Development and Application of Super Hard Tools

• 1
  Five things to settle before you buy the insert
• 2
  What makes a tool super hard
• 3
  When a super hard tool earns its price
• 4
  Speeds, feeds and depth of cut that hold up
• 5
  Machine and setup conditions that decide the outcome
• 6
  How super hard tools fail, and what to change
• 7
  Tool family versus workpiece and cutting data

Hardness is the whole story here. Diamond sits near 10,000 HV and cubic boron nitride near 4,500 HV. Coated carbide lands around 1,600–3,000 HV depending on the coating. That gap is why a super hard edge keeps its geometry after forty minutes in hardened steel while carbide has already rounded off.

The trade-off is toughness. Diamond is brittle and reacts with iron at temperatures above roughly 700 °C. That reaction turns the edge into graphite and the tool dies fast. CBN does not have that problem with steel, which is why CBN owns hardened steel and diamond owns aluminium, copper, brass and composites.

Binderless diamond and single-crystal diamond sit at the top of the PCD family. They give the best edge sharpness for mirror finishing non-ferrous parts, and they cost accordingly. Most production work does not need them.

Pick the family first, then the grade, then the geometry. Shops that start with geometry and hope the grade survives lose inserts on the first part.

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  PCDAluminium, copper, brass, graphite, CFRP, MMC. Not for steel or titanium.
• 2
  CBNHardened steel 45–65 HRC, Inconel, cobalt alloys, powder metallurgy parts.
• 3
  Coated carbideGeneral steel, stainless, titanium below 45 HRC. Start here.
• 4
  CermetFinishing steel and stainless at high speed. Sits between carbide and ceramic.

A super hard tool pays back when the same geometry runs across thousands of parts. Automotive valve seats, transmission housings, EV motor housings and hydraulic spools are the classic cases. The material is either too hard for carbide or abrasive enough that carbide wears out in a single shift.

Hardened steel above 48 HRC is the clearest call. Turning a 58 HRC bearing race with coated carbide means two or three inserts per part and a lot of scrap from size drift. A CBN insert holds size across the run and often removes the need for a separate grinding step.

Abrasive non-ferrous work is the second clear call. Aluminium with 20 percent silicon carbide, MMC brake rotors, and carbon fibre laminates chew through carbide edges because the reinforcement particles are harder than the tool. PCD shrugs that off.

High-volume aluminium is the third case. A PCD face mill running 4,000 m/min on 6061 or ADC12 produces a mirror finish and holds flatness without polishing. We run this on 5-axis centres with a Ø400 mm rotary table when the part needs both faces in one setup.

The fourth case is medical and aerospace work in cobalt chrome or titanium. Here the win is not speed but surface integrity. CBN and PCD leave less smeared material and fewer micro-cracks than a worn carbide edge, which matters for fatigue life.

• 1
  Volume mattersBelow a few hundred parts, coated carbide is usually cheaper overall.
• 2
  Hardness above 48 HRCCBN is the default. Below that, run carbide and check tool life first.
• 3
  Abrasive reinforcementPCD beats carbide on SiC aluminium, MMC and CFRP every time.
• 4
  Surface integrity rulesMedical and aerospace parts justify super hard tools even at low volume.

CBN on hardened steel runs at 100–250 m/min surface speed, 0.05–0.20 mm per revolution feed, and 0.05–0.30 mm depth of cut. Push the depth past 0.30 mm and the edge chips instead of wearing. The failure looks like a sudden bad finish, not a gradual drift.

PCD on aluminium runs far faster. Surface speeds of 1,500–5,000 m/min are normal on 6061 and ADC12. Feed per tooth sits at 0.05–0.20 mm. Because aluminium conducts heat away quickly, the diamond edge stays cool and tool life is measured in tens of thousands of parts.

Coolant depends on the material. CBN on hardened steel usually runs dry or with a small amount of air-blast lubricant. Thermal shock from flood coolant cracks the CBN layer. PCD on aluminium wants flood coolant to clear chips and control the built-up edge.

Rigidity is not optional. A super hard edge transmits every vibration into the workpiece as a mark. Keep tool overhang under four times the shank diameter, and never run a 200 mm boring bar on a small lathe.

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  CBN turning100–250 m/min, 0.05–0.20 mm/rev, 0.05–0.30 mm DOC.
• 2
  PCD milling1,500–5,000 m/min, 0.05–0.20 mm/tooth, 0.2–1.0 mm axial DOC.
• 3
  Overhang limitUnder 4× shank diameter for any super hard edge.

A super hard tool only performs on a machine that can hold it still. Ball screws with visible backlash, spindle runout above 5 μm, or a turret that repeats within 20 μm will ruin a diamond edge in a few parts. Check runout before blaming the insert.

Thermal stability matters as much as stiffness. A lathe that grows 30 μm over a two-hour run will drift out of tolerance no matter how good the edge is. For hardened steel work holding ±0.005 mm, we warm up the spindle and check size on the first part, then again after twenty parts.

Chip evacuation is the quiet killer on PCD aluminium work. Aluminium welds to itself, and a recut chip scratches the finished surface. High-pressure through-tool coolant, or at least an air blast aimed at the cut, keeps the edge clear.

Pre-balance every PCD face mill. An unbalanced cutter at 8,000 rpm shakes the edge and leaves chatter marks that look like a feed problem but are not.

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  Spindle runoutKeep under 5 μm at the tool holder taper.
• 2
  Warm-upRun the spindle 15–20 minutes before the first cut on tight-tolerance work.
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  Chip controlAir blast or through-tool coolant. Never let chips recut.
• 4
  BalanceBalance PCD cutters to G2.5 at the top running speed.

Micro-chipping is the most common failure. It shows up as a bright speckled edge under a loupe and a finish that suddenly gets rougher. The cause is almost always too much depth of cut, an interrupted cut, or a spindle that is not rigid enough. Reduce DOC by half and check runout.

Graphitization only happens with diamond on steel or titanium. The edge turns dull grey and wear accelerates in seconds. There is no cutting data fix. Change to CBN or to coated carbide.

Thermal cracking shows as fine parallel cracks perpendicular to the edge. It comes from flood coolant on a CBN insert that was running hot. Switch to dry cutting or air-blast lubrication.

Built-up edge is the aluminium killer. Aluminium sticks to the diamond edge when speed is too low or coolant is missing, then breaks off and takes a piece of the edge with it. Raise surface speed and add coolant.

• 1
  Micro-chippingCut DOC by half, check spindle runout, avoid interrupted cuts.
• 2
  GraphitizationDiamond on steel. Change to CBN, no data fix exists.
• 3
  Thermal crackingStop flood cooling a hot CBN edge. Run dry or mist.
• 4
  Built-up edgeRaise speed on aluminium, flood coolant, keep chips moving.