What is a CBN Insert (PCBN)?

PCBN (Nicknamed CBN) stands for “Polycrystalline Cubic Boron Nitride”, the second hardest material, second only to diamond. In addition to its high hardness[8] (4,500 HV[1]), it also has excellent hot hardness and chemical stability. Therefore, it can successfully machine Steel[9] up to 70 HRC[2] and other hard-to-machine materials.

CBN Insert
chart of The hardness of CBN relative to other cutting materials

What are CBN inserts used for?

  • Machine Steel heat treated to 50-70 HRC. (Coated carbide inserts cannot machine materials above 45 HRC)
  • Machine Cast Iron[10] 3 times faster compared with coated carbide inserts.  (~ 1,500 mm/min, 5,00 SFM[3])
  • Machine Titanium[4] and Inconel (HRSA[5]) 5 times faster compared with coated carbide (~350 mm/min, 1,100 SFM)

CBN inserts working materials and cutting speeds

Hardness [HV] 1,300-1,800 2,100-2,400 4,500-5,000 5,000-6,000
Price per Cutting Edge [USD] 4$ 10$ 40$ 160$
Machining Hard Steel
  • 10 - 45 HRC
  • 50 - 100 mm/min
  • 160 - 320 SFM
  • 40 - 60 HRC
  • 100 - 300 mm/min
  • 330 - 660 SFM
  • 50 - 70 HRC
  • 150 - 350 mm/min
  • 500 - 1150 SFM
Machining Cast Iron
  • 200 - 300 mm/min
  • 650 - 1000 SFM
  • 400 - 800 mm/min
  • 1300 - 2600 SFM
  • 500 - 2000 mm/min
  • 1500 - 6500 SFM
Machining Super Alloys
  • 20 - 60 mm/min
  • 65 - 200 SFM
  • 150 - 400 mm/min
  • 500 - 1300 SFM
  • 100 - 400 mm/min
  • 330 - 1300 SFN
Machining Aluminum
  • 300 - 800 mm/min
  • 1150 - 2600 SFM
  • 600 - 2500 mm/min
  • 2000 - 8000 SFM

Machining Hardened Steel with CBN inserts

Hard Steel ISO H

Machining hard steel is the most popular application for CBN inserts because, for hardness above 50-55 HRC, it is the only practical solution besides grinding. Despite its high price, it is still the most economical solution for these applications.


  • Excellent wear resistance.
  • Long tool-life
  • High cutting speeds
  • Maintaining close tolerances.
  • High surface finish[6]
  • Can replace grinding in many applications.


  • High price.
  • Low depth of cut. The depth of cut is limited by the size of the brazed CBN tips.
  • Solid CBN which enables heavier machining is far more expensive.

Machining Cast Iron with CBN inserts

Cast Iron ISO K

The more popular solution for fast metal removal on Cast Iron is with Ceramic inserts[7]. Ceramic inserts cost less and can work in a higher depth of cuts. CBN inserts are used only for finishing operations that require higher surface finish and accuracy that cannot be obtained with Ceramic Inserts.

Machining Super Alloys (Nickel Based) with CBN inserts

Super Alloys ISO S

The more popular solution for fast metal removal on Nickel Based Alloys is with Ceramic inserts. Ceramic inserts cost less and can work in a higher depth of cuts. CBN inserts are used only for finishing operations that require higher surface finish and accuracy that cannot be obtained with Ceramic Inserts.

Page Glossary Terms
1. Vickers scale [HV] ( HV ) Vickers [HV] is a common unit used for listing the hardness of carbide grades and other hard cutting materials such as Ceramics, CBN, and PCD. The test is done by measuring the surface area of the indentation created by a diamond in the shape of a square-based pyramid.
2. Rockwell scale [HRC] ( HRC ) Rockwell [HRC/HRB/HRA] is one of the most common units used for listing the hardness of machined materials. the test is done by measuring the depth of penetration of a sphere under a large load compared to the penetration made by a reference preload.
3. Cutting Speed [Vc] ( SFM ) In machining, the words "Speed", "Cutting Speed", "SFM" and "Surface Speed" all refer to the relative velocity between the tip of the cutting edge and the workpiece. The definition is the same for all machining operations turning, milling, etc. Opposed to feedrate which has a different definition for different applications (...)
4. Titanium (Ti) ( Titanium ) Titanium is a chemical element with the symbol Ti. Titanium alloy is usually made from about 88% of Ti with alloying elements, mostly vanadium (V) and aluminum (Al). What makes it a unique and useful metal are several properties not found together in other materials. It has an excellent strength-to-weight ratio. On the one hand, it is almost as light as aluminum, and on the other hand, it has a higher strength than most steel alloys. On top of that, it has superb corrosion resistance. This combination makes it popular in aerospace components and medical implants.
5. Superalloys ( HRSA ) Heat-resistance super-alloys (ISO S) are a group of materials engineered to have very high strength and superb corrosion resistance. These alloys must also preserve these properties at very high temperatures and chemically hostile environments. They are mainly used in jet engines, turbines, oil&gas equipment, and medical implants.
6. surface finish. Surface finish (Also called sometimes surface roughness) is a numeric representation of a surface’s “smoothness”.The number is obtained by scanning the surface with a needle that records the micro “hills” and “valleys” along a linear section. The measurement process produces a chart analyzed by a mathematical formula to yield a surface finish value.
7. Ceramic inserts. Ceramic Inserts are indexable inserts made from Aluminium Oxide Al2O3 or Silicon Nitride Si3N4. They have a hardness of 2,100-2,500 HV (About 40% above carbide), which enables them to machine Hard Steel up to 55 HRC. It can also machine cast iron and nickel-based alloys six times faster compared to coated carbide inserts.
8. hardness. Hardness is a measurement of the resistance to localized plastic deformation caused by force or abrasion. Materials with high hardness would generally be stronger and more wear-resistant, but on the other hand, more brittle and sensitive to fracture.
9. Steel. Steel (ISO P) is Iron (Fe), with the addition of 0.1 – 2.5 wt. % of Carbon (C). Besides carbon, steel may also contain many other alloying elements up to a total content of around 20%. Pure Iron is very soft. By “playing” with the mix and amount of the different alloying elements, Iron gains a range of unique mechanical properties. However, some of these additives also harm the machinability rating.
10. Cast Iron. Cast Iron (ISO K), is a group of Iron- Carbon-Silicone materials with 1.8-4% of Carbon (C) and 1-3% of Silicon (Si) content. The silicon pushes part of the carbon out of the solution, forming graphite flakes. The material is brittle at room temperature but has a low melting point and thus has excellent castability.
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