Cuts off by .025"

Mostly Printed CNC - MPCNC Troubleshooting
#1

Hey all, after months of it sitting idle, I’m back working on the CNC. I seem to have all the frame/gantry squareness issues sorted out now, and I’ve successfully cut some test pieces from plywood. Something is still not right, as my routed parts are not coming out the correct sizes. Take this piece for instance (see photo) - the square is supposed to be 5" on each side, with a 3.5" circle cut out of it. Instead, it’s coming out at around 4.975" and the circle varies from 3.5-3.52". The corners on the square seem to be 90degrees and corner to corner distance is good, so it doesn’t seem like it’s out of square. I don’t even know where to check, what would this be a symptom of?

This is with an 1/8 endmill, 1mm cutting depth, 15mm/s travel speed. The routed edges are all clean and straight.

I’m going to check perpendicularity of the DW660, but I wouldn’t think that would cause these symptoms. What else should I be checking?

Thanks,
Jason

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#2

3 thousandths is pretty good how precise do you need it?

Plywood is not a very good material to try and get precise measurements from. Plastic might have better results. Have you measured your bit diameter, vs the cut width?

Have a look at the faq’s about tolerance.

#3

I’d be happy with 3 thousandths, I’m seeing around 3 hundredths though. I’ll double check bit diameter.

#4

Sorry I wrote that very early in the morning. I am not saying it can;t get better but now you are in serious detail territory. It can literally be anything. belt tension, total flex in your machine, runnout in your spindle.

How big is the machine and how far out was the z axis hanging? The easiest test is move the the work all the way up as High as you can and run it again without changing anything else, same bit same material same gcode. That will check your z axis flex (most likely the issue I am guessing). If it was all the way up try cutting the speed in half just to see if it gets better, too fast and it will flex the system to far.

#5

End mill diameter was good. I was cutting near the bottom of my Z axis range, so I raised everything up 1.5" to be near the top of the range (I have about 4 inches of travel on Z) but I’m still having the same problem. The square is still consistently about 1/32 too small all around, and I have about .025 of variance all around. I’m going to try a different end mill tomorrow, but I’m doubtful that’s the problem.

#6

So when you cut a single pass slot with your bit it is coming out 1/8" wide, Or is the problem there as well?

Double the size see if the problem doubles or stays the same.

#7

Initial pass seems to be right at 1/8". I tried cutting some larger parts (rings at 18" diameter) and they seemed to be off the same ~ .025-.03" as the smaller circles, so it appears pretty consistent.

#8

Use a square so you can figure out if there is a problem on just one axis or both.

#9

Sorry, I’m not sure what I should be checking with the square. I’ve put a square on all corners of the square, and they all seem to be 90 degrees. , The square is consistently .03" short on both sides.

#10

Try just drilling a hole at each corner and measure, if the belts aren’t quite tight enough you could get slippage under pressure that wouldn’t show up when moving above the surface.

Though it’s also possible your bit is just cutting on the wrong side of the line… Cutting on the inside when you are doing a square like this would make it too small by twice the bit width, cutting right on the line would make it too small by half that.

#11

Thanks for the help guys. I re-tightened all my belt last night in case that might be the issue, but I’m having the same issue - actually it’s worse now since my latest test piece was out of square on one side too :frowning: The only thing I changed was redoing the belts, I have them a decent amount tighter than before, so I was really hopeful that might be the problem.

I did pause the print during the job and took some measurements. After the first pass, the cut slot was at 1/8", and the square measured right at 5" (5.09 or there about on my calipers). But the final piece still measures around 4.95ish, with the one out of square side now measuring near 4.9.

A new end-mill is supposed to be arriving today, but I’m fairly pessimistic it will resolve anything. Actually, I’m pretty pessimistic I’ll ever get this calibrated well enough to be useful for anything.

#12

You think being off in plywood by .09" makes it useless?

Have you used a finishing pass? It kinda sounds like you just verified you don’t. If you need your cuts that perfect you need to cut them very carefully. Make a roughing pass and leave some material then come back and make a finishing pass to perfect it. This is how high precision Cuts are made, never all in one pass. You can see how I do this in the intermediate ESTLcam walk through.

#13

Lets see a full picture of your machine with your material mounted.

Also sounds like you over tensioned your belts, you made it worse.

#14

I probably did over-tension the belts, I definitely cranked them down.

I haven’t been using a finishing pass, but I’ll reread your intermediate walk-through and give that a try. That makes sense, although I didn’t think it was necessary for cutting wood. And just to be clear, my frustration is fully with myself and not at all with the machine, which is unquestionably a solid design. The setup, calibration, and learning curve have just taken much much more time than I anticipated. I actually built the machine to make some parts for a different project that has now been languishing for nine months while I try to get this working, and I’m realizing now I was definitely being too ambitious.

Thanks again for your help, the support here is great, I’m always amazed how quickly you answer questions.

I’ll get some pictures of the whole setup later on today.

Thanks again,
Jason

#15

Let me try this another way. The thorough way.

  1. How big is you machine? x, y and Z.
    You said 4" of z travel but you only moved the material up 1.5" why not 3"? Chances are good your cutter is only .75" long. Every mm counts in z hang out. Every time to half the hang out distance you double the rigidity of the gantry.

  2. Can you use any other material?
    Plywood is the absolute worst to cut for many reasons. The only reason to test with it is if that is exactly want you want to use in your final project. If that is the case 0.02" is probably as good. pine is a really good test wood if you are planning on wood.

  3. Are you using my components bought from me or other?
    Different parts react differently. My tool mount?

4)What are you using as a spindle and what bit is in it?
To make sure it is the appropriate tool for the job.

  1. Exactly what tolerances are you needing and in what material?
    Just to see if we are on the same page.

  2. What software are you using to make you gcode? What version, and what firmware version?
    Major updates have happened recently.

  3. Are you using a finishing pass? How much material are you removing (what was left from roughing)? Full depth finishing pass or in steps?
    Proper gcode sequencing.

8)Is your spindle perpendicular?
This is also less of a factor the less your z axis hangs out.

#16

I am trying to help. I understand the frustration isn’t directed towards me. Early on I learned it is best for me to help trouble shoot one question at a time or people tend to change to many variables at once when I am just trying to get a clear picture.

We can get you there, just have some patience. you initially got your machine within .02" you did the assembly very very well. Most people would never even notice that. But if you are trying to get better results this could take some time. The finishing pass should get you much better results. If it does not answer the above questionnaire and we can go from there.