MPCNC is always skipping steps

Hey guys,

i’m running my MPCNC Primo since June 2021. As far i could manage to produce a nice bunch of work. But in the end, i am always running into errors like skipping steps (i do not think i had any other issues yet).
So, let me explain this a little bit: my last big issue was loose Pulleys. I lost several steps during the Jobs and get them fixed by tighten the belt more and more. But then i realised that the Pulleys were the problem. so i got new screws and my MPCNC worked without problems for 10 hours. But yesterday, i noticed again skipping steps. i could not investigate it further because i had to leave.

So, before i am wasting tons of time during the search, can you help me with your knowledge? I changed the little screws of every Pulley execpt one (they are totally wasted, can’t unscrew them. i need to drill them out and change to a new pulley). But as they are stuck, i do not think this Pulley can lead to the skipping steps, do they?

I worked yesterday 3h with my MPCNC, but on the last job i got aware of something like skipping steps. The next job was not able to be done - skipping steps after going more into the wood. I couldn’t fix it.

Another strange occurence: i never had problems with Z-axis, but during work on Saturday, i noticed the Z-axis motor did some steps during the z-axis (i held my finger to the rod and noticed the little movement every now and then). Instead of milling at -1.0 mm, it appears to be -1.0 mm at the start but may be -1.5 mm at the end. (at the end, it was like 5 mm off). I don’t know where to start to check this all.

To my setup: MPCNC Primo, Estlcam, 17 Nm Nema Stepper Motors

Thank you all for your help!

Cheers,
Patrick

We call those grub screws and they are often the problem and the last thing you think about. The right way to tighten them is to get one against the flat of the motor shaft (and wiggle it a little so it is in the middle of the flat) and then tighten the other. Blue locktite is a good idea. Most people have trouble with them loosening up.

That looks like skipping steps, but what I call skipping steps is when the motor is firmly held to the pulley, but the skip is inside the stepper motor. They are several potential causes. There aren’t any internal gears, so it isn’t grinding anything. But it sounds like that. Every machine can skip steps. But it should be unlikely.

What controller do you have? Rambo? Skr Pro? Ramps?

Yes, i hate these little grub screws! Here in Austria they are sometimes called “Wurmschrauben” (like worm screws). Thank you for the tip wth locktite, i might use this one in the near future. i wasted a lot of time finding out that these grub screws screwed up so much of my work.
I will try and tighten them all again with locktite next time i’m working on my MPCNC.

I do not know exactly, but i use the Tillboard. It’s quite famous in our corner:


can you work with that information or do i need to take the research deeper?

I use (black) fingernail polish instead of locktite. It works well and you can loose the screws later if needed…

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I recently read of another grub-locking technique where you put a second screw in on top of the first, after the first is appropriately tightened down. Haven’t tried it myself, but might be an option to consider if you’re having to purchase replacement screws anyway.

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I’ve done that in situations where it’s OK if the grub screw stuck out a little. I don’t think it’d work in this case unless you found a really really short grub screw.

Like a second nut instead of a locknut. I like it.

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The second grub screw is not an option in my case because i am already using grub screws that are longer than the thread. so i am going to buy the loctite stuff and give it a try. but the overall opinion here is that the grub screws are my problem? or is there any other suggestion where to look at?

It’s just that grub screws are so easy to check and so often the problem.

Once those are sorted, there are a lot of other things to check, but they’ll take more time and testing. It’s really no fun to go through all that and then find out if was the grub screws.

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This.

Skipping steps isn’t a subtle problem. The machine makes some loud noises when this happens. It’s not something subtle or hard to notice, and generally isn’t “constant”

A grub screw that’s cross threaded or otherwise giving trouble absolutely can be a problem. Just because you can’t turn it with an allen key or screwdriver doesn’t mean that it’s tight against the motor shaft.

Try with some soft material like foam. It might be something like the nut on the Z axis got upset by say dring to plunge into something that it couldn’t manage, then settling in as the cut progresses. Gravity is the main thing holding that dopwn. so maybe raise the Z axis up, and see if you can push it down by hand a little. Check the screws into the brass nut for the Z axis. Those shouldn’t need to be tight but they should be all the way in.

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Okay, thank you all for your advices. I will check and tighten the grub screws for sure. if that is not going to solve my issue, or if it is going to solve it, i will keep this thread up-to-date.

Just one other question: you are way more experienced with your MPCNCs than i am. I am wondering how long you can run your machine without problems? I’m just curious how reliable this machine really is in the long-term aspect.

I did a 6 hour cut one time.

I cleaned up the garage while it ran. Made for a very long day of wearing a dust mask and hearing protection.

