First, while printing the motor mounts, it's a good idea to do a google search with "mf70 cnc" and read some of the top results - many people have written better illustrated guides than fits here.
Take off the handwheels from the mill. For X- and Y-axis this is done by loosening the nut at the far end of the axis (note! the Y-axis has left-handed threads) and pulling the axis out far enough so that you can punch out the holding pin. After this, the wheel comes off. For the Z-axis, loosen the 4 M3 bolts at the top, lift the whole spindle off the machine and loosen the nut under the top cover.
Next thing needed is a coupler for each axis. The mounts are designed for 25mm long CNC couplers, available from Ebay. The mill end should be 4.5mm and the motor end 5mm. Since 4.5mm couplers are hard to find, a smaller diameter can be enlarged by drilling.
In the original design, the handwheels themselves push against the bearings. Since they are not used after the CNC conversion, something else must be fitted on the shaft to perform the same function. Others have used brass collars, but I used 20mm long PTFE spacers (milled from 15mm rod) and have the couplers take the load instead of a separate collar.
X- and Y-axis motor mounts can then be attached. They need longer self-tapping screws. 3.0x20 should be enough, but I wanted to use beefier screws, so I drilled the holes in the mill to 3mm and used four 3.5x22 stainless screws. The Z mount bolts on top of the mill with M3x35 screws.
After the mounts are in place, the couplers attached and the axis nuts tightened to take out the play, the NEMA17 motors with 20mm shafts can be mounted with M3x12 screws.
If everything still moves smoothly, it's time to hook up the electronics.
It's possible to use RepRap electronics. The firmware should be configured to have 200x16 (16 being the microstepping value) steps per mm, maximum speed 300 mm/min and acceleration 1000 or 2000 mm/s*s. If not using endstops, software movement limits should be disabled.
I'm using PyCAM for producing the G-code, and Marlin 1.0.0rc2 accepts it after a minor postprocessing step with a Perl script.
The zipfile contains all the three STLs. The mf70-cnc.hs is the ImplicitCAD source code that will output all the STLs when run. The attached STLs have additionally been checked and rotated in netfabb.
I have gotten much better results from Heekscnc as the CAM program. I recommed it instead of PyCAM. It also needs some postprocessing for repraps and the script is attached.
Also, the stock slide bearings should be replaced by MR126 ball bearings. These are a drop-in fit, but move the axis and the spacer so different motor mounts for X and Y are required. After shortening the spacers from 20 to 12 mm, the mounts marked with -mr126 can be used instead to get the proper alignment with the 25 mm couplers.
I replaced the MR126-variant X- and Y-mounts with 2mm longer ones, the fit proved to be a little too tight otherwise (the motor shafts extended too far into the couplers).
I crashed the mill into a work clamp and the X lead screw broke in two - even the NEMA17 motor was strong enough to break it. So I ordered some ball screws and will be publishing a ball screw upgrade sometime in the future. I'll also try to extend the Y work area at the same time. There are some modified versions out there with 86 mm Y travel instead of the default 46 mm.
I also concluded that the PTFE spacers are too much of a hack. Don't use them, install some sort of stoppers on the screw shafts instead (something with a 4.5 mm center hole and a grub screw to lock it in place should work.)