I’m building a polar sand table based on robdobson’s design using a BTT SKR2.

I’m hung up trying to calculate the steps per unit so I can utilize Sandify.

If anyone can baby step me through it I would appreciate it.

Did you look at this wiki?

You need to put the steps/mm into Marlin so a move 6mm long is one rotation.

Yes thats the page that was confusing me. Im not sure how to get to the numbers i need. The formula is confusing.

I’m being lazy on that page, and writing things with a keyboard, which doesn’t look good for this math.

What I’m doing there is called “dimensional analysis”, but it is a pretty simple way to figure things out like this. Here is a video about it, in case you aren’t familiar:

This is the equation:

200 full steps / 1 rotation * 1 rotation / 6 units * 16 microsteps / 1 full step = 533.3 microsteps / unit

200 full steps / 1 rotation: That comes from the type of stepper motor you have. If you have a 1.8 degree stepper motor, then it is 200 full steps / rotation.

1 rotation / 6 units: This comes from me, trying to get the final units to be something useful. These could be some made up unit, like jeffebs.

16 microsteps / 1 full step: This is based on the stepper motor driver. A a4498 can only handle 16 microsteps, a drv8825 can handle 32, but you can configure either for 16,8,4,2,1. TMC drivers are usually 16.

```
(200 * 16)
----------
(6)
```

That gives you 533.3 microsteps per jeffeb3.

Thats alot easier to wrap my brain around thank you!

Well something must be off in my marlin setup its taking 18 units to complete a full circle.

I have a 1.8 deg stepper and I’m using tmc 2209 drivers so it should be 533.3 same as your example.

loaded into marlin with M92 and saved. I even double checked to make sure it stuck.

Configuration_adv.h shows microstep modes set at 16.

Does anyone know of a preconfigured marlin for my setup that I could compare with mine?

I just adjusted the units per circle in sandify for now and its working beautifully.

Do you have a 3:1 gear ratio?

Awesome!

Do you have any pics to share?

Haha yea i bet thats it i didnt even think about the ratio.

The table is still pretty rough but its getting there.

Its 3ft across so im having problems with the arms sagging.

That is really neat. Great job. Thanks for sharing.

Thanks for your help and I never would have gotten this far without this wonderful site!