Milling Hardened Steel and Tool Steel (ISO H): Strategy, Geometry and Surface Quality

Milling hardened steel is not a "slower" form of milling soft steel; it is a separate strategy built on a different balance of force, heat and wear. This article covers the boundaries of the ISO H group, the hardness ranges of common tool steels, the engagement–speed–feed relationship in hard milling, the geometric behaviour of ball nose and corner radius cutters, the choice of coating and cooling, and the calculation of surface quality.

1. Boundaries of the ISO H group and common tool steels

In the ISO 513 classification, which defines the application groups of cutting tool materials, the H group covers hardened and tempered steels together with white cast iron. In manufacturers' technical publications the hardness range of the group is given as 45–68 HRC; the specific cutting force (kc1) varies with the material between roughly 2,550 and 4,870 N/mm². At the same chip cross section this means a markedly higher force than for the ISO P group. The general logic of the groups is covered in the ISO material groups guide.

Not every steel encountered in mould and die work falls into this group. Pre-hardened plastic mould steels still behave as ISO P at their delivery hardness; the transition to ISO H occurs when heat treatment takes the material above 45 HRC.

In supplier catalogues these ranges may be given with a difference of a few HRC; for 1.2344, values varying between 45–52 HRC and 48–54 HRC have been published. Parameter selection has to be based on the actual value in the material certificate. In 1.2767 the relationship between tempering temperature and hardness is pronounced: a level of 56 HRC is reached at low tempering temperatures, while at high tempering temperatures the hardness falls into the 40 HRC band.

2. The 45 HRC threshold: where hard milling becomes an alternative to grinding

Above 45 HRC two things change at once. The material does not merely abrade the cutting edge, it also strains it plastically; at the same time, because the chip cross section is small, the heat is largely concentrated at the cutting edge. The property expected of the tool material is therefore not "sharpness" but hot hardness and resistance to plastic deformation.

The reason hard milling has spread through industry is economic. When free-form mould surfaces can be completed in a single setup in the hardened condition, electrode manufacture for electrical discharge machining and profile grinding steps are shortened. Manufacturers' technical publications state that on the milling side coated carbide is the dominant tool material up to roughly 60 HRC, and that above this CBN-tipped grades take over in semi-finishing and finishing. The condition for CBN grades is clear: a very stable setup and dry cutting. A general comparison of tool materials is given in the tool materials guide.

3. Cutting logic: small engagement, high spindle speed, low feed

The three parameters of hard milling balance one another.

Small radial engagement (ae). Radial force increases directly with the width of engagement; in hardened material this force bends the tool and leads to dimensional deviation. A small ae keeps the radial force low, and the axial depth can be increased in return. With ball nose or round-insert geometry the entering angle grows together with the axial depth; according to the relation κ = arccos(1 − 2 × ap / D), the entering angle reaches 60° when the axial depth reaches a quarter of the diameter. Because the radial force rises rapidly beyond this point, markedly shallower engagements are preferred in hard milling.

High spindle speed. In hardened material the cutting speed raises the temperature in the cutting zone and lowers the flow resistance of the material. This allows the heat to be transferred to the chip in a controlled manner. Raising the spindle speed is only meaningful with a holder of high dynamic balance and low runout.

Low feed per tooth (fz). Chipping of the cutting edge is the most frequent form of sudden end of life in hard material. Keeping the feed per tooth low limits the impact load per edge. Since the total feed is calculated from vf = fz × Z × n, a low fz is usually compensated by a high number of flutes and a high spindle speed. The calculation method is set out in detail in the cutting speed and feed calculation guide.

4. Geometry: corner radius and ball nose cutters

In hardened steel a square-cornered end mill is as a rule not preferred. The sharp corner is the region of the cutting edge that is both thinnest and most highly stressed, and the first chipping starts there. A corner radius (toroidal) cutter spreads the load over a longer length of edge, distributes the heat and gives a markedly longer life under the same conditions. On three-dimensional free-form surfaces a ball nose cutter is used; the spherical tip keeps the contact geometry continuous even as the surface normal changes.

Effective diameter and true cutting speed

With a ball nose cutter the diameter engaged in the cut depends on the axial depth of engagement. At shallow engagement a spindle speed calculation made from the nominal diameter overstates the true cutting speed. The effective diameter is:

Deff = 2 × √(ap × (D − ap)) (for ap ≤ D/2)

With a corner radius cutter, where the axial depth is smaller than the corner radius:

Deff = D − 2r + 2 × √(2 × r × ap − ap²) (for ap ≤ r)

The table below shows the effective diameter for a 10 mm ball nose cutter and the true cutting speed arising at a constant spindle speed of 10,000 rpm.

The table makes visible the most common error encountered in hard milling: a ball nose cutter working with 0.2 mm of axial engagement, at a spindle speed calculated from the nominal 10 mm diameter, actually cuts at roughly one third of the intended speed. In this case the tool never reaches the temperature band that is efficient for it, and edge wear progresses faster than expected.

