Exactly. I can't speak for laser, extruders, knives or such, but for subtractive work (i.e. milling / drilling operations with some form of rotary tool) this design would be an exercise in frustration. Typically, you would have encoders on each axis to tell the machine where things are happening. These encoders are excellent, but any flex/slop in the chassis or moving parts is not caught by the encoder... so the software will not know that the cutter is in the wrong place. Thus, crappy parts. Any reasonably useful CNC design takes rigidity and backlash prevention seriously. Take a look at PocketCNC for a good (and yes, expensive) design.

could you use strain gauges to measure the deflection and fix it in software?

>could you use strain gauges to measure the deflection and fix it in software?

No. There's no substitute for rigidity in fixturing and of the machine itself. There will also be vibration motions that have unpredictable nodes, frequencies, and amplitudes.

Consider that a 2 flute mill at 20,000 rpm will induce vibrations with a fundamental frequency of about 670 hz. This would require a servo system with a bandwidth of about 2 khz to correct, a sampling rate of 8khz would be a bare minimum.

Next, what to do if the tool digs in, and shoves the part? You have to have backlash sufficiently damped, and rigidity enough to keep the tool or part from being snapped off.

There's a good reason machine tools are as heavy and rigid as they are, and why they must be on a thick foundation, and leveled precisely before use.

If you're going high precision, you'll also have everything in a temperature controlled environment, and allow the materials to soak at a day to achieve the proper temperature. Cooling is very important when you get into fractions of a millimeter, or "tenths" (1/10,000 inch) tolerances.

The best machine tools are old American cast iron machines from about 1940 to 1960, rebuilt with modern control systems.

The rotary axis is controlled by a stepper motor. This is not a usual turning lathe. The rotary axis can be made as slow as necessary and the only relevant analysis is a quasi-static one. Answering to parent comments: there are usually no encoders on these machines. The controller firmware just counts the motor steps. If a motor misses a step, everything will be off from then. For the cantilever design: it is already used by Prusa 3D printers which are well regarded (in the field of amateur 3D printing) so I do not think this is an issue for the targeted accuracy which is typically 10 to 50 micrometers.

The only issue I have with this design is that the accuracy decreases with the distance from the rotary axis, hence it is not uniform over the building volume.

>For the cantilever design: it is already used by Prusa 3D printers which are well regarded (in the field of amateur 3D printing) so I do not think this is an issue for the targeted accuracy which is typically 10 to 50 micrometers.

3d printers don't have forces pushing back on the print head, nor the vibration from a cutting tool to deal with.

This design might work ok for 3d printing, or laser cutting, or anything else where no contact between the working head and the material occurs.

As far as accuracy and repeatability, it's going to be about as good as the average 3d printer design. It's a nifty idea for a system, but it's only a decent machine for cutting if it's scaled up massively and built of cast iron or similar materials.

Depends on what you cut. It cuts wood pretty nice. I couldn't documented the wood cutting performance on prototyping. I will demonstrate it's wood cutting capacity in v1 release. It's a 3d printable desktop machine and never intended to cut precise metal parts. May be I can design a full metal body version with professional components like rail guides instead of plastic v-slot wheels.

It is 0.025mm on the outer edge of the table.

I don't know if there's any strain gauges designed for this application. I'd be worried about nonlinearity, creep and how you would calibrate it in the field. (If it requires a dial indicator and a surface plate, you'd probably be way better off getting a regular CNC mill)

Optical tracking might be easier.

I'm not sure how much deflection you could actually tolerate before deforming the joints in the arm. Even with deflection tracking you might end up taking very, very shallow cuts. A 3D printer that runs for 16 hours in your apartment is annoying, a CNC mill running for 16 hours will be genuinely intolerable.

Great question. Human arms are also very bad in rigidity, repeatability, etc. but we manage to do great work by adjusting. Of course we try to move and rest on things to get out of the error-prone postures (so in effect moving from this to a gantry type of configuration), and need to measure, create jigs and templates, etc. but there is something here to be covered by sw for sure.

Comparing human arms to CNC "arms" makes about as much sense to me as comparing a sparrow to an F16.

Yeah both humans and multiaxis cnc machines can articulate their arm to apply a tool to a work piece and both a sparrow and an F16 can fly through the air.

