There’s a lot of well-deserved excitement surrounding the RepRap 3D printer, and much of it focuses on the RepRap’s ability to make its own parts. The RepRap’s fabrication technique is additive — it uses a plastic extruder to “print” a plastic model, one layer at a time.
This contrasts with subtractive fabrication techniques, which start with a solid block of material and use a cutter to remove the excess.
Subtractive fabrication is far more common than additive, especially when working with metal and wood. Lathes, mills, saws, and drills are all subtractive tools. A CNC milling machine or router is the subtractive equivalent to the RepRap 3D printer.
For the hobbyist, milling is inferior to printing in numerous ways. The method inherently causes waste, and without any sort of dust control, that waste gets flung throughout the room.
Milling is more hazardous: while it’s possible that a plastic extruder might overheat and catch fire, I’ve already had a (minor) fire with my CNC router, and there’s the added danger of a blade, spinning at 20,000rpm, sending bits of itself, or even your workpiece, flying at you.
A mill or router is necessarily larger, heavier, and consequently more expensive and more difficult to move. It requires a positioning system that can maintain accuracy when encountering resistance, and motors powerful enough to drive it.
Software preparation is also more complex for milling. After drawing the object you wish to make in a CAD or 3D modeling program, it’s necessary to generate tool paths with CAM (computer-aided manufacturing) software. This involves specifying the dimensions and location of the stock material, the dimensions and characteristics of the end mill (cutter), and speeds for the axes and spindle.
The tools to do all this tend to be complex and a bit daunting for the first-time user. From the user’s perspective, CNC milling is much more complex than printing.
CNC milling does, however, have a significant advantage over 3D printing: the technology is mature. The RepRap is improving at a tremendous rate, but there’s still a lot of tinkering and experimentation involved in getting a good print. If 3D printing technology fascinates you, the RepRap is a great project to get involved in, but if your interest is in making things, CNC milling is the better option at this time.
Going with CNC milling, however, does not mean you have to give up on self-replication or on making your own machine.
Patrick Hood-Daniel’s Expandable CNC Kit (buildyourcnc.com) is a CNC router capable of making all of its custom parts. The router’s frame is built of custom-cut MDF (medium density fiberboard). Everything else is standard hardware. The aluminum angle, bolts, and screws are available from any hardware store.
The lead screws and anti-backlash nuts will probably have to be mail-ordered from McMaster-Carr and dumpstercnc.com, but you can get by with lesser hardware store parts in a pinch. The stepper motors and stepper drivers are completely generic and available from countless sources. The spindle is an ordinary wood router; I use a Porter-Cable 892.
As with the RepRap, the trick with Hood-Daniel’s CNC router is getting a seed unit. Fortunately, any CNC router that can handle a 2’×4′ sheet of MDF is capable of making the parts. Start looking around the forums at makezine.com or The Home Shop Machinist (bbs.homeshopmachinist.net), and you’re likely to find somebody local with the necessary equipment.
To get the files, fill out the contact form at buildyourcnc.com and Hood-Daniel will email you the CAD and CAM files for the parts. Videos on his website explain how to put everything together. The files are licensed under the Creative Commons Attribution-Noncommercial license, which essentially means you can make machines for yourself and for your friends, but you can’t sell them commercially.
The CAD files are in CamBam format. CamBam is proprietary, but there’s a free version available and it can export to DXF. Most good CAM software is very expensive, though, so if you don’t already have a favorite, stick with CamBam. It can do both CAD and CAM, and even the free version is tremendously full-featured.
If you can’t find anybody to cut the parts for you, Hood-Daniel sells a kit with the basic hardware for $1,100.
The first thing you’ll want to do, once you get it running, is make a set of spare parts. Next, start making replicas for your friends. While the MDF frame may look a bit light if you’re used to professional equipment, I’ve had no problem cutting wood with it. The design isn’t rigid enough to cut metal, but you could always cut out a new tool holder and mount a plasma cutter.
The platform could also support a RepRap plastic extruder — so when 3D printing does take off, you’ll already have a CNC platform ready with a 2’×4′ printing area.
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