MPCNC stutters

Hello MPCNCers,

I have a problem, I’m trying to do my first project but I keep failing…
I know I shouldn’t start in would, but I did anyway…
I’m trying to mill some pine wood, which is pretty solid.
I tried the feeds and speeds from this website and although it went pretty ok, it doesn’t sound like it and after a few passes of 1mm each (DOC) it went alle over the place and ruined my pieces.

Here is a short movie I recorded:

Spindle: Makita RT0700C
Router but: 6mm, 2 flute, Carbide (from AliExpress)
2D Contour in Fusion 360
Cutting feed rate 480mm/min
Plunge feed rate 180mm/min
DOC = 1mm

Some other info… the CNC has the biggest problem after the longest edge, so lower left and upper right corner.

Anyone has an idea?

Thanks in advance!

Did you check the pulleys and their grub screws? :stuck_out_tongue:

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That video doesn’t look bad. I’m guessing this doesn’t show the actual problem? Or are you concerned about the chatter?

I’m going to assume your description is right, and the video doesn’t show the problem. In the actual problem, the bit left the trough, and went off on it’s own somewhere.

Some possible sources:

  1. It could be skipping steps. The motors have strong “phases” and they have the same phase every few degrees. So when you skip steps, the stepper motor is pushed hard enough that it moves to the next place with the same phase. It sounds a lot like gears stripping, but there’s zero mechanical rubbing. It is all just the sound of it moving fast to the next step. The microcontroller has no idea this happened, so what it often turns out looking like is a second copy of the pattern offset from the first. But the easiest way to tell is if it sounds like crunching gears.
  2. As Philipp mentioned, if the motor isn’t rigid to the bit, then it can move around. If it moves a ton, something has given up. The little grub screws on the pulleys that attach them to the motor shafts get loose often. One should be on the flat, and the other on the side. Tighten the flat one first.
  3. If the belts are way loose, it could be jumping teeth between the pulley and the belt. That’s pretty rare though.
  4. One of the motors isn’t working anymore. If you have one motor go out, the other can do seemingly sensible things, and the rigidity of the gantry will make it hard to tell what is going on. Maybe it’s a wiring issue, or a pulley issue, or if you have dual drivers, maybe one of the drivers took a break.
  5. Overheating driver. If the driver gets too hot, it will just shut down until it cools off. This can take a little while (a few minutes) and in the mean time, that axis is just flapping in the breeze.
  6. Too much load. Too much load can end up in skipped steps. It is easy to think that because the CAM settings were conservative (I would call those conservative) it isn’t too much load. But if the bit isn’t the right geometry (using a gridning or a drill bit, for example) then that will cause high loads. If something isn’t rigid, and the bit just gets a full mouthful at a full depth, that can create huge loads. If the router RPM is too low, then the chips are too big and that can cause huge loads.

We can maybe narrow this down if we had a picture of the finished “part”. A few pictures of your machine will help us look for problems. A picture of your controller too, and it’s enclosure.

First of al huge thanks for the replies!
here’s 2 pictures: look in the lower left corner and the right side (and second picture)

image image

One thing I can see is a lot of screws (legs) and bolts where (really) loose. I thightend them.
The belts are not to loose, before I started the CNC I thightend them.

It stutters… did you listen to the video?
The video doesn’t show where it went wrong unfortunately… it went wrong after about half way (so the 13th pass)… look at the second picture.

  1. I’m not really sure what you mean but this could be a problem. What I can say is it has more trouble with the short edges, the long edges weren’t the problem and they look good also.

  2. I’m not really sure what you guys are talking about exactly. What I can say is there was a lot of screws and bolts really loose.

  3. This is not the problem, I thightend them.

  4. I have dual drivers for the X and Y. The motors seem to work correctly. After a restart everything moved oké like normal.

  5. I really don’t know… for now the MKS board is in the open (no enclosure or something) and the drivers have heatsink.

  6. Could be… some other guy from the cnczone forum told me he thinks the router bit is too long (and causes huge load) and he would prefer a 3/4mm singel flute bit for this job. And another guy says I’m going too slow which also isn’t good for your router bit and makes it dull. He said do it in 3 passes (26mm/3=9mm per pass)

That’s the downside of the cnczone forum, they all have (semi) professional machines.

Would going faster help? And are these settings too conservative?

I listened to your video. The changes in noise might be due to deflection, but you are also going to have changes when the bit hits the grain differently, or also you will hear some changes as the router slows to make turns. A major possible reason that your cut is “alle over the place and ruined my pieces” is a loss of steps…especially deep in a cut. So imagine you are down 10mm, and you lose some steps. As you turn a corner, your bit is no longer just cutting just 1mm, it is attempting to cut the full 10mm of material because the router is not where the firmware thinks it is.

There are a lot of possible reasons to lose steps, and the other people that replied list most of them. In my case having a similar problem, it was the grub screws on the pulley. So if you haven’t checked those, I’d start there. As for next steps, I would suggest you get a piece of scrap wood, and try a deeper and possibly faster cut to see if you cannot reproduce the problem long before for 13th pass.

