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Why my printed object is 1mm shorter?

Posted by yun 
Why my printed object is 1mm shorter?
May 04, 2013 05:50AM
I recently noticed that the objects printed by my Mendel90 is always 1mm shorter then design. For testing purpose I printed a 8mm x 8mm x 5mm block. The printed object I got was 7.98mm x 8.05mm x 4.05mm. I am pretty satisfied by the dimensions on X and Y direction, but on Z direction, it is almost 1mm shorter than my design.

I tried to print taller object, say 8mm x 8mm x 10mm. The object I got was 8.92mm height, which is again 1mm shorter than expected.

Does anyone have idea how could this happen? What should I do to fix this issue? Thanks in advanced.

Edited 2 time(s). Last edit at 05/04/2013 05:55AM by yun.
Re: Why my printed object is 1mm shorter?
May 04, 2013 07:27AM
First check the last layer in the g-code file to see what the Z of the last layer is. That will determine if the problem is in the slicer or the printer(firmware)
Re: Why my printed object is 1mm shorter?
May 04, 2013 11:07AM
Hi Banjer, thanks for the reply! I just checked the g-code and I can see the these commands at the end:

G92 E0
G1 Z5.100 F7800.000
G1 X-2.950 Y-2.950
G1 F1800.000 E1.00000
G1 X2.950 Y-2.950 F600.000 E1.08679
G1 X2.950 Y2.950 E1.17357
G1 X-2.950 Y2.950 E1.26036
G1 X-2.950 Y-2.887 E1.34622
G1 X-2.950 Y-2.950 E1.34714
G1 X-3.369 Y-3.369 F7800.000
G1 X3.369 Y-3.369 F600.000 E1.44625
G1 X3.369 Y3.369 E1.54536
G1 X-3.369 Y3.369 E1.64447
G1 X-3.369 Y-3.306 E1.74265
G1 X-3.369 Y-3.369 E1.74358
G1 X-3.788 Y-3.788 F7800.000
G1 X3.788 Y-3.788 F600.000 E1.85501
G1 X3.788 Y3.788 E1.96644
G1 X-3.788 Y3.788 E2.07787
G1 X-3.788 Y-3.725 E2.18838
G1 X-3.788 Y-3.788 E2.18930
G1 X-3.425 Y-3.579 F7800.000
G1 F1800.000 E1.18930
G92 E0
G1 X2.665 Y-2.398 F7800.000
G1 F1800.000 E1.00000
G1 X2.398 Y-2.665 F848.613 E1.00558
G1 X1.802 Y-2.665 E1.01439
G1 X2.665 Y-1.802 E1.03242
G1 X2.665 Y-1.207 E1.04123
G1 X1.207 Y-2.665 E1.07173
G1 X0.611 Y-2.665 E1.08053
G1 X2.665 Y-0.611 E1.12348
G1 X2.665 Y-0.016 E1.13229
G1 X0.016 Y-2.665 E1.18770
G1 X-0.580 Y-2.665 E1.19651
G1 X2.665 Y0.580 E1.26437
G1 X2.665 Y1.176 E1.27318
G1 X-1.176 Y-2.665 E1.35350
G1 X-1.771 Y-2.665 E1.36230
G1 X2.665 Y1.771 E1.45508
G1 X2.665 Y2.367 E1.46389
G1 X-2.367 Y-2.665 E1.56912
G1 X-2.665 Y-2.367 E1.57535
G1 X2.367 Y2.665 E1.68058
G1 X1.771 Y2.665 E1.68939
G1 X-2.665 Y-1.771 E1.78217
G1 X-2.665 Y-1.176 E1.79098
G1 X1.176 Y2.665 E1.87130
G1 X0.580 Y2.665 E1.88011
G1 X-2.665 Y-0.580 E1.94797
G1 X-2.665 Y0.016 E1.95678
G1 X-0.016 Y2.665 E2.01218
G1 X-0.611 Y2.665 E2.02099
G1 X-2.665 Y0.611 E2.06394
G1 X-2.665 Y1.207 E2.07275
G1 X-1.207 Y2.665 E2.10324
G1 X-1.802 Y2.665 E2.11205
G1 X-2.665 Y1.802 E2.13009
G1 X-2.665 Y2.398 E2.13889
G1 X-2.398 Y2.665 E2.14447
G1 F1800.000 E1.14447
G92 E0
M104 S0 ; turn off temperature
G28 X0  ; home X axis
M84     ; disable motors
G28 ; home all axes
; filament used = 40.5mm (0.3cm3)

So I think the Z value of last layer is 5.1mm. It seems ok since my layer height is 0.3mm, and there are 17 layers to print for the 5mm height block.

