CNC PCB milling

[NeverDie]

I was reading the cnczone primer on ballscrews, and the closest scenario to PCB milling was:

Situation 5: "I am scratch-building a small CNC bench mill for machining casting waxes and light metal work. Accuracy is important. I am going to machine jewelry prototypes, small components for turbine engines and R/C, and other small parts with close tolerances."

Solution: This will require a very accurate and tight system with 0 backlash. As the size of the parts go down, the need for zero backlash and a quality fixed bearing set go way up. 0.005" of backlash would ruin a fine filligree in wax for gold casting, or a turbine diffuser. I'd go with C3 or better ground ballscrews, fine pitch, servomotors, direct drive, THK/NSK linear rails and trucks. A commercial bearing block would ensure success, or you can create blocks on your own, but they'll need to be well-made. You'll also need a fast, high-quality spindle, but that is another topic entirely.

I did a quick survey of ballscrews on ebay, aliexpress, and amazon, and all of the ones I randomly sampled were of type C7 quality, which is a much lower quality rating than the "C3 or better" type that's recommended in the quoted passage. That may throw a wrench into the idea of buying a pre-made inexpensive CNC if to get the required repeatability I need to subsequently rip out and replace a lot of key components with better quality ones. Since the better quality ballscrews are relatively expensive, and the quality type is never even mentioned in most CNC machine listings, I imagine they're probably of type C7 (or worse). To put numbers on it, the least expensive C3 ballscrew listed on aliexpress is $170:
https://www.aliexpress.com/i/32798741066.html
and nothing of type C0, C1, or C2 is even listed.

That said, I do wonder whether a glass scale linear position sensor might allow a closed loop system to compensate using software. They're not hugely expensive, and they are commonly spec'd as accurate to 0.005mm, or 0.001mm for a modestly higher price. Since cutting depth is typically 0.05mm, I would think that either one should be good enough. I'm thinking that, at least for the z-axis, this might be worth exploring because for the z-axis all you typically need to do is reach the target depth-of-cut and then hold it there for relatively long time intervals while traces are cut, before picking up, moving to a new location, and repeating. I'm sure there are well studied algorithms for x and y as well, but z seems like it would be comparatively simple to implement. Well, actually, that's not entirely true, because the elevation map means the z-axis may need to continually adjust as well--maybe just not as quickly

Maybe what saves PCB milling is that part of the inaccuracy is related to distance travelled, and there's generally not much need for the z-axis to travel very far once the milling operation has begun (especially if the spoil board was milled flat beforehand by the same CNC). Maybe milling the copper cladding on a PCB flat prior to starting would be a feasible workaround that would throw the problem back into the idealized realm, so that then no elevation compensation would be needed. It might require thicker cladding to pull it off so as to avoid thinning it too much, but it just might work.

[NeverDie]

After doing a bit more reading, I think I'm on the right track as far as using a high precision linear gauge as the basis for the closed-feedback loop. There's a CNC manufacturer named accurateCNC which specializes in building CNC machines for milling PCB's, and as near as I can understand it, they take this approach and get good results with it: they use a linear guage that can resolve 0.1mm, and they claim 1 micron accuracy on their PCB milling: http://www.accuratecnc.com/designConsepts.php
Among other things, they also credit using the absolute best ballscews and a 60K RPM spindle that has negligible runout. As a result,

All new models listed currently on our web site are extremely accurate. The absolute accuracy is definitely 8 to 10 TIMES better than the competitors

If that is so, then I presume their machines literally are the state-of-the-art.

I think that for me, rather than develop custom software to handle the objective closed loop provided by the linear gauges, maybe I could simply translate the linear gauge data into the angular representation that the u-stepper already knows how to process as a closed-loop. I'm assuming there is no meaningful backlash or nonlinearity in a linear gauge. If (?) that is so, then I'm imagining that this approach should be a relatively easy hack to pull off, yielding pretty much flawless positional accuracy, because the closed loop based on the linear gauge measurements should smoothly correct for whatever backlash and non-linearity remains in the CNC mechanism, provided that the measurement resolution is fine grained enough that the corrections are continuous and not herky-jerky.

