Z Axis Printed Part Dimensions

Hi I have been using my MPCNC quite a bit to print non-dimensional random 3D parts from Thingiverse and it is has been great. I then created a 20mm cube on TinkerCAD sliced it on Repetier Host with the settings detailed on the screenshots from this site. The extruder is an E3D V6 clone with 0.4mm nozzle & heated build plate. The finish of the parts are great they are smooth and a lot better than my CTC Replicator clone , however the Z dimension of the cube is smaller by 2mm , X and Y are 0.5mm smaller , I’m not really bothered by 0.5mm but 2mm is a lot. I checked the Marlin RC7 MPCNC w/LCD firmware (it was originally set to 4535.44 ) and because it is M8 threaded rod I’m using the value for the Z axis is set to 5120. I then looked back at the slicing of this cube using Slic3r and there are 76 individual layers each with a layer height of 0.2752mm which when multiplied by come out at 20mm give or take a very small amount (the first layer is 0.32mm). Do you have any pointers , the only thing I can think is that the pitch of the threaded rod is less than the standard pitch of 1.25mm for M8 or it is inch rod which I don’t think it is.
Thanks -Jim

You just need to change your zaxis steps to match your screw. test is easy. move your axis (eg 100mm) with repetier and measure the actual distance it moves (eg 98mm).

new z steps = (current z steps * what you wanted it to move (eg 100mm))/what it actually moved (eg 98mm)

Don’t try to do this with 20mm. The longer the move the more accurate your axis will be. If you have a 200mm z axis use 200. Nothing less than 100 though.

Thanks for that I was suffering from brain fade. So when commanded to move 100mm the z axis only moved 89mm so new z steps = 5120 * 100 / 89 = 5752.8089

Well I found the issue!! It turns out even though I was changing the z axis steps from 4535.44 to 5120 in the firmware to suit my M8 threaded rod this number wasn’t being changed when uploaded for some reason. So to test I changed text MPCNC to MPCNCA in Configuration.h this to give a visual clue that the firmware was actually being updated when I compiled and uploaded to the RAMPS Arduino. When it was booted I was seeing the MPCNCA text on the LCD so it is definitely being uploaded but , when I used the menu on the RAMPS LCD I noticed that the value for z steps was still set to the original 4535.44 WTF! I manually changed this value to 5120 and saved it using the Ramps LCD Menu. After the step count was changed a perfect 20mm cube give or take 0.4mm was printed. It is a strange one but I remember reading on some forum somewhere that this issue has been reported before but I can’t find it to confirm. Anyway if anybody runs into this issue at least you are not alone…
Best Wishes -Jim

See here for details of why this issue happened.