Rotary Axis incorrect size of cuts

Hello all,

I recently bought a 3rd party 65mm rotary axis seen here.
They didn’t give very much information about the motor but I was able to wire it correctly and hook it up to the slave Y axis motor and get it working. Everything cuts great accept the sizes are off. I did 3 simple tests of rings on a cylendar in specific sizes to see if it was working. And although they cut very nicly, they were all off in diameter by 1.6mm.

I’m perplexed as to why and any insight would be greatly appreciated. I triple checked my zero axis height and the steps per rev. for the 1.8 degree motor converted to 0.45 degrees per step angle due tot he 4:1 gear ratio. When I cut it revolves 360 degrees so the step angle seems fine.

I’m using Aspire 10.5 and the converted rotary postprocessor found here made by Roger Treadway (Thank you Roger!)

Any thoughts or suggestions?
Thank you!

Are you getting that same diameter measured across the circles at different tangent locations? Trying to rule out issues with it being nonconcentric. If possible, try checking with a hardened rod and dial indicator. Also, just to confirm, all diameters were off by the exact same amount, no differences when measuring smallest to largest? Are you using the centre of rotation as your datum point for your 4th axis, or zeroing off the top of the cylindrical stock?

I am not sure if you have seen it, but I found this video extremely interesting and helpful:

Installation of a rotary axis on a CNC

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That’s a good idea to check and yes they are all concentric circles, no deviations. I’m pretty happy with the rotary except for this one issue.

And thank you I’ll check out the video. :+1:

Forgot to ask if bigger or smaller than expected (by 1.6mm). Was thinking if you did not zero (centre z axis directly above your stationary axis centre line) it may account for the discrepancy - they would be smaller diameters if my mind is seeing it correctly.

They are all smaller by 1.6mm BUT I did another ring around an axis and the ring was 1.6mm smaller but the axis was larger. Made NO sense to me. The axis was set at 4mm diameter but it came out at 6.35mm.

OK I figured it out. Like a fool I trusted the manufacturer’s information. The specification sheet said that from the base to the centre of rotation was exactly 54mm. Yeah… it wasn’t. hahaha

I have no tool to measure the height of my bit that well other than a small ruler, so through some math I figured it out. I measured the cut diameter that the CNC made, what it was supposed to be, divided by 2 (for the radius) and added it to the height difference and got 54.85mm (from base to zero axis). Then to verify visually, I placed a pyramid bit in the chuck and lined up my tool tip and sure enough, I think we have a winner. I just need to do a test cut next, but I’m pretty sure it’ll be correct now.

I feel silly but glad it was something so simple. Thank you for the CNC video from TMToronto, that really helped as well!