Blades for Super Hard Tools: Geometry, Wear and Cutting Limits

• 1
  Why Super Hard Cutting Materials Behave Differently
• 2
  Edge Geometry: The Real Variable on Blades for Super Hard Tools
• 3
  How These Edges Actually Wear Out
• 4
  Matching the Part to the Right Edge
• 5
  Parameters and Setup That Keep the Edge Alive
• 6
  Edge and Material Selection for Blades for Super Hard Tools

Diamond and cubic boron nitride sit at the top of the hardness scale. Diamond reaches roughly 8,000 to 10,000 HV, CBN sits near 4,000 to 5,000 HV. That hardness is why blades for super hard tools can cut hardened steel and abrasive composites at all. The same hardness makes the edge brittle.

Hardness is not toughness. A diamond edge chips before it dulls. CBN resists heat better than diamond but reacts with iron at high temperature. These two facts drive most edge failures in production.

Thermal conductivity also splits the two materials. Diamond pulls heat out of the cut zone fast, so the workpiece stays cooler. CBN holds heat near the edge, which is why flood coolant matters when turning hardened steel above 45 HRC.

• 1
  DiamondBest on aluminium, copper, composites and graphite. Avoid ferrous alloys.
• 2
  CBNBuilt for hardened steel and cast iron above 45 HRC.
• 3
  CarbideStill the default for mixed work and interrupted cuts.

The cutting edge is not a single angle. It is a rake face, a clearance face and a hone or chamfer where they meet. On a super hard insert, a 0.05 mm chamfer can change edge life by several times.

A structural edge at a positive rake cuts freely and lowers cutting force on aluminium. The trade-off is a thin edge that chips under vibration. A blunt circular edge spreads load over a wider contact area and survives interrupted cuts, but it rubs and raises temperature.

The chamfered edge sits between the two. A small negative chamfer, often 0.1 to 0.2 mm at 20° to 30°, adds strength without the full penalty of a honed round. This is the common choice for hardened steel turning.

Edge preparation is set at the insert grade level, not on the machine. Changing the chamfer means changing the insert, not the program. That is why the same tool path can perform differently after a grade switch.

• 1
  Structural edgeLow force, sharp. Good for aluminium and shallow finishing passes.
• 2
  Blunt circular edgeHigh strength. Use on interrupted cuts and heavy stock removal.
• 3
  Chamfered edgeBalanced. Standard for hardened steel above 45 HRC.

Super hard edges fail in four ways: flank wear, crater wear, chipping and delamination of the PCD layer. Flank wear is gradual and predictable. You measure it as a wear land on the clearance face and replace the insert before it reaches the limit.

Chipping is sudden. It starts from micro-cracks at the hone and grows under thermal cycling. A dull sound change and a spike in surface roughness usually appear first. If the operator catches it, one corner is lost. If not, the holder can be damaged.

Crater wear happens on the rake face where the chip slides. It is worse at high speed on steel and worse with CBN than diamond. Delamination is specific to PCD: the diamond layer separates from the carbide substrate when the braze or the substrate gets too hot.

Depth of cut and feed rate decide which mode dominates. Light passes favour flank wear. Heavy interrupted passes favour chipping. High surface speed favours crater wear and heat damage.

Start from the material, not the tool catalogue. Aluminium and copper alloys with high silicon content need PCD. The silicon particles are abrasive and will destroy carbide in minutes. A structural edge at a positive rake keeps cutting pressure low on thin walls.

Hardened steel above 45 HRC needs CBN. Choose a chamfered edge with a light hone. If the cut is continuous, a small chamfer is enough. If the cut is interrupted, go to a wider chamfer or a blunt circular edge.

Cast iron sits in the middle. CBN handles it well, and dry cutting is common because the graphite in the iron lubricates the edge. Flood coolant on cast iron can cause thermal shock cracks on the insert.

Composites and graphite are abrasive and dusty. PCD with a sharp edge gives the cleanest cut, but dust extraction matters more than edge geometry for tool life.

Surface speed is the first number to set. PCD on aluminium runs best between 500 and 2,000 m/min. CBN on hardened steel runs between 100 and 250 m/min. Going faster raises temperature faster than it raises removal rate.

Feed per tooth controls chip thickness. Too light a feed rubs the edge and accelerates flank wear. A chip must be thick enough to carry heat away. On a 50 mm face mill, 0.1 to 0.2 mm per tooth is a reasonable starting range.

Rigidity is not optional. Super hard edges have almost no ability to absorb vibration. A tool holder with 0.02 mm runout will chip a PCD edge within a few passes. Check runout, check the spindle taper, check the fixture.

Coolant choice follows the material. Flood coolant for CBN on steel. Air blast or minimum quantity lubrication for PCD on aluminium. Cast iron often runs dry. Mixing these up shortens edge life more than any parameter change.