What do I need to convert my Proxxon MF70 to CNC Milling device?

Hello all,

I am Ben (nickname Bjor) from the Netherlands and new on this forum. I’m also a novice when it comes to CNC and I have little knowledge of electronics. (Sounds pretty awfull, don’t you think?) So I can use some help with converting my Proxxon MF-70 to a CNC Milling machine. I wil use it for 90% for woodwork and for the rest for milling non ferro materials. After 7 to 8 years I’m tired from milling by hand. The MF70 is a small machine, but I am a N-Gauge modeller (1:160) and I am building modell houses in 1:12, So my machine is sufficient in 95% of the modelling work I do.

As far as I now know I need three steppermotors and a controller and drivers, CAD software and software to send G code to the Milling machine.
I think a Smoothieboard (preferably V2 once it comes available) is a good solution for me. Is it true that a Smoothieboard contains al the electronics my machine needs?
There seems to be lots of software that I can test, once I have the hardware installed. I have two PC’s with both Windows 10 and Linux and a Raspberry Pi model 4B with 4GB memory.

At the moment I have 2 questions (I think many will follow):

  1. Which brand and type of stepper motors will suit my MF70 best, given the sort of (hobby)milling I do?
    I thereby will refrain from cheap parts from Alieexpress and friends. I don’t need top quality (for top money), but just reliable and secure parts.
  2. Are all electronic parts for controlling the device integrated in a Smootieboard, or will I require additional hardware?

I hope I make sense and you can advice me on this.

Greetings,

Ben

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Welcome!

From a hardware perspective, the most unexpected change will probably be switching your axes from trapezoidal lead screw to ball screws. Trapezoidal lead screws resist back-driving so are good for manual machining, but must have some backlash (manageable in hand milling, bad for CNC) or they wear out quickly. Ball screws are relatively free of backlash, which is good for CNC, but back drive easily, which is bad for hand milling.

Therefore, you typically need to design ball nut mounting blocks for your mill, then have a mount for the stepper motor using a coupling to drive the screw. You don’t want to use the cheap spiral couplings that are common in 3D printers. Use Lovejoy / spider couplings, or diaphragm couplings, instead.

If you use the stepper drivers on the smoothie, as far as I know you will be limited to 24V. (@Arthur_Wolf can correct me if I’m wrong.) If you later discover a need for higher voltage, you can switch to external stepper drivers at that time.

You can do without adding limit switches, though if you make a mistake you will probably dislike the noise it will make. It should be harmless to push against hard end stops as long as they are solid.

Do you know yet about CAM tools and “toolpathing”?

For steppers, brand probably matters less than specifications. 470 oz-inch / 3.3 Newton-meter is probably the lowest torque you want. NEMA 23 (60mm square cross section) is probably the right size. You might need more torque for Z than X or Y. You can measure needed torque with a good digital scale and an arm and do the math. You’ll want substantially more rated torque than you measure. Probably at least three times what you measure. I am using 24HS39-4204D Dual Shaft Nema 24 Stepper Motor 4.2A 4Nm (566 oz-in) 60x60x100mm with external drivers and 36V, but my conversion is a larger mill than the Proxxon I think. You would need the v2-pro to exceed 2.8A on a channel with on-board stepper drivers. I think the pro will support 5A. Going too low on torque on your stepper motors risks losing stops and scraping your work.

I hope that is helpful!

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Yes, on-board drivers on smoothieboard ( A5984 ) are limited to 24v. Which should be perfectly fine for a Proxxon.

Or you can use external drivers ( like say a CW5045, see smoothie documentation on external drivers ) and those can go up to 48v or even 80 for some. Using external drivers is the “normal” / “industry standard” way of doing this.

For software, strongly recommending CamBam and Fusion360.

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About stepper motors, I would recommend https://www.linengineering.com/ in the dimension you need ( I’ll assume NEMA23 ), get the motors that have a current rating between 1.8 and 2A, that have the highest torque for that current rating.

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I didn’t realize how small the Proxxon MF-70 is — 7kg total weight! I retract my stepper recommendation in favor of Arthur’s which is better informed. :slight_smile:

Also maybe for modelling in wood a little backlash is not meaningful? Here I’m out of my depth.

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@ mcdanlj:
I did not know the axes need changing. I will check with Proxxon which type of axes they use in their CNC table: https://www.toolnation.nl/proxxon-micro-kruistafel-voor-mf-70-cnc-ready.html
(Which could also be an alternative should convertion prove to be to complicated for me.)
I have seen couplings on videos, but I know now what they are for. They will be on my shopping list.
About toolpathing I know there are 2D and 3D designs. The majority of things I will produce is 2D. But every now and then I have the need of making something in 3D.

The part about torque is latin to me. I presume it defines what kind of steppermotors I will need?
But Arthur and you clearified that later in your further remarks.
And you are right; the MF70 IS a small machine, ideal for the type of modelling work I do.

@Arthur_Wolf:
You mean something like this:

About software: I’ll look into the software you mentioned, but for me it is important in runs with Linux.

Thank you both for your answers. It clarified some things for me.

Ben

Torque is rotational force. The higher the torque, the stronger the twisting force.

Stepper motors are classified by their cross section. 60mm / 2.3 inches is a NEMA23 / 60mm stepper. NEMA17 are 1.7 inches / 43 mm. They have standardized mounting hole location and indicating cutout dimensions, though some of my NEMA17 motors take metric M3 hardware and others US #4-40 (IIRC) hardware, which drives me nuts.

