I have a 3’ x 3’ cutting area MPCNC using 1" OD X .045" WALL steel tubes instead of conduit. My core has just a tiny bit of flex that I haven’t been able to work out. But that’s not the main issue (I think). I made a board out of ash (~1200 janka hardness) and padauk (~2200 janka). Using the Dewalt Rotary saw, 3mm bit, 15mm/s feedrate, 1mm DOC. I got 10mm deep (10 passes) into a cut before the router hit something, is my best guess. It was cutting a straight line and stopped moving along the line, proceeding to jerk around in an ovalish shape and gouging the board.
. I asked around in a couple groups and the prevalent theory was that maybe the router hit a knot, and the frame wasn’t stiff enough to push through it. If this seems feasible, how do I fix it? I think building this machine just to cut MDF and plywood would be a shame. I have seen it cut aluminum so I am not sure what is going on. Do I need a router upgrade? Please advise.
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Does not seem feasible. The MPCNC does not really care about that when correctly build. With a few pictures it might be easier to help. 
I don’t know anything about the DeWalt you use though. Might be too weak? Maybe anyone can comment on that.
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That doesn’t sound like the load was too high. That sounds like the drivers got too hot. What controller do you have and how were you sending the gcode?
Did you hear a crunching sound (which is the motors skipping steps) or just the router noise?
It may also have been the grub screw came loose and the motor shaft started spinning in the pulley.
You said you are using 1mm depth per pass. What speed were you doing? Deeper and slower is usually better than faster and shallower.
A lot of CNC groups are way too concerned with the idea that everything has to be made out of steel and concrete. But that doesn’t explain this at all. Don’t get mad at them. They spent a lot of money on extra weight.
The frame on the mpcnc is strong enough to max out the motors. The motors are strong enough to chew through wood and any knots.
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What was jerking around, the bit, the router, the machine core?
Looks like you have a decent cut going into and coming out of the bad section. Did you do anything to fix it or did it sort itself out?
Been around wood a long time. That is very weird. Clean in and out, but torn right there. It is like it hit something it could not cut. Like maybe a nail. No evidence of a not, but even still would not do that. If it were machine I dont think it would be so unclean (torn fibers). It would just be wild cut paths. Did it maybe come loose? The wood that is.
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I cut 3mm deep through a screw. Should also not bother the CNC though it’s not too great for the endmill… 
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Wood didn’t move at all. It never exited the hole, the exit line is from the previous 10 passes. It just kept jerking around in the hole until I turned it off
It never made it out, the exit line is from previous passes. It kept jerking around in the hole. - I’m pretty sure it was the whole machine, both axis.
15mm/s x/y feedrate
DRV8825s (i think thats the order) on an skr v1.3 with an 80mm fan on them. I’ve run tge machine at 30mm/s and 2mm DOC for much longer (that was for ~4 hours, this cut went wonky about 25 minutes in) but I was cutting MDF
Gcode was made by estlcam, running off an sd card in the touch screen
Dont know about a crunching noise, but I dont think so.
Skipping steps isn’t actually slipping gear teeth, but it sounds like it. It is because the stepper motor moves from the magnet it is supposed to be aligned with to another one, a few steps away. If the drivers were still working, that’s what you would hear.
Have you adjusted the trim pots on them to set the current?
It sounds like they either overheated (they shut down for 30s or so to cool off) or there is a wiring problem.
That shouldn’t be a problem, even through a wood knot. If you want to shave time off of your cuts, go deeper instead of faster next time. But I don’t think feeds/speeds are the issue here.
Have you checked the grub screws? If they slip, this is also what that looks like.
The grub screws are fine.
I don’t think it is the drivers. I’ve run it much longer and harder before without issue. It was still testing to move, but just jerking around instead of continuing straight.
I ran into an issue before when cutting Patagonian Rosewood / Carupay (3800 janka) where the machine would come to a stop, trying to push forward but unable to. This was different in that instead of just being stuck in place it just started jerkng around.
I do have extra 2208s and 2209s if those would be better for some reason.