[Joerideman]

@NeverDie their tools are also $115+ voor 10 engraving bits.

Presicion comes with a heavy price.

[NeverDie]

@Joerideman Not just that, but their best machine costs nearly $18K.

One salient feature keeps coming up though, and that seems to be that PCB milling gets better results at higher RPM's. I've heard it suggested on the CNCZone forum that 24,000 RPM is a good lower bound for spindle RPM on a PCB etching mill. Why is it so? My current spindle tops out at a mere 12,000 RPM. There's a 12v 20,000 RPM spindle for sale on Amazon that's priced at around $30:
https://www.amazon.com/gp/product/B08DTHDSMV/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1
so I ordered it to see for myself whether higher spindle speed will make a noticeable improvement. If it's true then that's obviously a great thing to know. However, I'm still left wondering as to exactly why it leads to overall improvement. Might it simply be that spindles designed to run at higher speeds are built to a tighter specification, and so maybe as a result there's less runout, and so perhaps it is simply that which is what's truly responsible for the better results? Or maybe the higher RPM's result in more gyroscopic effect, which effectively renders the spindle a bit more rigid? I can imagine quite a number of different possible explanations. Does anyone reading this know the answer?

[Yveaux]

@NeverDie said in CNC PCB milling:

However, I'm still left wondering as to exactly why it leads to overall improvement.

Isn't it just that higher speeds lead to smaller amounts of material being removed with each rotation of the spindle? (given the milling speeds stays constant)
If that's the case you could exchange milling velocity for spindle velocity.

[Yveaux]

@NeverDie said in CNC PCB milling:

assuming there is no meaningful backlash or nonlinearity in a linear gauge.

Backlash no, nonlinearity yes, but that all depends on the quality of the ruler and the accuracy you are trying to achieve. At the nanometer level nothing is flat anymore :smile:

[NeverDie]

@Yveaux said in CNC PCB milling:

@NeverDie said in CNC PCB milling:

However, I'm still left wondering as to exactly why it leads to overall improvement.

Isn't it just that higher speeds lead to smaller amounts of material being removed with each rotation of the spindle? (given the milling speeds stays constant)
If that's the case you could exchange milling velocity for spindle velocity.

Maybe you're right. Somewhat rephrasing what you said: perhaps a higher RPM is needed to get a smaller chip size, and (for reasons unknown) a smaller chip size is what's optimal for PCB's.

I was starting to entertain an alternate theory, which is that a higher feedrate would serve to better avoid stiction, which at narrow pitches could be devastating. A higher feedrate needs to be matched by a higher RPM, and a higher RPM more or less demands a faster feedrate. Constantlly moving (and the faster the better) is maybe one way of mitigating against stiction. A corollary of this is that by having a high precision (to the nearest 0.1 micron) linear encoder, each axis is probably always moving and never completely stopping.

Anyhow, I suppose there might be a whole host of beneficial effects created by running at a higher RPM without any one of them being dominant.

Some of the higher end CNC PCB etching machines (such as accurateCNC's flagship model) have spindles which can spin as high as a 100,000RPM, or about 8-9x the maximum speed of my default spindle. If I recall correctly from an earlier discussion on this thread with @executivul , there tends to be much less vibration at higher RPM's, and so that too might be one of the positives driving the move to higher speeds.

