CNC Milling specifications: what the numbers mean
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, rail is used as the example: documented welds, coatings and fire smoke toxicity compliance.
What the numbers mean
A data sheet for CNC Milling is only useful if the numbers are read against the job. The usual entries, and what they tell you, are these.
- Typical tolerance – ±0.02 mm, ±0.01 mm with a finishing pass.
- Surface finish – Ra 1.6 µm standard, Ra 0.8 µm with a fine pass.
- Stock removal rate – 30 to 300 cm³/min depending on material.
- Cutting speed – 150 to 600 m/min in aluminium.
- Tool material – carbide, coated for steel and high temperature alloys.
- Coolant – emulsion or through tool high pressure.
Comparing offers
Two suppliers can quote the same typical tolerance and mean different things, because the figures are often quoted under the most favourable conditions. Ask what the value is measured over, what the material was, and whether it is a typical or a guaranteed figure.
| Specification | Typical value |
|---|---|
| Typical tolerance | ±0.02 mm, ±0.01 mm with a finishing pass |
| Surface finish | Ra 1.6 µm standard, Ra 0.8 µm with a fine pass |
| Stock removal rate | 30 to 300 cm³/min depending on material |
| Cutting speed | 150 to 600 m/min in aluminium |
| Tool material | carbide, coated for steel and high temperature alloys |
| Coolant | emulsion or through tool high pressure |
What to check before specifying
Make sure the value you need is the value you have called up. Use the largest corner radius the design allows, because it permits a bigger step over, and the difference between a typical and a worst case figure is where most disputes start.
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
CNC Milling is one operation in a sequence: roughing to leave even stock, any stress relieving the geometry needs, semi finishing, then the finishing operation that holds the tolerance. The order is what keeps the result stable as the machine warms up and the batch runs.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: Reaming and Boring, Honing, 316L Stainless Steel. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
