Can’t find anything really wring with the MPCNC. There is some movement but that is to be expected with moving parts.
Nothing more than 0.01-0.02 mm
For example: The trucks on the rails can move a little along the rails with the steppermotors powered. But only somewhere between 0.01-0.02 mm again. And when I stick an allenwrench in the setscrew in the pulley (to use as a dail/pointer) I can see that the movement is in the steppermotor. With the 1/16 microstepping that I use that is around what one microstep is. I have to use a lot more force to make it skip steps.
Would 1/32 microstepping make a difference here? But even if it does I can’t see how this would make a part 1 mm over or undersize with higher DOC’s.
For testing I have taken the cablechain and vacuumhose off.
No. The full steps send all current to one coil and none to the other. A 1/2 step sends cos(45) to one coil and sin(45) to the other (both are around 52% of max). When 1/2 stepping, the next step is 1,0. Then back to (.52,.52), etc. The numbers are hard for humans when you get down to 1/16 or 1/32, but half of the positions for 1/32 are the same as 1/16. The new positions are in between the other ones.
The whole thing is just trying to encourage the stepper to sit in a slightly different position between steps. You won’t actually get 1/32 accuracy unless you have zero load.
And, you are right that 0.02mm is not causing your 1mm of error.
Have you measured backlash? Does the machine go to the same position if you drive it from the left or right without load? If you have room in your dial indicator, try moving 10mm right and back, then 10mm left and back. Jaime has a test pattern generator that draws a ruler from both directions with a pen too.
Try doing your finish pass with step downs instead of full DOC if accuracy is your primary concern, also use the shortest bit you can get away with (eg, if you are cutting 1/8" material, use a bit with a 1/4" doc at most).
I have tried that. And always back to the same stop within 0.01-0.02 mm.
I will have a look at drawing that ruler. I doubt that will show an error as the machine is accurate under low loads. But I’m running out of ideas myself, so will try that.
No play on the spindle and hardly any runout. (a little over 0.01 mm)
I use this spindle mount posted by V1 Engineering. It feels solid to me. I can’t see any movement Printed it with 3 perimeters with a 0.6 mm nozzle and matching top and bottom layers.
At the moment I one have one type of bit. I will get different ones when I get the hang of this milling or when I feel I need some thing different.
I can do accurate cuts, even without finishing passes. But that would limit the DOC to 1-1.5 mm. That will take a long time to make some parts.
I started this topic because when I tried increasing the DOC the accuracy suffered a lot. There are plenty of videos around that show the MPCNC do really hard work.
Ryan has posted a few. I’m sure he has tuned it a lot to get that far and his MPCNC isn’t a big as mine.
I just want to figure out what causes mine to get inaccurate that quickly.
Just rebuild and adjusted the core again. Reprinted it because I worried I had messed to original one up a bit when trying to get the play out of that one. Haven’t had time to test .
I did a test cut again to make a video. I will try to post that here or at least part of it.
But I can post a small gif I made of the flex that I see now. There is no play in any of the bearings, but there is some movement somewhere.
With the X or Y close to the min or max there is much less. But the more I go towards the middle of one of the axis that is much more.
That makes me thing it is flex in the longer rails anyway. Those are 25 mm RVS with 2.5 mm wall thickness. The gantry x rail is 945 mm.
It looks really bad in the gif, but the is a 1/8 bit and it took quite a bit of force to move it that much. And with the camera really close.
I’m printing new feel and going to rebuild it on the spoilboard.
That way I don’t have to take the original feet off and can I easily put it back to the larger size.
Building it on the spoilboard will give a small machine with a workspace of 320x320 mm.
That way I will know for sure if the large build is the problem.
Looking at your video, something is loose, bearings not touching etc… i didn’t see if you mentioned the size rails you’re running. Check that all your bearings are touching the rails and pay close attention to the z rail bearings. My lower ones love to loosen up from time to time.
I want to say that that is not te problem. But as I can’t find a clear problem myself, I will have a look again.
Maybe that video wasn’t the best to post. It looks much worse than it really is.
I have now rebuild it with a 30x30 cm workspace. I’m going to do some test cuts this evening. But when I try to flex it by hand, it looks like not much has changed.
