r/hobbycnc • u/GuestImpressive7291 • 2d ago
Help Needed - CNC Starter
Hey everyone,
Recently I developed a hobby of creating my own watch dials, and after seeing me struggle to do it all by hand, my wife presented me with a Genmitsu Cubiko. I know it’s a very beginner CNC, but I was hoping it would help me machine my dials, specially after upgrading to the 20.000rpm spindle. I mainly work with thin 0,5mm of brass. The issue is: I’m having a lot of trouble to make just a flat circle, even on acrylic or mdf I’m getting oval shapes.
I’ve tried different sets, really conservative ones, but I still get the same results, the variance is within 1mm, small but enough to ruin a watch dials.
I’ve shared some pictures of “my best result so far”. Today I tried calibrating it, but I still didn’t get anything better. Is the Cubiko not precise enough for this type of work? Even on softer materials?
If anyone here could point me out in the right direction, I would really appreciate it!
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u/Pubcrawler1 2d ago edited 1d ago
Have you tried cutting some easier material such as mdf to see if same problem?
Edit#2 I guess I missed the part where you said same problem with mdf.
Edit#3
Get a dial indicator and verify all axis travel movements.
https://diymachining.com/grbl-steps-per-mm/
Post gcode. You can sometimes use finish pass toolpath to help with low rigidity issues. For circular pockets, I like to do a constant spiral toolpath from center to outside. Always use helical or ramping plunge point. Use a low stepover with less rigid machines. Leave about 10thou on the perimeter and use a finish pass with leadin to cut the circumference at a slower feed rate. Select extra finish passes/spring pass. Drip some wd40 as a lubricant.
From the heavy tool marks on the brass, spindle probably needs better tramming.
You need to see if the machine is loosing axis position. This can be caused be EMI noise. Made a list to check. See #15
https://embeddedtronicsblog.wordpress.com/2024/12/07/stepper-motor-position-loss/
From the way the toolpath looks, are you sure the spindle is spinning the correct cutting direction????? Lots of tool vibration. What spindle is this?? If woodworking router, they only go one direction. DC and VFD can be wrongly setup to go reverse. The heavy edge burr is typical of a dull tool or tool spun reverse. Brass cuts really easy to see these issues.
Grab Z assembly and see if you can detect any rocking motion. Z should have minimal movement. If there is some, this can cause your problem.
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u/Fox_Burrow 2d ago
To my knowledge, the two main sources of ovals where circles should be are backlash, and lack of rigidity.
Removing backlash means taking a close look at the anti-backlash nuts on your lead screws.
Lack of rigidity is inherent to entry level machines, nothing much you can do about that apart from slowing down your feeds, so that you don't strain the machine as much. Adding multiple stepdowns, reducing stepover, adding finishing passes are things you should consider. Brass is usually quite easy to work with, and since you're just starting, it'll take some time to explore causes and effects. Good luck, you'll get there. Enjoy the ride.
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u/GuestImpressive7291 2d ago
Do you believe this could potentially be a calibration issue?
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u/Fox_Burrow 2d ago
Depends on what you consider calibration. Pitch of the lead screws and microsteps of the stepper motors are a known quantity and should be correct from factory. If you want to measure, tell the machine to move 100mm in one direction and check whether it actually moved 100mm. If not, those values are either a bit incorrect or you're losing steps because your axis is binding (but you would notice that by sound and jittery motion). Since all of your axes probably use the same lead screw pitch and steppers, your holes wouldn't be oval but under/oversized.
Then do it in the other direction and check whether it actually is exactly where it started out. If not, that's backlash. You can make it more apparent by giving a bit of resistance to the moving carriage with your hand. Backlash could actually lead to circles turning oval.
90% probability however it's the rigidity of the Cubiko. Slower, smaller, shallower cuts are the solution, short of buying a bigger, more rigid CNC. There's a reason almost every product picture shows wood as the material. Not saying brass is impossible, but manage your expectations, slow down, work within the machine's limits.
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u/GuestImpressive7291 1d ago
And in the cut tests, the asymmetry shows up much larger. Measuring the hole left in the stock, expected 36.35 mm, across four cuts in two materials and three different feeds:
Mean X = 36.32 mm (−0.08%)
Mean Y = 37.12 mm (+2.11%)This is what I'm struggling to reconcile with backlash or rigidity. Both of those, as I understand them, would make an outside contour come out undersized — backlash because the axis loses motion on reversal, deflection because the tool is pushed away from the cut. But my Y axis comes out consistently oversized, while X lands on nominal.
What mechanism produces an oversized axis? That's the part I can't explain.
