Milling is the general purpose metal removal process: a rotating multi tooth cutter travels through a workpiece that is clamped still, taking material away in interrupted cuts. Almost any prismatic part can be milled, which is why it is the default answer and also why an unthinking design can be so much more expensive to mill than it needed to be.
How the process works
The cutter spins and moves along programmed paths while the part sits in a vice or fixture. Three axis machines move the table and spindle in three directions; four and five axis machines add rotation so the cutter can reach faces without the part being unclamped. Each unclamping is a new setup, and each setup is both a cost and a chance for features to shift relative to each other.
What drives the cycle time
Volume of material to remove, how fast it can be removed safely, and how much of the path is spent cutting air. Roughing is limited by machine power and rigidity; finishing is limited by the surface and tolerance required. Small internal corner radii force a small cutter, and a small cutter forces a slow feed, which is why one radius callout can dominate a cycle.
How shops build the quote
Programming and setup once, then cycle time per part, plus material and any inspection. That is why the first part is expensive and the tenth is not. Ask for a price break table rather than a single number, because the shape of it tells you what the shop is really charging for and where a design change would land.
Choosing a milling supplier
Match the machine to the part rather than admiring the machine list. A five axis shop is not automatically better for a simple plate. Look at whether they own the inspection equipment your tolerances need, whether they program in house, and whether they will tell you which feature on your drawing is driving the price. That last conversation is worth more than the quote.
Questions people ask about cnc milling
What is CNC milling?
Machining with a rotating cutting tool under numerical control, where the workpiece is held stationary and the cutter moves along programmed paths to remove material.
Milling or turning for my part?
Round parts belong on a lathe, prismatic parts on a mill, and parts that are mostly round with a few flats belong on a mill turn machine that does both in one holding.
How many axes do I need?
Three covers most work. Five axis pays for itself when the part has features on many faces, because it removes setups, or when a surface can only be reached with the tool tilted.
Why is a small internal radius expensive?
Because it sets the largest cutter that can reach the corner, and a small cutter must run at a slow feed with light cuts. Opening the radius where function allows is one of the cheapest design changes available.