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I have a weird problem with my old 3D printer, it is a Prusa/Mendel type. When I print a 20 mm cube, X and Y are correct, Z is resulting 16 to 17 mm. I have cheched the correctness of the movement on Z using the manual controls and there are no issues.

I played a bit with the layer tickness, I have a 0,4 mm nozzle, setting it to 0,12 (normally is on 0,16) but no changes in the result. I am printing PLA on a cold bed at 180 Degres C without any other particular defect.

I would appreciate some direction on how to solve such problem.

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It’s likely to be your steps per mm. Print a small cube that is 60 mm tall, and if its something like 50mm tall, divide 60/50 and multiply your steps per mm by 1 + the quotient. You may need to repeat this with a taller 100mm to confirm its accurate.

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Thanks for the hint. I will give it a try for sure and share the result

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Check the speed that your Z is set to move at. Manual movement speed may not be the same as the speed it is using during the print so you may not see a problem if you simply manually move the Z. If the print speed is too fast, the stepper will skip. Because the layer changes are so small, it is difficult to detect.

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If it were steps/mm being incorrect, the manual Z move would be the same as the print Z move. Correct??

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I think so.It could be missing some microsteps, so it would be good idea check out the current of z driver and threaded rods lubrication.

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I checked the z movement during printing, i have two motors on z and they move exactly in the same way during each layer step. I think would be difficult for two motors to move sincronous and loose the same steps unless the ramp, which is common, has a problem. What do you think?

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I printed a 40 mm cube and the Z is 30 mm.

when i print a 20 mm cube, the Z is 17 mm

it does not looks linear. So it is difficult to adjust the Z steps on those basis.

i have not yet tried to chnge the value because the eeprom does not accepts new value from Repetier and therefore i have to relod the firmware for which i do not hve the original one with settings. Will take few days but i i hope to come right to recompile the firmware.

meanwhile if you have some idea based on the bove results, it will be appreciated.

thanks

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What will happen if u print at a slower speed?

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If your steps are off the manual or print move of Z should be off the exact same amount.

Triffid hunter (search for it) explains how to use the M92, M501, and M500 to change them. If you type M501 in repetier it should show you the M92 results.

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I am already printing at low speed, 30 min for 20 mm cube

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Hi @FeliceM

For M5 drive screws and 1/16 microstep you should have:

Z steps/mm - 4000

Z max feedrate - 3mm/s

Z acceleration - 500mm/s^2

For other setups use this calculator: RepRap Calculator - Original Prusa 3D Printers

And check if steppers have enough power.

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Maybe your z axis had a lot of backlash when changing direction

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When was the last time you lubricated the Z axis ball screw? and alignments? The bed should come down very easily just by turning the Z axis with two fingers when the printer is off.

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Check the cooling of the stepper drivers. Install a fan if there is no one. You could also try to reduce the current a little bit.

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Thanks to the various hints given in this post, I decided to swap around the Polou drivers and I discovered that the problem is on the driver. It looses randomly some microstep. On the x axis was much easier to be detected because the printing clearly shifts away without leaving doubts.

Problem solved. I will replace the driver on Z.

Thanks for the help

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I would check your current setting then maybe the drive is set just a bit low the x would also need more current which would make this problem more evident. Also if you are powering down you z axis between movements there is a chance that its sagging a bit between moves