This repo holds design artifacts for my CNC conversion of a Precision Matthews PM-30MV bench mill using a ballscrew and motor mount kit from Arizona CNC Kits.
The base mill for this is a Precision Matthews PM-30MV, which is a medium sized (~200 kg) Chinese made bench mill with USA distribution and support. It is similar in size and cost to the Grizzly G0761, but the PM-30MV is a much better candidate for CNC conversion.
The PM-30MV has several salient features that are relevant to CNC adaptation.
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It has a variable speed 2HP (1.5kW) brushless DC motor that can be controlled via a normal 0-10VDC analog voltage. The speeds are noticeably quantized in the electronic controller (seems like 32 steps) but that doesn't hurt much in practice. It's trivial to implement a switch on the 0-10V signal to transfer control from the potentiometer on the stock control box to the CNC controller. The BLDC gives better torque at low RPMs than typical VFD AC motors.
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The E-stop button that comes with the PM-30 is a clone of the standard Omron/IDEC type with stackable contacts, so you can easily add a contact module to provide the E-stop input to the CNC controller.
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The spindle has a pulley drive that provides 1500rpm and 3000 rpm ranges, so you can get more torque at lower speeds if desired. In practice, the BLDC motor makes this feature almost redundant. When using carbide tooling on aluminum I've never encountered a need to go to the lower speed pulley.
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The column has a square flat mounting surface to the base casting and is not cantilevered, unlike many small mills. That allows the column to be shimmed on the base if needed to adjust head nod. On my unit the verticality of the column is very good and I have not had to shim it. That said, the dimensions of the column mount faces are fairly small, which would make accurate shimming pretty tough as you'd need sub-0.001" shims.
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It's possible to extend the Y travel in the outward direction by around an inch via a simple modification to the base casting. This doesn't really increase the net Y travel wtihout resetting the work on the table, but does allow some additional flexibility in positioning larger workpieces on the table.
I used the PM-30 conversion kit from Arizona CNC Kits.
The owner Dave Clements is fantastic to work with and very conscientious.
When my Y mount plate turned out to be too thick for my particular unit, he
made me a new one immediately.
The kit with the mechanical parts - ballscrews, ballnuts, and NEMA-34 motor mounts - will cost around $1K.
In addition to the mechanical kit, you'll need motors and controllers to drive the ballscrews. Here you have options:
- Closed-loop stepper motors
- DMM servos (Canada)
- Clearpath servos (USA)
The closed-loop steppers weren't a realistic option when I built my system in 2019, but they have come a long way and now provide speed and torque approaching that of servos at lower cost.
I used the 750W DMM 86N-DHT-A6MD1 (NEMA flavor) servos from Dynamic Motor Motion with their DYN4 series drivers. A motor+drive pair costs about $USD 560 on the 2023 price list. It would have been OK to use the next smaller size servos for the X and Y axes, but I consciously chose to keep them all the same type for interchangeability. Overall I've been happy with the performance, but I've had all three original servos burn out in the four years I've been running the system, despite only moderate levels of use. In all cases it was the motors rather than the DYN4 drive units that failed.
The Z axis has some tendency to sink under its weight, especially when you de-power the servos, so the next time the Z motor fails I'm going to consider replacing it with a 86N-DHT-A6MDB that has a 24VDC holding brake. However that feature does require continuous 24V current to hold the brake, so if you want to shut down the system completely you still need to physically hold the Z axis up at final shutdown.
If I had it to do over, I'd probably go with the Clearpath servos based on the failure history with the DMM units and the fact that Clearpath has an extensive series of integrated step/direction servos that don't require a separate driver, though you then pick up a need for a beefy 70-80VDC supply. The Clearpath product line is extensive and has many options for motor size and power options.
The PM-30MV stock motor leaves much to be desired in the way of spindle RPM, topping out at 3000. That is enough to get most milling operations done, but not necessarily very fast.
All of the upgrades worth considering will give up the manual quill function on the Z axis, but provide considerably better runout overall, and switch the tool holding taper from R8 (or TTS) to ISO30.
CNC Depot has a series of ISO30 taper air-cooled spindles with German-made motors that will generate much higher RPMs than the stock PM-30MV spindle.
3HP 18krpm This upgrades the available power from ~2HP to a full 3HP and provides 18,000 RPM, which will enable fine cutting and engraving of aluminum with small tools, as well as high speed routing. It will also increase the realistic material removal rate by at least 50% over the stock PM-30MV.
This motor uses a 24V cooling fan, for which power must be provided. The base version costs $3200 (2023), and provides 18krpm with steel bearings. A version with ceramic bearings is also available for $3500 and will hit 24krpm.
To use one of these motors, you'll need to build a sturdy motor mount box to go between the back face of the motor and the head tilt interface of the PM-30MV Z carriage.
Here is a block diagram of the power section of the DMM servo system. This design follows the DMM recommended setup almost exactly, except for inclusion of separate fuses on the controller logic AC power connections.
Note: browser dark mode will make this hard to read; use light mode!
The current digital section is based around an Ethernet SmoothStepper coupled to a CNC4PC C82 breakout board, driven by a touchscreen Windows PC running Mach4 CNC software. A VistaCNC hand controller is used for manual motion.
Major components
Here is the block diagram for the digital section:
This approach is workable, but the PC parallel port emulation architecture of the SmoothStepper and breakout board is archaic, and there are enough problems with Mach4 (e.g. unsafe with probes due to unexpected rapid moves, bug regressions, etc.) that I am already in progress with a breadboard for a Centroid Acorn based control system. Someday my breakout board will again die, and I'm concerned that by the time that happens, parallel port emulation breakout boards will no longer be available - the ecosystem is diminishing.
If it were not for the great work of the developer who does the Ethernet Smoothstepper driver for Mach4, a change away from the original control design would have been forced quite a while ago.
There are a few basic choices for non Mach4 based CNC on a mill:
- Get a commercial CNC mill with a dedicated control system. The options that
could possibly be within reach are
- Tormach PCNC series with PathPilot (based onLinuxCNC)
- A small Haas vertical milling system (proprietary SW)
- Centroid Acorn (or Oak but that's quite a bit more money)
- LinuxCNC
Tormach PathPilot is really cool but they don't sell it for non Tormach machines. It's based on LinuxCNC and would theoretically be open source, but there are ways to structure your CNC software so that critical things would reside in separate processes that would not incur source code publication obligations.
LinuxCNC is interesting because it's open source, but you need to configure it yourself for your control hardware and do the integration. There are a few pretty good UI's available, but lately there has been a shortage of parts for the Mesa I/O boards typically used with LinuxCNC. I could deal with all that, but I need to choose where to spend my time.