CNC machines are computer-controlled machines that cut a part out of a solid block of metal or plastic. CNC stands for computer numerical control. The machine follows a program, so it cuts the same part again and again, without a person guiding the tool.
There are two common kinds. A milling machine holds the block still and moves a spinning cutter over it. A lathe spins the block, and a cutter that does not spin moves along it. This suits round parts such as shafts. A machine moves its tool along axes, which are the directions in which it can move or turn. A common milling machine has three axes: left to right, front to back, and up and down. A five-axis machine can also tilt or turn the part, the tool or both, so it can cut complex shapes, such as an aircraft part or an engine part, in one set-up. Some newer machines also combine cutting and printing, as in a 3D printer, in one machine.
Take, for example, a machine shop in Belagavi that must make 60 aluminium valve bodies for hydraulic presses. A worker clamps an aluminium block in the machine, in a holder called a fixture. The CAM software has already written the program. The machine starts: a 12 mm cutter removes most of the metal, and a 6 mm cutter finishes the shape. A liquid coolant is sprayed on the cutting point to keep the tool and the part cool. Each valve body takes 14 minutes. The cutting tool wears out and is replaced after a number of parts. Last, a coordinate measuring machine (CMM) measures a few valve bodies, to confirm that their sizes are within the tolerance, the allowed difference from the drawing.
A CNC shop charges for machine time, usually by the hour, and for the set-up, which is the time to fit the fixture and test the program. Set-up takes the same time for 1 part or 100, so it matters more for small orders.
CNC machines are the main way that parts for on-demand manufacturing platforms are made. They suit single parts and small batches, since they need no special tooling. For thousands of identical plastic parts, injection moulding is cheaper per part.