Z-axis unreliable zeroing

[zhangxueyou]

I am out of mothods. You must be followed by the ghost.
Are you sure the z motors did not move when you are heating the bed only? Because the bed and extruder has been firmed, it is impossible that either of them changes their position automatically.

[TheDreamMaster]

I am no making this up. It is happening.

The motors are not moving at all. I think the problem has to do with a heat related problem, something is structurally changing when the heat is applied to the bed.

The problem is so bad that If I calibrate the printer while cold, then the first layer wont stick because the extruder is far away from the heat bed.

And if I calibrate it when hot, then when it cools down the extruder hits the glass.

IT IS NOT A MOTOR PROBLEM, something is changing when the heat bed gets hot. I cannot possibly be the only person experiencing this.

I JUST WANT TO PRINT!

[zhangxueyou]

Please take it easy.
I am thinking if it is expansion and contraction caused by temperature. The glass expands when heating the bed and it press the bed downwards, so the distance between glass and nozzle increased. Please keep rotating the 4 butterfly nuts until you can not rotate them, then re-heat the bed, see if the distance still changes, assuming the change is not caused by extruder.
You are really the first one who meets such an issue.

[TheDreamMaster]

Yes, as I mentioned in one of my previous posts it seems like the heat bead is warping due to heat (dilation, expansion).

I was not able to work on the printer last week but tonight I tried to run some more tests.

I had to calibrate the extruder head with the heat bed at 95 degrees, due to the problem, and then I tried to run a test print. The first layer printed perfectly, but as the second layer was being printed, all of the sudden, the x-axis lost control AGAIN and started to jump all over. This is after I had already exchanged the x-axis driver for the driver for the second extruder. I will post this problem in my post about the x-axis motor problem...

[zhangxueyou]

To solve the issue of unreliable zeroing, please rotate the 4 screws on bed as tighly as you can, then calibrate it slighty.
As for the motor driver, it is the most easily dead component especially x motor driver, that why the selling of motor driver in every company is always high.
Did you stick heatsink on motor driver? Did you wear static gloves?

[TheDreamMaster]

I lowered the heat bed all the way by tightening the screws as far as possible. After doing that, it became apparent that the holes on the corners of the heat bed are slightly farther apart than the holes in the acrylic base. This makes the heat bed bulge upwards. I think this might be the source of the problem, because the heat bed is not flat.

I have included a picture that shows the bulging. I took the clamps off so that the glass is unbent, so you can see how the heat bed corners bend down (pushing the center up).

Corner or heat bed bends down, from the pressure from screws.

HeatBed-Bulge-1.jpg (254.85 KiB) Viewed 11752 times

[TheDreamMaster]

Ok so even with the bulging problem shown in my previous post, I tried to calibrate the bed the way it was suggested. It does not work. Because the bed is not flat, not even with the glass clamped down, the extruder head is at different heights on different places (closer to the bed at the center, and farther away at the corners).

I tried the opposite, I loosened up the screws as much as possible, to allow some leeway for the difference in hole registration between the heat bed and the acrylic plate. It made the heat bed flat, but the height change problem still shows up: Extruder is closer to the bed when cold, and away from the bed when hot.

I don't know what else to try.

[zhangxueyou]

It's really hard to guarantee all the glass are flat, there is a slight error that can be ignored or does not affect the printing. I suggest you clamp the 4 corners of the glass.

[TheDreamMaster]

I am sorry, possibly my previous two posts were not sufficiently clear.

The problem is not the glass, the glass is flat. The problem is the heat bed. When the screws are tightened, the head bed bends up, making a bulge in the center.

This happens because the holes from the bed are wider apart than those in the acrylic base, so when the screws are tightened the bed is forced to bend. The corners are fixed by the screws, so they can't move, but the center is pushed up. It looks like the corners are bending down with respect to the heat bed.

I always clamp the glass, but of course this does not help because the bed pushes the center of the glass up.

Please be kind to read my posts again.

I think what I might have to do is drill the holes on the acrylic base, so the holes are at the correct positions and the heat bed does not bend.

[TheDreamMaster]

I removed the screws and springs and aligned a corner of the heatbed with one screw. Here are the pictures of the misaligned holes:

Bottom Left corner aligned using a screw

BottomLeft.jpg (264.47 KiB) Viewed 11740 times

Bottom Right corner

BottomRight.jpg (214.44 KiB) Viewed 11740 times

Top Right corner

TopRight.jpg (231.07 KiB) Viewed 11740 times

Top Left corner

TopLeft.jpg (221.31 KiB) Viewed 11740 times

Afer measuring the heatbed and the acrylic platform I made this sketch to show the discrepancies:

Sketch showing discrepancy between hole centers.

Sketch-1.jpg (115.2 KiB) Viewed 11740 times

The hole center distance along the X-axis:

Acrylic: 20.77mm Heat bed: 20.87mm

The hole center distance along the Y-axis:

Acrylic: 20.84mm Heat bed: 20.87mm