Hi,

The link is interesting. Problem is, all the spirals mentioned
on the page would require a formula entered into the milling
machine.

I have a 1976 Anilam Crusader II. No serial port, no parallel
port. It has a tape drive. I have to manually enter any
program I want to run. As far as I know, you can only
program x,y,z movements (lines), and arcs (startx&Y,
endx&y, centerX&y).

I can't even have a CAD program draw a true spiral,
convert it to a series of x/y movements (straight lines)
and move it to the machine. No way to transfer data
from the PC to the crusader.

The "half circle" method that Fred (FF) mentioned
is *perfect* for my application. I only have to
program approximately 16 instructions into the
machine.

16 of these:
ARC CCW X,Y to X2,Y2 center = X3,Y3

easy and quick.. I understand how the
half-circle-spiral diameters work. I am just
trying to figure out how the center points
relate to eachother for programming into the
machine.

Rich