Where Do I Go From Here? - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 199

CNC Machine
The first tool I want to discuss is called a CNC machine. Before I show you what
one looks like (and many different shapes and sizes exist, just like 3D printers),
I'll go over briefly what a CNC machine is and what it can do for you. I'm going
to use what you know about 3D printers to compare and contrast with a CNC ma-
chine.
First, a 3D printer performs what is called an additive process . Addit-
ive—something is being added. Pretty straightforward. What's being added is
melted plastic, layer by layer, to create a solid object.
A CNC machine performs what is called a subtractive process . It takes something
away. And that something depends on the material you are working with. A CNC
machine uses a milling bit in lieu of the hot end found on a 3D printer. The milling
bit looks like a drill bit, and it spins at a very high speed. If you can imagine a
3D printer with a spinning drill bit on the end instead of the hot end and leave the
motors in place to control the movement of the spinning bit, you've got the basic
idea of a CNC machine.
The milling bit removes material instead of adding material. Place a small piece of
plywood on the worktable, and a CNC machine (using the familiar g-code) can be
instructed to move the milling bit to and fro over the plywood. The Z axis controls
how deep into the wood the milling bit cuts. Using a combination of movement
along all three axes, you may be able to start picturing how a CNC machine can
remove wood (in small amounts), leaving behind an object.
On the simple side, a CNC machine can be used to engrave your name, for ex-
ample, into a variety of materials—plastic, wood, and metal being the most popu-
lar. (Note that you must select the proper milling bit for the material you want to
work on.)
On the more advanced side, a milling bit can be used to create intricate designs
and even cut out objects from a larger piece of material.
Take a look at Figure 11.1 and you'll see one example of a CNC machine. This is
the ShopBot Buddy from ShopBot, a manufacturer of CNC machines.