A milling machine (subtractively) shapes solid materials using both the tip and sides of a rotating cutter.
Unlike most other machines, a mill both
- (a) pushes a toolhead in all directions through the raw material, enduring significant forces in the X, Y, and Z axis (often all three simultaneously), and
- (b) requires precise positioning in the X, Y, and Z axis.
This is technically difficult. Roman Black has a review of "Good CNC Designs", which shows a few ways of meeting those requirements.
There are a variety of machines that sacrifice one or more of these capabilities in order to improve the precision or power or speed of the other capabilities or to reduce the cost of the machine.
- a CNC Router precisely pushes a rotating cutter with significant forces in the X and Y axis, but uses a flat sole plate to support all the Z forces and to maintain a single precise fixed Z position.
- a drill press pushes a drill bit only parallel to the Z axis, but only requires precise positioning in the X and Y axis (and has relatively little force along those axis). ( PCB through-hole drilling requires a small drill; manufacturing grid beam requires a somewhat larger drill).
A high-quality milling machine requires a stiff, heavy, and therefore expensive frame in order to maintain precise positioning in the face of strong forces at high speeds.
An additive thermoplastic rapid prototyping printer like Mendel requires precise positioning in X, Y, and Z, but has only a tiny fraction of the forces seen in a milling machines. Therefore a typical RepRap or RepStrap has a much lighter, lower-cost frame than a typical CNC mill, and so it typically moves at much lower speeds when doing CNC milling on order to maintain precise positioning.
- Cartesio is a reprap printed CNC machine.
- Eiffel is the self-reproducing CNC router/mill project. (Eiffel will also be a 3D printer.)
- Mendel is able to move a lightweight spindle around, like Flex Shaft milling-toolhead.
- MegaRap is designed to cut a full-sized sheet of plywood.
- Cyclone PCB Factory is a RepRap-alike CNC router/mill intended for PCB manufacture.
- BuildersBot  is a Arduino Controlled CNC / 3D Printer that uses a RAMPS on an Arduino Mega with standard RepRap firmware. The heavy-duty welded stainless steel frame and NEMA 23 motors are designed for CNC milling and CNC drilling. The 500 x 600 x 200 mm work volume is designed for 3D printing with a RepRap plastic extruder.
- Mostly Printed CNC is an inexpensive printed CNC multi-machine based on 608 bearings and EMT conduit.
- Your Project Here - we are eagerly soliciting all RepRap-> subtractive spindle and self-built CNC machines.
To use the Reprap electronic for the CNC Mill you have to modify the G-Code because the reprap controller dont know many of the Milling commands ...
- CNCGcodeController Useful java Tool for Optimising, Auto Leveling, Art support, Controlling a reprap Mill
- CNC GCode to RepRap converter to convert pyCam and HeeksCNC (Based on emc2b post processor) G-Codes into Marlin-Firmware readable Machine-Code
Commercial benchtop and large CNC Mills are popular, effective, and economical RepStraps - RepRap bootstrap machines. See the extensive MillStrap and EMCRepStrap pages if you are interested have a mill or are planning on buying one.
self-manufacturing CNC mill
It is still an open question whether the total doubling time of a self-manufacturing CNC-capable RepRap is minimized by (a) adding material to the design to make the machine stiffer, so that it can make a given part at a given precision faster, or (b) taking away material from the design, so that all the parts of the design can be printed out faster.