§1Not like steel
Steel is the reference for cutting data, but most of what a shop machines is not steel. The non-ferrous metals and the non-metals each depart from steel’s behaviour in ways that change speeds, tool geometry and technique.
Two properties drive the differences. The non-ferrous metals — aluminium, copper, brass — are far softer than steel, so they cut with a fraction of the force and tolerate far higher speeds (§2), but their softness and ductility invite a built-up edge (§3). The non-metals — plastics and composites — are soft too, but they are poor conductors and either melt (plastics, §5) or abrade and delaminate (composites, §6). The result is that each material class wants its own approach: high speed and sharp positive tools for the ductile metals, controlled speed and keen tools for plastics, and hard, sharp, delamination-aware tooling for composites. This page takes them in turn.
Contents§2Aluminium at speed
Aluminium is the ideal material to machine fast: soft, low-force and free-cutting, it runs at many times steel’s speed and removes metal prodigiously.
Aluminium cuts at roughly 10 times the speed of steel — around 300 m/min against steel’s 30 with high-speed-steel tooling, and far higher with carbide — because it is soft and sheds heat well. Its specific cutting force is only about 700 N/mm² against steel’s 2500, roughly a third, so the same cut draws far less power and force. At ten times the speed and the same feed and depth, aluminium’s metal removal rate is about ten times steel’s — which is why aluminium is the material of choice where fast, high-volume machining matters. The tooling wants sharp, highly positive rake and polished, open flutes: aluminium’s soft, sticky chip must be sheared cleanly and cleared fast, or it welds to the tool (§3).
§3Built-up edge
The characteristic trouble of machining ductile metals at the wrong speed is the built-up edge — work material cold-welding onto the tool tip, then breaking away and ruining the finish.
At low and moderate cutting speeds a soft, ductile metal such as aluminium or mild steel can weld itself onto the cutting edge under the heat and pressure of the cut, forming a built-up edge (the hero). This lump then acts as the cutting edge for a moment before it breaks off — carrying away part of the tool and depositing on the work — so it cycles on and off, leaving a torn, rough surface and wearing the tool. The cures follow from the cause: cut faster, since above a threshold speed the chip flows too quickly to weld; use a sharper, more positive rake that shears rather than crushes; and use lubrication to stop the chip sticking. Built-up edge is why aluminium is machined fast and sharp, not slow — the opposite of the caution a hard material would call for.
Contents§4Copper and brass
Copper and its alloys split sharply: free-cutting brass is one of the easiest materials to machine, while pure copper is one of the stickiest.
Free-cutting brass, with a little lead, machines superbly — it cuts cleanly at high speed with low force, breaks into short chips and leaves an excellent finish, which is why turned fittings and electrical parts are so often brass (the materials pages). Pure copper, by contrast, is soft, extremely ductile and gummy: it tears rather than shears, smears over the tool, forms long stringy chips and a built-up edge, and takes sharp, highly-positive, polished tools and generous lubricant to machine at all cleanly. Bronze sits between, generally machining well. The rule mirrors aluminium: the free-cutting, leaded alloys are a pleasure and run fast, while the pure, ductile metals fight back and need the sharpest tools — so where a choice exists, the free-machining grade is specified for anything to be turned in quantity.
Contents§5Plastics
Plastics cut easily but melt easily, so the governing concern is heat: a plastic is a poor conductor and softens at low temperature, so the cut must stay cool.
Because a plastic cannot conduct heat away — the opposite of a metal — the heat of cutting stays at the tip and can melt or char the surface, so plastics are cut with sharp tools (to minimise rubbing heat), high positive rake, and often moderate speed with light cuts so the material shears and clears before it softens; air or coolant helps carry heat away. The materials pages’ warnings apply directly at the tool: plastics’ high thermal expansion means a part heats and grows as it is cut, so it can bind on the tool and finish undersize once cool, and their low stiffness means thin sections deflect away from the cutter and must be supported. Sharp tools, controlled heat and support for flimsy sections are the essentials — cut a plastic like a soft metal and it melts and moves rather than cutting clean.
Contents§6Composites
Fibre composites such as carbon-fibre laminate are the hardest of this group to machine well — abrasive to the tool and prone to splitting along their layers.
A composite is fibres in a resin, and both fight the tool. The fibres are abrasive — carbon and glass fibres wear an ordinary edge rapidly, so composites are cut with carbide or diamond tooling kept very sharp. And the layered structure delaminates: as the tool exits a hole or an edge, it can lift and separate the surface plies, leaving splintered, frayed damage, so cutting is arranged to press the plies together rather than peel them — sharp tools, backing support behind the exit, light feed, and sometimes cutting from both sides. Heat must be watched too, since the resin, like any plastic, softens. Composites reward the same discipline as plastics — sharp, cool, supported — with the extra demands of a hard, abrasive, splittable material, which is why machining them is a specialised skill rather than a scaled-down metal cut.
Contents§7Quick reference
The working core of the page on one card rack.
Aluminium
~10× steel speed · ~⅓ force
sharp positive rake, open flutes
Built-up edge
low speed + ductile → welds
cure: faster, sharper, lube
Copper/brass
free-cutting brass easy
pure copper gummy
Plastics
melt-prone · sharp, cool, support
Composites
abrasive · carbide/diamond
delaminate at exit