CNC Router calibration

Thanks to some careful “noticing” and “paying attention” and a stubborn refusal to accept a shrug (from me) as an answer, @mgrossbauer got to the bottom of some measurement errors he was experiencing on the CNC router. Parts were consistently coming out slightly oversize.

After ruling out software errors and operator error, he found that the machine was in fact out of calibration. He measured about 1/4" of error over a 50" span, consistent between the X and Y axis. We got it dialed in last night to eliminate this.

The scale error was at least consistent between the X and Y axes, so circles were still coming out circular, and squares were still square, although about 1/2% oversize. There was also a small Z axis error. If you took home the little stool we make in the checkout class, you’ve been sitting about 1/2% higher off the floor than planned. We apologize for this!

Probably this came about when the old round stepper motors were changed out for new square ones a good while back. All stepper motors have the same number of steps per revolution, but there may be a new pulley of a slightly different diameter than the old one in there somewhere. It’s also possible it was off before changing the motors.

In any case the Mach 3 software has a feature to account for this. It’s now reproducing dimensions as accurately as we can measure (by middle aged guys squinting at a tape measure, at least).

Many thanks to Mike for digging into this!

I remember Carl Lance and I counting teeth and going over these numbers a few times. I’ll check for our old notes and see if we can compare and get these updated to the correct values.

We should document this somewhere.

This is what we had going back and forth in the email: It may NOT be correct! I think this was for the z-axis.

Care to check me on this. I think I see a similar slight difference on the Z Axis. Took the heads off and found:

Stepper connected by Shaft to 18 tooth gear
Belt connecting 48 tooth Gear.
Shaft connects 24 tooth Gear.
Belt connects 80 tooth Gear.
Shaft connects 30 tooth Pinion.
6.4 Tooth per Inch Rack.

18/48 Ratio: .375
24/80 Ratio: .3 (total reduction to .1125)
6.4/30 Track: 4.6875 (.527344 Inches per Rotation)

Half Stepping Stepper @400 steps/rot = .001318 inches / step
or inverted as 758.5185 Steps per Inch.

Was at 754.512. Small difference to 758.52 rounded.

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