Dimensions are off

So I successfully created a reproduction artifact (see Archeo-build in Builds and Built) but because of the irregular nature of the artifact, I did not notice that my dimensions are off until cutting a few squares. In AutoCAD I created 50 mm and 100 mm squares and then used EstlCam to cut them out of 1/4-inch plywood. When finished, both squares are square (exactly same diagonals within 0.1-0.2 mm measured with some plastic digital calipers). The smaller one was only very slightly off. However, the larger one (100 mm) was off by 2-2.3 mm in both x and y axes. The x and y dimensions were 97.5-98 mm on a side with equal diagonals with acceptable error (0.1-0.2mm). So, over 100 mm, I lost 2 mm in each direction. Angles are 90 degrees at corners and edges of cuts appear to be perpendicular/vertical. I’ve repeated the larger square 3 times with similar results. I’m going to try two larger squares (200mm and 300mm) tomorrow

I was using a 1/8" 2 flute bit on Dewalt 660W with I believe 20 mm/sec feed rate and 1mm DOC on 1/4-plywood. I was getting nice chips and the spindle did not seem to be straining against the wood (too fast) or heating up and burning (too slow).

Any ideas or help would be greatly appreciated. I want to try to solve the dimensional inaccuracy so I can cut parts for the MP3DP.

Follow Up:
Just cut a 200 x 200 mm square. The X-axis is 198.6mm and the Y-axis is 196.1. Again, the diagonals (277.8mm) are exactly the same (at least within my ability to measure them) so it is cutting square, just loosing distances along the axes.

I also cut two circles which both came out circular. The 50 mm circle came out at 48.7 mm and the 100 mm circle was 98.7 mm. I cut a second set and the two circles each matched their respective other circle exactly so the MPCNC is cutting consistently, just of by about 1.3 mm in 20 cm

I got that as well on my test parts. I put up a picture in the new center thread. Mine weren’t off by that far .2mm on inside and outside cuts. Equal on both the x and y axis. I could adjust the bit diameter but that I wouldn’t get equal dimension on inside and outside cuts. I am not sure if it is a machine or a software thing. With the correct bit diameter I got equal inner and out .2mm small, that leads me to believe it might be a resonance (machine thing) like the vibration adding .2mm, .1mm on either side of the bit. This is on the new center, with 55% infill, I am almost done printing a set with 70% infill. I want to see if infill has a direct effect.

Sorry that was kinda off topic.

For you, if it is what I am thinking. Try the same cut with the same settings but make sure you have the z axis at a minimum length (raise your material to as close as you can possibly get it to the gantry. If your tolerances get tighter I was right. How to solve that…not sure the best route yet. Maybe its rigidity, maybe belt tension, maybe a sloppy tool, so many variables.
If you had it all the way up already, lower it and try again, if they get worse I am still on the right track.

Thanks Ryan! I’ll give that a try tomorrow when I have some time to play around with the machine and post results. I think you may be right about the vibrations since the 660W beast does kick off some vibration. I am also using the universal mounting system which given the design could allow for more vibration movement. Looking forward to the new center design and the new “quick” changing mounting system that looks like it’s more rigid. Keep up the good work. I’m now considering building one for home once I get the department’s finalized and calibrated. You never knew how much you need a cnc until you have one :slight_smile:

Just for giggles, I tried cutting a 40mm square. Got 36.35 on the X and 36.5 on the Y. I’m still using rotozip bits, so they’re kinda long, and downcut 2 flute, so that may have something to do with it. Real endmills get here maybe Friday. My lower outside bars are 5.25 inches from the table, with a half inch spoil board on top of that. If I want to go lower I’m going to have to cut the leg tubes, they’re touching the bottom panel of my table.

Wow! Real end mills really make a difference! 40mm square comes out 39.5mm now!

Barry- What end mills did you order if you don’t mind me asking?

Sorry, totally missed your post…
Got these.