I completed the build of my MPCNC a week ago. I drew the crown and it looked good so I moved on to cutting some stuff. Originally I was using the smallest bit I had (1/4" straight flute router bit) and once I got the steps/mm dialed in I was able to cut 1" test squares that were consistently ±0.010". About half the time they would even be ±0.005". I was happy with that so I moved on.
I got some new bits and today I was using the smallest bit (1.5mm spiral up 2 flute) and just messing around. I tried to cut some finger joints that came out a bit loose. The fingers were about .020" undersized. Out of curiosity I expanded my digital caliper and set it on the table with the head butted up to the cutter bit (cutter was not running). I zeroed the caliper and used the manual controls in repetier to jog the head 10mm over toward the caliper. I did this a number of times (zeroing the caliper each time). When telling the machine to move the head 10mm I was getting anywhere from 9.90mm to 10.29mm.
I tightened up the belts because the seemed a little bit loose and tried cutting some more 1" test squares. They were also about 0.020" under sized. I adjusted the steps/mm again (added 3.5 steps/mm on x and 4.5 on y) and was able to get them consistently ±0.005".
After all of this, I tried checking the motion with the digital caliper again. Now when telling the machine to move 10mm I am getting anywhere from 9.91mm to 10.19mm. It is slightly improved but I would assume that with almost no load (just enough to push the head of the caliper) the motion should be more consistent.
Am I expecting too much or is a variance of ±0.014mm per 1mm of programmed movement normal?