Do you have idea where is the problem? Thanks a lot.
Re: Why my printed object is 1mm shorter?
May 04, 2013 11:25AM
sounds like your firmware settings are wrong or your z-speed is too high, more than likely the later if this is a nophead kit, try again with limiting z-speed to 1.5 in your slicer
Re: Why my printed object is 1mm shorter?
May 04, 2013 12:28PM
Are your Z-axis pointers staying in sync? If not, you could be losing steps on the Z raises as well, though I doubt it would be so consistent.
Re: Why my printed object is 1mm shorter?
May 04, 2013 12:28PM
Hi hexitex, thanks for the suggestion. Yes it is a nophed kit, so I would like to change the z-speed in slicer. I am using Slic3r, I can see some settings for speed under the "Print Settings" tab, but could not find the setting for z-speed. Could you let me know where is the configuration of z-speed? Thank you.
Re: Why my printed object is 1mm shorter?
May 04, 2013 12:34PM
Hi Fl0yd, thanks for the idea. I observed (by eyes) the z axis pointers and I think they were staying in sync.
Re: Why my printed object is 1mm shorter?
May 04, 2013 12:45PM
You could take an object of a known height, zero your Z height, then tell your Z to rise by the object's height.

Example for 30mm object:
M28 - home all
G1 X0 Y0 Z0 F300 - middle of the bed, Z touching the bed (if you've leveled well)
G1 Z30 F300 - raise Z by 30
Re: Why my printed object is 1mm shorter?
May 04, 2013 12:48PM
Slic3r expects the firmware to limit the Z speed, which it does, so it shouldn't be a problem with the firmware supplied with the kit.

The Z pointers should be set to point to the bars when the axis is homed and levelled. Then any time it is homed, if they don't both point to the bar one has slipped.

Having said that I can't think how the object could be consistently 1mm short. It would have some very squashed layers which should be obvious looking at the sides.

Try printing something with a 45 degree slope and see if it ends up with steps in it.

Re: Why my printed object is 1mm shorter?
May 04, 2013 02:46PM
Hi Fl0yd and nohead, thanks a lot for the suggestions. They are very helpful and I have found the problem by following the direction you suggested.

It seems the problem is on my X ideler, the ideler bracket does not follow the M6 brass nut all the time. It is ok after moving the idler up:

However after moving the idler down, the brass nut is pulled out of the slot, so the extruder is always higher than the expected postion.

I didn't notice this issue when calibrating Z axis, so I had to increase the Z_HOME_POS value about 1mm to make sure the extruder can touch the glass. By doing so, the extruder is actually too low for printing. When the extruder moved up, the M6 brass nut will be pushed back into the slot and "eat" some height. I believe that's what happened.

This picture shows the first few layers are squashed.

What will be the best way to fix this issue then? Can I glue the M6 brass nut with the bracket together? Or should I put a heavy object on the X idler to make sure it always follow the nut?

Any idea will be appreciated.
Re: Why my printed object is 1mm shorter?
May 04, 2013 04:06PM
The weight of the axis normally keeps the nut in place. If it doesn't the most likely thing is the axis is not level or the Z rods are not parallel and the so the Z axis is binding.

If you drive it to the bottom and then manually slide it up and down (leaving the nuts at the bottom) does it move freely? If not drive it to the top and loosen and re-tighten the right bar clamp the drive it to the bottom and loosen and re-tighten the Z motor bracket. That should make the bars exactly parallel.

When the axis is lowered to the bed the base of each X end should be the same height above the base of the machine. Also when at the top the distance to the bar clamps should be the same.

If it isn't and the axis has been levelled to the bed it implies the bed is not level for some reason. That can be fixed by adding washers under some of the bed pillars.

I had a machine behave like that. The reason was the Y carriage was bent on the side with one bearing. I think I bent when I dropped a spool on the bed. The solution was to add washers on one side, front and back to get it level to within a fraction of a millimetre, then adjust the Z axis to get it spot on.

Edited 1 time(s). Last edit at 05/04/2013 04:08PM by nophead.

Re: Why my printed object is 1mm shorter?
May 05, 2013 07:39AM
Hi nophead, thanks for the help. I adjust the Z rods again and now it works fine.

I printed the 8mm x 8mm x 5mm block again and this time I got 8.03mm x 7.92mm x 4.82mm. Although it is still 0.19mm shorter, I think it is acceptable and I am satisfied with the result. Problem is solvedspinning smiley sticking its tongue out

Great thanks to the kind people here :-)
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