Bent Moai Mystery: solved after 2 years

This post is for those who have run into this situation, and for those who don’t want to run into this situation. As a preface, I maintain that the design of the Moai is still superior to many platforms out there, and that the design work put into this machine was substantial. However, this is a long term effect that was hard to spot.

Issue: If you’re Moai has developed a wobble along the vertical walls of your print, read further.

When my Moai was new, I savoured every minute in assembling it. I employed every precision technique there was to ensure my machine was absolutely perfect. For a time, that remained true.

However, the Moai was put on a shelf — for when I had more time to play with it. During that time, the Moai did something I didn’t expect: It bent itself.

You see, the vertical rail with linear bearings is very stiff and stays perfectly tight at all times. Also, the lead-screw for the Z-axis is also perfectly straight, at the beginning. The mount for the stepper motor is a 90˚ bracket that allows the motor to point upwards. What is not apparent is the bracket is not perfectly 90˚. If you assemble the bracket with the stepper motor, and then install it in the back of the Moai, you should check to see if the lead-screw points up perfectly.

To see if this is the case, disconnect the black flange screw from the arm that holds the build plate. The arm will drop to the bottom (make sure there is no vat or build plate installed). You will notice the lead-screw passes through the centre of the opening of the support arm. If you raise the support arm to the top of the z-axis rail, the lead-screw should still be in the middle of that opening. Mine wasn’t, it leaned outwards.

Over time, while the Moai was not used, the pressure on the lead-screw remained in one direction. This would not change even if you used your Moai occasionally, because the build plate would always park in the same place every time.

Add a couple years of pressure and something will change: The angle of the lead-screw. Yes, the linear bearing on the z-axis rail is very stiff, but not stiff enough to cancel out the twisting from the lead-screw. As a result, the build plate shimmies around as the screw raises your build plate. The result is a print with wavy walls.

What I’ve done afterwards is not something I would recommend: I removed the lead-screw nut at the top of the support arm and turned the lead-screw so I could see the direction the lead-screw was bent. I then applied pressure (not much) to the lead-screw to straighten it. Multiple adjustments were needed, but now the lead-screw does not wobble when turning freely.

This does not mean my lead-screw is fixed, far from it. I found that inside the stepper motor are two damaged spacer rings made of aluminum. For whatever reason, these were crushed and had created plenty of shredded bits of aluminum. I cleaned all that out and made new spacers out of brass tubing. Let’s hope it holds.

More importantly, I loosened the bracket that holds the stepper motor to the chassis of the Moai. Yes, this is not great, but given that the linear bearing is responsible for guiding the build plate properly, the stepper motor does not need to be completely rigid. It only needs not to move up or down. So I adjusted it enough that the lead-screw could tilt into a position where there wouldn’t be lateral forces applied to it. Seeing how this worked, I wonder if the lead-screw was perhaps not as good a solution compared to a toothed belt on a stepper with a reduction gear. The reduction gear could allow for finer layer hight control, and the job of lifting the build plate would be the same - without the unwanted bending forces.

Unfortunately, my Moai is far outside of warranty, and if I need another stepper motor (which is very likely), I will install it loose - to prevent it from flexing the chassis and applying an offset to the build plate.

2 Likes

Thank you - this is something to work with.

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