Tool grade selection is one of the cheapest levers a small workshop has for improving productivity, and one of the most commonly ignored. The right grade for the material in front of you can double tool life without a single change to the machine.
Match the grade to the workpiece, not the habit
Many workshops run one general-purpose grade across every job because it simplifies stores. That convenience is paid for in edge life. As a starting point:
- Steels (P group) — favour a tougher substrate with a thicker CVD coating. Steel machining generates heat at the rake face, and coating thickness buys you crater resistance.
- Stainless (M group) — needs a sharper edge and a coating that resists built-up edge. Work hardening punishes a dull tool quickly.
- Cast iron (K group) — abrasive rather than adhesive wear dominates. A harder, less tough grade generally wins.
- Aluminium (N group) — uncoated or DLC-coated, polished flute, high positive rake. Coatings meant for steel will gall.
Read the wear, not the calendar
Changing inserts on a fixed schedule wastes edges that still have life and lets others run past failure. Pull a worn insert and look at it under magnification:
- Flank wear that is even and gradual is what you want. If it reaches your limit too quickly, reduce speed before touching feed.
- Cratering on the rake face points to excess heat — a coating or speed problem.
- Chipping suggests the grade is too hard for the interruption, or the setup is not rigid enough.
- Built-up edge usually means the speed is too low, not too high.
Change one variable at a time
When a tool underperforms, the temptation is to change grade, speed and feed together. Then you cannot tell what helped. Move one variable, cut a known number of parts, and record the wear. Three disciplined trials will teach you more than a month of guesswork.
Where ToolClinic fits
If you are unsure which grade suits a job, our experts have spent careers making exactly these calls. A short conversation is usually cheaper than a box of the wrong inserts.