MPCNC is always skipping steps

Mostly Printed CNC - MPCNC Troubleshooting
21

I can try that. But before, i just saw that there are 4 tiny screws, which are responsible to hold the Z-Motor in position through that little piece of plastic - and when i checked it, i saw there is one completly missing and one is terribly loose. i tigthened the third one, but don’t have a replacement for the fourth tiny screw - will check on that later. But now i see, the Z-Axis is way more rigid than before. The router is held firm in its position, even if i apply some pressure on it above. (before it was able to go down with minimal force).
I suppose that was my Z-Axis problem - but i will confirm that later.

Yeah, i’m sure it’s firm. i can’t move the router in the mounting tool, and i cannot notice any wobbling in the bit.

I measured it with a caliper and no, not in the same place. the only way to have such differences is a warbed surface (of the work piece or work area). but i am pretty sure that’s not the problem. I really think the Z-Axis went down every inch a little bit.

Once i read you should go max. 50% DOC of your bit diameter. I guess that’s not correct? My cutting speed varies beteen 1200 - 1500 mm/min. What parameters would you suggest with a 3 mm bit?
Nice idea with the foam. I am going collect some of it!

I postet a picture in #20, you can see on one side i do have a nice perpendicular plane, but the other side has stairs.

not easy to check; i dont have a measuring tool for such short distances. i need to think of another solution.

No i don’t think it’s wobbling. router and bit are held firm and securely.

I am using the black GT2 belt; yesterday i noticed that the belt on one axis is damaged: if i move the axis over the damaged part, the machine starts to rattling and grinding. i need to replace it. Could this also result in my problems? I can upload a picture of that later. I tried to avoid the room where the belt is damaged without luck, but i dunno if the belt can cause troubles in a wider range?

that’s something i need to talk about. what is the right tension for the bearings especially on the trucks? i feel like i don’t get the rigth tension on them.

yeah i could live with some heigth errors. but in future, i might need a perfect depth in some occasions. so i would like to get a nice and decent working machine.

Thank you very much for your help. I appreciate it a lot; i am going to work it out and post an update here with my knew knowledge.

Also, what about ambient temperature? My MPCNC is in a shed, not very hot in here. we had overnight like -10 °C and now it’s -4 °C. I can’t heat the shed in a sufficient way, but temperature should be above 0 °C. This is my first winter with the MPCNC, and i am wondering how the temperature will affect the machine especially the plastic stuff on the bearings?

22

You may be right, but even on a perfectly built machine, the spoil board will not be perfect to the XY plane. Once you have a repeatable machine, you can surface the spoil board by taking off a few mm and then the Z will be much more consistent. You need to consider the fact that this is always a source of error.

But if you’re right and the Z is dropping, the. You can tell by cutting a square. When it gets back to the start, if it is at 4mm, then yeah, it’s dropping. If it is back to 1.2, then it is just A being Z.

There are some very conservative numbers in the milling basics. I have gone 8mm deep with a 3mm bit and no one got hurt. Going deeper and slower really helps, because stepper motors aren’t as strong as faster rotations and the bit will wear more evenly. The same load can be had at quadruple the depth and quarter the speed.

The other major benefit is that if you are going slow and trying for 4mm DOC, but your Z gives you 6mm, you are only off by 50%. If you are going fast and shooting for 1mm, and you get 3mm, you’re getting 300% load.

I would start with 8mm/s (480mm/min) and 2-3mm DOC. When you have a machine that behaves, you can do some tests with deeper cuts (up to maybe 6-8mm) and then increase the speed. If you want to minimize your total job time, you can play games, because 12mm can be reached by 3x4mm or 2x6mm. It doesn’t save you time to do 8+4mm.

Yeah. I can’t really tell. But I think that is flex?

There is a perpendicularity tester print somewhere that Ryan made that attaches to the collet and sticks out 6 inches and has a screw sticking down. When one end is touching, you rotate it 180 and see how high it is off of the table. I have also seen a wire coat hanger in the collet, bent out 6inches, and then bend down a bit to do the same thing.

That’s not good. Those belts have fibers inside, and then the teeth are made from rubber. If the fiber breaks, the rubber can stretch. Otherwise, the belts don’t stretch. My fear is that end of the belt is stretching underload, but the other end isn’t. I would love it if that were the issue.