I like your analogy, but just to clarify, which is the human being in your comparison: the sparrow or the F16?

A sparrow can take off and land on a branch, and it regularly weaves through thick foliage no wider than its wingspan. So, I assume the human is the sparrow?

Yeah I was thinking of the human as the sparrow.

You could probably use it for ECM or EDM, but yeah, not stiff enough for traditional hogging cuts as a lathe or mill.

I would like to see software compensation for frame flexibility.

Either open loop (ie. use mechanical models to predict frame flexing and compensate for them), or closed loop (have sensors to detect realtime head position).

The whole idea of having a frame which is 100% rigid seems like a simplifying assumption that makes everything more expensive, with knock on effects too (eg. making stuff stronger makes them heavier, which in turn means a bigger motor is needed, which uses more power, so a bigger power supply is needed, etc)

The Shaper Origin handheld CNC router makes an impressive attempt at compensating for imperfect frame rigidity, as it's designed to be handheld. The Origin operates in only two axes and uses consumable tape placed on the workpiece as an optical reference, but its progress is noteworthy.

I expect it would be easy to compensate position (closed loop is already table stakes in the professional tier) and extremely difficult to compensate chatter. What would it take to drive the motors of a 5 axis CNC to responses well into the kHz? You would need that, to compensate chatter. You would also still need to handle the same peak force, whether or not you do it with negligible deflection.

There are some industrial machines that compensate for chatter by slightly varying the spindle speed when chatter is detected. However machines so equipped are fundamentally very rigid and usually experience chatter occurs when pushing end mill performance to its limits. I suspect the adaptive spindle speed strategy would be much less useful on a machine like the Polar Bear.

Right, resonance dodging and deflection compensation aren't going to cut it here, not by a long shot. I still think a sufficiently good 5 axis control loop could damp acoustics: for one, the (fractional) millisecond timescale of the acoustic oscillations are geological by compute standards, and while I'd expect limitations from sensing, actuation, and power electronics, I don't see anything fundamentally prohibitive. I'm pretty sure this is an engineering problem, not a physics problem, and that eventually someone is going to crack this nut. It won't be the Polar Bear.

One way to compensate chatter is to have a small mass (say 50 grams) on an X-Y-Z voice coil on the cutting head.

Then you can use that to apply large brief forces in whatever direction is needed to stop chatter for a few milliseconds with each turn of the cutting tool.

It's the same mechanism used in iphones "taptic engines" to make 'clicks'.

The same mechanism can be used to actuate the head at far higher frequencies, allowing cutting fine detail at high speeds.

I would think it would have to be closed loop. Material density is not always uniform (especially with something like wood) and that has one of the biggest effects on how your tool behaves.

Even then, compensating for sudden release in tension, where the tool tries to spring back, seems perhaps insurmountable.

The problem isn't so much the rigidity for absolute positioning, it's the high-speed vibration/chatter than a rotary cutter will induce. You can't really damp that out in a system like this (remember, you're spinning your cutter at thousands of RPM). This results in additive errors. Think of the washboard pattern that arises on dirt roads in dry areas. It's the equivalent, but your cutter doesn't have shock absorbers. If my limited experience is any guide, you'll eventually start snapping off your cutter, breaking your stock free from the mounting, and/or getting mysterious gouged cuts into the stock that destroy your detail.

Not pragmatic IMO - flexible usually means resonance and vibration which leads to poor finish and deafening the operator/upsetting the neighbors.

If a lighter frame was the design goal (but low cost wasn't!) then carbon fiber has an interesting stiffness/weight ratio and could have viscoelastic damping elements incorporated too. Realistically though, lightness isn't a prioritized design goal though I certainly see the appeal in marketing for home usage where you wouldn't need a crane to move the machine about.

It could work to have two towers. The first carries the cutting tool. The second carries a measurement device and is subject to negligible forces. Alternatively some form of photogrammetry, possibly with a structured light field.

Are there similar systems- CNC with multiple heads- that don't have this problem?

I played with a wood router CNC in high school and loved it. I've also played around with 3D printers a bit. The idea of having (and building?) a reasonable system that can do both strong appeals to me.

