Z axis Calibration Very Poor

I have some serious issues with the accuracy of the Moai. I’ve done what I can to correct them, and I’d like to know if there is anything else that can be done to make the machine usable.

I did the full XY axes advanced calibration, then printed the advanced calibration test part. The various steps were in error between -0.45 mm and +0.87 mm. I created a calibration file as instructed, and found the XY steps were now within 0.08 mm.

I found my parts were still up to +0.7 mm in error over 42 mm, mostly in the Z axis. A box and lid printed in different orientations were not even close in size.

I printed some 20 mm test cubes at 4, 7 and 10 mm off the build plate. The X and Y dimensions were close enough to 20 mm, but the Z dimension was horrible. The bottom to top face distance was 20.70 mm in all cases, and had a pronounced rounding of the bottom edges:

As near as I can tell, the vertical ( Z) edges are close to 20 mm long. At the point where the outermost supports attach, the distance to the top surface is about 20.6 mm. The “Z” imprint is 1 mm deep.

I milled off the bottom (nearest build plate) face until the Z dimension was 20 mm:

The corner rounding is almost (but not entirely) gone. However, the “Z” imprint and the drain slot edge thickness is not 1 mm as it should be, but probably about 0.3 mm.

I don’t know what to make of that. I measured the piece before and after post curing (no change), and the support raft was completely flat. The top surface (away from the build platform) is completely flat. The vertical edges being the correct length (nearly) indicates no Z scaling error, but the placement of the bottom face with “Z” imprint suggests a Z scaling error. The effect is not dependent on the Z offset height, so it doesn’t look like it is related to peeling.

There is another issue. Both of the faces parallel to the door (Y axis?) are not straight but are a constant distance apart in X. They bow towards the back of the machine about .05 mm. This looks like a Z axis straightness issue.

If it matters, I am using the Peopoly tough resin at 31C and FEP VAT, with default settings, except the power is 59, as my Z levelling tests would not print otherwise.

I’d like to hear what might be the cause of, and fix for, the “bulge” on the lower Z face, as clearly a surface 0.7 mm out of flat is totally unusable.

Somewhat off the main point, I was surprised to find that the machine itself cannot be corrected, but requires a calibration file in the Asura slicer software. This destroys portability of code, as well as tying us to Asura, which is very buggy, crashing at the least provocation. Moreover, there is no provision for Z calibration, merely a scaling factor. I understand that there are X size and Y size settings, but the description makes no sense to me.

I should explain what I meant about the X and Y size not making sense.

"X Size and Y Size

Both have to deal with the size of the scanning area. This is where you adjust when your print size is different than the STL file. 900 is the default value, and when you increase the value, the scanning area for the X-axis expands. For each unit in X/Y Size, it represent a change of 0.02mm."

If the units are 0.02 mm, then 900 represents 18 mm. Where is this 18 mm dimension? The bed is 260 mm wide. “… when you increase the value, the scanning area for the X-axis expands.” What is meant by the scanning area?

“… print size is different than the STL file.” This sounds like it is a scaling factor, for when the print is coming out larger or smaller than the value it should have in the STL file? But if if is a scale factor, how can it have dimensions of mm?

Hi Jed, I think you see here a behavior called bleeding. The laser is curing more resin as wanted because the laser beam doesn’t stop exactly at the first layers. Unfortunately some resin stays still over the first layers, the gravity doesn’t help. The laser cures the next layers, is shining partial through the first layers and cures unfortunately the resin. Transparent resin is showing that effect more than the darker resins.

raise up may help with the accuray since first few years layer is both compressed and overcured.