Stepper motor variations include single-shaft (shaft comes out one end only) or dual-shaft (really a single shaft that sticks out both ends), round or D profile shafts, shaft diameter (often different on one end from the other), how many steps per revolution (typically 200, 1.8⁰/step, with greater torque; or 400, 0.9⁰/step with less torque, all other things being equal; but as you found you can get 800, 0.45⁰/step as well), max current per phase (ø), voltage (voltage which, if constantly supplied, will result in the max current; this is not a max voltage that can be used), torque (typically measured in newton-meters and/or ounce-inches), and inductance (lower allows moving faster).

Note that degrees per step is degrees per full step. Modern stepper drivers use microstepping which holds the stepper at an intermediate position, with a corresponding reduction in torque. 1/16 microstepping is common in CNC; that’s 16 microsteps per step, which has a bit less than a tenth the motor’s rated maximum torque.

You need more torque than you think you need. The steppers you are looking at are “open loop” which means that they have no way to directly measure whether they actually moved. This means that if the machine resists the motor turning hard enough, they can fail to turn where they are commanded to move and the system has no information about that, and now doesn’t know where the mill end really is. It will continue to drive it as if it were where it “thinks” it is, but there will be a shift. This is called “skipping steps”. Because of that, you need substantially more rated torque (after accounting for loss of torque from microstepping!) than you might measure that it takes to move the axes on your mill.

The voltage rating is really a maximum only when not using a modern stepper driver. If you want to know I can go into more detail. But the net of it is that you might have a stepper motor rated at 2.5A and 1.3V, and supply it 24V through a stepper driver programmed to limit to 2.5A current, and you are not operating outside the stepper’s design parameters. This would be typical of your use with the smoothieboard which contains such drivers. (The current limitation on the smoothieboard is set in its configuration file.)

For running on Linux, what software are you running or do you consider running for CAD?

Michael,

Thank you for the explanation. This helps a lot. It will take a bit more reading about steppers and their caracteristics before I fully understand it all, but over time, it will.

Do I understand you well that the stepper I mentioned in my last posting will comply with Smoothieboard V2 and is suitable for my MF70?

About software: I just looked into only 1 or 2 CAD solutions and downloades Open Scad 2019.05 the other day. Do you have an opinion about that CAD program?

Ben

Since you are a Linux user, you might want to know that Linux Weekly News just the past few days has run a couple of features on open source CNC.

https://lwn.net/Articles/828037/

(The second article, covering FreeCAD, is currently available only to subscribers until the middle of next week.)

The first LWN article I linked has links to several programs for toolpathing.

I use OpenSCAD extensively for designing parts to 3D print (additive manufacturing) but not for milling (subtractive manufacturing). You can use it, though. FreeCAD has some fairly extensive CAM capabilities, but it’s not the tool I’d use first to generate toolpaths from STLs. It has a fairly steep leaning curve. That said, it has been my primary tool for generating toolpaths. But you could try some of the other programs he links to, like the browser-based Camlab.

For whether that particular stepper motor will work, probably… Certainly within the limits of the Smoothieboard. Honestly, the steppers are the last step of the build. The fixtures to mate the stepper is the same for any stepper of the same size class. I would figure out everything else before buying any steppers!

I seem to have forgotten to point out that there are two kinds of torque: a single torque rating (140.2 oz/in for the stepper you linked to) is holding torque: it’s how much force it can hold still at when it’s being held to a full step. The torque it can apply while moving is lower, and the higher the RPM the lower the torque it can apply; see the torque curve at the bottom of the page you found. The best you get is about 100 oz/in at around 40RPM. That will be with full stepping, and you will want to use microstepping for smooth movement. Full stepping, 1/2, and even 1/4 microstepping sound horrendous and make the machine vibrate. 1/16 microstepping is what you really want to use in most cases.

You might need a larger stepper for Z than for X and Y. Usually you can put up with Z moving more slowly than X and Y, though; moving Z at lower RPM will be OK.

For those steppers, you would need 1/4" to something (6mm? 8mm? 10mm) couplings between the shaft and the screw. Diaphragm couplers are typically the best; lovejoy/spider couplings second.

You’ll have to design and make fittings that go between the stepper and the mill. This should have a central hole to indicate against the round section around the shaft. This is what keeps the stepper perfectly centered. Note the tolerances for the motors themselves: for NEMA23, it’s 1.5" ± 0.001"! Then you’ll need four mounting holes. On NEMA17 motors, the mounting holes are usually threaded M3 and the screws go through the part to which you are attaching the motor. On NEMA23, the holes are typically 5.1mm holes intended for M5 screws to screw into the part you are mounting to, with the screw typically a hex socket head screwed into the part to which you are attaching. (This is typically more convenient for a CNC machine.)

Or, a better idea, you buy something someone else made. For example:

I wrote lots of advice on all the things you would need to do, and then discovered this kit:

That uses an 8-bit arduino-based Synthetos gshield (see #controllers:tinyg) controller instead of the Smoothieboard, but it shows what components are there. The Smoothieboard is a good idea; 32-bit processors are better for cutting arcs. The kit is basically adapters like the amazon link, three stepper motors, electronics, and a housing.

Here’s another writeup:

The instructables author purchased from mbbilici. I don’t see a MF70 kit on his site, but I have purchased from him before and recommend his work, and he has responded to email. You could ask.

Thanks a million for your extensive answers. I like your idea of a complete conversion set, apart from the controller, because I’m afraid of picking the wrong parts myself. I’ve looked for such sets available in Europe, but much of it is sold out. As well as the Smoothieboard.
I think I’ll wait until the Autumn. I expect stockpiles will be replenished by then.
In the meantime I have a lot to read.

Sincerely yours,

Ben

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