The closest that can do both is probably MPCNC (https://docs.v1engineering.com/mpcnc/intro/).

it is built with that modularity in mind but as other commenters have pointed out building something for rigidity makes it slower and heavier. a cheap 3d printer like an Ender 3 in addition to the MPCNC makes more sense, especially since you can save on the MPCNC by 3D printing the components yourself.

The main benefit to swapping a 3d printer head onto the MPCNC would be the pretty massive XY size you can build the MPCNC out to. If you need to print really large, flat objects it is probably cheaper than a lot of alternatives. But at that point it probably would make more sense just to mill it out of wood anyway.

no. anything that is strong enough to, say, CNC will also be very slow for 3d printing or laser. different processes demand different designs.

Depends on the price class, of course. If you spare no expense, you can employ lots of iron and lots of power to throw around the iron.

https://youtu.be/RcansEmVknA?t=150s

For context, that DMU50 mill is about $275,000 new.

>with a horribly flexible frame that denies any serious usage

What's the risk from this? That CNC work will be affected by resistance from materials, causing the frame to bend? What kind of scales would that effect be noticeable on?

For any real cutting, you need rigidity or nothing will be accurate.

You will not be doing any real cutting on this machine.

But to what extent will it be inaccurate? It seems unlikely to me that anything used for machining hard materials can be 100% rock-steady. Presumably, industrial machine are simply inaccurate to very small, tolerable degrees.

Accuracy and Precision would be problematic with this design, but, lack of rigidity is more insidious. Most of these high speed spindles will use carbide tooling, which is highly susceptible to breakdown under vibration. A flexible horrible frame like this is going to vibrate a lot even in materials like delrin or HDPE.

Also, with a vertical spindle and CNC control, there is no reason to have a vertical rotary axis. The reduced motion doesn't buy any capability upgrade. The 4th axis that is helpful would be angular. (you could put a horizontal spindle on this, I guess, but this machine is not up to the task.

Obviously there is no perfectly accurate machine for any application.

The extent it will be inaccurate depends on a lot of factors, such as tool length, material parameters, feeds/speeds, etc. You can probably cut aluminum somewhat accurately on this if you take extremely slow cuts, with a tiny tool spinning at an extremely high RPM (which keeps cutting forces to a minimum). Surface finish would still be pretty bad simply due to vibration of the arm. Likely going to break a lot of tools too. The majority of metal cutting setups you would do on a mill are simply impossible on this machine.

For a few tiny cuts, it might be handy to have one of these, but it definitely seems more useful as a 3D printer, plotter, etc. Though even most 3D printers are significantly more rigid than this design.

I have a small mill, it still weight around 1250kg… to be accurate at 0,01. And for that I would need to lock the unused axis and respect the power of the machine. This is not CNC, where the forces are often higher because they can work faster, with curve and some other things I’ll have a hard time doing.

The frame will vibrate resulting in chatter and in turn your cut will have waves and dips.

The risk is that you will only be able to mill materials that are very soft; materials that could be cut with a pair of scissors (card, foam). Anything harder will be an exercise in frustration (friction rubbing instead of cutting, frame flexing, motors stalling, terrible tolerance).

You have limited use for subtractive CNC work. Materials it can work on will be like chalk, charcoal, graphite. It might be usable on machine wax but I doubt it.

Whats the definition of "serious" here? Is there a standard measure of tolerance or material compatability that it can be rated against to see if it would work for a specific need?

CNC (and mfg in general) will have different tolerance needs. And the flex on this machine will depend on the material and work speed.

I'd probably argue that anything worse than 0.5mm (~20 thou) will start giving you headaches if your making parts that are supposed to interact with each other. That said, if you're making basically oneoffs and are willing to handfit, that tolerance is probably tolerable (ha!).

It's not hard to make a mill that can cut shapes into a block of styrofoam. If I stretch I can maybe imagine some industrial uses for a styrofoam-only CNC mill. (Moldmaking?) But what would a regular consumer use it for?

"Serious" just means the ability to cut more useful materials, like wood or steel.

Serious = harder (hence more useful) materials. As it stands I would say only materials that could be cut with a pair of scissors (card, foam) could be used on this safely without frustration.