Hello
I have a circle problem. I make it in Aspire and I want a circle with a diameter of 57.5mm, I get 56.2mm. When I make a 70x70mm square, it comes out to be 70.02x70.2mm. I cuut insisde profile. I use a 3 mm router. I have Ramps and DRV 8225 and use Marlin. Steps per mm is set to 199. Why is there a difference?
Hi my initial thought is that your steps per mm are not quite correct.
I wound recomend cuting a square part and a square hole of the same dimentions and same bit, the actual error of each you measure should be the same either side of the nominal design dimension. If that is not true then the steps per mm needs tweaking.
You should not need to adjust the steps per mm. I see you already set them to 199. So you probably thought to adjust them for the square? The belts stretch a tiny bit, but everything else is integers (16 teeth, 200 steps per rotation, etc).
If you want precision parts, you should enable a full depth finishing pass and choose something like 0.1-0.3mm for the finishing pass. The bit will flex during the roughing pass and then just kiss the surface in the finishing pass. The FP will have zero load and just remove to the line.
You can also verify the machine mechanics with a pen. There is a nice tool for drawing a ruler in both directions by @jamiek:
https://vector76.github.io/gcode_tpgen/
Just make sure your pen and paper are set up for accurate marks or you’ll be chasing your tail.