Surface finish achievable with CNC Milling

CNC Milling is a cutting process where a rotating multi tooth tool removes material from a stationary workpiece. It is used for producing flat faces, pockets, slots and contoured surfaces on prismatic parts. This entry explains what it is, what the numbers on a data sheet mean, and what to put on a drawing.

It is written from the point of view of the buyer and the drawing office: the values that are realistic in production, the ones that cost money, and the way the result is verified. Where an industry sets the requirement, oil and gas is used as the example: materials that work harden and still have to be certified.

What finish is achievable

The finish left by CNC Milling is Ra 1.6 µm as milled. It is measured with a stylus instrument and reported as Ra, and the lay – the direction of the pattern – matters as much as the value on a face that seals or slides.

How it is improved

A finer finish is a separate operation, not a setting: a finishing pass, a dressing pass, or a secondary process. Machine time and the number of setups, so the call out belongs only on the faces that need it.

How it is checked

CMM, height gauge and roughness gauge. Where the drawing gives a value, the report records where and how it was measured, because the same face can read differently in different directions.

Applications

CNC Milling is used for producing flat faces, pockets, slots and contoured surfaces on prismatic parts, and in practice it appears most often in enclosures, brackets, mould plates, machine frames.

How it fits into a machining route

The route is fixed before the first part is cut, and it is the route rather than the individual setting that holds the result: machine time and the number of setups. Where a drawing calls for something the route cannot hold economically, that is raised at quotation rather than after the parts are made.

Standards, documentation and what is issued

The documentation requirement is in-house approval with dimensional reports. Material is supplied against a certificate where the application calls for it, inspection results are recorded with the job, and the records are kept so that a repeat order can be compared with the first instead of being measured from scratch.

CMM, height gauge and roughness gauge is the verification step. Where a customer format or a standard report is required, it is agreed before the batch starts, because a report written after the event is a report nobody can use.

Frequently asked questions

What tolerance should be specified for CNC Milling?

±0.02 mm is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.

Which material is usually used?

6061-T6 aluminium is the usual choice, supplied against in-house approval with dimensional reports.

How is the result verified?

CMM, height gauge and roughness gauge, with the results recorded so a repeat order can be compared with the first.

What is the most common mistake?

Climb milling used on a machine with backlash, which shows as a step on the wall.

How long does it take and what does it cost?

A realistic cycle is 10 to 60 minutes per part, and the cost driver is machine time and the number of setups.

Related capability

If you are specifying CNC Milling on a drawing and want a second opinion on the route, send the model and the quantity. The first response is a manufacturability review with the likely risk points and a quotation, so the decision can be made on facts rather than on a price alone.

Machining, inspection and documentation are handled in one place, which is why the dimensional report and the material certificate arrive with the parts rather than after them.

Related entries

These entries sit next to CNC Milling in the encyclopedia: Deep Hole Drilling, Reaming and Boring, Duplex 2205 Stainless Steel. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.