Make sure your router is trammed. If it’s leaning a bit, the deeper you go, the more of the cutting faces engage the wood, so you end up biting off more than the machine can handle.

Is the 4 supposed to be there?

Sorry, I am used to much larger (in terms of error) issues where it takes off and ends up inches away from where it should be. Those bullets were all for that problem. If it ever recovers and ends up in the right place, going the right way, then it isn’t losing its place, it is just flexing.

I did hear the chatter in the video. But “all over the place and ruined my project” means something different to me.

Chatter is caused by deflection. The load is too much for the rigidity. Either because the load is too high, the the machine isn’t rigid enough.

The grub screws are what keep the pulleys attached to the motor shafts. They are little black screws with no heads. They loosen up and the pulley can wiggle back and forth, but stay on the flat, or it can come off entirely and then just one end is pulling…

Most of those items are assuming skipped steps. Since it isn’t that, I would point at the bit, maybe Barry’s idea of it not being trammed, maybe you have a huge build (what size is it, xyz?), maybe not enough rpms.

You should check that the bearings are all touching. Probably post some photos of the machine. Maybe something is on backwards.

I’ll preface all my comments with “I don’t have a lot of experience with this and I am probably just showing my ignorance” however a couple of questions came to my mind…

  • Are you sure this is pine? Maybe its quarter-sawn or rift-sawn pine but the micro-grain looks a lot like oak or hickory to me. It doesn’t look like my typical yellow southern pine. To my understanding different wood requires different feeds/speeds

  • In the second pic there is a bunch of z “chatter” with all the circles in the bottom of the cut. What would cause that? Is this where the next layer starts? Maybe the z speed is too fast? The cuts just before/after this point looks really good from what shows in the photo

  • You mention that

I would assume that is because you have the longest edge running with the grain. Generally when the bit starts across the grain it has a harder time. Is it possible the bit isn’t sharp? The right side of the first photo also makes it look like a tramming issue but it may just be a weird photo angle.

Anyway just my inexperienced .02

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Power the machine and try to move the middle assembly by hand, it should show you how rigid it is and whether your belts are loose or something in your assembly needs fixing. If possible, try single flute instead and maybe your workpiece is too low. If you can add a 3/4 mdf and then your material, it’ll bring the router closer to it’s most rigid point in the z-assembly. I dont know if you are using conduit but check if any bearings are riding on a flat spot on the pipe, if it is then rotate the pipe so all three planes of bearings engage pipe. The taller your machine, the easier the bit can deflect the lower the z goes. As far as testing your cutting speeds and feeds, use MDF because wood grains can add variance in cutting precision. Do a few simple shapes so you can see if a certain axis is affected or if your drivers are hitting thermal shutdown. Check for debris on your bearings and pipes, pieces or dust can cake on and trap in between.

Wow a lot of replies… really apreciate it!

I hear when I listen to the video (and live), but ofcourse I’m not the expert, that the router bit is not steady (enough).

After like 10 passes it lost a step and then it has to move a lot of material. After that the fail happend.

I don’t know what you mean by ‘trammed.’ Do you mean it’s steady and goes down ‘perfectly’ vertical?

Yes, I did it after I ended the contour pass, by pulling the plug.

Well it’s not that terrible because I pulled the plug when it happend, it was going all over the place for a second or 2 and because it was at the depth it was wondered in the let’s call it valley (do you know that I mean?)

I think it’s not rigid enough, because things like the gantry was too loose and a lot of screws where loose cause of vibration I think.

Now I know what you guys mean, I think, I will have a look tonight. I didn’t think of this.

Yes the bearings where not all touching, so I thightend those. And I will post some pictures. BTW my machine is not too big, like roughly 400x300mm working area.

Sorry for the confusion, if I translate ‘Grenen’ from Dutch it says it’s pine. But I’m not reffering to the lumber pine. The density is more like Oak, and not a soft wood. It has pretty dens grain.

That was after the long edges at the top, so it wasn’t plunging or something. The plunge feed rate it 180mm/min. I took those values from the milling basics page.

That could be a problem, I have another one (few) of those. I will try another one.

I’m not sure what you guys mean by this?

In what direction, horizontal or vertical (the gantry) or both

I have 2 single flute bits also 6mm, I will try those.

I was thinking of adding a MDF as a spoilboard on top where I will mill some holes in for a clamping system.

Yes that sounds logic to me (is that English?)

I will try cutting/milling some MDF

Oke I did a search for ‘tramming’ on the forum, so it basicly means making sure your router is perpendicular to the X and Y axis (perfectly vertical). So how can I check this?

Easy way to tram or surface is to pocket a box and check if there are ridges, it should be relatively flat and if it is angled slightly, youll feel high and low points along the cut. start real shallow like .05 - 0.1mm depth. Can you post what electronics you are using and cam program? There are a few techniques that will greatly improve your cutting ability but I am only familiar setting it in Estlcam. For example instead of plunging vertically 90, estlcam can be changed to 60 so it wont load the bit so much, which is visible in your pics that there is a lot of movement going to the next depth. Always try to climb mill so your chip load is low and youll have better quality cuts. Adding a spoilboard will help definitely. If you are using arduino + cnc shield as electronics, you can adjust voltage to prevent skipping steps. Check middle assembly for all axial movement, any play will force the bit to dig more in that axis as seen in your second pic. The problem with 2 flute is when you plunge, the chips cannot escape the center of the bit, that bit is better to enter cut from sides, or plunge at angle along the cut direction instead. And 2 flute youll need a faster movement speed or it will burn or cause friction.

