Choosing a Material for Machined Parts: Aluminium, Stainless, Brass and Plastics

The same drawing, switched to stainless steel, comes back at more than double the price. Switched to engineering plastic, the price drops — and three months after installation the dimensions have moved. Neither case is the supplier's fault: the moment the material is chosen, tolerance, finish, lead time and cost are already fixed.

This is not a material handbook. It covers what four common materials actually do on a machine: aluminium, stainless steel, brass and engineering plastics. The focus is where buyers and design engineers really get caught — which tolerances the material grants you, which costs hide in the process, and which four lines on a drawing cause rework when they are missing.

1. Decide the function first, the material second

Material decides more than whether the part is strong enough. It decides whether the drawing can be held repeatably, whether the finish can be applied at all, and where lead time and price land. Choosing the other way round — pick a material that sounds advanced, then let the process catch up — usually means paying twice for one drawing: once for machining, once for rework and sorting.

The order that works is: load and environment first, which sets the material family; then fit and precision, which sets the tolerance band; then appearance and corrosion, which sets the finish. Reverse that order and every later step is compensating for the earlier one.

2. What the four materials really do on a machine

The table below is the rule of thumb we quote against. Relative cost uses aluminium as 1x and covers material, machining and a standard finish for the whole part; it moves with batch size, geometry and tolerance band, and it is a planning reference rather than a pricing formula.

The row most often misread is brass. It machines best and holds tolerance most easily, but the material is expensive per kilo and dense, so the finished part is not cheap. Plastics are the mirror image: cheap stock and light cutting, yet the moment the tolerance tightens to ±0.01 mm the scrap rate swallows the saving.

3. Aluminium: fast, but three things must be said up front

3.1 Thin walls move, and no amount of care stops it

Aluminium has roughly a third of the elastic modulus of steel. Below about 1 mm wall thickness, clamping force and cutting force both push the wall away and it springs back when the fixture is released. The workable answer is lighter radial cuts in layers, symmetric material removal, and a stress-relief step between roughing and finishing — not an operator cutting more gently.

3.2 Anodizing has a dimensional cost

An anodic layer grows outward, typically fifteen to twenty-five micrometres, which lands on a mating face as 0.01–0.03 mm. Bores, journals and threads therefore need either a drawing note stating "dimension after anodizing" or a clear instruction that the face is masked. Otherwise the part simply will not fit and has to be reworked.

3.3 Threads in aluminium are weak

A tapped thread in aluminium with too little engagement strips easily, especially where the joint is opened and closed. There are usually three structural answers: increase engagement length, use a thread insert, or convert to a through hole with a nut. Stating the number of assembly cycles on the drawing is more useful than writing "thread must be strong".

4. Stainless steel: the cost is in the tool and the time

Stainless is expensive, but only up to a point; what really lifts the quote is cutting it. Low thermal conductivity and strong work hardening mean that a tool rubbing instead of biting hardens the surface layer, and the edge dulls within minutes. For the same geometry, stainless needs more passes, more tool changes and more coolant than aluminium, and machining time typically runs 1.5 to 2 times longer.

The other point that gets skipped is passivation. Machining leaves free iron on the surface, and untreated it pits in damp conditions. Passivation is not optional — it affects corrosion resistance more than the choice of grade does. If the drawing only says "stainless steel", the supplier will use the most common grade and condition and usually land where you wanted; if it names a grade but not a condition (solution annealed, cold drawn, annealed), both the quote and the delivered part can drift.

5. Brass: best machinability, two other bills to pay

Free-cutting brass produces clean chips and a bright surface, so small parts come off the machine looking good and holding tolerance. The price comes in two places. First, stock cost and density: the same volume weighs about three times as much as aluminium. Second, leaded grades are restricted in drinking water, children's products and some export markets, so the compliance requirement has to be settled early and lead-free brass specified where needed.

Brass is also soft on visible faces and marks if parts knock together between operations. Appearance parts usually need individual packing or protective film, and that cost is easy to leave out of a quote.

6. Engineering plastics: dimensional stability is the whole problem

Almost nothing about a plastic part goes wrong during cutting; it goes wrong afterwards. Polyamides absorb moisture and swell, so a change in ambient humidity moves the dimensions. All plastics creep under sustained load. Excessive clamping force makes the part spring back once released. Those three facts mean plastic tolerances cannot be written the way metal tolerances are.

What works: leave margin on critical fits, anneal after roughing, let the part stabilise at controlled temperature before finishing, and state the measurement condition — free state, and how long after machining. A drawing that says ±0.01 mm without a measurement condition forces the supplier into the most conservative quote, and nobody is happy with the result.

7. Where the price of the same tolerance differs by material

A tolerance is an instruction that applies to the whole drawing. Writing ±0.01 mm everywhere means every operation runs as a finishing operation and cost rises in multiples, while assembly usually needs tight tolerance on only two or three fits. The rule holds for all four materials, but it is amplified most on stainless and plastics.

• Stainless steel: tight tolerance means more finishing passes, more tool changes and more inspection — the largest cost increase of the four.
• Plastics: tight tolerance means annealing, temperature control, controlled clamping and possibly a dedicated fixture, which does not amortise on small batches.
• Aluminium and brass: tight tolerance mostly adds machine time, which is manageable, but it should still be specified feature by feature.

8. Four things the drawing must state

Leave these four out and the quote will be wrong and the delivered part will be argued about. They are material-related and far more consequential than notes like "finish as required".

1. Material condition and heat treatment: solution annealed, annealed or T6 changes both machinability and final strength. Without a condition, the supplier ships from stock and the result is uncontrolled.
2. Stock form: plate, bar, forging or casting differ in cost and in grain direction. The same drawing in plate or in forging can differ by thirty percent.
3. Finish and coating thickness: whether the process affects fits, which faces must be masked, and the thickness range. Those three sentences decide whether stock has to be left on.
4. Whether substitute material is allowed: when lead time is tight, whether an equivalent grade may be substituted and who signs it off. Writing this removes an email round trip.

9. When to change material

Changing material is rarely about saving money; it is forced by three signals: yield will not come up, lead time will not be met, or the finish cannot be applied. When one appears, do not switch immediately — first check whether the tolerance callouts, the stock form or the finish requirement are the real cause. Fix those and many "we must change material" conclusions disappear.

If a change is genuinely needed, recalculate four things at once: whether the fits have to move with it, whether the finish still applies, whether inspection has to change, and what the cost curve looks like at volume. Changing only the material usually moves the problem from one operation to another rather than solving it.

Material is not better because it costs more, and tolerance is not better because it is tighter. Both are cost, and both take effect the moment they are written on the drawing.

Closing

The right order is function, then material, then tolerance, then finish — not the reverse. State those four on the drawing and the quote usually settles in one round, with far less rework.

Our approach is straightforward: we read the drawing and choose the material and process it needs, and we do not redesign a part to suit a preferred material; we quote within 24 working hours with a material recommendation and a plan for the critical features; the shop runs to ISO 9001 and first articles ship with an FAI report. If you are unsure about a material or a tolerance, send the drawing over and we will give you a recommendation before we talk price.