New 100Krpm spindles seem to be priced around $1,500 (and up), whereas a new 60Krpm spindle can be had for around $300. I'd speculate that either one, almost by definition, needs to be relatively well made just to avoid ripping itself apart, but I can't say that I know that for sure. Just a weak hunch. On the other hand, maybe they're built only just good enough to last through their warranty period and then promptly die, just like Salmon after spawning. But then again warrantees may be meaningless in the first place if the spindle is sourced from some no-name manufacturer. And then there are "pre-owned" spindles, which I'm unsure how to evaluate, and for that reason I so far haven't even considered them. Maybe after I learn how to properly measure and characterize a spindle I'd know which spindles won't be good enough. This might be the way to go, because if it works when I first receive it, that would imply it has already survived Part 1 of the MTBF bathtub model, and I'd probably use it so seldom (well, compared to the industrial user that it's probably designed for) that it would probably outlive me. According to the bathtub model, those are likely better odds than buying a new spindle that has only a (probably worthless) warranty to stand behind it--of course, that analysis relies upon the very large assumption that it's not being sold for the precisely the reason that the seller already knows it to be defective or otherwise impaired, much like many/most of the cars on a used car lot. The oddsmaking works only if blind chance alone governs whether a particular pre-owned spindle is defective or not. Well, good luck getting the seller to disclose that information if he figures you can't tell the difference. That is, if I don't know enough to evaluate and check acceptance criteria on a new purchase. But if I do, then it shouldn't matter (well, not as much anyway) even if the seller is lying about the true condition of the spindle or his reason for selling it, as in that scenario I hopefully know enough to run the right diagnostic tests upon receipt of the spindle, and I would then document the issue, reject it, and send it back to the seller, being sure to avoid that particular seller on all future purchases.

Yup, I think that sorts the medium to long-term strategy. In the short-term, though, until I develop that skillset, I'd either roll the dice and accept the risk or, more likely, I'd probably buy a reputable brand with a trustworhy warrantee, even if it means paying more (as it surely would). For instance, in my case that probably means I'll be buying linear rails and ballscrews and related parts directly from Thomson or Misumi, both of whom will sell to individuals. For my next machine, that may also rule out buying a heavy, fully integrated machine from China, as the parts are who-knows-what and any return shipping would probably cost quite a lot (and be yet another thing that could go wrong). But that's me and my particular baggage. I'm not saying others shouldn't do that if so inclined.

[executivul]

Regarding feedrate vs rpm the last engravings I ran at about 40k rpm, was to lazy to balance the endmill (turn spindle on, if it hums loudly then stop, undo the clamp, rotate 20-30 deg the endmill in the clamp, retighten clamp, rinse, repeat). I just tend to find the poles of harmonics: start spindle at full speed, hums badly, lower rpm slightly less hum, lower even more starts humming again, lower again, even more quiet... so find the sweetspot with least humm (vibration). If you go down enough at about 35-45k rpm you will get the quietest.
For best feedrate I've manually written a simple zig-zag gcode with different speeds: F1000 G1 X20Y5 G1 X0Y10 F1100 G1 X20Y15 G1 X0Y20 F1200... and inspect with the microscope for the best, burr free, engraving speed.
I normally run at 1400mm/min.

[niallain]

@NeverDie said in CNC PCB milling:

@niallain said in CNC PCB milling:

non-linearity is measured only on 1mm, with calliper or glass scale it would be possible to measure whole screw. (linuxcnc can use glass scale to map screw and then compensate, but I still don't have all components for it yet)

That's quite an interesting result! I would not have imagined it would be so bad. What exactly is happening that causes this? Is the screw being wound up almost like a spring and some kind of stiction is causing the carriage to jump about like that? Is the anti-backlash nut causing it? Or is the stepper in fact not advancing as much as it should be due to no closed-loop feedback? What's the cause? How repeatable is it? i.e. does it jump about to the same amount in the same places each time it's cycled through the test, or does it vary each time the test is repeated?
It's a great advertisement for DRO's.

It might be acceptable non-linearity for trapezoidal screw. But on Y axis, at one place it was far worse (don recall exactly but around 0.1-0.05mm). I wouldn't blame steppers though as load in test case is only inertia of Z-assembly and I didn't see lost steps. Yes, It's repeatable.

