Hardness Guide (HRC)
Hardness is the single number that most changes how a part must be machined — and the one most often quoted without saying which scale it came from.
General engineering reference Standard, publicly established machining knowledge — formulas, ISO classifications and industry practice. This is not EUROCUTPRECISION-specific cutting data and no value here is a test result obtained with a EUROCUTPRECISION tool.
What HRC measures
The Rockwell C scale measures resistance to indentation. A diamond cone indenter is pressed into the surface under a defined minor load, then a major load, then back to the minor load; the hardness number is derived from the permanent depth increase. It is fast, leaves a small mark and needs no optical measurement, which is why it dominates production inspection of steel.
The C scale is one of several Rockwell scales. Its useful range runs from roughly 20 HRC to the low 70s; below about 20 HRC the geometry loses resolution and the B scale or a Brinell test is used instead. Reporting "45 Rockwell" without the scale letter is ambiguous — 45 HRB and 45 HRC are not remotely the same material condition.
Typical hardness bands
| Material condition | Typical range | Milling implication |
|---|---|---|
| Mild and low-carbon steel | Below 20 HRC (measured on HRB) | Soft; built-up edge is the usual problem, not hardness |
| Quenched and tempered alloy steel | Roughly 28–38 HRC | Conventional milling; standard geometry and parameters apply |
| Pre-hardened tool and mould steel | Roughly 30–45 HRC | Machinable in the hardened state; the usual mould-making condition |
| Hardened tool steel | Roughly 45–58 HRC | Hard milling: light engagement, high rigidity, short stickout |
| Fully hardened die and cold-work steel | Roughly 58–65 HRC | Specialist hard-milling geometry; light passes; deflection dominates |
| Case-hardened surfaces | Surface above 55 HRC, soft core | The hard case is a thin interrupted layer — the worst of both |
| Austenitic stainless (as supplied) | Below 25 HRC, hardens rapidly in cut | The as-measured figure understates what the edge meets |
| Titanium alloys | Roughly 30–36 HRC | Hardness is not the difficulty; heat and reactivity are |
Bands are broad, publicly established figures for orientation. The hardness of a specific batch is a property of that batch and its heat treatment, and should be taken from the material certificate.
Converting between scales — and why it is approximate
Brinell (HB), Vickers (HV) and Rockwell (HRC) measure the same physical property with different indenters, loads and geometries. Published conversion tables exist and are useful, but every one of them carries the same caveat: conversions are approximate and material-dependent, and are strictly valid only for the material family they were derived on — commonly steel.
A rough orientation, for steel only: about 200 HB corresponds to roughly 20 HRC, and about 400 HB to roughly 43 HRC. Above roughly 650 HB the Brinell test itself reaches its practical limit and HRC or HV must be used directly.
The practical rule: convert for orientation, never for acceptance. If a specification is written in HRC, test in HRC.
What a hardness figure changes in milling
- Cutting speed falls as hardness rises. The edge is doing more work per unit volume and generating more heat in a smaller zone.
- Engagement falls sharply. Hard milling is characterised by light radial and axial engagement at constant load — the material will not yield, so any compliance in the system becomes tool deflection and dimensional error.
- Edge preparation matters more. A very sharp edge chips in hard material; a controlled edge hone survives. This is a geometry decision made when the tool is manufactured.
- Interruptions become dangerous. Cross-holes, previous passes and cast skin subject the edge to mechanical shock it cannot absorb at high hardness.
- Rigidity dominates everything. Shorten the stickout, use the most concentric holder available, and prefer a shorter tool over a longer one at every opportunity.
Work hardening — the hardness that was not there before
Austenitic stainless steels, nickel superalloys and some aluminium alloys harden significantly in the surface layer as a result of being cut. A material certified at 20 HRC can present a considerably harder skin to the following tooth.
This creates a specific trap: reducing the feed "to be careful" makes it worse. A light chip skims the hardened layer, hardens it further, and the next tooth meets a harder surface again. The correct response is the opposite — a chip load high enough to cut beneath the affected layer, a sharp positive geometry, no dwelling, and no re-cutting of a surface that has already been lightly passed over.
The recommended workpiece hardness range for EUROCUTPRECISION tools is a pending manufacturer field and is not published on any product page. This guide describes the hardness scale and general hard-milling practice; it makes no claim about the capability of any specific tool.
What determines the final approach to a hardness
Every figure on this page is a starting point for a calculation, not a setting to type into a control. The value that is correct for your job depends on:
- workpiece material
- workpiece hardness
- tool diameter
- radial engagement (ae)
- axial depth of cut (ap)
- holder and tool-assembly rigidity
- machine rigidity and spindle power
- coolant strategy
- spindle capability (speed, torque, runout)
- the actual machining conditions on the job
The manufacturer’s own starting parameters for the Z5 series are published, per workpiece material and per operation, on each variant’s product page. Start from those rather than from anything you derive here: this page explains a calculation, it does not describe a EUROCUTPRECISION tool. Either way, verify on a test cut and adjust from the behaviour of the chip, the sound of the cut and the finished surface.