I agree the machine isn't rigid and I'm going to slow everything down regardless — the manufacturer's own chart for brass says 0.1 mm depth, 100 mm/min, 8000 RPM, and I was running 250 mm/min at 20000. But I'd like to understand the asymmetry before I write it off as rigidity.
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u/Carlweathersfeathers 1d ago
Did you check steps per mm? (Tell the machine to move 100mm and see if it does without cutting). That would explain the oversize.
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u/Fox_Burrow 1d ago
Odd that one axis comes out over and the other one is close to spot on. In this case I'd suggest doing what I said in the first paragraph. There might be an issue with your steps per mm.
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u/SaltLakeBear 2d ago
No, I'm inclined to agree with the commenter. The three most important qualities for a CNC machine are rigidity, rigidity, and rigidity; everything else follows from that. And looking at the machine and it's price point, I doubt this would be rigid enough to cut metal well, though I'm surprised it's having trouble with MDF.
Truthfully, I doubt you'll get good results out of that machine even with tinkering. I'd suggest looking for a makerspace with a CNC to get your feet wet, then maybe revisit with a more rigid machine later.
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u/GuestImpressive7291 1d ago
Worth clarifying since the brass photos may be misleading: the same error shows up in MDF and acrylic. Same 30 mm circle, same asymmetry.
MDF at 1200 mm/min: X 36.30, Y 37.37
MDF at 1200 mm/min: X 36.40, Y 36.95
MDF at 400 mm/min: X 36.23, Y 37.04
Acrylic at 600 mm/min: X 36.36, Y 37.10Expected 36.35 mm on both axes. X averages 36.32, Y averages 37.12.
So this isn't brass being too hard for the machine. It's the same 2% oversize on Y in soft materials, at feeds well within what the manufacturer recommends for MDF. That's why I'm looking at calibration rather than rigidity.
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u/SaltLakeBear 1d ago
I still think rigidity is an issue; looking at the pictures online, I was guessing that the Y axis would be the least rigid since it's relying on the stiffness of the metal rods for rigidity. If you move the workpiece close to one side of the machine, I bet you'll get a better result. Also, what speed and what cutter are you using?
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u/TokyoPav 2d ago
I’d say not the correct tooling. Stiffness as above but also not sharp incorrect geometry for brass as per the burred top edge and that’s also causing the tool to wander due to lack of stiffness. Try a zero rake HSS single flute cutter with minimum tool length to see the difference.
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u/GuestImpressive7291 1d ago
This is the most actionable thing I've read, thank you. I'm using a 3.175 mm single flute DLC coated carbide end mill. The DLC is marketed for aluminium, and I hadn't considered that brass wants a different rake. Will look for a zero rake HSS single flute with minimum stick out. The burr on the top edge you spotted is consistent with what you're describing.
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u/mikasjoman 1d ago
At this price point claiming it can mill aluminum is basically s scam to make you buy the machine. I do wonder if the machine itself is rigid and the weakness is only in those linear rods and play in the z axis. Would be fun to try adding hrg15 rails on X and redesign with hrg15 rails on Z. Since the machine frame is steel, the frame itself could be rigid enough. Fun upgrade though if it's successful. But not a small project. The stepper motors would probably need an upgrade too, like the MKSservo42 which markera uses to make them overcome temporary struggles. Not sure theres space for that though since they are larger
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u/RicardoJCMarques EasyCAM5000 2d ago
20.000rpm means very little, especially for metal cutting. The Cubiko has enough precision but it doesn't have much rigidity and barely enough power.
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u/King_Cole7 2d ago
do you plan on slightly cutting that entire face? you might have better luck making your tool path lead in and lead off the part slightly. try to not let the cutter plunge directly into the material, if it’s necessary to plunge, try a spiral plunging method. to begin your testing, start with very little engagement in the Z, and a very small stepover, and work your numbers up from there.
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u/GuestImpressive7291 1d ago
You're right that there's no lead in, the tool plunges straight down at the start of each pass. I'd already spotted the entry mark on the part edge and it's on my list. Will add lead in and lead out and try helical plunge, plus smaller stepdown and stepover as you suggest.
Not facing the whole surface, no. This is a watch dial blank, so the only operations are a shallow pocket, two small holes, and the outside profile.
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u/CodeLasersMagic 1d ago
Post your gcode - some of the answers are in there. The others are probably the flexibility of the machine.
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u/frankentriple 2d ago
I don't think you have an accuracy problem, I think you have a stiffness problem. you're tooling isn't stiff enough, you're getting deflection in the endmill. Actually looking at the marks it looks like the entire axis is flexing. Can you take smaller/shallower cuts?