Learn the basics of hard machining without grinding with CBN Inserts: The Complete Guide
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Manufacturers have been replacing carbide tools with CBN inserts since it was discovered that carbide tools could burn out half-way through a batch of hardened steel parts. Cubic Boron Nitride, the second hardest substance in the world after diamond, is the answer for shops requiring to cut hardened metals quickly and with precision, but not having to grind every part first.
In this guide, you'll learn what CBN inserts are, how they operate, their role in your process, and how to select the optimal insert for your application.
CBN Inserts are what?
CBN inserts are tips of cutting tools that are fabricated from polycrystalline cubic boron nitride (PCBN), a synthetic super hard material embedded in a tungsten carbide base plate. The PCBN layer is extremely hard and heat resistant, and the carbide base is tough enough to endure the shock and vibration in actual machining.
CBN inserts, unlike conventional carbide or ceramic tooling, are specifically designed to take on one task: to efficiently machine materials that are too hard. These include hardened steels, chilled cast iron, powder metals, and some superalloys.
Hardness alone cannot be the answer
Hardness is a significant virtue, but it is the combination of properties that makes CBN inserts valuable.
Thermal stability — CBN is hard at cutting temperatures which would quickly soften or degrade the carbide.
Chemical inertness — It will not react with iron based materials which decreases crater wear on the cutting edge.
Wear resistance – Edges remain sharp much longer than carbide, thus less tool change and constant part dimensions during a production run.
How CBN Inserts Work in Hard Turning
The biggest shift CBN inserts have driven in manufacturing is the rise of "hard turning" — machining parts after they've been heat-treated, at hardness levels of 45-65 HRC, using a lathe instead of a grinder.
Traditionally, hardened parts had to be ground to reach final dimensions, because standard cutting tools couldn't survive contact with material that hard. CBN changed that equation. A properly selected CBN insert can turn a hardened shaft or bore to final tolerance in a single setup, cutting out the separate grinding operation entirely.
The Practical Payoff
Replacing grinding with CBN hard turning typically delivers:
Fewer process steps — One machine, one setup, one operator instead of a turning operation followed by a separate grinding cycle.
Shorter cycle times — Turning removes material faster than grinding in most hardened-part applications.
Lower fluid and energy use — Grinding is coolant- and energy-intensive; dry or near-dry hard turning uses a fraction of both.
Better geometric flexibility — Complex profiles, grooves, and contours that are difficult to grind can often be turned directly with the right insert geometry.
Where CBN Inserts Are Used
CBN inserts show up wherever hardened or abrasive materials need precision machining. Some of the most common applications include:
Automotive Manufacturing
Camshafts, gear hubs, brake rotors, and brake drums are frequently hardened before final machining. CBN inserts hold the tight tolerances the automotive industry demands, run after run.
Aerospace Components
Landing gear components and turbine shafts often involve hardened alloys and superalloys that punish conventional tooling. CBN's thermal stability makes it a reliable choice for these high-stakes parts.
Die and Mold Production
Hardened tool steel molds require accurate, repeatable finishing. CBN inserts let moldmakers machine post-heat-treatment without relying entirely on EDM or grinding.
Energy and Heavy Industry
Bearing races and valve components benefit from the surface finish and dimensional accuracy CBN inserts provide directly off the lathe.
Mining Equipment
Wear parts and slurry pump components involve abrasive, hardened materials where tool life directly affects uptime and cost per part.
Choosing the correct CBN Insert
There are various CBN inserts and if the wrong insert is selected, it may lead to early insert wear, poor surface finish or edge chipping. Here is an easy way to make the right selection.
Step 1: Identify the Material to Be Machined
Begin with the part to be worked. Know its hardness, abrasiveness, and thermal conductivity — all other aspects of tool choice (CBN grade, cutting parameters, etc.) depend on these three.
Step 2: Click on Insert Shape and Size
Select a shape (square, triangle, diamond or round) according to part geometry and machine set up. The size of insert and nose radius should be in accordance with the depth of cut and finish requirement.
Step 3: A student matches the Grade to the Application
There are two basic types of CBN inserts:
High-CBN-content grades – Tougher, for interrupted cuts and rougher cuts.
Harder and more wear resistant, low-CBN content, ceramic bonded grades for continuous finishing cuts on hardened steel.
Among the most important selections is the matching of the grade with the cutting condition (continuous or interrupted).
Step 4: Run the Test.
After choosing a grade and geometry, perform a controlled test cut. Observe tool wear, surface finish and chip formations, and then modify speed, feed, and depth of cut prior to going into full production.
Typical cutting parameters for CBN inserts
Specific parameters are dependent on the material and insert specification, but most CBN inserts will operate best at a cutting speed between approximately 100 and 400 meters per minute. Feed rates and depth of cut should be adjusted to the particular material hardness and interruption level required for the part - a good tooling supplier will be able to help you narrow it down based on your material and machine.
Request a quote for CBN Inserts, Carbide, and Ceramic.A quick comparison for CBN Inserts, Carbide and Ceramic
CBN inserts typically result in lower production-unit scrap and fewer process steps, as well as significantly longer tool life on hardened part production runs, making up for the initial higher cost.
How to Make Your CBN Inserts Last Longer
Not all shops reap the rewards of CBN tooling but there are a few habits that set them apart:
Rigid setups – little to no vibration, loose setups – edge chipping.
Heat treatment consistency- hardness variation between parts makes it more difficult to maintain consistent tool life and finish.
Correct edge preparation – a honed or chamfered edge for the application, as it is a balance of toughness and sharpness.
Routine wear monitoring - detecting progressive wear as it is happening rather than when it reaches the chipping stage means the quality is maintained in a run.
Final Thoughts
CBN inserts have revolutionized the cutting of hardened material, eliminating the need for a grinding operation by enabling the material to be easily turned in one single pass with efficiency. The extreme hardness, thermal stability, and long tool life make CBN a logical improvement over hardened steel, cast iron, and abrasive materials wherever they are found in the automotive, aerospace, die/mold, and heavy industry applications.
The key to selecting the right insert is knowing the material, the grade and geometry of the insert and the application, and finally, testing the insert in a real test cut to ensure that it performs as needed. Do that right and CBN inserts are capable of reducing cycle times and producing a finish quality that is comparable with grinding straight off the lathe.
Frequently Asked Questions
Do CBN inserts take the place of grinding? Yes. In many applications, hard turning of parts is possible using CBN inserts, thereby avoiding the need to grind parts after they are machined.
What will CBN inserts cut? CBN inserts are designed for the very hard materials, such as hardened steel, chilled cast iron, powder metals and some superalloys, generally between 45 and 65 HRC.
What CBN grade should I use for my task? Correlate the grade with the hardness of the material and cutting conditions. The harder the grade with a higher content of CNC, the better it will be for interrupted cuts; the harder the grade with a ceramic bond, the better for continuous passes
Does it cost more to use CBN inserts or carbide? Upfront, yes. However, when it comes to hardened-material applications, the cost savings from grinding in comparison to CBN inserts is typically outweighed by the longer tool life and reduced grinding requirement over a production run.
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