CBN cutting tools are cutting tools made from cubic boron nitride, a man-made material that is almost as hard as diamond. Machinists use them to cut hardened steel and cast iron.

Here’s the quick version:

  • CBN keeps its edge at high heat, where regular carbide gives up.
  • CBN works best on steel that has a hardness rating of over 55 HRC.
  • It can often replace slow grinding with faster turning.
  • It costs more per tool, but it can cost less per part.

What are CBN cutting tools?

Cubic boron nitride is a synthetic material. Makers press it under extreme heat and pressure until it forms a super-durable crystal. People write it as CBN or cBN. Tools made from the sintered form are often called PCBN.

How hard is it? CBN is rated above 7000 Hv, next to natural diamond. To cut hardened steel of about 700 Hv, the tool needs to be at least three times harder, around 2100 Hv. Regular carbide can struggle to meet that. CBN clears it easily. 

One quick note. “CBN” also stands for a cannabis compound and a bank in Nigeria. This page is only about the cutting tool material.

Why CBN works on tough steel

Hard steel makes a cutting edge hot. Really hot. A carbide edge can soften, wear fast, and then fail.

CBN doesn’t have that problem. It stays hard at high heat, and it moves heat away from the cut well. It also doesn’t react much with iron. CBN has a lower affinity to iron than diamond does, so it handles steel and cast iron without breaking down.

That’s the big difference from diamond tools. Diamond is nearly pure carbon, and above about 700°F, that carbon reacts with iron, and the diamond collapses. So diamond is for aluminum, plastics, and composites. CBN is for steel and iron. 

Types of CBN cutting tools

Not every CBN tool looks the same. Here are the main kinds:

Type: What it is: Best for Solid CBN inserts The whole cutting tip is CBN. Heavy-duty hard turning Tipped CBN insert A small CBN wafer is brazed onto a carbide base,CBN cutting tools which cuts tool costs. Finishing work: Multi-edge (half-solid) insert A CBN layer on a carbide base with several usable corners Lower cost per edge CBN endmill: a milling cutter with a CBN edge Mold and die steel finishing CBN grinding wheel Abrasive grains bonded to a wheel Precision grinding CBN cut-off blade Resin-bonded blade Cutting hardened parts of 

Multi-edge inserts are a clever idea. One maker’s patented design puts four CBN edges on styles that normally carry only one. More edges per insert means a lower price per cut. 

Looking for a place to start? Check our CBN inserts and CBN grinding wheels.

Low-CBN vs. high-CBN grades

This part confuses a lot of people, so here’s the simple version.

  • Low-CBN grades are about 40–65% CBN. A ceramic binder makes up the rest and adds wear resistance. 
  • High-CBN grades run from 85% up to nearly 100% CBN.

Which one lasts longer? It depends on the job. Higher CBN content adds fracture and thermal shock resistance, while lower content gives better wear resistance. A study on 56 HRC steel found the lower-CBN grade lasted longest, even on interrupted surfaces. Edge shape mattered too. A chamfered edge won in smooth cutting, and a rounded edge won in interrupted cutting. 

So there’s no “best” grade. There’s a best grade for your part.

What can you cut with CBN?

CBN’s sweet spot is hardened steel and cast iron. Common targets:

  • Hardened tool steel and die steel
  • Case-hardened gears and shafts
  • Brake rotors and drums
  • Cylinder liners and engine blocks
  • Sintered metal parts
  • Hard rolls used in steel and paper mills

Hardness decides the tool. A handy guide from one major maker:

  • Carbide: up to about 45 HRC
  • Ceramic: about 45–60 HRC
  • PCBN: about 55–70 HRC

Hard turning is any single-point cutting of parts above 45 HRC. Above 55 HRC, CBN is the only cutting tool that can replace traditional grinding. That’s why shops pay attention to it. 

CBN vs. carbide, ceramic, and diamond

Picture a shop turning a gear at 60 HRC. Carbide chips in minutes. Ceramic lasts a while but wears off fast. CBN holds the size for far longer.

Research backs this up. One study found CBN tools beat ceramic tools on both tool life and surface finish in continuous and interrupted cutting. Another test on D2 tool steel reached the same result. 

But let’s be fair. Carbide isn’t always a loser. One study on 48–49 HRC steel found that at low to medium speed and feed, coated carbide performed about as well as CBN. So if your steel is on the softer side of “hard,” carbide may be all you need. 

Here’s the simple rule:

  1. Softer than 45 HRC? Try carbide.
  2. Between 45 and 55 HRC? Compare carbide, ceramic, and CBN on cost per part.
  3. Harder than 55 HRC, or a long production run? CBN usually wins.
  4. Cutting aluminum or other soft metals? Use PCD tools instead.

The cost question

Yes, a CBN insert costs more. Maybe a lot more. So is it worth it?

Don’t compare the price of the tool. Compare the cost of the finished part. Count tool changes, machine downtime, scrap, and the grinding step you might skip. One manufacturer says its multi-edge CBN inserts save 30–50% compared with carbide inserts and can let you turn a part instead of grinding it. That’s a maker’s own claim, so test it on your parts before you trust it.

A simple example. Say a grinding step takes six minutes per part, and CBN turning does the same job in two. Over 10,000 parts, that gap adds up fast, even if each insert costs more.

How to choose the right CBN tool

Ask yourself these five questions:

  1. What material is it, and how hard? Give the exact HRC.
  2. Is the cut smooth or interrupted? Cuts with gaps, slots, or keyways hit the edge hard.
  3. Is it finishing or heavy roughing? CBN is best for light semi-finishing and finishing. One maker’s guide puts the sweet spot under 0.8 mm depth of cut. 
  4. How stiff is your machine and setup? CBN is hard but brittle.
  5. How many parts will you make? Longer runs favor CBN.

Tips for longer tool life

  • Keep the setup rigid. Shake or chatter can chip a brittle edge.
  • Don’t use CBN for heavy roughing. Rough with something tougher first.
  • Match the edge shape to the cut. Chamfer for smooth cuts, rounded for interrupted ones.
  • Watch for wear early. On hardened steel, the main wear on CBN inserts comes from oxidation, so heat control matters. 
  • Store CBN and PCD separately. Mixed-up inserts and stray diamond dust can cause trouble.

FAQs

What are CBN cutting tools used for?

They cut hardened steel, cast iron, and sintered metals. Shops use them for gears, shafts, brake parts, molds, and dies.

Is CBN harder than carbide?

Yes, by a wide margin. CBN is second only to diamond. Carbide sits far below it, which is why CBN holds an edge on hard steel.

What’s the difference between CBN and PCD?

Both are super-hard, but PCD is a diamond material. It can’t cut steel because the carbon reacts with iron at high heat. CBN can. Use PCD for aluminum and CBN for steel.

Can CBN cut aluminum?

You can, but it’s the wrong tool. Aluminum is soft, and PCD gives a better result at a lower cost.

Are CBN inserts worth the money?

They are when you cut steel above roughly 55 HRC or run long batches. For softer steel or short runs, carbide may cost less per part.

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