HI,
I have recently built a MPCNC and really enjoyed the process still have a few things to do but was really eager to test out the machine.
I drew a 30mm circle in estle cam its 30mm and in CNC js but its 37mm when the machine draws it.
The only thing I can thin of is that i used 20 teeth pulleys instead of 16 teeth pulleys.
Please help.
That might be, did you check it? 
HI Philipp,
im not sure where to check and fix that
If you count the teeth on your pulleys, how many are there?
Apologies, i didnt explain what i mean. I know i have 20 teeth pulleys but im not sure where in the firmware i need to make changes.
What controller board did you use for you MPCNC?
Skr 1.4 with TMC2209 stepper drivers
Send an M503 to the controller and save the output so we know what you had.
Your steps are wrong by 20/16, so we want to reduce your steps/mm for X and Y so they are only 3/4 of the current value.
You send M92 to set a new value and then M500 to save it.
If you post what your firmware shows for M503 we can confirm the correct values for you to use.
Assuming GT2 timing belts that are actually 2mm pitch (this is unfortunately not guaranteed):
16T pulleys are the specified, and you get 100 steps/mm at 16× microstepping with a 1.8° stepper motor.
With the same parameters, you get 80 steps/mm from the 20T pulley.
For the SKR, you should be able to adjust the steps/mm in the Marlin firmware. Since this is not a V1 supported board, your me us may look different, but it is one of the settable parameters.
Use M500 (or save to EEPROM) to save it so you don’t have to reset it every time.
For 20T pulleys, you basically cut the steps/mm by 20%. So from 100, go to 80. 50 goes to 40 (jackpots are default set up for 8× microstepping.) Change for both X and Y
Z steps/mm is usually 400 (T8 4 start lead screws) – don’t change that.
I ran the M503 command and it returned this.
09:36:25.920 : echo:; Linear Units:
09:36:25.920 : echo: G21 ; (mm)
09:36:25.920 : echo:; Temperature Units:
09:36:25.920 : echo: M149 C ; Units in Celsius
09:36:25.920 : echo:; Steps per unit:
09:36:25.920 : echo: M92 X200.00 Y200.00 Z800.00
09:36:25.920 : echo:; Max feedrates (units/s):
09:36:25.920 : echo: M203 X50.00 Y50.00 Z15.00
09:36:25.920 : echo:; Max Acceleration (units/s2):
09:36:25.920 : echo: M201 X180.00 Y180.00 Z80.00
09:36:25.920 : echo:; Acceleration (units/s2) (P R T):
09:36:25.920 : echo: M204 P180.00 R3000.00 T180.00
09:36:25.920 : echo:; Advanced (B<min_segment_time_us> S<min_feedrate> T<min_travel_feedrate> J<junc_dev>):
09:36:25.920 : echo: M205 B20000.00 S0.00 T0.00 J0.04
09:36:25.920 : echo:; Home offset:
09:36:25.920 : echo: M206 X0.00 Y0.00 Z0.00
09:36:25.920 : echo:; Endstop adjustment:
09:36:25.920 : echo: M666 X0.00
09:36:25.920 : Y0.00
09:36:25.920 : echo:; Stepper driver current:
09:36:25.920 : echo: M906 X900 Y900 Z900
09:36:25.920 : echo: M906 I1 X800 Y800
09:36:25.920 : echo:; Driver stepping mode:
definately the steps i did the M503, M500 and changed it to 160 for X and Y and was getting 29mm on a 30mm circle. Changed it to 162 and im getting 30mm.
going to have a look at the firmware.
Anytime the steps don’t match the mechanical characteristics of the machine, I worry there is an underlying issue. Your need to change to 162 vs 160 may have a different cause than the teeth on the pulley. It could looseness of your core, deflection in the bit, the size of the bit as specified in CAM being wrong, tramming issue, speed and feed issue, CAM settings, etc.
I would test cutting out some squares of wood of different sizes and with different bits. Be sure to use a finishing pass when making the cuts. If the sizes are inconsistent, go back go 160, and look for other causes for the final 1 mm of discrepancy.