X and Y dimensions off and by different amounts

Mostly Printed CNC - MPCNC Troubleshooting
#1

I just tried cutting out a closed shape with a cut out circle in the middle. Both the X and Y dimensions of the resulting piece are off, but by differing amounts. The width (X) of the piece should have been 292 mm, but was 287.3 mm. The length should have been 343 mm, but was 332 mm. So I’ve lost 1.6% in X and 3.2% in Y of the overall dimension. The circle cut out was 71.5mm in Y and 73.5mm in X when it should have been 70 mm in diameter. I was cutting in 1/8 inch birch plywood with a 1/8 inch down cutting end mill feed rate of 20 mm/s with a 3.5 mm pass (so entire thickness of plywood). Designed in AutoCAD, brought into EstlCAM 8 to create gcode. Controlled MPCNC with RepetierHost

  1. Did you buy everything from here? YES. Original 23.5mm parts purchased back in April/May 2016
    a)If you didn’t or changed some things please don’t leave out any details.
    b)What firmware? that which came on the board back in April/May 2016.

  2. Are you using end stops? NO

  3. Are you using all my recommended parts? ALL EXCEPT
    a) Using HICWIC mount for the Dewalt 660

  4. Include a picture so obvious errors might be spotted.

#2

How long is your Z axis?

Try a pen and see it it’s better.

That is fast and deep. Both your cuts are small meaning your tool didn’t catch up to the pressure you put on it dragging it through the material so hard. I’m sure a pen will show much closer dimensions. Slow down or cut it in 2 passes and your dimensions will be better. You can have fast or precise not both at the same time. Also shorten your z axis by moving the material up and I bet the dimensions will be closer as well.

How big is your overall machine?

A picture would eliminate some of the guesses I just made. And I could narrow it down a bit but those are the things everyone seems to be doing wrong at the beginning.

#3

You’re right, must have been the cut speed and depth of cut!

I ran the same gcode with a pen and the work surface raised about 3/4 inch. Overall dimensions within less than 1 mm when accounting for the 1.5 mm offset on either side because the gcode was built for a 3.0 mm diameter end mill. I recalculated the gcode for engraving along the center line and used a 0.5mm pen. Overall dimensions both X and Y within less than 0.75-1 mm short and circle was right on.

I need to learn patience! I’ll dial back the speed and cut depth per pass and raise the work surface even more.

Do you think 20 mm/sec is reasonable with a 1-1.5 mm cut depth per pass or should I reduce the speed/cut depth more? I’m using a 1/8 inch end mill I purchased from your Store.

As soon as I get the speeds correct, I’m back to making my 5 yr old daughter a four string “medieval lute.” I have no idea if it will actually sound like anything but she’ll have fun making “music” with it!

I’ll post pics when done

Thanks for an awesome tool!!

Oh, for the record I built it out at 30 x 30 inches so about a 19 x 19 inch work area. Z-axis is 7 inches from work surface to bottom of lower gantry rail

#4

The shorter your machine the faster it can go, or deeper.

This is why I don’t put speeds or depth of cut anywhere. so many variables. I suggest always making test cuts. make a smal parts with all the right features and use it with every material change. I do it to make sure my bits are sharp. When they start to dull your dims will suffer, becuase eventually you will have to go too slow. There is such a thing.

Practice makes perfect…experience makes better parts? Need a slogan…that one didn’t work