Common problems with Hard Milling and how to avoid them
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, medical is used as the example: validation evidence that matches the drawing exactly.
The problems that actually occur
These are the failures that show up on a real job, listed in the order in which they are usually discovered.
- Using a strategy that lets the tool dwell, which work hardens the surface. The cause is usually upstream: a datum that is not the functional one, a fixture that does not repeat, or a specification that cannot be measured the way it is written.
- Expecting a soft machine to hold a hard milling tolerance. The cause is usually upstream: a datum that is not the functional one, a fixture that does not repeat, or a specification that cannot be measured the way it is written.
What causes them
In most cases the cause is upstream of the operation, not in it. Using a strategy that lets the tool dwell, which work hardens the surface is usually a symptom of one of the three causes above rather than a problem in its own right.
How they are prevented
Prevention is duller than diagnosis. Design for a tool with the shortest possible overhang, and the result is confirmed by CMM and roughness gauge before the batch is released.
| Item | Note |
|---|---|
| Failure | using a strategy that lets the tool dwell, which work hardens the surface |
| Prevention | design for a tool with the shortest possible overhang |
| Tolerance to hold | ±0.01 mm |
| Finish to check | Ra 0.4 µm achievable |
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
Hard 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: 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, Duplex 2205 Stainless Steel. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
