Metals in the workshop: steel, stainless, cast iron and aluminium
How the four major material groups behave under the cut
A CNC technician must not only be able to operate the machine but understand the material in it. Machinability — how easily a material can be cut — varies enormously and governs cutting speed, tool selection, cooling and chip shape. Knowing the character of materials is the difference between a programme that just runs and one that runs well. Here are the four groups you most often meet.
§Steel: the workhorse
Unalloyed and low-alloyed steel (ISO group P blue) is the most machined material and behaves predictably. Machinability decreases as carbon and alloy content increase and the material becomes harder. Ordinary construction steel produces long continuous chips that need to be broken and carbide runs well at moderate to high cutting speeds. Steel is a good material to learn on precisely because it is forgiving and well-documented in all cutting data tables.
§Stainless steel: tough and sticky
Stainless steel (group M, yellow), especially the austenitic types such as 304 and 316, is notorious for being difficult. It is tough, it work-hardens — becomes harder if the cut rubs instead of cutting — and it conducts heat poorly, so the heat stays in the cutting zone. It requires sharp tools, sufficient clamping (so you come under the hardened layer), plenty of cooling and not too high a cutting speed. If you run too carefully in stainless, it punishes you with a hardened, dull result.
§Cast iron: short chip, lots of dust
Cast iron (group K, red) behaves very differently. It is brittle, so the chips are short and crumbly rather than long ribbons, and it can often be machined dry. In return, it develops a fine, abrasive and health-damaging dust — graphite and silicon compounds — which requires good extraction and cleaning. The abrasive dust also wears on the tool. Cast iron is easy to cut, but places demands on the working environment around the machine.
§Aluminum: fast, but sticks
Aluminum (group N, green) is soft and conducts heat well, so it can be cut at very high cutting speeds – often several times faster than steel. The challenge is not hardness but stickiness: aluminum tends to build up on the cutting edge (built-up edge) and smear itself. Therefore, sharp, very positive cutting edges with polished or diamond-coated (PCD) surfaces and abundant cooling or minimum oil are used. With the right tools, aluminum is a rewarding and extremely fast material.
| Material | ISO group | Cutting speed | Typical challenge |
|---|---|---|---|
| Steel | P (blå) | Moderate–high | Long chips should be broken |
| Stainless | M (gul) | Lower | Work hardening and heat |
| Cast iron | K (rød) | Moderate | Wearing, harmful dust |
| Aluminum | N (green) | Very high | Built-up edge and adhesion |
- 01Always look up cutting data for the specific material — not from the last job
- 02Stainless will need solid clamping and good cooling
- 03Cast iron requires extraction against harmful dust
- 04Aluminum will need sharp, positive blades and high cutting speed
“The machine is the same — it is the material that determines how the day goes.”