Router oscillating

Today I had a problem with my MPCNC when making deep cuts. Well, I don’t know if they are really deep, but you guys can probably tell me (and hopefully also what I am doing wrong).

The size of my machine is 1000 x 1000 x 200 mm^3.

So this is what I did: I wanted to cut a circular shape with a radius of about 12 cm out of a 21 mm board. The wood was pine, so pretty soft. I used a brand new 6 mm two-flute milling bit. 700 mm/min feed rate, 24000 rpm, 2 mm z feed per pass. The z axis was probably 80 mm extended down at the start of the run (sorry, I’m neither a mechanical engineer nor is English my mother tongue, so sometimes I’m not finding the right terms).

The first 16 mm down into the wood were going well, but on the next pass, at some point, the router started to oscillate itself into a resonance catastrophe. My entire setup was dancing for a second, and the 6 mm groove turned into a 12 mm hole in that spot. Luckily, the machine wasn’t damaged, but it was scary.

I suspected that the z axis might not be stiff enough, and sure enough, I noticed that I am an idiot and that I hadn’t completely tightened all the bolts on the z assembly of the gantry. So there was a tiny little bit of play. That is a huge smoking gun, of course.

After fixing that, I tried again with a new piece of wood. It did turn out well, but in some places along the trajectory, now and again, it sounded slightly as if the oscillations were still happening, on a smaller scale, though.

So now my questions for the experienced users: Have you ever experienced something like this? Are my cutting parameters bad? Is 21 mm to deep for these parameters?

PS: the router bit was long enough, meaning the blades are at least 21 mm tall.

I’m not sure I understand what you mean by “oscillations”. Pine is indeed soft, but the grain is hard enough to push the endmill around and makes a distinct sound when cutting. So if you are hitting grain, no grain, grain, no grain, you would hear it.

2 flutes at 6 mm is not too bad but a single flute 1/8" or 3mm will be better. The cutting forces can deflect the Z axis (worse when it’s not stiff) and the deflection can create new cutting forces that set up a bad feedback system. With a single flute there is less opportunity for the cutting loads from one flute to cause the other flute to dig in.

I have the same problem: i’ve tried to cut a telecaster and there is no way that I can cut an entire body all the way down, the router starts shifting half way through binding and rouining the body, is there any way to stiffen the z axis?

When I’m contouring, I use roughing passes. Not sure how estlcam handles it, but fusion let’s me take a pass around the object at a larger size than final, then takes another closer, also not at final. That’s usually enough, but you can program more. Gives the chips more room to evacuate as I cut down, and keeps them from binding up on the tool. Then a finish pass at final depth.
Think that might help you here?

thank you, I’ will see if estlcam has such an option

I did some more milling today and I tried different things. For beech wood, the most aggressive parameters I can do are diving 1 mm deep at 500 mm/min with a 6 mm two-flute bit and 25000 rpm. If I go deeper or faster, my z axis starts to vibrate itself into a frenzy, and the results then look awful in the best and disastrous with a damaged MPCNC (ripped a belt from the tensioner) in the worst case. If I take it easy as described above, smooth sailing. I can go 20 mm deep no problem, because nothing vibrates. It just takes 20 passes.

My MPCNC has a 200 mm z-axis and I had it extended all the way down to the workpiece on the bed. Could it be that that is too much for it to be stiff enough? I might experiment some more and raise the work piece as high as possible, that should give it more stability. Then I’ll try what speed and feed I can give it to still work comfortably.

It could still be, of course, that I still haven’t adjusted/tightened everything correctly. Can you guys let me know your expereinces and maybe some speed/feed rate/depth parameters so that I can get a picture of what is possible and of where I am. I have no experience with other machines at all, so I wouldn’t even know what is fast and what is slow. It seems to me that 1 mm z feed per pass is not very much, but I’m still happy that I have the machine and that I can do stuff that is impossible without it.

Here

you can see a video of what I mean. You can hear that the router vibrates when it digs into the wood. This is z 2 mm and 600 mm/min. For one pass not much happens except that the edge of the contour looks ripply. When going deeper, the vibrations lead to binding up of the tool. Of course, when the vibrations are audible in the first pass already, I don’t go deeper. But it has started to happen to me all of a sudden when I was already 16 mm in.

Yes 200 mm is to far a reach block your work up or shorten your legs it will work a lot better

Ah, sorry, it is not 200 mm but 120. But anyway, yes, I will try with a shorter reach.

Wow that is very large. 120 is still large. This needs to be as short as possible so if you could prop up your workpiece as much as possible so it just clears the bottom of the center assembly you will have much better success.

The deflection (of tilting the Z axis away from vertical) increases with the square of the height. Twice the height will have four times the deflection. At full extension it would probably feel pretty soft if you push on it with your hand.

The overall size of 1000 x 1000 is also going to reduce your stiffness somewhat. The gantry rails can deflect sideways and the belts can stretch, which gets worse as the machine is larger. It’s not ideal and it will also be a factor in limiting the aggressiveness of the cuts you can take but it’s not as big a problem as the long lever arm on the Z axis.