Carl Mikkelsen wrote:
A decent parabola would require a rather large billet to start with, and a lot of material to be removed. I agree that it would be a good test.
The proposed test came out of a discussion of quantization error, which intrinsically prevents a hexapod from cutting a smooth plane, unlike a Cartesian machine which could cut a perfect plane.
What I was curious about is how to measure the small-scale deviations from the desired shape. Isn't this property called surface roughness? What is a simple way to measure surface roughness?
There are instruments that do this, surface roughness indicators.
But, one other way is to do this backwards. Maybe the plane is the easiest, here.
You mount a dial test indicator in the spindle, lock the spindle and sweep the indicator
across the surface by CNC control. The deviation of the indicator tells the error.
Sweeping an indicator across a granite surface plate would sure tell you whether the
machine could move in a straight line.
To bring it up to multiple axes, you could obtain a new bearing ball of the largest size
possible, like maybe 2 or 3" diameter, and sweep the indicator around that surface. You'd
need to make a heavy "nest" to hold the ball so it doesn't move.
Jon
