Hard Milling vs Reaming and Boring: which one fits your part
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 short answer
Hard Milling and Reaming and Boring overlap, but they are not interchangeable. Choose Hard Milling when the requirement is finishing hardened tool steel and bearing steel without grinding or EDM; choose Reaming and Boring when the requirement is holding hole size and position more closely than drilling can.
Side by side
| Point of comparison | Hard Milling | Reaming and Boring |
|---|---|---|
| What it is | milling material in its hardened state, usually above 45 HRC, with coated carbide or ceramic tools | finishing a hole to size with a multi tooth reamer or an adjustable single point boring head |
| Used for | finishing hardened tool steel and bearing steel without grinding or EDM | holding hole size and position more closely than drilling can |
| Typical tolerance | ±0.01 mm | H7 |
| Surface finish | Ra 0.4 µm achievable | Ra 1.6 µm reamed |
| Cost driver | tool cost, because the cutters are expensive and short lived | the number of size adjustments and the gauge check |
Which one fits your part
Work from the feature that has to work. If the requirement is finishing hardened tool steel and bearing steel without grinding or EDM, Hard Milling is the safer call; if it is holding hole size and position more closely than drilling can, Reaming and Boring wins on cost or on lead time. Where the drawing does not make it clear, that is the question to settle before quotation rather than after.
A note on specification
Write the drawing so that either route can be considered where the function allows. That keeps the quotation competitive and stops a process being specified where a result should have been.
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: High Speed Machining, Thread Milling, Inconel 718. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
