A Brief Discussion on Tool Material Classification

Every cutting tool sits between two limits: how hot it can run before it softens, and how much it wears before the edge geometry changes. Most classification schemes are really a ranking of those two properties. A grade that holds hardness at 1,000 °C can cut fast but tends to be brittle. A grade that survives impact will not take the same surface speed.

Hardness at temperature, transverse rupture strength and thermal conductivity are the three numbers that matter. Thermal conductivity decides how much heat leaves through the chip instead of soaking into the workpiece. Low conductivity pushes heat into the part, which moves the dimensions you just held.

Speed and feed tables are written around a specific grade. Swap a coated carbide insert for an uncoated one and the same program may rub instead of cut. The chip tells you which is happening. Thin, curled chips mean the edge is shearing cleanly; blue powder or a dull rumble means you are burning the edge.

High-speed steel still earns its place. It bends before it breaks, it is cheap to regrind, and it takes an edge that carbide cannot hold on interrupted cuts. Taps, reamers, form tools and small-diameter drills are the usual homes for it. M35 and M42 cobalt grades add hot hardness for stainless and 4140.

Carbide covers most production work. Grain size drives the trade-off: fine and submicron grades hold a sharp edge for finishing at Ra 0.8–1.6 μm, while coarser grades survive roughing in 4130 or 17-4PH. Uncoated carbide suits aluminium and non-ferrous work, where a coating would only add friction.

Carbide is brittle, and that is the failure mode to plan around. A 6 mm end mill with 40 mm of stick-out in a 5-axis cut will snap on a sudden direction change long before the edge wears. Shorten the gauge length or drop to a smaller stepover.

Tool steel sits outside the cutting family, but the name causes confusion. It is a workpiece material for punches, dies and wear plates, not a cutting tool substrate. We machine it after hardening only when the drawing calls for it, and usually grind the critical faces instead.

Ceramic inserts run dry, at speeds that would destroy carbide. Silicon nitride handles grey cast iron and Inconel roughing; alumina grades handle hardened steel finishing. The catch is rigidity. Any chatter at 500 m/min will break the edge in seconds, so keep tool overhang short and check the spindle taper.

CBN is the answer for hardened steel above roughly HRC 45. It turns a 60 HRC die insert in one pass where grinding would need a fixture and a long cycle. It does not like soft, gummy material, and it does not like interrupted cuts.

PCD handles aluminium, copper and carbon fibre. The diamond edge stays sharp far longer than carbide, which matters on abrasive composites where carbide edges round off in minutes. Keep PCD away from steel and titanium. The cobalt binder reacts with iron at cutting temperature and the edge fails quickly.

A coating is a thin layer, usually 2–10 μm, that raises hot hardness and cuts friction. CVD coatings are thicker and better for roughing. PVD coatings are thinner, sharper and better for small tools and finishing. On a 3 mm end mill, PVD makes the difference between a usable edge and a rounded one.

Match the coating to the workpiece. TiAlN and AlTiN for steel and stainless, TiCN for harder steel and cast iron, DLC for aluminium and copper where built-up edge is the problem, and diamond coating for graphite and composites. Wrong pairing usually shows up as built-up edge, not as wear.

Coated tools do not fix a bad setup. If the part moves in the vise, no coating holds tolerance. We check workholding before we change grades, and we run the first part slow until the chip shape looks right.

Start with the workpiece. Aluminium and plastics go to uncoated carbide or PCD. Stainless and titanium go to sharp, coated carbide with a positive rake. Hardened steel above HRC 45 goes to CBN. Cast iron can go to coated carbide or ceramic.

Then look at the feature. Deep pockets and long reach push toward stiffer carbide grades, not harder ones. Threads and reamed holes often go to HSS because the tool needs to flex slightly, not snap. Thin walls push toward lower radial engagement and smaller tools, which changes the grade you can use.

Finally, look at the quantity. One prototype does not justify a CBN insert. A 10,000-piece run does. We hold ±0.005 mm and inspect 100% before shipment, so the grade also has to hold size across the run, not just on the first part.

When a job comes in, we run a free DFM analysis and flag the tooling before the first chip. If a feature needs a grade we would rather not run, we say so at quote stage instead of after the first article.