Plus when I milled something, the first few layers would often go nicely, but over time the motors always skip a few steps, ramming the end mill into full material depth.
In the past I experimented with the marlin config, setting Motor current to different values, only now finding out that the correct value to set #define DIGIPOT_MOTOR_CURRENT { 120, 120, 120, 120, 120 } // Values 0-255 (RAMBO 135 = ~0.75A, 185 = ~1A) to is 211, using Wv = (VRef / 1.66) * 255 = (1.44V/1.66)*255 = 221 for 1.8A. (Stepper Motor Datasheet below and Guide here )
And I wanted to ask you guys if I will need a heatsink and a fan to cool the drivers because last time I tinkered with it in january I got the burning-electrical smell (granted I used random settings so… oof). I never touched the setting since then and left it at default.
Has anyone experienced the same problem? And how did you fix it? Will this setting fix my problems?
Sadly I can’t try it out myself right now (studying away from home).
Any suggestions what else to try?
Parts I use:
Rambo 1.4
Marlin
Repetier Host
Nema 17 17HS19-2004S1
Dremel with 2 flute end mill
Thanks for everyone who replies, I really wanted to flesh out this post more and make it kinda like a better guide than what you find elsewhere but sady I dont have the patience and time to write up everything again.
That video isn’t showing skipped steps. When the machine skips, it doesn’t know it skips. It will continue going as if it had not skipped, so shapes end up distorted. In that video, it looks like something is springy. Something is building up pressure, and then releasing it to continue to the right place in the part. That video’s problem is probably in the belts springing around or something being loose. Maybe the zip ties are too long, IDK. Large builds also suffer from this, if the EMT of the gantry bends.
You shouldn’t be skipping steps with the rambo on Ryan’s settings.
Does the machine move smoothly when not milling and pushing it around by hand? Eliminate the physical resistance here first.
Are your motors wired in series? Wiring them in parallel with divide the current between the motors, making your drivers work twice as hard.
Are you going too fast or too deep? Picking some conservative settings first will help.
The PWM_MOTOR_CURRENT will convert into the DIGIPOT_MOTOR_CURRENT and should be used instead. There are too many possible errors that are solved by letting the compiler be your calculator. A setting of 900 is 900mA, and that should be plenty strong for most applications. Above that, you’ll want a fan on the drivers, at least. But I wouldn’t mess with that until you’ve eliminated other sources of error and you just want to go faster/deeper.
I dont think I made this clear enough but my Machine IS skipping steps when faced with an actual job. This can range from 1/2 milimeter to about 1 to 2 cm cutting besides the line it should. With a pen attached everything looks and works fine except for the stuttering seen in the video.
My axis are moving fine when the motors are off and I’ve redone my belts several times now, every time thightening them differently. I have even redone my axis themselves. First I’ve used Steel conduit (where tolerances werent that great) and now I’m using cold drawn round steel (which is very heavy but the tolerances are very good). This didn’t change anything.
Please keep in mind that my stepper motors are rated for 2A max and the ones that Ryen recommends are rated for 1.5A.
My motors are wired according to this diagram. That should mean that each of my motors got their own driver, and therefore full current
[attachment file=“Rambo diagram.png”]
By buildarea is 600x600x90mm, so not the largest one and I’ve seen multiple people who got a bigger build working properly.
Thanks for your response, it really helps getting some additional input.
Does it do that when running the Crown Gcode I made?
Post your Gcode. It really could be a lot of things at this point, if it does actually ever skips steps that is a symptom of something very very wrong. Router spinning the wrong way, wrong bit, too deep, a stepper not moving, so many things. Dremmel could have severe run out and is kinda skipping around, not perpendicular, what about carving foam or wood instead of plastic?
re 1: You’ve got to find that manually. It’s going to be easiest to find with just your hands and eyes. Something will always flex, which is why this is a hard test to describe, but if you’re seeing that flexing during milling, then it’s probably too much. In my builds, it’s either been from the tubes flexing or the zip ties changing shape. It could be the tool mount, or the bit to spindle to mounts. I can only guess from here. When you get it right, stuff will still flex with enough pressure, but it won’t be jerky like the video when it’s milling.
Just to be clear on the belts. On each end, the zip tie that is harder to get to should be basically as small as you can get it. There should be no loop. The closer side should be a small loop, but still adjustable. The total tension should be similar to a 3D printer, something that makes a tone when you pluck it. The tone will depend on your size, and I’m tone deaf anyway. But the tension doesn’t have to be perfect.
re 2: This could be from problem #1. If the bit isn’t where it’s supposed to be, it can grab a lot more than you told it to, and it will dig in and the load will spike. It might also be something with your feeds and speeds. This isn’t a $5k CNC machine. Starting from somewhere like 8mm/s 1mm DOC and 3/4-full power on the router should be a conservative start. After that’s fine, increase the DOC. You should be able to go much deeper than 1mm, but probably not 10mm. If you find that the only problem is that you’re OK at 3mm and not at 5mm and you want to push it, then that’s when I would look at more current.
