Hard Milling FAQ: the questions buyers ask

Hard Milling is milling material in its hardened state, usually above 45 HRC, with coated carbide or ceramic tools. It is used for finishing hardened tool steel and bearing steel without grinding or EDM. 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, aerospace is used as the example: traceability and a first article before anything enters the batch.

The questions buyers ask

What is Hard Milling used for?

It is used for finishing hardened tool steel and bearing steel without grinding or EDM. Milling material in its hardened state, usually above 45 HRC, with coated carbide or ceramic tools.

What tolerance can be held?

±0.01 mm is typical, held on the features that carry the function and confirmed by CMM and roughness gauge.

Which material should be specified?

H13 tool steel at 52 HRC is the usual choice; the certificate requirement is in-house approval with dimensional reports.

What finish comes as standard?

Ra 0.4 µm achievable. A finer finish is a separate operation and is quoted as one.

How long does it take?

A realistic cycle is 2 to 10 hours for a cavity, and the cost driver is tool cost, because the cutters are expensive and short lived.

What information is needed for a quotation?

A STEP or IGES model and a PDF drawing with tolerances, material and finish called out, plus the quantity and the expected repeat frequency.

Can the same result be guaranteed on a repeat order?

Yes, because the fixtures, tool lists and verified programmes are retained for the life of the part number.

What is the most common mistake on the drawing?

Using a strategy that lets the tool dwell, which work hardens the surface.

Applications

Hard Milling is used for finishing hardened tool steel and bearing steel without grinding or EDM, and in practice it appears most often in die and mould work, hardened wear plates, bearing components, cutting tools.

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: tool cost, because the cutters are expensive and short lived. 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 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 Hard Milling?

±0.01 mm is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.

Which material is usually used?

H13 tool steel at 52 HRC is the usual choice, supplied against in-house approval with dimensional reports.

How is the result verified?

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

What is the most common mistake?

Expecting a soft machine to hold a hard milling tolerance.

How long does it take and what does it cost?

A realistic cycle is 2 to 10 hours for a cavity, and the cost driver is tool cost, because the cutters are expensive and short lived.

Related capability

If you are specifying Hard 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 Hard Milling in the encyclopedia: Wire EDM, Sinker EDM, Inconel 718. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.