Both this issue and your Rosewood issue are not typical. Your machine, without modification, should be able to cut both. I’ve had something somewhat similar to your issues happen twice to me. The first time was due to the Z feedrate being too high causing lost Z steps. That resulted in a much deeper cut that the router could not push through. The second time was caused by a broken core clamp that allowed the core to be sloppy. This resulted in the router getting off track and also trying to push through a lot of material.
Solving intermittent problems like this one can be frustrating. I think the starting point is to assume (until proven otherwise) that your machine is perfectly capable of doing this task, knots and all, but something is failing. Here is a list of things to consider. Some are more likely to cause your problem than others.
- Grub screws not tight
- Z feedrate set too high so you are losing Z steps. Note this will cascade to problems with X and Y if in a deeper cut.
- VRef voltage not set right on one or more of the DRV8825 drivers. It could be set too high resulting in overheating, or it could be set too low resulting in not enough torque. Note that you could have a Z driver failure, and the X and Y would continue to move.
- A failing stepper driver.
- Broken core clamps
- Intermittent wiring problem.
- Router rubbing or snagging on the core
- Lead screw friction issue caused by lack of lube or by tension on the lead screw
- Failing power supply.
- Mechanical binding or rubbing issue causing lost X and/or Y steps.
- Electrical noise corrupting the g-code
- Failing stepper motor (highly unlikely).
- Bit running backwards if you are using a spindle (highly unlikely since I don’t see burning)
- Extremely dull bit (unlikely since your cuts look clean)
Edit: If this was happening to me, and if none of the list of items above seemed to be a concern, I would start with some serious stress tests to see if I could figure out a quick and repeatable scenario to the failu8re.
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I do mostly hardwoods, full reliefs, cutouts, profiles, etc.
My experience with hardwoods in the 1200 Janka and higher range is this: Use the best downcut bit you can afford. Low, very low, step-down and step-over, I usually run at 0.7mm on both. With that, said speed becomes relevant, I usually can run hard maple, Janka 1450, at 45 in/min. Any higher and results as you have shown.
It takes more passes in step down, but no fails
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Of these I think it could be the vref voltage or possibly the router wire snagging. My z feedrate is only 3mm/s and it had already stepped down for the cut so the Z wasn’t / shouldn’t have been moving at all. The Grub screws are fine, core clamps look fine.
I still think it just physically hit something. After the straight line it cuts upwards in a curved line (I was cutting the shape of Pennsylvania and the picture was the bottom straight edge of the state). I think possibly it was trying to finish the straight edge but physically couldn’t, then thought it was on the curvy right hand side of the state and that’s what caused the jerking around. But I’m not sure.
Do you think replacing the drivers entirely with 2208s / 2209s and updating the firmware is worthwhile?
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I’m probably not the right person to ask, but assuming you have your vref voltages set correctly on the DRV8825 drivers, and assuming all the drivers are good, I don’t think replacing the drivers will help. Both types of drivers are going to limit the current similarly given similar settings.
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Snagging wire will definitely do it every single time. Believe me, I know.
The only other thing i see not mentioned (probably because you seem sharp enough to have noticed it) is that sometimes trash gets under the bearings and binds them up.
If you suspect a snagged wire, I’d say clean that up, then devise a torture test as suggested earlier. You can get a lot of deep cuts out of not much wood, and that should put your mind at ease for future jobs.
I’m not sure that replacing the drivers would have any effect, nor would reflashing the firmware unless the driver voltage is set through firmware (which you might also be able to adjust through user settings), but I don’t recall whether thats true. Of course, I’m far from an electronics expert here, so definitely listen to an expert if one chimes in.
The driver vref is set by hand using a trim pot on the DRV8825 drivers, but it is set in the firmware (the way V1 has the firmware and drivers setup) for the TMC2209 drivers.
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Your first post said stopping. If it is still trying to drive forward, then the drivers didn’t overheat.
Yes, it was cutting the straight line and in the middle of the line it stopped going straight and started jerking around in / creating that pocket in the picture. My bad if it was confusing.
Ah yes, I remember now. Been so long since i set one of those. Thanks.