--

Dan