Proxxon MF70 conversion

(4/08)

As a way to get started with CNC machining, I converted a Proxxon MF70 micro-mill. I had help from 10bulls, of CamBam fame. He's been trying to tempt me into his world of CNC machining for years.

The machine is lightly built but seems fairly precise. And it's cheap. I found a couple of other well-documented conversions that were very helpful: Tim Goldstein and Graham Stabler . Since then, I have helped Russ with his conversion which might be worth a look if you are planning one of these. I was briefly tempted by the pre-converted Usovo machine, but in the end it was more interesting to convert it myself.

I bought steppers and controllers from Motion Control Products Ltd, specifically NEMA 23 steppers (FL57STH51-1008B, datasheet here) and MSD415 Microstepping Drivers. Motors are wired as parallel bipolar, taking 1.4A and giving a nominal 1Nm of torque. That cost about 35 quid per axis, plus 200 for the MF70. There's no shortage of torque in the steppers, and I suspect I could have used much smaller ones.

I'm using an ncPod USB stepper controller, which was around $100, and has been no trouble at all. Software is Mach3 and CamBam.

Conversion was straightforward enough. I had one wrinkle when dismantling the thing, which is that the Y axis leadscrew is left-handed! I started undoing the nut on the rear of the leadscrew without noticing, and bent the Y axis endplates out of shape. But they were easily bent back. The knobs are removed from the leadscrews by punching out 2mm rollpins, and you need a punch of the right size to do that.

I replaced the original thrust washers with cheap sealed ball races from Bearingboys (MR126-2RS) - those ones are 12mm OD, so they drop into the recesses the thrust washers occupied. I turned up brass adapters to go through the bearings and fix up the overall length of the leadscrew, and attached them with the original roll pins.

My original shaft couplers were bits of PVC pipe pushed over the leadscrew and motor spindle, but they tended to slip and twist under torque. I replaced them with fuel hose from Halfords and little Jubilee clips, and they have performed much better. Motor mounts are long M5 bolts going into tapped Perspex plates. The Z plates is held down by self-tappers into the plastic endplate on top of the column. The X and Y plates are held on by M3 bolts - I ran an M3 tap into the channels which had previously held self-tappers. Those bolts do not fill me with confidence, so at some point I plan to replace them with M3 studding epoxied into the channels.

Power supply is currently 12V from an old PC switchmode, though I think I would get better speed from a higher voltage. And the ncPod needs a 5V supply of its own, currently from a variable bench supply, but I shall regulate it from the stepper supply in the future.

I got a supply of endmills and engraving cutters from CNC Plus in Germany who were cheap and efficient.

The original handle and spacer on the Y axis.

The new spacer and bearing arrangement.

Spacer.

The machine!

At work.

PS - April 2010

I'm at work on my new machine - a Taig - and I want to record the setup details here before I derange my Mach3 setup

The steppers are powered by a 70W 24V switchmode, sold as a universal laptop PSU. Greenweld (http://www.greenweld.co.uk/) part ST661081

The ncPod is now powered via a dc-dc converter (from the stepper supply), a Recom "R-78HB5.0-0.5" RS part number 416-874, 10.

My hose-and-jubilee-clip shaft couplers didn't work out - too much lost motion - and in the end I relented and bought proper couplers, something like RS part 814-607

The drivers are set to 8 microsteps, giving 1600 steps/rev

The setup in Mach3 is:

Units - mm

Motor tuning - X,Y, and Z - 1600 steps per mm, velocity 250 mm/min, acceleration 20 mm/s/s (~0.002G), step pulse 0, dir pulse 0

Port setup - Port 1, 0x378, kernel speed 25000

Motor outputs:

X step 2 dir 6

Y step 3 dir 7

Z step 4 dir 8

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