Guide

3D printing – Additive vs. Subtractive Fabrication

There’s a lot of well-deserved Excitement surrounding 3D printers, and for the avid DIYer much of it is focused on their ability to “self-replicate” by making their own parts. But is a 3D printer the right tool for you? A 3D printer’s fabrication technique is additive — most of them use a hot plastic extruder to “print” a plastic model. This contrasts with subtractive fabrication tools, 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 3D printer. For the hobbyist, milling is inferior to printing in numerous ways:

  • Milling inherently causes waste, and without some sort of dust control, that
    waste gets flung throughout the room.
  • Milling is more dangerous — 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 and 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 toolpaths 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 this tend to be complex, and a bit daunting for the first-time user. From the user’s perspective, CNC milling is a much more complex process than printing is. CNC milling does, however, have a significant advantage over 3D printing: the technology is mature. Home 3D printers are improving at a tremendous rate, but there’s often still a lot of tinkering and experimentation involved in getting a good print.

What Do You Want to Make?

If your interests tend toward larger and more structural creations, go with milling. Also consider that it’s cheaper to work in wood than in plastic, and that you’re likely to get substantially superior results. On the other hand, making complex 3D objects is a lot more complicated with CNC milling than with a 3D printer. There are free tools for doing 2.5D milling (the z-axis never moves when the x- and y-axes are moving), but CAM software for 3D milling is very expensive and difficult to use. Doing 3D work for 3D printing is much easier. You can design your models in a free program like SketchUp or Inventor Fusion, and then export an STL file. Skeinforge software converts the STL file automatically to tool paths in G-code, then Printrun software sends the G-code directly to your 3D printer. With 3D printing, there’s no need to tweak cutting paths, and no worry about the tool crashing into your work. If creating small 3D objects is your goal, a 3D printer is the right choice.