Alternatives to the Printrbot Simple - 3D Printing: Build your Own 3D Printer and Print your own 3D Objects - page 194

opened the door to other useful “maker tools,” namely a CNC machine and a laser
cutter. You may not be familiar with these tools, but you'll be happy to know that
learning to use them will be considerably easier if you've gotten comfortable with
CAD and using a 3D printer.
Entire topics have been written about CNC machines, and the reason for the lack
of topics on laser cutters is probably because of their high price. I don't have room
in this chapter to go over every technical detail about CNC and laser cutters, so I'll
explain them relative to the 3D printer knowledge you now possess.
A 3D printer creates objects by building them up in layers. This is called an addit-
ive process because something (plastic, for example) is being added over and over
to create a three-dimensional object.
Now compare this to the subtractive process of a CNC machine. CNC stands for
Computer Numerical Control machine. Just as you use a computer to control the
motors of a 3D printer to deposit molten plastic in specific spots, a CNC machine
uses a computer to control motors that move a milling bit around to cut away ma-
terial at specific spots.
Note
A CNC machine is all about taking away, not adding. A 3D printer
places molten plastic (or other material) onto the work surface to create
an object. A CNC machine takes away material (such as wood) so that
what's left behind becomes the desired object.
Take wood, for example. If you've seen a master carver use a knife to whittle away
bits and pieces of wood to create a sculpture, you're getting the basic idea of a
CNC machine. Because a computer is capable of much finer movements (using
special motors, like servos), with the right milling bit, amazingly complex and de-
tailed items can be created in wood, plastic, metal, and dozens more materials (de-
pending on the strength of the milling bit and other factors). Take a look at Figure
10.9 and you can see an example of the kinds of objects that can be made using a
popular CNC machine called a ShopBot ( www.shopbottools.com ).