Your CBN Inserts Are Breaking. The Cutter Is Not the Problem.

Date: 14 June 2026

If your ceramic or CBN milling inserts are breaking on every indexing, the first thing most shops do is blame the insert grade. Wrong grade, wrong geometry, wrong supplier. The cutter gets swapped. The inserts get changed. The problem comes back.
That is exactly what was happening at a machining shop running an Ø80mm milling cutter with CBN inserts on a VMC.

What Was Happening

Inserts were breaking during machining and during indexing. The team kept replacing them. Every time a new insert went in, it broke again within a few cycles. The cutter body itself started showing pocket damage because broken insert fragments were hitting the pockets on the way out. Nobody had connected the breakage to the program.

The Root Cause

The cutter’s maximum depth of cut is 3.2mm. The program was running it at 3.5 to 4mm on every pass.

On top of that, full tool engagement was never reduced at initial entry. When a milling cutter enters a fresh cut at full engagement and above-rated DOC simultaneously, the load spike at entry is severe. The CBN insert, which is a hard and brittle grade, cannot absorb that impact. It fractures.

Spindle load was hitting 125%. At that level the machine is telling you something is wrong. The insert was the first thing to give way.

The cutter pocket damage was a secondary consequence. When an insert fractures mid-cut, the broken fragment passes through the pocket face on exit. If this happens repeatedly and goes unidentified, the cutter body itself becomes unusable.

What Was Done

The Sigma engineer identified the root cause on site and implemented the following corrections:

Corrected the program and reduced depth of cut to within the cutter’s rated limit
Reduced full tool engagement at initial entry so the cutter ramps into the cut rather than plunging at full width
Optimised cutting parameters at initial cutter engagement
Monitored spindle load continuously over two days of production

The damaged cutter pockets were inspected, marked, and separated. The cutter was kept under observation before being cleared for full production.

The Result

Spindle load came down from 125% to 55-60%.
Zero insert breakage was observed over two days of continuous production. Insert life doubled without any compromise on cycle time.
The cutter was not the problem. The program was.

The Rule

Before running any CBN or ceramic milling insert, check three things:
First, confirm the depth of cut in your program is within the cutter’s rated maximum. Not close to it. Within it.
Second, reduce tool engagement at initial entry. CBN and ceramic grades are hard and brittle. They perform well in stable cutting conditions. They do not tolerate impact loading at entry.
Third, watch your spindle load. If it is consistently above 80 to 90%, the program needs correction before the insert pays the price.
A five minute program check prevents days of insert breakage and cutter body damage.

We Have Seen This Before

This is a common mistake across machining shops running hard milling operations. The insert gets blamed. The grade gets changed. The supplier gets questioned. The program never gets looked at.
We have seen this in 600+ shops across 19+ countries. Same overload. Same breakage. Same fix.
If your CBN or ceramic inserts are breaking and you are running out of grades to try, let us check your program before you change the cutter.

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