How does a milling machine follow a shape hidden in a metal block?

A computer-controlled milling machine makes a part by moving a rotating cutter along planned paths through a securely held workpiece. The desired shape is not physically hidden inside the block. A digital design defines which material must remain, and a toolpath specifies how to remove the surrounding material without cutting into that shape.

A milling cutter has sharp edges that repeatedly enter and leave the work as it rotates. Each edge shears off a small chip. Motors move the cutter and workpiece relative to each other along controlled axes. On a basic three-axis machine, these motions cover left–right, forward–back and up–down directions. Combining them lets the cutter make pockets, faces and curved outlines.

The computer cannot simply move the cutter's center along the finished boundary. The tool has a real diameter, so its center must be offset from a wall by the tool radius. A round cutter also cannot leave a perfectly sharp internal corner smaller than its own shape allows. Designers and manufacturing planners account for those limits before metal is removed. The illustration shows a rounded pocket corner as a consequence of the tool, not an error in the drawing.

Roughing passes remove much of the unwanted material quickly while leaving a small allowance. Finishing passes then establish the final surface more carefully. The amount removed per pass, rotational speed and feed rate depend on the material, cutter and machine. Too much load can deflect a tool or cause chatter, a vibration that marks the surface and reduces accuracy. Chips and heat must also be managed.

The machine needs a known reference position for the workpiece and a known tool length. If a block is clamped in a different place without updating those references, an otherwise correct program can cut the wrong location. Sensors and feedback help control movement, but the setup still matters. Measurement after machining establishes whether the actual part meets its requirements. CNC turns a geometric plan into repeatable motion; producing a reliable part also requires understanding the physical cutter, material and machine.