I ran a couple of 15 hours jobs that ended up being more like 20. Left the machine paused and the steppers on overnight. After the first hour, everything is pretty much as hot as it’s going to get, so unless you wear out (burn up) a tool or you had some good that were only just BARELY tight enough, there is no reason to expect anything to go wrong.

The only real issues I’ve ever had were caused by a gcode streamer. Marlin is used in 3d printing for DAYS at a time, so even that should be rock solid.

3D printers have a number of firmware protections built in to cut power and interrupt a print if the heaters begin to misbehave, because the temperature sensors provide feedback and the controller manages the electricity to the heaters.

Do not expect the CNC machine to run unattended. There are lots of examples of fires starting in literally seconds due to friction of a bit or collet that is rubbing in an unexpected way. These fires can escalate extremely fast as there is often a ready supply of fuel (the workpiece and chips) and air (being moved by the dust collection system if present). The controller has no way of knowing there is a fire, so there’s no way to build in similar protections short of adding a full fire detection subsystem.

Responsibility for recognizing and reacting to a fire, or other dangerous situation, then falls on the operator. I strongly recommend the following:

  • Constant vigilance on the part of the operator. This gets harder the longer the job runs, so find ways to fight complacency.
  • Emergency stop that removes all power from the controller, motion electronics, and cutter (spindle, laser, etc.)
  • Having an appropriately rated Fire Extinguisher standing by
  • A fire blanket (used to smother any flames) may be an attractive alternative, as the extinguisher is pretty messy, but I’d still have the extinguisher in case the blanket didn’t get the job done.

Pausing the cutter while keeping the steppers energized is one way to allow the operator to take a break. Another is to break a large complex cut into smaller jobs. A third is to include end stops that provide sufficient precision to restart the job (or begin the next) after re-establishing machine and workpiece locations following a full power-down of the machine.

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I’ve usually kept individual jobs to less than a couple hours, but overall, I’ve run my machine for over a year with several hours/week running time. I’ve had to tighten things (Bearing bolts, belt pullers) a couple of times, and fix wiring harness issues. In that time, I’ve replaced the control board, but that was more to upgrade to 32bit electronics, and make a firmware change.

I’m confident in the ability of the machine to manage any job that I want to sit through (Never leave the machine unattended!) I do some routine maintenance, cleaning the bearings and checking that screws are still tight on a fairly regular basis.

Always a good reminder. Couple times, I’ve heard a cut…change…and find the workpiece had shifted and started to char. Once, i smelled it before i heard anything.
If i were in another room, never would have known.

As long as you’re listing options, though, i definitely vote for breaking up the job. Pausing the machine worked well for the job i mentioned above, but part of why it took so long was restarting after skipped steps, burnt tools, etc. I had to go find the correct spot in the correct operation. No fun.

Plus, getting to the end of a program just FEELS better to actually reach a finish line of any sort vs “guess I’ll pause it here and hope for the best tomorrow”.

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It looks like it uses drv8825s for drivers (or maybe a4498). The driver current needs to be set by the little potentiometer screw on the top. I hope the till oard instructions have made you do that already.

If the current is too low, the machine can skip steps. If the current is too high, the driver can overheat and then it just turns off for 30s and ruins the job. The motors are totally loose when it overheats. There is also a region where the drivers are ok, but the motors get too hot and make the mounts soft and the motor mounts deform. The rule of thumb is to keep the motors under 50C.

There are a lot of successful users (in the thousands now, for sure) and in my personal experience, it does not skip steps unless it crashes or I am pushing it way too hard. I try to keep my jobs under 1.5hrs because I am doing this for fun. The motors and drivers actually use the same current and get about as hot if you are moving or not.

Dear guys,
thank you so much for your help in first place, and sorry for the long radio silence from my side, i was on vacation and also for a long time not at my CNC place. I am right now here and - having again - troubles. I already put on some glue on the grub screws, they must not get anymore loose i suppose.
And with that, i hoped a got the solution for my skipping step problem. but it was not.

With that said, i do not think this is about skipping steps, it’s a mechanical problem. But i don’t know anymore where to look at. I think i tigthen and loosed every available screw, but no change in my misery. i am loosing my mind with this issue.

Well, let’s start with the first issue: Z-Axis has its own will
I start the CNC programm with Estlcam, and with the ongoing cutting, the z-Axis gently starts to lower itself (my settings: cut 1.2 mm in Z every round). what did i measure? beginning: 1.2 mm and after some seconds: 1.5 mm, just before finishing the round: about 4 mm in depth.
Interestingly, i tried cutting without a mounted router. and the same thing happened (i held my finger against the Z-Rod and felt the movement during the “cutting”).
My first thought was the CNC router (Makita) was too heavy, so the Z-Motor can’t hold the mass, but it occurs only when cutting and also without a mounted router.

