Almost all the work in a Spanish machine shop falls into three families: aluminium, carbon steel and stainless steel. Each has different physics and all three punish you differently when you get it wrong. This article is about understanding why, not about copying a table.
The variable that rules: cutting speed
Cutting speed (Vc, in metres per minute) is the speed at which the edge passes through the material. It is not spindle rpm: the rpm come out of the Vc and the diameter. Orders of magnitude with carbide tooling:
- Aluminium: 200–1,000 m/min. It takes an enormous amount.
- Carbon steel (S235, C45, 42CrMo4): 120–250 m/min.
- Austenitic stainless (AISI 304, 316): 80–150 m/min.
These are ranges, not recipes: the insert maker's table always wins, because it depends on the specific coating and geometry. But the proportions tell the story: stainless is machined at less than half the speed of ordinary steel, and aluminium takes five times more. If you want the calculation from Vc to rpm and feed step by step, it is in cutting speed and feed.
Aluminium: the problem is adhesion, not hardness
Aluminium is soft, conducts heat very well and hardly wears the edge at all. Its problem is a different one: it sticks. The material welds to the cutting edge and forms what is called a built-up edge. When that stuck-on lump breaks away it tears material off the part and leaves the finish marked.
What prevents it:
- A very sharp geometry and a large rake angle. Two- or three-flute cutters (not four), with a high helix and a wide flute so the chip can get out.
- A polished tool or a non-stick coating. DLC and diamond work very well; coatings designed for steel do not.
- A generous feed. Against intuition, going too slowly in aluminium makes things worse: it encourages sticking.
- Coolant or air. Plenty of emulsion, or minimum quantity lubrication (MQL). What you cannot do is machine aluminium dry at high speed and expect a good finish.
Here you can afford to be aggressive: aluminium is the material where high-speed machining makes most sense and where you gain the most parts per hour.
Carbon steel: the reference material
Steel is the material most tools and parameters are designed around by default. The chip breaks well, wear is gradual and predictable, and the edge warns you before it fails.
What needs watching is heat. A 1,000 N/mm² steel generates a lot of temperature in the cutting zone, and the edge wears by abrasion and by cratering. TiAlN coatings, plenty of coolant and constant depths of cut are the way through. A varying depth of cut heats the edge in bursts and cracks the coating.
In steels hardened above 45 HRC the game changes again: you are into hard machining, with ceramic or CBN inserts, low speeds, very fine feeds and frequently dry, because coolant causes thermal shock at the edge.
Stainless: low conductivity and work hardening
Austenitic stainless — 304 and 316, the two most common — is the material that eats the most tools in an ordinary shop, for two reasons that reinforce each other.
It conducts heat badly. In carbon steel a good share of the heat leaves with the chip and through the part. In stainless it stays at the cutting edge. The tool runs much hotter at the same speed, which is why the Vc has to come down.
It work hardens. This is what really breaks tools. If the edge passes through the material without cutting — because it is worn, because the feed is too fine, or because the tool rubbed on a badly made entry — the surface hardens. The next pass meets a layer harder than the original material and the tool is destroyed in seconds.
Hence the three rules of stainless:
- Enough feed, and a steady one. Never "stroke" the material. The edge has to really cut on every pass.
- Do not stop the feed with the tool in the cut. No pauses, no hesitation, no air passes that still touch.
- Change the tool early, not late. A worn edge in stainless does not give you a bad part, it gives you a broken tool and sometimes a lost part. The signs that it is time are in tool wear.
Add coolant under pressure and well aimed — in stainless, pointing the jet properly matters more than flow rate — and inserts with a positive geometry and a keen edge. On which insert to pick for turning, there is how to choose a turning insert.
The chip tells you everything
You do not need instruments to know whether you are on track. The chip is the best indicator there is, and it is free:
- Spring-shaped chips or long strings: the feed is low or the geometry is not breaking the chip. Risk of it wrapping and scoring the part.
- Dark blue or purple chips: too much heat. Drop the Vc or improve the coolant.
- Chips like dust or very small fragments: a worn edge, or material harder than you expected.
- Comma-shaped chips, straw-coloured, coming off on their own: you are on track.
And a fourth case: mixed jobs
If a part goes from roughing in steel to a fine finish, or the order mixes aluminium and steel in the same batch, what you pay for is the change of parameters and tooling. A shop with plenty of tool magazine positions and in-machine gauging does this without stopping production; a small one does it just as well but takes longer. It is worth asking before you place the order. The workshop directory lists the processes and materials of each shop so you can filter for what you need.
Need a part machined? The directory of Spanish machine shops lists more than 700 workshops across Spain with their processes, materials and certifications verified, filterable by province. And if you are learning, the CNC Path takes you from G-code to your first part.