I just put together an Open Builds CNC machine and I am using a

Controllers Smoothie
,
1

I just put together an Open Builds CNC machine and I am using a Smoothie to control it. I just flashed the latest version of firmware (firmware-cnc.bin). I am running a v1.0b board. I have a Tr8*8-2p (8mm-2mm pitch) screws. My stepper motor is a standard 1.8 degree, 200 step device for the X, Y, and Z axis. The current is set to 2A. I set my alpha_steps_per_mm to 400 by using the calc:

step/mm = ((fullsteps per rotation) * (microsteps per step)) / (mil per rotation)

which should equate to:

400 = ((200 * 16) / 8)

I read that a v1 board uses 16 microsteps vs 32 for the newer boards. This appears to be the case for me. When I move the X axis stepper 1mm, I get .91mm as measured by a micrometer. When I move 10mm, I get 9.97mm. If I move several hundred mm’s (say 500 as measured with a meter stick), I will be over by several mm’s (approx 503 to 504mm). Something doesn’t quite add up. Any suggestions of things to check? Could other settings effect the scaling? I am obviously missing something.

BTW - I have a TinyG board that I swapped in and with it’s recommended settings, I get the expected measurements as per the micrometer - both at 1mm (within .01mm) and 500mm.

2

Hum… with a Leadscrew TR8 with 2mm pitch running 200 step, you should be around 1600 in the setting ?

3

Move 100mm and do this formula :

New Steps/mm = (Current steps/mm) x [100 / (measured distance traveled)]

4

@Sebastien_Plante I moved it 300mm to make it easier to measure. The actual distance traveled (as best as I can eyeball) is 302mm. By the formula you provided, that comes out to 397.35 mm

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@Sebastien_Plante I replaced 100 with 300.

6

So you did
300/302 = 0.99338
400 x 0.99338 = 397.35

Change your config from 400 to 397.35, reset and try again :slight_smile:

7

@Sebastien_Plante will give it a shot tomorrow. Thanks for your help.

8

If it is a true tr8x8 then there is no fudge it will be exactly 400 or 800 steps/mm depending on your microstrpes (1/16 or 1/32) the advice to use some fractional steps/mm is bogus. You need to find out why it is not moving the correct distance. It could be they are not tr8x8, or there could be some mechanical issue. fudging the http://steps.mm when they should be exact is not the solution

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@Phil_Aldrich I agree that “fudging” is not the best answer but using 397.35 seems to make it much more accurate when I run the X axis out to 300+ mm. It just seems odd that when using a TinyG board, I can set it exactly to 400 and it is dead on at 1mm or 300mm.

10

I suspect you are losing steps by having acceleration too fast and/or feed rate too fast. Although I would think it would stop short if pulses were lost.

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@Wolfmanjm Got something there, maybe compare/copy the jerk, acceleration and others setting from the TinyG.

And I saw I miss readed the threaded rods pitch is 8 et not 2 :slight_smile:

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I’ll double check the settings against the TinyG. Thanks all for the help.