Z-axis moving too little?

Dear fellow RepRappers,

I have built a RepRapPro Mono Mendel which is my first ever 3D-printer. I print PLA at various temperatures (I have tried 180-225°C) on heated glass using a 0.5 mm nozzle. The software I am using is Slic3r and Printrun.

The problem is that when I print something the first layers works alright but later layers (after about 1 cm, for some models earlier) collides with the nozzle. The distance between the printed object and the nozzle seems to get smaller and smaller until it is basically zero. I have set the homing position so that a sheet of paper barely fits between the nozzle and the bed. The threaded bars which moves the carriage in the z-directions is of type M5 and when I command the printer to move 0.8 mm the bars turn 360 degrees. The parts of the prints before the collision comes out with somewhat the correct height. I estimated a part that was supposed to be 1 cm to 9.95 mm using a caliper.

Since it seemed unlikely to me that the z-axis could be moving the wrong amount I started checking the extrusion. I previously calibrated the filament feeding and measured the filament diameter with a caliper but even so I tried setting the extrusion multiplier in Slic3r to different values all the way down to 80%. The only difference was that the solid layers was not always completely solid for the lower values. Next I tried setting the Z-offset to 0.2 mm (one layer) and indeed the print failed a few mm later but it still failed the same way as before. In case it had something to do with thermal expansion I have also tried with and without a fan directed at the nozzle, no difference whatsoever was seen. I have checked that the generated G-code seems to make sense.

I have cleaned the bars (they were not that dirty to begin with) and measured how much the carriage moved with a ruler and detected no change from the expected values (I moved the carriage in 0.2 mm steps to simulate a print). I plan to get an indicator to make more precise measurements.

I am starting to run out of ideas? Is is possible that the z-axis is moving too little? Can I check somehow if the motor has missed any steps? Can it still be an extrusion problem? What can I tweak and/or test?

I appreciate any help!

If the potentiometers on the stepper motor drivers are not adjusted correctly, after a while they will heat up and give you some strange errors. I would suggest feeling them while the printer is running to see if this is the case.

Checking your pots is a good start as you just may be missing steps.

Also make this check (hopefully you have calipers...a ruler can get you pretty close)

Home your Z axis. Then, use your manual controls to raise it by 5mm. Check with your calipers to see that it in fact raised 5 mm. Raise it another 10mm and see that its up 15mm. Another 5mm up to 20mm.

If it's not, you'll need to adjust your Z-steps in your firmware until you get it right on. Or troubleshoot any missing step issues. You really need this right on.

If you only get 9.5mm, then it will start smashing into the print as you go up. And when you print something 50mm tall, it will come out at 47.5mm. So a half a millimeter can be a big deal.

Once the Z-axis movement is locked in, then you can work on your extruder. Using the extruder multiplier can make pretty quick changes, but it affects a lot of things so the preferred method is to do a fine calibration on your extruder (E-Steps) so you can run your multiplier at 1 and let the software do all the work.

Let us know how your Z-measurements work out and if you need help on the other stuff, we can expand a bit more...

Edited 1 time(s). Last edit at 08/21/2014 06:20PM by ltklein.

I have the same problem and have not solved it yet. Interested in replies to your questions.

Thanks you for your suggestions. I have got myself an indicator now so if everything goes well I should be able to measure the z-axis with a precision of 0.03 mm.

Hopefully time will allow me to performs these checks tonight before going away for the weekend.

I made some quick measurements tonight. I will make more next week but here is my current findings:

1. The motors do not get warm.
2. The carriage is homing to the same height consistently.
3. If I, after homing the z-axis, gently push on the printed pieces that slide on the smooth z-rods they (and the carriage ofc) drop slightly.
4. The first ~0.2 mm the z-motors move does not raise the carriage at all.
5. The carriage moves less than it should in the beginning, after a few mm the steps are close to correct, they seem to fluctuate a bit though (I hope to make more precise measurements next week).

So at least part of it seems to be some kind of hysteresis problem, unfortunately it does not only causes an offset in the z-axis coordinates. Do you think a second nut on the threaded z-rod (on the other side of the spring) would help?

It's the stepper motor driver on the circuit board that will get hot if the potentiometers are not set correctly.

Yes ofc, sorry. They are not much hotter than room temperature.

I have now installed two nuts instead of one nut and a spring on each threaded rod. The backlash (hysteresis) problem seems slightly better but the steps are still wrong.

I tried moving from the homing position to z=0.5 mm using different number of steps. With only one step the axis moved 0.0.315 mm, with 5 steps it moved 0.3 mm and with 25 steps it moved 0.28 mm. It's small differences but the numbers are reproducible.

After that I let the printer go from z=0.5 mm to z=1 mm. Using one step the z-axis moved 0.48 mm, using 5 steps it moved 0.46 mm and using 25 steps it also moved 0.46 mm. So the missing height seems to be dependent on the number of steps (or step size).

After that I let the printer go from z=3 mm to z=3.5 mm. Using one step the z-axis moved 0.47 mm, using 5 steps it moved 0.47 mm and using 25 steps it moved 0.46 mm.

I tried setting the acceleration lower but it did not improve anything. I also tried turning the potentiometer on the board a bit in both directions but no change.

What now?

Another update:

I have tried increasing the steps/mm just to see what happened. I increased it with 80 steps/mm which should give me an additional 0.2 mm after 1 cm. Surprisingly the test print I have used fails at exactly the same height as previously.

Seems like it could have been an adhesion problem. I did not expect the print head to drag the model with such force. I started using hair spray and now it seems to work.

For anyone reading this and having similar problems: make sure the hair spray contains vinyl polymers otherwise the PLA will not stick at all.