From 11SMnPb30 to titanium Grade 5 – which material for which part.

Material selection is, alongside tolerancing, the second big lever for the unit price of a turned part. If you blanket-specify V4A (1.4571) when V2A (1.4305) is enough, you pay unnecessarily. If you use titanium because the part is supposed to be light, you often overlook that a heat-treated aluminium alloy does the same job. This article walks through the most important material groups for turned parts.

Free-cutting steels are the workhorses of CNC turned-part manufacturing. 11SMnPb30 (former name: 9SMnPb28, material no. 1.0718) has been the standard for decades for parts without corrosion requirements. Very good machinability, good surfaces, low material price. Disadvantage: it contains lead – unsuitable for medical or food applications. Alternative: 11SMn37 (1.0736, lead-free).

Stainless steels divide by application. 1.4305 (V2A, AISI 303) is the standard for parts without aggressive media – good machinability, corrosion-resistant in non-extreme environments. 1.4571 (V4A, AISI 316Ti) is the choice for aggressive media (chloride, acid, seawater) – harder to machine, significantly more expensive as a material. Rule of thumb: if no chloride and no acid are involved, 1.4305 is enough – V4A costs a premium with no functional gain.

For medical parts with implant contact, material selection is strictly regulated. Standard: 1.4441 (for temporary implants), titanium Grade 5 (for permanent implants), PEEK for radiolucent requirements. Here price is not the deciding factor, but biocompatibility and documentation.

Brass and copper alloys are the choice for electrical contacts, hydraulic components with sliding function and parts in areas with thermal requirements. CuZn39Pb3 (material no. 2.0401) is the standard. For highly loaded plain bearings: CuSn8 (bronze, 2.1030). The material price is high, but often unrivaled for the function.

Aluminium alloys are rarer in the turned-parts sector than often assumed. AlCu4MgMn (2017A) or AlZnMgCu1,5 (7075) are the typical choices for highly loaded, lightweight parts. Advantage: weight. Disadvantage: susceptibility to corrosion – problematic in industrial environments without anodizing.

A pragmatic recommendation from 77 years of practice: involve your supplier early in the material selection. We regularly see customers fixated on a particular material number even though a cheaper alternative fulfills the same function. A 30-minute material review in the design phase often saves 10 to 20 percent in material costs on larger series – with no loss of function.

The machinability index relates a material's workability to the reference free-cutting steel 11SMnPb30, which is set at 100 percent. This index translates directly into machine time and thus into unit price. 1.4305 reaches only about 50 to 60 percent depending on the batch, austenitic stainless steel such as 1.4404 often below 45 percent. Concretely: a turned part that runs in 30 seconds in free-cutting steel quickly needs 60 to 70 seconds in 1.4404, because feed rate and cutting speed must be reduced.

Plastics are a turning material in their own right, not a stopgap. POM-C is dimensionally stable, machines well and is the first choice for precision parts such as plain bearings or nozzles; PA66 is tougher but tends to absorb moisture and thus change dimension. PEEK carries loads up to over 250 degrees Celsius, PTFE is chemically almost inert but creeps under load. In turning, the low thermal conductivity is critical: the cutting heat stays in the part, so sharp tools and adapted clamping prevent distortion. Sensible instead of metal wherever weight, electrical insulation or freedom from corrosion matter.

Material availability is one of the underestimated selection criteria. Stock grades such as 11SMnPb30, 1.4305 or CuZn39Pb3 are available at short notice in common diameters, while special grades or unusual dimensions trigger a special procurement with several weeks of lead time. Add minimum order quantities at the material supplier, which noticeably raise the material share per part for small series. If you factor the material into project planning early, you avoid a supposedly minor material question later becoming the schedule bottleneck for the entire series.

The form and quality of the semi-finished stock is the prerequisite for tight part tolerances. Drawn round bar exhibits diameter scatter and internal stresses; peeled or ground bar stock, by contrast, offers tight diameter tolerance and a defined surface. Straightness is decisive: bent bar stock wobbles in the chuck and in the guide bushing, which directly limits the achievable roundness and dimensional accuracy in Swiss-type turning. For parts in IT6 or finer, peeled material is therefore mandatory, not optional, since the stock quality caps the achievable part tolerance.

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