The crown gcode works fine (tested with a pen), the lines are just a bit jittery (?) from the movement shown in the video I linked earlier.
I’ve done “dry runs” with my gcode (basically a square, nothing complicated, h=4mm, 0.25mm cuts, 18000 RPM, Single Flute (said dual flute earlier, my bad), 1/8 inch, End Mill , 900mm/min). The “dry run” works as it should, at least there are no visible skips. And the actual run works fine too, just until it somehow doesn’t after a few layers. Sooner or later it starts go get off track. I made the Gcode in fusion 360 and it shows up correctly in repetier host.
When I cut my plastic the cut looks clean, no melted plastic anywhere and the end mill is not blocked. The dremel “looks” like its spinning perpendicular but you can never know for sure. Thats something I can’t measure. I strapped it on there to test the MPCNC before I invest another 300$ into a spindle that is actually a spindle.
Regarding other materials: I’ve ran some manual tests (manual gcode creation) to cut a circle in (ply)wood and that worked without a problem (minus the jittering which made the end mill crash into the walls, but only slightly. It didn’t break anything), no major skips here.
The MPCNC is not perfectly squared by default, but an offset is used in the firmware to get it to square correctly. Distance travelled is also correct if that matters.
Hm this is definitely something I didn’t do this way. Reading this makes me think that I always left too much zip tie space at the end of my belts. I never changed the lenghts of the belts.
Sadly I cant tinker on it right now, but I will look forward to do so in a week or two, giving you guys an update. I’ve found a photo of my MPCNC and I will just include it here, its not pretty, but maybe you see something I don’t.
That pen is never going to cut through wood. No wonder you are skipping steps!
0.25mm DOC and 15mm/s should never skip steps (in wood). 4mm DOC and 15mm/s might, sometimes. I’m guessing the root problem is that the bit is grabbing a mouthful at the deeper layers and it’s too much.
I hate to immediately blame the dremel, but there are a few things which could potentially be causing this from the dremel:
shaft stiffness. The bit is always trying to grab the material and pull away chips. If it’s allowed to move, then it will pull itself into the work and grab bigger pieces of wood. This will add a lot to the load.
runout: If there is runout, even if it is still, then it will also tend to grab the wood, especially when it’s deeper.
non-perpendicular: If the router bit isn’t perpendicular to the XY plane, then as it plunges, it won’t be plunging straight into the slot it just cut. As it’s leaning to one side, that side will grab the whole 4mm at a time.
Power: Even set to 18krpm, if it’s hitting resistance, that speed will drop, causing bigger chips, and higher loads. A 600W DW660 is much more powerful than a dremel and can keep the speed even when it’s hitting some resistance. This helps it blow through problems.
My immediate suggestion is to look closely at the dremel. You can just enable the steppers and push the end of the bit with your finger. If the dremel is moving in it’s mount, or the bit is moving relative to the dremel, then that’s a problem.
You can put a piece of wood on the bit, and spin it around. If the far end of the wood touches the board only on one side, then it’s not perpendicular. If the far end of the wood moves easily, then it’s not very stiff.
I don’t think 0.25mm DOC in wood should cause the zip ties to flex. I think you’ve got something else wrong. FWIW, deeper is better than faster, so I would drop the speed in half and double the DOC. But you should still be able to do twice that DOC, no problems. Slower lets the steppers have their max torque and when/if you do get a full bite, the machine will be moving slower and might just be OK.
The Dewalt DW660 sadly isn’t available in Europe, so thats a bummer. Im thinking about going all out and buying a 800W watercooled china-spindle from ebay.
But first I will see what I can accomplish by doing 0.5 to 1mm DPC, lower feed rates and redoing the belts.
I was mainly cutting Acrylic with the 0.25mm DPC and experienced allmost all my skipped stepps with acrylic. But I can’t say I have much experience processing wood in the first place.
Man it really bothers me not being able to tinker with the MPCNC as much as I would like to.
Do my heavy axis have any effect on those things? maybe thats also a problem?
The weight will make it take more force to stop and start it. So if you tried to go to 15mm/s instantly, then it might skip a step when starting. Or when you go from 15mm/s in +X to -X. You can adjust the max acceleration in the firmware, or in the LCD to be a bit smaller. You can try something like G1 X30 F1800 and see if it skips steps while doing that, without milling. If not, then it’s probably fine.
The weight should actually help you a little, because as it’s moving along, it will want to continue moving.
My guy though is that no, the weight isn’t a problem, and it’s very little force compared to the other factors.
But its the only one I had at hand now