Ideally, zero tension. But everything should touch. If every bearing is touching, you don’t need any more tension. I don’t have a primo, so I won’t give advise on which bolts to tighten. I will warn you that those bolts are picked for their surface area and not their strength. They can easily destroy the plastic.

I don’t think this makes much difference. I would be more concerned with the wood movement in your table than the CNC.

23

Well, hello there again. I was just last weekend at my MPCNC’s place and was able to perform tests.
According to your recommendations, i changed the damaged belt, and looked on every bolt end nut for tensions. none of them are very tight anymore, so that the have enough room to roll around.

Yeah, and i also changed the DOC from 1.2 mm to 4 mm and cutting speed from 1600 mm/s to 600 mm/s.
And i think this was the real issue. because i tried some stuff, but the cuts were all perfect. no shift, no stairs, just as it should be. this is really a game changer. i had no more issues.

well, only now a new “problem” occured. when cutting in a specific direction (i suppose through a hard part of the wood like a branch or the tree rings), the machine starts to vibrate like hell. i don’t know what causes this resonance, but what are possible solutions for that? lower rpm of the router?

Anyway, thank you for your advice and help. i’m back at the game now and can use my MPCNC in a proper way.

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That’s chatter. It is caused when there is something flexing, and the standind frequency matches the router/bits. You can adjust the router speed sometimes to fix it. If not that, you can adjust the feedrate. But it also may be that there is something cracked or flexing that shouldn’t be flexing.

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Here i go again. Thanks to your help Jeffe, i was able to have much more fun with my MPCNC than ever before. But due to the chattering i mentioned before it’s again a little frustrating. i have to watch out really carefull on the machines behavior during cutting. hardwood is almost a no-go yet.

During the built-up of the machine, i already noticed a flexible gantry. i suppose this is way too much, which would cause the chattering. but i yet, i don’t know how to minimize the chattering. once i tried tighten some screws, which helped in a way, but the gantry wouldn’t move anymore. I made a video where you can see there is a lot of wobble in the gantry. you can observe this here: MPCNC Primo Gantry wobble - YouTube

You can see in the pictures above that there are some minor cracks in the core also. should i print a new core? but why is there a flexible gantry in the first place? It is for sure the right version for the tubes, but otherwise i don’t know what could cause this?

i also suppose, when i minimize this movement, the chattering will also minimize.

thanks again for your help guys.

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Are the 3 bearings in contact with the tubies i can only see 1 but the angle is bad just making sure they are there. It all 3 are there take it clear loose and reinstall them again.

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which 3 bearings? the one in the clamps?

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Okay…

  1. There is a good reason Ryan recommends attaching nothing to the top of the Z tower. There is a lot of leverage on the core from there so a little force has a lot of effect.

  2. If that is clicking, chances are your core bearings need some tightening. I’m sure its in the build documentation, but those bearings are adjusted/tightened with the vertical bolt through the clamp part. Each one should have a total of 3 skate bearings to it, and you want to adjust it so that all 3 just touch the steel. With a fingertip, you should not be able to spin any of the bearings easily. They should also not have a lot of drag moving the core, it’s a matter of getting it just right. A tiny turn of the bolt makes a fairly large difference in tension.

  3. I don’t know that thise cracks are significant, but anything that allows stuff to move there isn’t doing your machine any favours. All of the screws and bolts that hold this machine together are way more strength than anything needs and enough to crush stuff. It does all need to be toght enough not to let stuff move, but it is also very easy to overtighten things. Try the adjustment that is available, and see if it helps.

  4. While you are at it, check the bearings on the trucks. All of the bearings should be in contact with the steel. Again nothing too tight, but none of the brearings should rotate with just a fingertip pushing it.

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Yes they are in the one in contract with the X and Y there should be 3 in each so there should be 12 all together correct. I ask because my BIL did this he left off the bearing in the middle so its not unheard of

30

Had recently experienced similar issue and had to slightly tighten clamps/bearings on core. Had second issue also. After the slight wobble was eliminated, there was still jerking motion as core moved around. Found dust clumped on bearings and rails. Clean rails and bearings periodically now and no more issues.

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