My second even more bigger problem is, that i can’t cut through wood without a shift in “layers”. I hope you get what i want to say - my router bit is 3 mm in diameter, but after cutting, the cut path is about 5-6 mm thick in some directions.

I tried to think about a reason on the this two problems. I am quit sure my machine is not 100 % rectangular, so the x- and y-axis must relieve this “misfit-energy”, and the easiest way to do so is two lower the z-axis - because this only happens during movement!

I also thougth about wasted bearing to be the problem, but they don’t seem to bad. only the ones in the Core look a bit wasted, and the one on top of the trucks. Also, i think there something broke at the core, might this also be a reason? I will upload some pictures of my setup below. Let me know if you need something else.

Maybe the printed parts are completly out of alignment anyway, because i was also new to 3D printing and i printed the Primo parts without 3D printer calibration?

I hope you can help me - i am desparate for solutions, and my motivation for solution searching and tigthening and loosing screws and producing firewood is at the bottom.

Thank you so much for your help!




The Z axis only has so many parts involved. The motor shaft connects to the coupler. The coupler connects to the leadscrew. The leadscrew to the nut.

The router is in the mount, the bit in the router collet.

If you’re skipping steps, the motor shaft is moving when it should be held rigid. Use a marker and mark the shaft on the top of the Z motor. Take a look and see if it is moving when. It shouldn’t.

Also mark the coupler, motor and leadscrew. If that is slipping, then you should be able to see it.

How is your router mount? Is it firm? Is your bit held securely? A loose collet can be an annoying problem.

How are you measuring that? Is it in the same place? There are many reasons why a cut line from 0,0 to 300,300 would be deeper at the end than the beginning. But it should be exactly the same depth back at 0,0. Fluctuations in Z of even 3mm is pretty common. And it isn’t a big deal. You just have to compensate.

1.2mm DOC is not very much. What speed are you cutting at? Going deeper is better (at this range) than going faster, due to the nature of the stepper motors (among other things). Cutting in HD foam is a good way to check out the machine, without testing it with load. If you have a problem in foam, then it may be easier to diagnose than in wood.

Do you have “stairs” on one side of the cut?

Is your bit perpendicular to the XY plane? If your router is held at an angle, it would cause a triangle shaped cut (but it would have to be pretty dramatic to get to 6mm). Is the bit wobbling?

Skipped steps shift the whole pattern one way, and they never return to alignment. The machine has no idea where it is. It is counting steps. When some are skipped, it thinks it is in the right place, but it is off by a specific number of steps. If you manage to get good cutting after skipping steps, then it looks like a duplicate pattern, but offset from the original. If you skipped in one direction by 3mm, then it would cut on the inside on one end and the outside on the other. If your workpiece shifts (because it is not well clamped), that looks the same as skipped steps.

Flex is something that is stretching and returns to normal when the force is removed. We’ve seen some cracked parts or flimsy router mounts do that. The steel belt’s core breaks and the belt starts stretching (don’t use white belts). The workpiece can flex. Cutting in foam will remove all resistance, so any flex problems go away. Reducing the speed and depth of cut will reduce the flex problem.

Backlash is something where the machine is free to move for a short distance. If the grub screw wasn’t tight and it moved back and forth along the flat of the motor shaft, you would try to move 10mm to the right, but the first 1mm would just be the shaft slipping. The next 10mm would be perfect, until you moved back, then you would lose 1mm again. If the belts were very loose, they would pick up the slack before starting to move the machine. If there is space between the bearings and the tubing, you can have the machine click clack back and forth instead of staying rigid (that’s especially annoying because it is an angular error, so hard to figure out). Backlash will result in small errors, but they will be centered around the right thing. The bit will try to follow a track, but be off by a mm or two back and forth.

Alignment errors are things where the machine, or workpiece are not perfectly at right angles and are just systematically wrong. You said your machine isn’t square in XY. If you try to draw a square with a pen, it will end up as a parallelogram. If your Z axis isn’t perpendicular to the XY, then your bit will cut triangles. If your work piece is 12mm thick on one end and 16mm thick on the other, the machine can’t cut it evenly throughout. If your feet on one side are higher than the other, same thing. Alignment errors are measurable and you can tune the machine to make them better. You can also design your projects and cutting to minimize them. Things like cutting an extra few mm into the spoil board will solve the fact that the distance isn’t the same everywhere.

Post some pictures of the bad cuts Maybe we can help figure out which category to look at.

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Hey Jeffe,

thanks for the extended answer. i will reply to that later, i’m on my smartphone right now. i am doing some wood working :slight_smile:

but i can upload the pictures. you can see on one spot, there is a straight line in depth, but on another spot i have a huge layer shift.

thanks in advance!