A Maker's guide to CNC cutting...

General Notes:

• This is not to teach you how to use a CNC machine, but how to design with CNC cutting in mind


• It uses ShopBots as its examples, but the concepts are the same for all CNC tools


• CNC cutting is different than laser cutting, and requires a different perspective

Materials:

• The materials that you use have thickness, which you have to consider in several ways


• Thicker materials require either larger diameter bits or multiple passes to cut


• Materials might not be what they seem...3/4" plywood is rarely 3/4" thick. It's usually .03" undersized, but not always. If you have parts that slot together and have to fit precisely, you need to know how thick the actual material you're using is.


• A CNC tool can more precisely control cutting depth than a laser, so you can more accurately cut grooves and do intricate 3d carvings


• The factory edge of most materials is rarely straight or at 90 degrees to the other edges, so don't count on using them as finished edges if you need to have accurate parts. Instead, move your parts in from the edge a little so that ALL edges are cut by the tool


• When cutting plywood or other lumber products, generally the grain run parallel to the longer edge of the sheet. Sometimes grain direction can make a difference aesthetically in a project and a part needs to be laid out in a specific orientation on the sheet, but that can sometime cause an in-efficient use of the sheet. Keep that in mind


• Many types of sheet materials have one side that's better than the other. If you will need to have grooves cut that will be on the inside of the project for instance, you might want to have the parts cut with the good side of the sheet facing down on the table. If that matters to your project, make sure you specify which side is going to be up.

Cutters:

• Unlike a laser, the edge of the cutter is usually doing the cutting so when cutting the cutter usually has to cut on the outside of the outline of the part (if you're cutting something out) or the inside (if you're making a hole). You have to consider this offset when fitting parts on the material and leave enough room.


• The cutter leaves a path (or Kerf) that's as wide as the cutter you're using, so you have to leave space between your parts when you lay them out on a sheet

File formats:

• For 2d and 2.5d cutting...cutting at multiple depths...the ShopBot operator will need your design in a "vector" file format... a file with lines, arcs, and curves... rather than a "raster" format that is made from dots of color like most graphics programs supply. Though CAD programs are most often used, some drawing programs can supply vector fomat files as an option. The ".dxf" file format is the most common, but ".eps" and ".ai" are usually options as well


• As design software evolves, file formats tend to get "muddy" and add things that CNC cutters don't need, or modify their format so that they are unreadable by other software packages. If you're given an option you're generally better off using an earlier version to export as...the "Release 14" format of the dxf format is much more universal than the "AutoCAD 2009" version.


• True 3d cutting..an artistic relief carving or a fan blade...requires a solid model of the part in either "3ds" or "stl" formats

Techniques and Methods:

• The cutter will exert a side force on the material when it's cutting, much like the resistance you feel when cutting with a hand router. This can make holding the cut parts a challenge, especially if they are small. The ShopBot Operator may need to use small tabs of thin, uncut material to retain the parts in the sheet, similar to the way that model car parts are held to their plastic manifold. This can easily be cut away with a knife or with a hand router and a trimmer bit


• Undercuts...sections where the back of a part is cut away more than the top surface... usually needs to be done by flipping the sheet over and cutting from both sides. This require extra thought to how the parts will be cut out, and almost always extra expense.


• You may be lucky, but usually the first version of any project ends up needing some modifications before it's "just right". So sometimes it's better not to get too wrapped up in details until you're sure the basics are right.