Thanks Brian!

Oke before I did this project, I surfaced some Oak with a surface bit. After it was finished it had some ridgesand wasn’t completely flat. Now that I have thightend everything I will try to pocket a box. Do mean a DOC of 0.05 to 0.1mm or 0.05 to 0.1mm total depth.

I’m using a MKS Sbase board, which has DRV8825 with 32 microstepping. With a 24V Meanwell power supply.

For software I’m using Fusion 360 and Repetier Host. I’ll probably replace Repetier Host by CNCjs.
I can’t check at the moment, but I’m pretty sure it doesn’t plunge at 90 (but not 100% sure).

Based on the video I posted, is that climb milling or conventional? I think conventional right?
I have to look into this more.

Yes I will check this… I already thightened the bolt on the gantry.

I think it does plunge at an angle, I will check this when I’m home.

Oke so I will up the feedrate. Keep the plunge rate low right?

That’s not very big at all. What size Z?

It shouldn’t matter a ton. MDF is very hard too. In English a “softwood” is any coniferous tree, the ones that keep their leaves in the winter. Pine trees are common in many parts of the US, so are fir trees. They grow fast and have low density grain (usually). Construction grade lumber is made out of softwoods (and it varies by region. No SYP here in CO). A “hardwood” is a deciduous tree, like an oak, maple, birch. Softwood and hardwoods aren’t always soft or hard.

But I haven’t found that to matter much. The softwoods splinter more and the hardwoods can burn if you’re going slow. But the load seems similar to me.

There is a “perpendicularity tester” Ryan made and put on TV. But it is basically just a 6" stick attached to the router collet. As you spin the router (manually) the end of the stick goes up and down if you are not perpendicular. I have seen people chuck in some wire and that seems fine too. You don’t need it perfect.

I think that is conventional. But in a slot, it won’t change this problem. The waste is being cut in climb. It might make a difference to the size of the part (are deflections out or in), but not to the load in this case.

I would still like to see some photos. Just to make sure there’s nothing obviously wrong. Check the grub screws, change the bit (do you have a 1/8" bit/collet?). I would also do some tests that are just some squares so you can easily measure what happened. Maybe a square donut with an inside and an outside square.

In my opinion, tramming does not make much sense until your spoil board has be surfaced. And, based on a discussion on another thread on this forum, I’d wait to surface the spoil board until you have some hours on your machine. Spoil boards can be uneven. MDF conforms to the surface it is resting on, screws for mounting the spoil board pull it down unevenly or warp the surface, and there can be variations in manufacturing. Tram readings are a distance to the spoil board, so if the spoil board is uneven, you may do more damage by shimming the router or changing the leg lengths to fix a problem that wasn’t really there.

As a start, take a piece of scrap wood (MDF or plywood preferred) and run it the length of your machine. Do a shallow surfacing pass with runs along the length of the scrap wood using your largest bit and running at slow speed. Are there scallops in the wood? How large? Are there portions of the wood that the bit did not reach? Repeat the process running the width of the machine. This does not give you definitive answers but it gives you a starting point for further investigation. Note that deflection, especially if there is looseness in the rig, can be a cause of scalloping. And if the issues appear to be minor, put some hours on the machine then go back and fine tune things.

Note there are a couple of tram arms on Thingiverse.

I will get back at this… it’s like 90mm or so? Look at the picture.

I don’t have a 3D printer… I could machine one maybe or use a measuring clock (is that English) if I can borough one.

Here some photo’s:
image image image image image

This is the grub screw right? I checked those and thightend the black hex srecws

My smallest bit is a 2mm (6mm shank) HSS coated Chinese bit

BTW I just tightened the gantry bolts, there was a lot op play in it… now there’s not much. Way better.

I have a surfacing bit of 57mm, I posted a picture. I will try this first.

Oké I did a surfacing test on MDF… and although it’s not completely flat, I’m really happy with the rigidity of the machine now.
Although it’s not a fair comparison to the sign I was trying to make.

Here’s a video of the start:

Oke so here’s the info:
about 10.000RPM
cutting feedrate 600mm/min
Feed per tooth 0.03 mm
0.5mm in total, 0.1mm DOC

Geez! With a bit that size, that’s a really good surface. Do it with a 6 or 4mm bit, and (aside from taking a bit longer), it would probably be imperceptible. Heck, just use a stepover, any stepover, and it would smooth it out a bit.

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Thanks, yes you’re right… because of the bit size it’s more obvious. I didn’t think of it this way.

That’s why the tramming tutorials often have you putting a 4"-6" arm in your spindle to check that it hits the same level all the way around. the wider the diameter, the more it exaggerates any error. Just be careful chasing zeros that really aren’t meaningful. :slight_smile: