This document collects Chris's rules-of-thumb for doing CNC work.
You can break these rules, but knowing when and how to do so takes experience. The idea is that if you do follow these rules, you should be able to get consistently good results as a beginner.
A CNC router is used for cutting soft materials like wood or plastic. It has a high spindle speed (usually 12,000-20,000 RPM).
A solidly-built CNC router can machine soft metals like aluminum or brass, but aluminum will need coolant and due to high RPM you may need carbide tools. A CNC router that is not solidly built will do a bad job and might even be damaged if used to machine metal.
FIXME: Links to the shop's routers go here.
A CNC mill ("milling machine") is used for cutting hard materials like metal (aluminum, brass, steel). It has a lower spindle speed (usually 4,500 RPM or lower) and is built like a tank. The area being cut needs to be sprayed with coolant (unless cutting brass).
A CNC mill can be used with soft materials like wood or plastic, but due to the low spindle speed, the quality of the cut may be poor.
FIXME: Link to the almost-a-mill goes here.
Square end mill from McMaster-Carr.Ball end mill from McMaster-Carr.
An end mill is a cutting tool used with CNC mills and routers.
You want a "center-cutting" end mill with "spiral flutes" in the "up-cutting" direction. Two flutes is a good number for most materials.
For the FIXME: name of small router router, the end mill needs a 1/4" shank.
End mills are made from "high-speed steel" or from "carbide".
Carbide can cut at higher RPM but is more expensive and easier to damage (it'll chip if dropped on a table). It's a good idea to use it when cutting metal, and it's needed when cutting steel.
High-speed steel is less expensive and works fine for wood and plastic. It can be used with soft metals like aluminum and brass, but will wear out more quickly with them. They often have coatings (titanium nitride or other) to make them last longer.
A square end mill has a flat bottom. It's used when you want to leave a flat surface with vertical walls. A ball end mill has a rounded bottom. It's used when carving 3d surfaces.
Summary
If you're cutting wood or plastic, use a router. If you're cutting metal, use a mill if you can.
If you're doing a complicated 3d surface, use a ball-end mill as the cutting tool. Otherwise, use a square-end mill.
