Start with the job, not the datasheet

Every material decision starts with a question that has nothing to do with materials: what does this part actually have to do? A bracket that holds a sensor in a workshop jig has a very different set of demands from a spindle spacer running at 8,000 rpm, or a wear pad sliding against a hardened rail. Before you open a stockholder's catalogue, write down four things: the loads the part will see, whether it needs to resist wear, corrosion or heat, how many you need, and what your machine can realistically cut.

That last point trips up a lot of small workshops. A material that machines beautifully on a 15 kW machining centre with through-spindle coolant can be miserable on a manual mill or a small benchtop CNC. If your spindle is modest, your vice is small and your coolant is a squirty bottle, machinability matters more than a marginal gain in tensile strength you will never use.

Steels: the default workhorse

For most structural and mechanical parts, steel is still the sensible starting point. It is stiff, strong, weldable in many grades, and predictable. Two families cover the majority of small-batch work.

• Low-carbon free-machining steel (EN1A, 11SMn30 or similar): the easiest steel you will ever turn. It produces short chips, takes a fine finish and is cheap. Use it for spacers, bushes, brackets, jig plates and anything that is not highly stressed. It does not harden well and welds poorly, so keep it for parts that are purely mechanical.
• Medium-carbon steel (EN8, C45): noticeably stronger and tough enough for shafts, pins and load-bearing brackets. It machines well enough in the annealed condition and can be flame or induction hardened afterwards if a surface needs wear resistance.

Stainless is worth mentioning because it is often specified for the wrong reasons. Grades such as 303 or 304 are corrosion resistant, but 303 machines far more freely than 304, which work-hardens aggressively if you rub it rather than cut it. If you need stainless, plan for slower feeds, sharp tooling and a rigid setup — or accept that the part may cost twice what the same shape in EN1A would.

Aluminium alloys: light, fast and forgiving

Aluminium is the natural choice when weight matters, when you need good thermal conductivity, or when you simply want to remove metal quickly. It cuts at high speeds, accepts sharp detail and is far kinder to small machines than steel.

• 6082-T6: the general-purpose structural grade for UK workshops. Strong, corrosion resistant and widely stocked in plate and bar. Ideal for brackets, housings, manifolds and machine frames.
• 7075-T6: significantly stronger, closer to mild steel in some respects, but more expensive and harder to source in small quantities. Reserve it for aerospace-style parts or where stiffness per kilogram really counts.
• 2011 or 2007: free-machining grades that produce lovely finishes and break chips cleanly. Excellent for small turned parts, but poor corrosion resistance and not weldable.

Remember that aluminium is roughly a third of the stiffness of steel. A part that is strong enough may still flex too much, so if deflection is your problem, a thicker aluminium section often beats a change of alloy.

Plastics: when metal is overkill

Plastics are frequently dismissed as a compromise, which is a mistake. For electrical insulation, chemical resistance, low friction or light weight, an engineering plastic can outperform metal and machine in a fraction of the time.

• Acetal (POM): the machinist's favourite. Dimensionally stable, tough, low friction and superb for bushes, rollers, spacers and gears at light loads. It cuts like a dream and holds tolerance well.
• Nylon (PA6, PA66): tougher and more abrasion resistant than acetal, but it absorbs moisture and moves. Great for wear strips and impact parts, poor for tight tolerances.
• PTFE and PET: PTFE for chemical resistance and very low friction; PET for good stiffness and electrical properties at a modest price.

The catch with plastics is heat. They expand roughly five to ten times more than steel for the same temperature rise, and they soften if you push the cutting speed. Take light passes, keep the tool sharp and expect to deburr by hand.

Machinability, cost and small-batch reality

In a small workshop, the cheapest material is rarely the cheapest part. What you are really buying is cycle time, tool life and the risk of scrapping a component halfway through. A free-machining grade that costs 20 per cent more per kilogram but halves your machining time is almost always the better deal.

Availability matters just as much. Buying a full 3-metre length of an exotic alloy to make four parts ties up cash and shelf space. Where you can, standardise on two or three grades you keep in stock — a free-machining steel, a 6082 aluminium plate and a bar of acetal will cover an enormous range of work.

Also price the finishing early. Anodising, zinc plating, blacking and heat treatment all add lead time and can distort thin parts. If a component needs a hard-wearing surface, it is often cheaper to choose a material that already has the properties you need than to add a process later.

A quick decision route

Work through it in this order and most choices make themselves.

• Needs stiffness, strength or weldability at low cost? Choose EN1A or EN8.
• Needs low weight, good finish or fast machining? Choose 6082-T6.
• Needs insulation, chemical resistance or low friction? Choose acetal, nylon or PTFE.
• Needs corrosion resistance? Consider 303 stainless, but budget for slower cutting.
• Still unsure? Machine a test piece in the free-machining equivalent and prove the design before committing to the expensive grade.

Get the material roughly right, make one part, measure it, and let the result guide the rest of the batch. That approach beats agonising over datasheets every time.