From what I've read glass scale is only good enough for positioning starting point accurately (i.e. various measuring tasks), trying to add it into control loop (of linuxcnc) as encoder were considered not useful as it were too slow.

Ball screw for Y axis has finely arrived, now it's time to drill holes for new supports
and modifying moving table so that supports won't get in a way. After that it would be interesting to compare precision with X axis.

[NeverDie]

@niallain said in CNC PCB milling:

From what I've read glass scale is only good enough for positioning starting point accurately (i.e. various measuring tasks), trying to add it into control loop (of linuxcnc) as encoder were considered not useful as it were too slow.

Maybe if it were magnetic encoding there might be some latency, but if it's optical I would think that there'd be no meaningful lag. Anyhow, I'll be finding out after mine arrives: I just now ordered a Sino KA300 optical linear encoder with 1 micron resolution to try out. I was leaning toward a magnetic encoder until I read your post, so thanks for the heads up. Priced at $64 on Aliexpress, including FedEx shipping.

[NeverDie]

Closing the loop on the earlier topic of best inks and substrates to use for printing a UV mask, I can report what seem like solid answers based on the findings of two well known youtubers who did actual compare/contrast experiments:

The applied science guy measured the amount of uv passed by vellum paper compared to transparency and found that, even though vellum is translucent, it actually passes more uv (see time index 11:00):
https://youtu.be/NS8Q9LUIKA8

So, for that and other reasons, he recommends using vellum instead.

Meanwhile, bigclive compared different kinds of printer inks and found he got the best results using a pigment ink (see time index 12:35):
https://youtu.be/W_oDBp_wgJQ

[NeverDie]

I finished the first-pass of new upgrades on my CNC:

Shown above are the UStepper-S closed loop drivers on each stepper. Each stepper was also upgraded to a higher torque Nema 17 than the more basic Nema 17 which came with the original kit.

In the back, the Woodpecker CNC controller has been replaced by a plain Arduino Uno together with a generic GRBL shield. Clearly I'll need to find shorter RJ-45 cables. Each stepper will be powered by its own separate 24v DC power brick. By using power bricks instead of a shared fan-cooled power supply, the axis power supplies will be dust-proof.

Speaking of dust, I removed the previously applied wet lubricants from the rails and screws and then applied A-9 as a dry film lubricant, which claims to both repel dirt and to not pick up dust. The only significant source of "dust" is the very fine sawdust created by leveling the wasteboard. The wet lubricants were picking up that sawdust and caking it on the rails, so hopefully A-9 won't.

I'll be resuming with the same spindle that I was previously using so as to better gauge the amount of improvement (if any) afforded by the new closed-loop stepper drivers. After establishing a reasonable baseline, I'll swap in the much smaller 20,000RPM spindle motor I recently acquired to see whether or not it's an improvement.

I have no idea how I should install the z-axis linear encoder when that arrives. It might push me toward a different design, whereby the z-axis is mounted on a fixed column , and the X and Y axi are merged into an XY table that moves around beneath it (more akin to a typical mill than than a typical CNC router). I happen to think that would also make the z-axis more rigid and so would be, in itself, a worthwhile upgrade. To that end, I may borrow the column from the openmill design, as it looks like it would be fast and easy to build (and rigid as well?):

though I would aim to use better rails than the wheeled approach used by openMill. With that as the column, I would likely use:
https://www.aliexpress.com/i/4000527851100.html

in order to quickly validate the proof-of-concept for the new z-axis, though I'm aware the tolerances are probably C7 or worse. Even so, I'd wager the tolerances would be tighter than the status quo.

[NeverDie]

One problem with picking an inexpensive power brick as a power source for a stepper motor is that even though it may say it's rated for 6a at 24v, there's often no datasheet, and so that "6 amps" it's claiming may not be for continuous duty, and there's no guarantee that you can rely on even 80% of 6a. Yet, "6 amps" is the only info that's written on the brick, and so adequate qualifying information just isn't available, unless maybe you generate the data yourself using an electronic load test setup. Or, just run it and see how it performs, which is the quick and dirty rout--which is the path I chose.

