Layers not aligned .. Sometimes

I can not reconcile what is going on. If I print a plate with, lets say 8, small parts like an action figure, layers misalign. See pic

So what do you do? Print a 40mm tall calibration cube. And it comes out PERFECT! See pic

Mendel 90 MDF
Ramps 1.4
Printing from computer using Pronterface
Slic3r
Nophead Marlin optimized firmware

Belts tight. Extruder and steppers perfectly calibrated.

Any insights?

- Jim

Hi Jim,

it looks like you have a slipping belt or pulley. Check if they are o.k.

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MfG / Regards

Stefan

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Blog / Gallery / Wiki / Mendel90 kit since Sep 2013 from Nophead / Original Prusa I3 MK3 since Feb 2018 /
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Double checked. Everything tight. Even marked the pulley and shaft to be sure.

In which direction is it misaligned?

The second photo looks like Z wobble because it is very periodic as the build height increases. How tightly constrained are your Z rods. They should be allowed to swing freely, and let the smooth bars and bearings maintain vertical alignment.

Is the belt on the x-motor sitting centrally on the motor gear and the idler? The first picture looks like x-belt slippage to me.

If the belt skips a tooth the error will be a multiple of 2.5mm. If the motor skips steps it will be a multiple of 0.8mm. Other values imply a loose pulley.

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