Carbon and alloy steels
- S235
- C45
- 42CrMo4
- 16MnCr5
- 11SMn30
The backbone of general engineering. S235 is a plain structural steel — cheap and forgiving. C45 can be quenched and tempered. 42CrMo4 usually arrives already in the quenched-and-tempered condition, with uniform hardness through the section: it turns predictably, but demands tougher tooling than C45 and punishes a dull edge. 16MnCr5 is a case-hardening steel — machined soft and hardened only at the surface after carburising, so final dimensions are cut before heat treatment with an allowance for grinding. 11SMn30 is a free-cutting steel with sulphur: it cuts easily with short chips and suits large runs of small parts, but it cannot be welded or hardened.
The grades listed are typical of current production — to be confirmed with the client.
Stainless steels
- 1.4301 / 304
- 1.4404 / 316L
- 1.4542 / 17-4PH
Austenitic grades (1.4301, 1.4404) are non-magnetic, tough and poor conductors of heat — the heat stays at the cutting edge instead of leaving with the chip. They want a firm feed and generous coolant; too light a pass work-hardens the surface and makes it harder to cut than it was. 1.4404 contains molybdenum and so resists chlorides better. 1.4542 (17-4PH) is a precipitation-hardening martensitic steel: machined in the solution-annealed condition, then hardened at low temperature with little distortion — which is why it is chosen where both strength and close tolerances are required.
Tool and hardened steels
Above roughly 55 HRC, cutting with carbide inserts stops being economical: the tool wears faster than the part advances. The shape is therefore produced before heat treatment, with an allowance, and the final dimensions afterwards — by grinding and wire EDM, where hardness no longer matters. On such parts the sequence of operations decides the result more than the speed of any single one, and distortion from quenching is absorbed by the allowance left behind rather than corrected later.
Machined by grinding and wire EDM after heat treatment.
Aluminium
- EN AW-6082
- EN AW-7075
- EN AW-2024
It cuts fast, but the result depends on fixturing stiffness and on the stresses locked into the stock: a thick plate roughed on one side only will bow before you get to finish it. EN AW-6082 is the general-purpose alloy, weldable and anodisable. 7075 and 2024 are strength alloys — they machine well, resist corrosion less well and are not routinely welded. In all of them the surface owes more to edge geometry and chip evacuation than to the alloy itself.
Brass, bronze, copper
Brass cuts easily with short chips and holds fine dimensions — hence the classic choice for small valve and fitting parts. Bronze wants genuinely sharp edges, as it tends to smear onto the tool. Pure copper is the hardest of the three to finish: soft, and it drags under the edge. All three conduct heat well, so the part stays cool while being machined but expands noticeably in the hand — where tolerances are close, the measuring temperature belongs on the drawing.
Engineering plastics
They machine fast, but dimensions behave differently from metal: thermal expansion is several times greater — for POM, close to ten times, and PA6 takes up moisture from the air and keeps changing size for days after machining. POM is the most stable of the three and the right choice for close tolerances. PEEK withstands heat and chemicals but is expensive and needs sharp tools, since it melts readily at the edge. For all of them, tolerances and measuring conditions belong on the drawing rather than being assumed.