It appeared I was able to drive the y-axis at a nominal 10,000 mm/min, but going faster than that yielded wildly unpredictable results. Plugging in a 10a 24vdc power brick instead of the 6 amp power brick seemed (so far) to avoid the problem, so I'll be replacing the 6 amp power bricks with 10 amp power bricks for about 4x the cost. All this just to avoid dust incursion into a single, centralized, standard, high current, "open to the atmosphere" power supply. :face_with_rolling_eyes:

That said, I'm unsure how well even the 24vdc-10a power brick will hold up over time to the very spikey current demands of a CNC-driven stepper. That's the lingering doubt that accompanies going down the quick and dirty path with no datasheet. Given the unknowns that come with power bricks, maybe a fan-cooled , ordinary power supply isn't such a bad trade-off even if it sucks in a lot of dust... :rolling_on_the_floor_laughing: Or maybe a Redundant Array of Inexpensive Power Bricks is yet another alternative. Meh,....

Another, unrelated thing I'm noticing is that the Dupont and RJ45 connectors and barrel jack on the uStepper boards are a bit loosey-goosey. They may be conceptually neat and convenient but in reality they don't seem particularly good at handling vibration and/or physical strain without tending to wiggle loose from their connections. Not sure yet what, if anything, to do about that. Even if I were to solder the connections, it may still need some kind of additional physical anchoring to hold everything firmly in place and provide sufficient strain relief.

Unlike the original nema 17 steppers that came with the CNC kit, these higher torque nema 17 steppers don't work at all on just 12v. However, the good news is that the upgraded higher torque steppers are pretty snappy when running them at 24v.

[NeverDie]

I have the closed loop CNC nominally working now with 16 microstepping. I'm finding that at each stage as I push it to 32 microsteps, then 64 microsteps, then 128 microsteps, and finally 256 microsteps, I have to ease off the acceleration and velocity or else an alarm happens. At 128 microsteps, it's really quite slow, and at 256 microsteps it seems unuseable. My hypothesis is that at each stage the microstepper is becoming weaker and weaker, and so, to compensate, I thought I might drive the system with higher voltages and higher currents. However, I don't want to burn-out the drivers, so I'm waiting on a reply from the folks at Ustepper before proceeding. It turns out they have very little documentation.

[NeverDie]

I received useful guidance from the guys at uStepper:

1. They advise that I can go up in voltage, no problem--all the way to 42v if I want. Even at 42v there's still some safety headroom leftover. However, they advise against increasing the current, unless absolutely needed to avoid stalling, as that may lead to overheating, unless active cooling (a fan) were to be added.
2. "10mm/revolution a 32 microstepping input will give you 0.0016mm resolution. " This is a good point. Actually, with the current lead screws I'm at 8mm/revolution, and if I switch to ballscrews it will be either 4 or 5mm/revolution, which would be even better. Hence, maybe even the current (default) 16 microstepping is good enough.

So, I guess once I get limit switches installed, then before I get fancier it'll be time to do some milling and see if I notice any improvement brought by just the uStepper drivers alone. Then I may upgrade to the 20,000 RPM motor to see what, if any, difference that makes.

I still have my eye on a 60,000 RPM spindle. They seem not much more expensive than the 36,000 RPM spindles. I've found ER8 60,000 RPM spindles for less than $200 delivered, but ER8 seems like it would be too tight a fit (?), especially for regular router bits like the ones with 1 inch diameter heads used to flatten spoilboard. For some reason, ER11 60,000 RPM spindles seem to cost around twice as much, though I have no idea as to why. Is it really that much harder to make an ER11 spindle than an ER8 spindle that's otherwise the same?

[ncollins]

@NeverDie Mind posting links to the spindles you're looking at?

