# Steps per unit for polar sand table

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.

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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.

2 Likes

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!