Manufacturing Process
CNC Milling
CNC 铣削
Machining faces, pockets, slots, holes and complex contours with rotating end mills on a CNC machine.
Process Steps
- Fixture setup
- Rough milling
- Finish milling
- Drilling/tapping
- Inspection
What is CNC milling?
Direct answer: CNC milling removes material with a rotating multi-tooth cutter that is moved along a programmed path. Where a lathe turns the workpiece, a mill turns the tool — which is why milling is the process for pockets, slots, flats, contours, bores and any shape that is not a surface of revolution.
Which flavour of milling
| Type | What it does | Typical use |
|---|---|---|
| Face milling | Flattens a face with the cutter axis perpendicular to the surface | Datum faces, die plates, mounting surfaces |
| Peripheral / slot milling | Cuts with the side of the cutter | Slots, keyways, open profiles |
| Pocketing | Clears an enclosed area in passes | Recesses, weight-saving pockets, seal grooves |
| Contour / 3-axis milling | Follows a 2D outline at a fixed depth | Plates, brackets, tool details |
| 4- and 5-axis milling | Adds rotary axes so the tool reaches more of the part in one setup | Complex contours, ports, once-and-done features |
What milling holds, and what decides it
| Property | Typical | What limits it |
|---|---|---|
| Dimensional tolerance | ±0.01–0.05 mm | Machine geometry, tool deflection, thermal drift |
| Surface finish | Ra 0.8–3.2 µm | Cutter geometry, step-over, feed per tooth, rigidity |
| Internal corner radius | Equal to the cutter radius | A sharper internal corner needs a smaller cutter (slower) or EDM |
| Minimum feature | Roughly 3–5× the cutter diameter deep | Reach and chatter, not the machine's resolution |
Where milling sits in a metal-parts route
- Tooling. Die plates, punch holders, stripper plates and inserts are milled before grinding and wire EDM finish them.
- Machined features on stamped parts. A drawn or stamped part often needs one flat face, one bore or one thread. Milling and turning add it as a secondary operation instead of forcing the die to do something it cannot.
- Prototypes and low volume. Below the volume where a die pays for itself, milling is the direct route to a metal part.
Where the cost comes from
Cycle time dominates, and cycle time is set by how much material has to be removed and how hard it is to remove. Three levers usually decide the price:
- Number of setups. Each re-clamp costs more than the cutting it enables. A 5-axis machine that finishes the part in one setup is often cheaper than a fast 3-axis job spread over four.
- Tolerance and finish. Going from ±0.1 mm to ±0.01 mm means a finishing pass, a smaller step-over and more inspection.
- Material. Aluminium cuts fast and freely; stainless work-hardens and needs lower feeds with more rigid tooling; hardened steel is usually ground or EDM'd instead.
Common problems
- Chatter. Vibration from a long tool or a light setup, visible as a rippled surface. Fixed by shortening the tool, changing the spindle speed, or supporting the workpiece better.
- Thin-wall deflection. The wall bends away from the cutter and springs back. Fixed with light finishing passes, support or a different process.
- Tool wear and drift. The first part is on size and the hundredth is not. Managed by tool-life monitoring and in-process gauging rather than by re-cutting.
- Burrs. Every milling cut leaves one. Removed by a deburring operation, which should be planned in the route, not discovered at the end.
Frequently asked
What is CNC milling used for?
Cutting pockets, slots, flats, contours and bores into metal parts, and making tooling. It is the standard answer when the shape is not a surface of revolution and the volume is too low to justify a die.
CNC milling or CNC turning?
Milling rotates the tool and suits prismatic features; turning rotates the workpiece and suits cylindrical parts. A part with a turned outside and milled flats is often finished on a mill-turn machine in one setup.
How tight a tolerance can milling hold?
Around ±0.01–0.05 mm on a typical part, and tighter on a rigid, simple feature. Beyond that the answer is grinding or wire EDM.
When is milling cheaper than a stamping die?
Below a few thousand pieces a year. Above that the die amortises and the stamped part is both cheaper and more repeatable. Many programs use both: a stamping for the body, milling for one or two critical machined features.
Related: CNC milling capability · CNC turning · wire EDM · metal machining services · deep drawing