[NeverDie]

@ncollins Sure. For example, an ER8:
https://www.aliexpress.com/item/32651625780.html?spm=a2g0o.cart.0.0.7a203c00aR74s0&mp=1

and an ER11:
https://www.aliexpress.com/item/32948530784.html?spm=a2g0o.cart.0.0.7a203c00aR74s0&mp=1

I tried to make it apples to apples. Prices are constantly fluctuating, so at the moment this particular ER11package seems priced lower than a lot of the other ER11 60,000 RPM packages, so that will contradict my earlier 2x statement (which is more of a running average across different vendors). At the moment, the price looks fairly attractive, if the quality is there. If the quality is poor, I'd probably be stuck with it, so there's a lot of risk in buying from sites that don't list the tolerances.

Marcos Reps suggests that buying used industrial equipment might be a safer path, because on his theory most industrial equipment gets replaced while it still has reasonable life left on it, and the quality tends to be high to begin with. But in his case, because of his vlogging, a manufacturer actually just gave him a high quality spindle, so he never really tested his theory.

Which 60,000 RPM spindle manufacturer is the best value? It may boil down to who makes it and what the tolerances are. That sort of info is often missing from the aliexpress listings. If the etchings on the spindle cases can be believed, then in this particular comparison the manufacturer is the same (namely, Changsheng).

Anyhow, Wegstr, for instance, seems to get great PCB results without relying on such massive spindles, so maybe I should be looking elsewhere.... But where, exactly?

Also, who makes the best VFD? I've read that certain brands are problematic, but I haven't kept a list (yet) of which ones to avoid.

These spindles are quite heavy, but maybe the extra weight would just amount to more preloading, which maybe (?) could be a good thing. I do like the portability of my CNC, which a Marco Reps style machine has clearly lost, despite its other advantages.

[NeverDie]

@ncollins Funny thing: when I look now at the photo for the 60000RPM ER11 spingle, the marking on the spindle itself shows only just 24000rpm. Similarly, the specs contradict themselves as well: the title says 60000rpm, but the speed (RPM/Minute) table shows 24000. It's this sort of inconsistency by Aliexpress sellers that gives me the heebie jeebies. For that reason, I'm guessing it's not really 60,000 rpm after all, which would explain why its price is considerably lower than the other 60,000 rpm spindles on Ali Express. If the Aliexpress seller were to send me a 24,000 rpm spindle instead of the 60,000 in the headline, I'm really not sure how that would play out for buyer protection, given the inconsistency portrayed in the description. I would try hard not to buy from a seller like this one, whose listing so badly mixes up the product data, as it seems like a heartache just waiting to happen.

[executivul]

My 60k rpm is ER8, I guess the lighter collet/nut combo is less prone to vibrate, never seen a decent priced 60k rpm ER11.
ER 11 good is for heavy endmills, big collets, more than 3.175mm (1/8") shaft diameter, maybe 1/4" for table flattening.
The 60k rpm spindles are usually 300W, not a lot of power at lower rpm to do tough milling, so no use for larger endmills, ER8 is a good choice for fine milling with 1/8" shafted endmills.
My VFD is a Sunfar one, pretty decent I might say, I ran it for a few dozen hours now without issues.

PS. The best way to check for runout is the sound test, run the spindle empty, then with collet+nut, then with the endmill inserted. If I "tune" the position of a new endmill I can reach 60k rpm without any vibration at all, so I get almost no runout.

[ncollins]

@NeverDie Once covid started, my aliexpress luck really took a dive. Two missing shipments, one partial shipment, and very little recourse. If I absolutely have to, I'll buy from banggood. I don't think I would make such an expensive purchase on aliexpress, especially when shipping is a significant portion of the cost. OR, I would want to talk to somebody who has purchased from that seller and has the exact product I'm looking at.

Contrary to my 3D printing and PCB/component experience, where I do okay with cheap/counterfeit parts, the tolerances and high cost of mistakes in my CNC experience are leading me to pay the premium for quality parts.