Hard Milling tolerances: what is realistic and what it costs
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, automotive is used as the example: a fixed takt, because a late delivery stops the line.
What is realistic
The tolerance that can be held on Hard Milling depends on the feature, the material and the batch size. A value held once on a first article is not the same as a value held across a thousand pieces.
| Aspect | Typical value |
|---|---|
| Process tolerance | ±0.01 mm |
| Measurement | CMM and roughness gauge |
| Surface finish | Ra 0.4 µm achievable |
| Batch effect | capability, not a single measurement |
| Documentation | in-house approval with dimensional reports |
| Cost effect | tool cost, because the cutters are expensive and short lived |
What it costs to hold
Tolerance and cost are not proportional; they are closer to exponential once a process has to be controlled rather than simply performed. Tool cost, because the cutters are expensive and short lived, which is why the drawing should say which features carry the function and which do not.
How it is verified
CMM and roughness gauge. Calibration dates and measurement uncertainty are recorded with the result, so a later order can be compared with the first instead of being measured from scratch.
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: Thread Milling, Deep Hole Drilling, PEEK. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
