From Roughing to Finishing — the Red Lines for Hardness, Coating and Flute Count
An injection mould travels from blank to ejection through five cutter types: a high-feed mill for roughing, a corner radius cutter for semi-finishing, a ball nose cutter for finishing, a chamfer cutter for edge work and a taper cutter for the draft angle. Each has its own hardness, coating, flute count and parameters, and running them on one shared setting typically ends with a scrapped cavity. Understanding the division of labour along that chain matters more than memorising any single recommendation.
The underlying logic: hardness as a duty statement
Solid carbide end mill hardness is expressed on the HRA scale, generally between 89 and 94. The band looks narrow, but every single point adds roughly 15 % wear resistance and roughly 20 % brittleness. Hardness is therefore not a case of higher is better; it is a balance between wear resistance and impact resistance set by the application. Roughing absorbs interrupted impact and takes the lower value; finishing needs stable edge geometry and size, and takes the higher value.
Coating follows the same duty logic. Typical temperature ratings are: TiN around 600 °C as the basic general-purpose layer; TiAlN around 800 °C for steel and cast iron; AlCrN around 1100 °C with strong oxidation resistance for high-speed finishing; and DLC as a low-temperature coating used mainly to prevent adhesion in aluminium and copper. The selection criterion is cutting-zone temperature, not appearance.
Cutter one: the high-feed mill (cavity roughing)
The high-feed mill uses a special edge geometry to combine a shallow depth with a very high feed: 0.3–0.8 mm depth at 3000–5000 mm/min, removing material several times faster than a conventional end mill.
Hardness is set at HRA 89–91, on the tough side, because roughing is an interrupted cut with heavy impact loading and higher hardness sharply raises the risk of fracture. Coating is AlTiN at around 900 °C, since roughing generates substantial heat and the coating must hold hardness at temperature. Flute count is 3–4, balancing core rigidity against chip room; beyond five flutes chip room runs out and re-cutting inside the groove leads to edge failure.
Reference data: 0.3–0.8 mm depth, 3000–5000 mm/min feed, 6000–12000 rpm. The red line is a 1 mm depth of cut — beyond it the load model changes abruptly and the failure mode shifts from wear to fracture.
Cutter two: the corner radius cutter (semi-finishing)
The corner radius cutter carries a flat bottom with an R transition, combining the flat-milling efficiency of a square cutter with the contouring capability of a ball nose. It is the bridge between roughing and finishing.
Hardness is HRA 90–92, the middle value: enough wear resistance to hold the R geometry, enough toughness for interrupted cutting. Coating is TiAlN at around 800 °C as the best value option, upgraded to AlCrN for stainless steel. Flute count is 2–4: two flutes give chip room for aluminium and copper, four give rigidity and finish quality in steel and cast iron, and semi-finishing favours four. Reference data: R at 10–20 % of the bottom diameter, 0.5–2 mm depth, 800–2000 mm/min feed.
The red line is that the corner radius must be equal to or larger than the smallest internal radius on the workpiece. An undersized R jams the corner into the workpiece fillet, stress peaks immediately and the edge chips — and once the R is deformed, every subsequent part cut with that tool is out of tolerance. Measure the workpiece fillet before choosing the tool.
Cutter three: the ball nose cutter (contour finishing)
The ball nose cutter generates a smooth three-dimensional surface through the continuous motion of its tip. It carries most mould cavity, automotive surface and medical contour finishing work, and it is the cutter most likely to go wrong.
The ball nose has one structural weakness: as the ball rotates, linear speed falls towards the centre point, reaching zero at the tip. That zone is not cutting but compressing, and eventually the tip chips. The cutter must therefore be used at a tilt — around 15° of spindle inclination — to keep the tip clear of the zero-speed dead zone, with step-over held within one third of the radius.
Hardness is HRA 92–94, on the hard side, to preserve the edge sharpness and dimensional accuracy finishing requires. Coating is AlCrN at around 1100 °C with strong oxidation resistance, holding edge condition at high speed and light chip load. Flute count is primarily two: the ball form limits chip room, and three or more flutes leave a channel too narrow to evacuate chips, causing re-cutting and edge breakage — a far larger risk than any finishing gain.
