Newbie: Successful build but need help with final tweaks please

Hi all
I have just completed the build of my RepRapPro Mendel. Took me just over a week to complete and I am very pleased with the results, especially as I have never done anything like this before. It printed first time, but have a couple of issues which may be related.
1) some layers keep shifting along the x axis. This has happened on several prints, but not all, and varies from 2nd layer to much later on. On the very first test print I did of a cube, the cube slanted along the x axis. I have since tightened the belt on the x axis and the prints are now straight, but the can suddenly jump 1-2mm left or right when printing.
2) my test print of a 20mm cube highlighted that prints along the x axis where over scale. For example the 20mm cube was 24mm along the x whilst y and z were fairly close to 20mm. I also printed the fan mount and this resulted in the hole spacing being incorrect. I have temporarily overcome this by.rotating 90 degrees in slic3r so the the critical dimension was along the y axis.
Any advice on how to fix this would be greatly appreciated so I can get on with enjoying the world of 3D printing!

Thanks
Dave

Hi Dave

It sounds like you may have the wrong pulley on the X axis; this would explain the incorrect movement on the X axis, and the jumping. Check that your X axis pulley looks like your Y axis pulley. I'm not sure how this could happen. The only other 'pulley' is on the extruder, though this is a gear, and quite a bit different.

Ian
RepRapPro tech support

Ian

The x axis pulley is the correct one. (i.e. it looks like the y axis one, and not the gear that is on the extruder motor).
Could the belt be too tight or too loose? Could this be anything to do with voltages to the stepper motor?
I am a bit lost as to what to try to fix this issue.

Dave

Hi Dave

The voltages shouldn't cause this issue, though you can check them if you want; I set the reference voltage (0.6V), while flashing them with the latest firmware, before we pack them into kits. See: [www.reprappro.com]

Can you post a picture or two of your machine? It might make it obvious what the problem is. Also, send M503 to the printer, and copy the resulting output from pronterface's log window into your reply. This will let me check that the steps per mm haven't changed, amongst other things.

Ian
RepRapPro tech support

Here is the M503 data:

SENDING:M503
echoteps per unit:
echo: M92 X92.635 Y92.635 Z4000.000 E650.000
echo:Maximum feedrates (mm/s):
echo: M203 X300.000 Y300.000 Z3.000 E45.000
echo:Maximum Acceleration (mm/s2):
echo: M201 X800 Y800 Z30 E250
echo:Acceleration: S=acceleration, T=retract acceleration
echo: M204 S1000.000 T1000.000
echo:Advanced variables: S=Min feedrate (mm/s), T=Min travel feedrate (mm/s), B=minimum segment time (ms), X=maximum xY jerk (mm/s), Z=maximum Z jerk (mm/s), K=advance_k
echo: M205 S0.000 T0.000 B20000 X15.000 Z0.400 E15.000
echo: M206 X-40.000 Y-10.000 Z0.000
echo: M208 X210.000 Y210.000 Z140.000
echoID settings:
echo: M301 P12.000 I2.200 D80.000 W125
echo:Thermistor settings: M304 Hh Bb Rr Tt, H0=Bed, H1..n=nozzle, b=thermistor beta value, r=series resistor, t=thermistor resistance as 25C
echo: M304 H0
B4126 R4700 T10000 M304 H1
B3960 R4700 T100000
FPU Enabled no

I will take some photos of the build too and post them later.

Dave

Scaling issue now solved.
I sent the command: M92 X81.258 to the print to rescale the x-axis and then printed the 20mm cube perfectly.
I found the calculations here but then had to search the internet on where to put this info. Finally realised that it was the M92 command. Maybe this should be included in the setup instructions in the troubleshooting section?
Strangely it also seems to have solved most of the shifting problem too.
Now to get printing !!

Dave

Dave

I think you have a recent kit, with black belts? And the X and Y pulleys are the same?

We currently use MXL belts (pitch = 2.032mm) with a 17-tooth pulley. The steps per mm should be the same for both axes, and are calculated as follows:
(Motor steps per revolution x microstepping) / (number of pulley teeth x pitch of belt) =
(200 x 16) / (17 x 2.032) = 92.635
Josef Prusa's calculator here [calculator.josefprusa.cz] can help calculate this for you.

We don't cover this in the build instructions as, generally, it's only the extruder that needs calibration, which we do cover: [www.reprappro.com]
This also includes the instructions about using M92!

I think, for some reason, your X motor is not working correctly. Either it is not stepping correctly, the motor current isn't high enough, or there is some other issue. By fudging the steps per mm, you may store up problems for later. Try swapping the X and Y motor axes (swap the wires at the screw terminals on the Melzi), and see if the movement is consistent.

Ian
RepRapPro tech support