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PCBN Inserts for Gray Cast Iron Machining

Gray cast iron is where PCBN earns its place most clearly. The material's graphite flakes make it free-machining, but its abrasiveness and the sand inclusions at the casting skin destroy carbide edges quickly. At speeds carbide cannot survive, PCBN delivers both tool life and surface finish in one pass.

Why gray cast iron suits PCBN

The graphite flake structure acts as a built-in lubricant and breaks chips cleanly, which means low cutting forces and good chip control. That free-machining behaviour allows high cutting speeds without thermal overload — precisely the condition where PCBN's hot hardness becomes an advantage rather than a cost.

Because the material is abrasive and contains hard inclusions, tool life with carbide is short enough that insert changes become a throughput problem rather than a cost problem. PCBN's wear resistance addresses the throughput constraint directly.

Typical components and operations

The highest-volume applications are automotive and general engineering castings: brake drums and discs, cylinder blocks and heads, flywheels, clutch housings, pump bodies and impellers, and valve bodies.

Operations are normally continuous or lightly interrupted external and internal turning, often with a boring pass. Because many of these parts are finish-machined in a single set-up, the ability to hold finish in the same pass that removes the casting skin is a significant process advantage.

Grade and geometry direction

For continuous turning at high speed, a high-CBN-content grade gives the maximum wear resistance and the longest edge life. For operations where the cut crosses the casting skin, a port or an inclusion, a tougher lower-CBN-content grade with a ceramic binder is usually the better choice — the wear resistance sacrificed is more than recovered by eliminating chipping.

Recommended cutting conditions by grade (Koves catalogue)

Official PCBN conditions for cast iron turning and milling, quoted from the Koves catalogue (KBN910 / KBN980 / KBN800).

If your operation is not listed, send us the workpiece drawing — we will specify the grade and geometry.

Cast iron — recommended cutting conditions (Koves catalogue)
OperationWorkpiece materialHardness (HB)GradeVc (m/min)f (mm/rev)ap (mm)
TurningGray cast iron< 230KBN910500–2000 (650)0.1–1.0< 5.0
TurningGray cast iron< 230KBN980500–20000.1–1.0< 5.0
TurningGray cast iron< 230KBN800200–10000.1–0.5< 3.0
TurningAlloy cast iron> 200KBN800200–10000.1–0.6< 3.0
TurningNodular cast ironQT450–QT550 / QT600–QT700KBN98080–2000.1–0.4< 3.0
TurningNodular cast ironQT450–QT550 / QT600–QT700KBN800150–4000.1–0.4< 0.5
MillingGray cast ironHT200–HT300 (HB < 200)KBN910800–20000.1–1.0< 5.0
MillingGray cast ironHT200–HT300 (HB < 200)KBN980800–20000.1–1.0< 5.0

Parameter guidance

Start conservative and increase speed until surface finish or tool life becomes the limiting factor. Increasing speed beyond the point where thermal cracking appears means the limit has been passed and speed should be reduced, not feed.

Gray cast iron starting parameters (industry reference values — verify on your machine)
OperationHardnessVc (m/min)ap (mm)f (mm/rev)
Continuous external turningHB 170–220400–8000.5–3.00.20–0.50
Continuous internal boringHB 170–220300–6000.3–1.50.15–0.35
Turning through casting skinHB 190–250150–3501.0–3.00.15–0.30
Finishing passHB 170–220400–7000.1–0.40.05–0.20

Frequently asked questions

What cutting speed should I use on gray cast iron with PCBN?
Gray cast iron in continuous cutting with a high-CBN-content PCBN grade typically runs in the range of 300 to 800 m/min, with depth of cut from 0.3 to 3.0 mm and feed from 0.15 to 0.50 mm/rev. Casting skin and sand inclusions require more conservative speeds. Verify against your own workpiece, machine rigidity and fixturing.
How do I handle the casting skin?
The chilled casting skin and sand inclusions are highly abrasive and are the main cause of edge chipping. Options are to take the skin in a separate, more conservative pass with a tougher grade, to reduce speed and feed through the skin, or to select a lower-CBN-content grade with a ceramic binder that tolerates the interrupted nature of the cut.
Need help choosing a grade?

Send us your workpiece material, hardness and machine parameters. Our engineers will recommend the right grade and cutting data.

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