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Dimension error in Z

Posted by ChicoCyberGoth 
Dimension error in Z
October 23, 2018 09:08PM
Recently assembled a 3d printer (similar to Prusa Mendel I2), I have done some prints.
Yesterday I was doing my first test with ABS, I was able to print, the piece has printed nicely in the second try.
When measuring, in X and Y sizes the value conferred very well, but in Z I had practically 1mm of difference for less, the piece should have 14.5mm but it was 13.5mm.
I checked again the movement of the Z (steps per mm), made a homming in Z, measured the distance (49mm), had the machine move 101mm up and measured again, measured 150mm with a margin of error very small, perhaps negligible.
This is normal or is something wrong? What do you think?

Thank you very much.
Re: Dimension error in Z
October 24, 2018 12:07PM
Do your Z-screws turn easily using your hands? There might be some binding there. What kind of anti-backlash springs did you use? The Z travel should be exact just like the X & Y.

Prusa I2
Gen7-ARM 2.0 / Teacup
Genuine J-Head Mk V-BV

RAMPS 1.4 / Marlin
E3D V6

Piper 1 Version 2
Gen7-ARM 2.0 / Teacup
E3D V6

RAMPS 1.4/ Marlin
Re: Dimension error in Z
October 25, 2018 01:38AM
Shank man
Do your Z-screws turn easily using your hands? There might be some binding there. What kind of anti-backlash springs did you use? The Z travel should be exact just like the X & Y.

Do your Z-screws turn easily using your hands?
I believe yes.
I dont believe it is losing steps, I cant see any layer inconsistency in the printed part.

What kind of anti-backlash springs did you use?
I dont know how to answer this, I just bought a generic spring

The Z travel should be exact just like the X & Y
Strangely the print height does not match but the Z movement looks ok.

Thanks for your answer.
Re: Dimension error in Z
October 25, 2018 06:31PM
The problem will become more pronounced when you do taller prints. 14.5mm isn't very tall. Remember
your extruder will continue to feed the same amount of filament. and if your Z isn't moving up the proper
distance, the topmost layers of taller prints will become mashed down and eventually your hotend will start scraping the top and
might cause a layer shift.

I only mention the backlash springs because on my first build I used springs that fit but
weren't necessarily the proper springs for the job. I sourced as much as I could from
McMaster-Carr and the black spring on the right was the only one I could find that fit. The right I.D. to fit over the 8mm rod and
12mm OD to fit in the X-end and it was the perfect length. I put it in and It worked great when I turned the
rod by hand and it moved the proper distance when I sent commands from the host. The
problems didn't occur until I started doing actual prints. The motors didn't have enough
torque when microstepping to overcome the amount of tension the springs were applying to the hexnuts
All of my prints came up short. I later switched over to the lighter gage
silver spring that has a lot less of compression tension. I can pinch the silver spring closed with my thumb and forefinger. The black one would only move about a millimeter when I pinched it.

Re: Dimension error in Z
October 26, 2018 05:30PM
Hmmmmm I think I got your idea Shank Man.
I will try change that springs and see what happens, thank you.
I didnt know which type of spring to use and saw a video saying to compress it to make it very tight, so tought a strong spring would be better.
Anyways, looks like my spring is not hard as yours.

I'm attached 2 images, what do you think about the problem and overal print quality?

Thank you again.
open | download - IMG_20181025_224504535.jpg (141.6 KB)
open | download - IMG_20181025_224433336.jpg (195.6 KB)
Re: Dimension error in Z
October 26, 2018 08:45PM
Did you set your layer height to 80% or less, of your nozzle diameter? Those are some tall layers. Those layers aren't bonding together like they should. I don't see any evidence of your Z height coming short. Looks like you might have to adjust some speed settings and the X-Y jerk. Overall they look pretty good to me.
Re: Dimension error in Z
October 26, 2018 10:03PM
I'm using 75% (0.4mm nozzle with 0.3mm layer height), this is correct?
Also, I have fine adjusted the extruder recently (steps/mm).
This is same as I tought, the Z looks clean in the print, maybe the spring is not the problem but I will change it anyway.
I never dealed with jerk, dont even know what it does but I will try to learn something about it, thanks for the indication.
Thank you smiling smiley
Re: Dimension error in Z
October 27, 2018 02:22AM
After taking your measurements, did you change your Z steps using the formula "(expected length/measured length)*current steps per mm"? 80% layer height is an absolute maximum but its a good idea to keep it low, like 0.1-0.2 mm or 25-50% to achieve better quality and layer adhesion, but if you need a part quickly then do what you have to do. That X carriage print should work just as well with low detail/high layers.
Re: Dimension error in Z
October 27, 2018 09:14PM
The Z steps looks 99,9% ok.

About the Z springs, I need some help from you.
I dont understand much about springs, so I have "weighed" it, I put it in a scale and compressed the same much as when installed, the one I as using is around 500g, is it ok?
I have other one but it is 700g, I can cut it if I know how much I need but I dont have a clue! Over internet it just says the diameter and lenght, this is not enough.

Thank you very much.
Re: Dimension error in Z
October 28, 2018 12:16PM
don't get too hung up on these springs. If you ordered a kit and the springs came with it then chances are they are the right ones.
If your prints are coming out the right height than those might not be the problem at all. If you can squeeze the springs closed with your fingers they
should be okay to. I have read on some posts that anti-backlash springs on the Z-screws are a waste.The weight of the motors,rods,x-carriage help to overcome the backlash. The X & Y Axes are the ones that are rapidly and constantly changing direction are more subject to backlash than the slow moving Z which is only moving up. I don't want to give bad info so it would be best to pose this question on the General or Reprappers section.
Not too many people in the know post over here anymore.

Edited 1 time(s). Last edit at 10/28/2018 12:17PM by Shank man.
Re: Dimension error in Z
October 28, 2018 11:35PM
That is the problem, it is not a kit, I chose a model from the internet, ordered the parts, my friend printed the printable parts for me and I'm assembling it, so, I have no clue on what kind of spring I should use and all info I can get from mine are this compression measured in a scale.
The Z moves up and down when the extruder retracts, so, it changes its direction a little more than just layer by layer, anyway it is much less than X and Y for sure.
I will check that sections, thank you.
Re: Dimension error in Z
October 31, 2018 11:39AM
I've been working with a Prusa i2 for years.

I've only seen slight mention of z-steps. You shouldn't need to "calibrate" the z-steps. The exact z-steps can be obtained by knowing the following.

Pitch of the z-screw(s): Dependent on what you have
Eg. 0.8 mm per rotation for M5, 1.0 mm per rotation for M6, 8 mm per rotation for a T8 8 mm 4-start lead screw, the motor step angle

Motor step angle: Dependent on what you have
Commonly 1.8deg or 0.9deg

Driver microstepping: Dependent on electronics and how you've set them
Generally RAMPS and 1/16, make sure you have all 3 jumpers fitted under each of the stepper drivers on the board or know what you have these set to

Then plug those details into https://www.prusaprinters.org/calculator/ and let it do the math for you.

That should be all you need, you needn't be concerned with backlash or anti backlash springs.

The only thing you need is for the mechanics to move freely. If you remove the z-screws the entire assembly should move up and down smoothly without much resistance. If not, your guide rods most likely aren't parallel. Then you'd want to fix that.

You'll also want to set the default_max_feedrate for z-axis in your firmware to anywhere between 0.5 mm/s to 2 mm/s. The default of 5 mm/s is too fast and could cause the screws to bind and throw them out of sync.
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