Axial chip thinning

The same geometry also affects chip thickness. In the tip region of a ball nose cutter the cutting edge lies at an angle relative to the feed direction; the programmed feed per tooth is therefore greater than the actual chip thickness. This is called axial chip thinning. The result is that the cutting edge removes an excessively thin chip, rubs against the material instead of cutting it properly and wears rapidly. The correction is made by raising the programmed fz in proportion to the thinning ratio; the calculation method is given in the chip thinning guide. Where the radial engagement falls below half the diameter, radial chip thinning also comes into play and the two effects are assessed together.

5. Choice of coating and cooling

On hardened steel the fundamental property expected of a coating is oxidation resistance at high temperature. PVD coatings with a high aluminium content therefore come to the fore:

• AlTiN / TiAlN. Forms an aluminium oxide layer on the surface at cutting temperature and acts as a thermal barrier. Widely used in dry and high-speed cutting. The upper working temperature reported in the literature varies between sources; a band of roughly 800–900 °C is commonly given.
• AlCrN. Oxidation resistance is higher; manufacturers' data state values approaching 1,100 °C. Academic studies have reported that chromium-containing aluminium-based coatings (AlCrN, TiAlCrN) show more stable wear behaviour than AlTiN in difficult-to-cut and hardened materials.

A general comparison of the coating families is given in the coatings guide.

On the cooling side the established practice in hard milling points towards dry cutting or compressed air. Milling is an interrupted process; the cutting edge heats and cools on every revolution. Flood coolant increases the amplitude of this cycle and can lead to thermal fatigue cracks (comb cracks) in coated carbide. An air blast is sufficient to carry the chips away from the cutting zone and keep the temperature stable; re-cutting of chips is among the principal causes of edge chipping in hard material. As an intermediate solution, MQL (minimum quantity lubrication), in which a very small quantity of oil is carried by the air flow, is used.

Where fluid is used, the flow has to be uninterrupted. Switching the coolant on and off during the cut is one of the fastest causes of tool breakage.

6. Surface quality and scallop height

On a surface machined with a ball nose cutter, ridges known as scallops (cusps) remain between adjacent passes. For a ball radius R and a stepover ae the scallop height is:

h = R − √(R² − (ae/2)²)

At small stepovers the practical approximation takes the form h ≈ ae² / (8 × R). This relation makes one consequence immediately visible: halving the stepover reduces the scallop to roughly a quarter, while the machining time doubles.

The table shows that a larger-diameter ball nose cutter gives a lower scallop at the same stepover. In mould finishing the choice of diameter is therefore made not only according to the smallest internal corner radius but also according to the targeted surface quality. The relationship between scallop height and the measured Ra/Rz is not direct; the detail is explained in the surface roughness guide.

There are two further established practices that improve finish quality. The first is keeping the feed per tooth close to the stepover (fz ≈ ae) for an even and symmetrical surface texture. The second is tilting the tool axis by 10–15° relative to the surface; this takes the centre of the ball, where the cutting speed is zero, out of contact, and both chip formation and tool life improve. Manufacturers' publications state that when the stepover becomes very small (in the super-finishing band) the cutting speed can be raised appreciably, because the contact time is shorter.

7. Thermal stresses, sequence of operations and rigidity

Hardening and tempering leave volume change and residual thermal stress in the material. As material is removed, these stresses redistribute and the part distorts. In heat treatment practice the established sequence is roughing → stress relieving → finish machining → hardening and tempering; stress relieving is applied after roughing in order to absorb the movement before the dimensions are finalised. On parts machined in the hardened condition, sufficient stock has to be left between the roughing and finishing passes to accommodate the stress relaxation. Dimensional deviation is frequently seen on hard surfaces completed in a single pass with very small stock.

Rigidity comes before parameters in hard milling. The tool has to be clamped at the shortest possible length, the overhang/diameter ratio kept low and the runout minimised; runout creates an imbalance of load between the cutting edges and leads to the early chipping of a single flute. Shrink-fit and hydraulic holders are therefore common in hard milling; manufacturers' data state runout values of 3 µm and below for hydraulic holders and emphasise that hydraulic holders extend tool life through their damping effect. The holder families are compared in the tool holders guide, and the sources of vibration in the chatter solutions guide.

The ball nose and corner radius tool sizes used in mould finishing are listed in the ball nose end mills and corner radius end mills categories.

Sources

• ISO 513:2012 — Classification and application of hard cutting materials for metal removal with defined cutting edges — Designation of the main groups and groups of application (iso.org)
• ISO 8688-2:1989 — Tool life testing in milling — Part 2: End milling (iso.org)
• Sandvik Coromant — Workpiece materials: ISO H — hardened steel (sandvik.coromant.com)
• Sandvik Coromant — Milling inserts and grades for hardened steel (sandvik.coromant.com)
• Sandvik Coromant — Profile milling: effective diameter, true cutting speed and surface texture (sandvik.coromant.com)
• Harvey Tool — End Mills for Hardened Steels (48–68 Rc): technical data and application notes (harveytool.com)
• Kennametal — Choose the right toolholder for your operations (runout and damping data) (kennametal.com)
• ASM International — ASM Handbook, Volume 4: Heat Treating (stress relieving and heat-treatment distortion)
• G. S. Fox-Rabinovich et al. — Design and performance of AlTiN and TiAlCrN PVD coatings for machining of hard to cut materials, Surface & Coatings Technology, 2009 (sciencedirect.com)