Reference data: 0.05–0.3 mm depth, 8000–20000 rpm, 15° tilt. The red line is that tilting is mandatory; a flat tool path with insufficient speed will destroy the tip.
Cutter four: the chamfer cutter (edges and deburring)
The chamfer cutter uses a negative rake to reinforce the edge and handles edge chamfering, deburring and weld prep. Chamfer quality feeds directly into sealing performance on sealing faces.
Hardness is HRA 90–92. The negative rake already reinforces the edge, so ultra-high hardness is unnecessary and only adds brittleness. TiN at around 600 °C is sufficient because chamfering generates limited force and heat; upgrade to TiAlN for stainless steel to resist adhesion, or DLC for aluminium and copper to resist built-up edge. Flute count is two for rough chamfering and four for finishing — sealing-face chamfers must use four flutes, because a two-flute surface carries chatter marks that leak under pressure test.
Reference data: 45° / 60° / 90° included angle, 3000–8000 rpm, 200–500 mm/min. Two red lines apply: sealing faces must use four flutes, and speed must not exceed 8000 rpm because the chamfer cutter runs at long overhang and vibrates at higher speed.
Cutter five: the taper cutter (draft angle)
The taper cutter carries an included taper that generates the mould's draft angle directly. Whether the part releases cleanly comes down to this tool.
Hardness is HRA 90–92, balancing wear and toughness: too hard and it chips against locally hardened areas, too soft and the edge wears quickly enough to lose taper accuracy. Coating is TiAlN at around 800 °C because mould steel cuts hot, upgraded to AlCrN for hardened mould steel. Flute count is 2–4: two flutes for deep cavities because the wider channel keeps chips off the taper face, four for shallow cavities where rigidity and surface quality matter. Chip evacuation is harder on a taper cutter than a flat cutter because the tapered face pushes chips inwards, which makes the two-flute advantage particularly clear in deep cavities.
Reference data: 0.5° / 1° / 2° / 3° taper, 0.5–3 mm depth, 500–1500 mm/min feed. The red line is that the taper must equal the draft angle, verified with an angle gauge before machining rather than trusted from the box label.
Three selection principles
- Hardness by duty: roughing on the tough side (HRA 89–91), finishing on the hard side (HRA 92–94), general work in between (HRA 90–92).
- Coating by temperature: AlTiN for heat in roughing, AlCrN for oxidation resistance in finishing, TiAlN as the general choice, TiN as the basic layer.
- Flute count by chip evacuation: two flutes where evacuation is difficult (ball nose, deep cavities), four where rigidity dominates (high feed, finishing).
Beyond carbide: where ceramic and cermet step in
The HRA system tops out at around 94, and once work moves into high-hardness mould steel, superalloys or high-speed deep cutting, carbide's margin on wear and heat resistance is consumed quickly. At that point changing the material system is more effective than tuning parameters.
In contour finishing and cavity forming, a precision super-hard ceramic ball-nose milling cutter for aerospace and automotive mould work takes over from the carbide ball nose, using ceramic hot hardness for high-speed, light-depth, long-travel finishing. For radius and transition work, a custom ceramic corner-radius milling cutter for high-speed deep cutting in aerospace is built to the workpiece profile for aerospace high-speed deep-cutting conditions. For flat faces and side walls, a super-wear-resistant solid cermet flat-bottom end mill for precision engineering and mould machining balances wear resistance with transverse rupture strength for continuous cutting on precision parts and moulds.
All three share the same prerequisites: machine rigidity, spindle speed capability and cooling must be adequate. Ceramic and cermet are less tough than carbide and more sensitive to runout and vibration, so they are not roughing tools. What they cover is the finishing and high-speed segment of the chain — not the whole chain.
Conclusion
Five cutters form one chain, and any single failure interrupts the whole sequence. Apply the three principles — hardness by duty, coating by temperature, flute count by chip evacuation — and then decide, on workpiece hardness and linear speed, whether to move up to ceramic or cermet. That is what makes the chain dependable.
Post time: Oct-23-2026