A milling machine is different than a mill used for sawing. Rather than cutting through material, it cuts material away using rotary bits, or it spins metal stock so a stationary cutter can remove material. A milling machine is essentially a vertical lathe or a machine that operates as both a lathe and milling machine. It is a workhorse tool for machine shops and fabricators. There are a number of basic projects to get you started on a milling machine.
Bearing Diameter: Outside
Bearings are a critical element in the machine world. Regardless of the type of machine where there is contact and friction, a bearing is often used to reduce the friction, wear and heat. Examples are roller bearings and needle bearings. Often, a bearing has an inside diameter and an outside diameter. The outside diameter allows the bearing to be snugged into place. The inside diameter usually accepts a spindle or axle of some type. A simple milling project is using a horizontal mill to cut a cylindrical void in metal stock to accept a bearing.
Bearing Diameter: Inside
Just as metal stock must be milled to accept a cylindrical bearing, the axle the bearing supports must be cut so it fits the inside diameter of the bearing. An example of this is a camshaft, crankshaft or motorcycle axle. The milling machine mills the outside diameter of the spindle equal to the inside diameter of the bearing, so the bearing can be snugly slipped over it.
A chamfer is a bevel. When parts are machined, angles can leave sharp edges. Even if they're 90-degree angles, they can have a sharp edge and burs. The edge of a piston, for example, can be sharp if not chamfered. Casting flash (residual ridges from molten casting process) might be cut down with a file. But a mill is great for making a precision chamfer.
One way to increase the compression ratio in a combustion engine is to make the combustion chamber smaller. This can be done by decking or shaving the head. Decking effectively shaves a thin, uniform layer of metal from the cylinder head. With wood, this might be done with a block plane. With metal, decking is done with a mill and it's a simple, straightforward project.