Inaccurate Z steps?

First of all, I currently do not have Z end stop.... It's a long story... But anyway, in order for me to establish Z=0 point, I have to send in a g code G92 Z0 after locating the nozzle tip at Z=0 position.

So, with this setting, I have to trust (?) my printer that if I move the head 100 mm up from Z=0, and then move it down by 100 mm, it will come back to Z=0 position. I thought that's not too much ask...

However, it appears not to be the case....


I recently had an opportunity to have a small "production run" of some object that I have designed. The object dimension is 100 W x 65 D x 15.56 H (all in mm).

So, when the printing is complete, the bottom of my hot end nozzle has been raised to 15.56 mm (or should have been) and that's what I also read on LCD.

However it seemed like it only had moved up to 15.52 mm position, instead of 15.56 mm.


How do I know? How did I find this out?

After a couple of prints, I've realized that I had to re-establish Z=0 point. If I start the next print without doing so, the nozzle tip gets too close to the heated bed. As a result, the first layer is pressed down too much to the bed and some plastic gets squeezed out to the side - basically the first layer gets very messy.

After a few trial and error, I've learned that if I change the Z height to 15.52 mm after printing is done (= send in g code G92 Z15.52) then the first layer of the next printer would come out okay. Well, at least for a while - I still needed to make further adjustments every now and then. In other words, I think the difference between the actual Z position and reference Z position was not exactly 0.04 mm, but ever so slightly off.


Well, 0.04 mm tolerance over 15 mm is quite small - it's like 0.27%. I actually think I may not have been able to notice this if it wasn't for the fact that this particular PLA filament is very picky with the first layer height. I believe that's because it doesn't adhere to the heated bed as well as other filaments (which I have been using so far) - it won't allow me to set Z=0 position too far away from the bed, while I would've had more tolerance with other filaments...

Anyway, while it may be an acceptable margin of error, I wonder if this is something normal/to be expected indeed, or an indication that I need to perform some adjustments/calibration.

Also, I would think this kind of inaccuracy would come out somewhat randomly (within the error margin, that is), like the sometimes the height is 0.27% lower, sometimes higher, sometimes lower but not as much as 0.27%, and etc. But, my error is quite consistent; the actual Z position is "always" 0.27% smaller than the reference Z position.

Since it's repeatable, predictable, and consistent, I wonder if it's actually fixable..?

Any ideas?

Where's the 15.52 coming from? Is it a measurement (e.g. you physically measured your printer) or the firmware is telling you it is at 15.52? Also, have you tried other distances? E.g. go to Z=20, 30, 50 and see what happens.

If it's a difference between measured and what firmware gives, then there's something physically wrong with your printer or your steps per mm setting for Z. If the firmware gives you 15.52 when you really want 15.56 there may be some rounding error going on, but we'd need to know the firmware and your software host...

Also ANY plastic extruded by a printer (your PLA, my PLA, ABS, whatever) is very dependent on the first layer being right It's just a different set of problems if it isn't.

15.52 came from... trial and error basically. That's why I don't think it's exactly 15.52, but close.

When I started the next print without adjusting Z height, as I have described, I was getting symptoms of "Z=0 set too low" - extrusion line is wider than normal because the plastic gets squeezed out to the side (because nozzle head is pressed down too far against the bed).

How much too far? Well, I just tried various distances.

In the beginning to make this adjustment, I brought the nozzle head down to Z=0 (G1 Z0) and tried to measure the distance/gap, but it was just too small of a value (under 0.1 mm).

So, instead, I moved the nozzle head to Z=5 position, and told the printer that nozzle head is actually at a position which is slightly lower than 5, for example at 4.98 (G92 Z4.98), which will effectively adjust Z=0 by 0.02 mm.

I tried a number of different values a number of times, and it just so happened that 0.04 mm was the value that would give me the best result.

Now I know my Z height will be off by 0.04 mm after the nozzle traveled up 15.56 mm, I don't really have to bring the nozzle head down to Z=0 or Z=5 to make adjustment. After each print I could simply tell the printer the nozzle head is actually at Z=15.52 mm, instead of where the printer "thinks" it is (15.56 mm).


I "think" this error is proportional to the amount of distance the nozzle head traveled up along the Z axis. So, if I print something twice as tall (i.e. 30 mm) then the Z hegith is probably off by 0.08 mm... Again, even though I got this number (= magnitude of the error) from the actual data (= trail and errors), I could not actually measure this small distance. I was just going with whether the first layer "looks good" or not. So.. who knows? Maybe the actual discrepancy was 0.035 mm or 0.043 mm (again these are just some arbitrary numbers ) instead of 0.04 mm.

This actually makes me think twice about installing the Z end stop again. In the past I had some issues with Z end stop - I had to keep making the adjustment to the Z end stop every day for whatever reason. If I can get the Z end stop work consistently, then I can rely on it to reset Z=0 point for me, instead of having to do that myself with g code command...

Edited 2 time(s). Last edit at 09/19/2013 09:10PM by Yamster.

I have to constantly adjust my Z endstop as well. I don't know what it is, but it seems that before every print, I have to adjust it. It's much easier with an adjustable endstop though. I'm thinking maybe an optical endstop would be better.

Oh yeah.. optical. Not just an optical switch, but optical measuring device that can find out the distance between the tip of the hot end and the heated bed. That will be sweet. With a proper firmware, it will be able to make height adjustment (if needed) during the print, like compensating for blobs or irregular extrusion height/thickness (if any) of the previously layer.

Anyway, if I can't get my end stop working without daily adjustment, then I will continue to set Z=0 manually. It's just simpler that way, since I have to set the nozzle tip at Z=0 position in order to adjust end stop anyway. Then why bother trying to adjust the end stop switch to make it toggle at that position... I can just tell my printer that is Z=0 position myself by issuing g code G92 Z0...

If (and only if) I can have the end stop work consistently without frequent adjustment, however, then it will definitely be worth the effort.

This time, I am going to try to install the end stop on the X motor side - previously it was on the X idler side. I've read it somewhere that it's more desirable to have it on the X motor side since that side is heavier (therefore it will press down the end stop switch more consistently).

Also, from some other thread I read someone suggesting to get rid of the metal lever/fulcrum - to remove the springiness. So I might as well try that, too.

I've been OK with a micro switch as my Z endstop. By micro switch I mean the type with 7mm spacing between 2mm mounting screws. I'm also using a pretty simple end stop holder for them: [www.thingiverse.com]
Print the end stop holder out of PLA instead of ABS if you can so you can get a tighter grip without breaking it.

I only adjust my Z (usually the springs on the build plate rather than the end stop) if I did something dumb to unjam my extruder like pulling violently to get stuck PLA out.

As for optical measuring devices, let me know if you find something cheaper than those hundreds / thousands of dollars laser devices. At the distances printers work at though, a caliper permanently fixed to the side of the printer riding along Z would work too. (The trick would be getting the firmware to read the thing.)

use full step on Z, you dont need microsteps. this is common error in reprap community, 2 Z motors on one driver with microsteps, that cant be good.

0.04 mm error in Z height is nothing to worry.