I can see some flex between the core and the core clamps. To me it looks like the bolts that go through the core, bearings and core clamps should be much tighter. But I don’t want to mess up the PLA parts (again)
Try to get the whole core and z axis in the video. It will give us a better idea of what the issue may be. My working area is 600x900 and there is nowhere near that much flex from the rails. The only time i see flex like that is when the bearings in the core have loosened up. Normally flex like that is the z axis bearings after several long jobs. Contrary to popular belief, nylock nuts can and do loosen up. I’d try to figure out the issue you are having instead of tearing it apart not knowing what the actual problem is.
Just my 2 cents…
This isn’t my usual way of diagnosing a problem. But I have no experience at all with the mpcnc or any cnc for that matter. I know I had over tightened some things trying to get the play out, that is why I replaced some parts. And making sure the large size isn’t the main problem crosses that of the list too. The small build did the same. But gave very little room to measure and feel/move things.
It is back to the original size and build according to the manual.
That should be a good base to work from trying to find the problem.
I was looking for a way to show the flex I see. But somehow I can’t see it in a video or it looks much worse than it really is.
I needed a bit more control. This is what I came up with.
I put a weight (2.5 kg) on a piece of rope and hang it from the end of the end mill over the edge of the table.
I put the dial indicator on 0 without load and it moves 0.23 mm with the 2.5 kg load.
In the build instructions for the Burly MPCNC I remember reading something about belt tension and cutting forces. That was about cutting alu and according to Ryan it takes about 2.4lbs of force to cut aluminum
Now I found some info about side load in an old thread here. Maximum side load
In that thread Ryan says most MPCNC’s (Burly or Primo?) can do 1.9 kg.
That gave me another idea. I put the same weight and rope on a piece of MDF and see what happens when I let that weight pull the MDF against the end mill. Very little cutting and a burning smell. I have to push hard to get it cutting. That was with 6 mm DOC. With 2 mm it will cut and with just 1 mm DOC it cuts cleanly and quick.
To me that sounds like the cutting forces are way to high. And the only thing I can think of is that I have crappy and mills.
It looks like the forces build up during milling and get released when the mill gets to the end of the grove.
Also the video you posted shows what I see over here.
I did a new test with lower RPM and I think I over did it a little. I have to guess, but I thing it was around 7000 rpm. And that was to low and increased the forces even more, resulting in missed steps. First in X and later on in Y too. All with a 1/8 bit and 6 mm DOC. Clearly the forces got to high even with a small endmill like that.
Before I go try different thing I’m going to get some new and hopefully better endmills.
I calculated cutting forces on my LR3 with the parameters I use, and reach 7,27Kg when cutting Plywood with 6mm single flute endmill, 6mm DOC.
I didn’t try these values on the Primo, but I thing it can reach similar values, maybe lil lower.
At this value the LR is at it’s limit i think, because when the endmill get worn starts to appear some oscillations, but it doesen’t skip steps yet
The lower RPM increases the forces only due to the increasing of the chipload.
Reducing the rpms decrease the cutting TORQUE, but this is related to the power of the spindle, not on the steppers.
Got the new bits today and did a quick test. It was a bit noisy and didn’t sound great.
But the 50x50 square was accurate, even with an 8 mm DOC.
The finishing pass now sounded like it was hardly doing anything (like it should I guess if the rough cut is close to the needed size)
No funny movement visible anymore.
The new bit is a single flute 1/8" bit. When cutting full width over 3 mm DOC is sounds like it is struggling.
Like it is vibrating/bouncing. When widening the slot with around 50% step over it does fine. Even with an 8 mm DOC.
I have to start playing a bit more with feeds and speeds. But the few test I did didn’t help much so far.
To me it feels like it need higher RPM’s, but the spindle is already maxed out at 12000 rpm. Lower RPM’s sound worse.
At least it is working better now and I know I have part of the problem fixed.
Thanks Philipp. I couldn’t any bits locally for a reasonable price. I found the bits you ordered in the link you posted. Even with the little higher shipping to The Netherlands it was still a lot cheaper than what I had found. And with the bits from Sorotec I knew someone had success with them.
