CNC print only 79-80% of actual Size

Hi just got my CNC machine running a SKR 1.4 Turbo with 5160 driver, Marlin 2.x bugfix version. I’m using a licensed version of Fusion 360 and everything works perfectly except the print is only circa 80% of the actual size. I’ve tried both the prost processors on this site and both produce identical results. I’m baffled, I can’t find a scaling function in Fusion. Any ideas.

Have you tested and measured that a manual move is the proper length? This will tell you if the issue is in the machine configuration or somewhere in the CAM process.

Good point, no i haven’t i’ll try that now. thanks :grin:

Once you have confirmed Tom’s suggestion work backwards.

Using the screen (do you have a screen?) or your connected pc running your chosen app, zero the coordinates and the send

  • a G0 Z20, did it move 2cm up,
  • then a G0 X100, did it move 10cm along X
  • then a G0 Y100, did it move 10cm along Y

If these all worked then draw a box in Fusion the same size and do a trace mill (bit centred). Export that with the post and edit the GCode. You should see almost the same GCode with stuff around it and feed rates.

Last ideas, if you are not working in mm then change to mm. Both PP work better in mm.

Thanks, I’ll get the feed rates sorted and test it. Many thanks :grin:

Thank you for your very quick and insightful help. You were right it was the steps per mm on the x & y axis. The Z axis was ok. I should have realized this myself but i think I was a bit overwhelmed with learning how to CNC and not looked at it logically. Lesson learned (again!) first principles. I’ve adjusted these and now working perfectly with a very high degree of dimensional accuracy. Thank you. :grinning:


Glad it was that simple. Before anyone else mentions it, I’ll also say “did you locktite the grub screws on your pulleys?” It’s pretty common for them to work loose, and the excessive backlash that results is sometimes mistaken for other issues (lost steps, changed steps/mm, etc.) If you didn’t use threadlock to start with, you might consider having some standing by when (not “if”) the situation arises.


What did the steps end up being?

X steps = 101.3
Y steps = 100.5
Z steps= 400

1 Like

I got the same result when trying to measure my steps per mm but then someone said that “Usually the measured number is larger due to belt stretch” in my thread about it: Calibrate steps per millimeter, or not?

1 Like

Interesting. What were they before, I wonder.

They were originally set to 80, 80, 400.

Huh. Ok. Where did that firmware come from? The MarlinBuilder releases should all be 100,100,400 or 200,200,800 (for drv8825 varieties).

Not a big deal. It looks like you got it figured out.

Its a good question. I created the firmware myself from Marlin 2.0 bugfix based on a post from a guy called Saberlod I found on Youtube. However he was putting in an SKR 1.3 and i have gone for a 1.4 turbo. Also his firmware changes were for an earlier release of Marlin so not quite 100% suitable. I remember changing the feed rates but I can’t remember why! I thought it was one his suggestions but i’ve looked back through and can’t find a reference to it. I’ve obviously had a brain fart somewhere along the line lol!

The 80 is usually for 20T pulleys. The recommended is 16T (which you have).

1 Like