4g PRT axis unit values

[mgilley]

I am using my indexer for the first time with the upgrade. When I move the a axis 3.6, it doesn't move a full revolution with the unit value set at the 2291.831. If i change the value to 4000, it almost makes a full revolution. Does anyone know the correct value?
Thanks,
Michael

[Brady Watson]

Michael,
Your UV will depend on the gear ratio you are running on your indexer. The math is pretty simple. Make sure that your indexer is assigned to the B-axis, and not the A-axis. The B-axis is set up for rotary movement, not linear movement. The A-axis is generally reserved for a 2nd Z axis.

If you are running a 4G board, then the values will be as follows:

A 3.6:1 stepper will be: 2000 steps/rev * 3.6/360° --> Unit Value = 20

A 7.2:1 stepper will be: 2000 steps/rev * 7.2/360 --> Unit Value = 40

Make sure that you assign your indexer in SB3 to run as a B axis & set it up so that it matches the appropriate channel for your machine. Then enter in the correct UV for the gearbox that you have.

A quick way to tell what ratio you are running is 3.6:1 motors typically have 0.5°/step on the motor nameplate and 7.2:1 have 0.25°.

-B

[mgilley]

Brady,
I have been running on the a axis and it worked fine before the upgrade, I use visual mill and the bit travel the x axis, the routine is call parallel finishing. The Y axis stays at 0 and the a axis turns an increment with each pass. I am using the 7.2 stepper. I tried a value of 4000 and it left about 0.05 uncut. Hope this makes sense.
Shopbot is going to send me a chart of values Monday.
Thanks,
Michael

[Brady Watson]

Michael,
What were your previous unit values for your indexer? Providing that you were 1/4 stepping using the standard dip-switch settings on your original PRT board, then you will be exactly 2.5 times greater in resolution & your unit value number.

So...If your previous value was, let's say, 733.3859 - then your new UV would be 1833.4647, which is just about the value for a 3.6:1 gearbox motor running a 25T pinion on an X or Y axis, with a 4G microstepping driver. Since you have a 7.2:1, you need to double that value since the motor turns 2X the 3.6:1, so it should be 3666.929 (or close to it) - I have a similar tool running a 7.2 in rotary for linear unwrapping & my UV is 3666.866...

I am not familiar with how VM writes it's post processor, but keep in mind that your indexer, with these values, will be running as a linear axis. This means that it will essentially unwrap a flat 3-axis file as it is cutting, essentially substituting the Y axis for the A axis. This is not true indexing with the indexer moving incrementally by degrees as it steps over for your parallel finishing raster toolpath. But...if this is how you were doing it before, then go for it!

BTW, the 2291.xxx number is for a 4G PRT Z axis running a 20T pinion. Note that the Z axis UV is exactly 20% greater than the XY 25T value, since it needs to turn 20% more to achieve the same distance on the rack.

-B