The influence from hardness of tool life in finish hard turning 00:18

If I don’t get the same tool life every time I run a CBN insert, what can the reason be? Everything is the same; the component, the machine, the insert!

Have you ever asked yourself this question? If so, you are not alone.

Let’s analyse the machining process in more detail. The term “process factors” is used to describe the input to the machining operation (ie all that is needed to be able to generate chips), and includes insert, machine, material etc, see figure.

Many automotive components are case hardened, and on the drawings, the required hardness after case hardening is usually specified to 60 ± 2 HRC. So, if the components are 58 HRC or 62 HRC, will this influence tool life?

To study this, 5 different materials were produced, one reference material and five different variants. After quenching, the reference material was tempered at 160 degrees C. By tempering the material at other temperatures, the hardness is changed.

For each of the workpiece materials, machining was done at the following data:

 - Insert

• CNGA120408S-01525-L1-B, CBN060K 

- Cutting data

• Cutting speed 200 m/min 
• Feed rate 0.15 mm/rev 
• Depth of cut 0.10 mm 

- Tool life criterion

• Breakage 

Tool life in the reference material (60.5 HRC) was set to 100%. When increasing the hardness to 62 HRC tool life is decreased slightly, while when decreasing the hardness, tool life increases.

 As can be seen in the graph, tool life is relatively stable in the 58 – 62 HRC range, but changing the hardness from 62 to 58 HRC, will increase the tool life. A further decrease in the workpiece material hardness increases the tool life even more.

So, to answer the question, tool life of an insert depends on the cumulative effect of all the “process factors”. If any of these factors change, like to hardness of the workpiece material (even though it is still within the approved range) there can be a change in tool life. In the test above, tool life is changed by 20%, but the actual value will always depend on the specific application.

For more information, please contact your local Seco representative, or our Advanced material (PCBN/PCD/CERAMIC) team.

Author: Henrik Sandqvist Product Specialist SECOMAX PCBN/PCD/CERAMIC SECO TOOLS Shanghai CO., Ltd. Level 2, Building 8, 456 HongCao Rd., Shanghai 200233, China Office: +86 21 5426 5849 Mobile: +86 185 1520 6942 e-mail: henrik.sandqvist@secotools.com