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The make vs. buy decision looks like a cost comparison but it's really a strategic question about where your time, capital, and capability should be focused. Unit price is just the starting point. Total cost of ownership, core competency, and supply security tell the real story.
The purchase price of a component or system is often the smallest part of what it will cost over its life. Lifecycle cost analysis (LCA) accounts for acquisition, operation, maintenance, downtime, and disposal — revealing total cost of ownership and enabling comparisons that unit price alone cannot make.
The project management triangle — speed, quality, cost — applies to engineering design decisions as much as it does to project schedules. A structured trade-off analysis makes the implicit explicit, prevents decisions by default, and gives teams a shared basis for revisiting choices when conditions change.
The material selected at the design stage locks in cost, weight, machinability, and supply reliability for the life of the product. A bracket specified in mild steel when aluminium would perform equally well at half the weight carries that decision into every unit ever manufactured. Changing it later means new drawings, new tooling quotes, new supplier qualifications, and a production disruption. The leverage on material selection is highest at concept stage and drops sharply as design matures.
The previous post examined whether you should make a component or buy it. This post steps back to the earlier decision: what should it be made from? The method is systematic — define what the material must do, screen the candidates by function, rank by cost and availability, then verify against the complete specification. This avoids both the common failure of specifying by habit and the opposite failure of over-engineering the selection into a months-long analysis.
Material Selection Is Locked In Early
At 20% design completion, material choice is still open. At 70%, tooling is quoted and suppliers are engaged. Changes after that carry significant cost and schedule risk. Run material selection as a deliberate step at concept phase, not as a default that hardens by inaction.
Most materials in most products are chosen by one of three processes: habit (the last product used this), convenience (the supplier stocks it), or imitation (a competitor uses it). None of these is a selection process — they are defaults. Defaults produce adequate results when the design is a close variant of prior work. They produce poor results when loading, environment, volume, or cost targets differ from the precedent.
Material selection is a design decision because:
It determines unit cost. Material cost plus processing cost (machining, forming, welding) dominates manufactured part cost. The same geometry in aluminium versus steel has different raw material cost, different cycle time, and different tooling wear.
It determines design space. Choosing a material before completing geometry analysis constrains the design unnecessarily. Some geometries only make sense in certain materials; locking in the material early forecloses lighter, simpler alternatives.
It drives supply chain risk. A material specified to a proprietary alloy, a single country of origin, or a minimum order quantity that exceeds typical run size creates supply risk that persists indefinitely.
A structured selection process takes two to four hours at the concept stage and eliminates a class of problems that would otherwise surface during production ramp-up, cost reviews, or supply disruptions.
The method attributed to Michael Ashby at Cambridge — and formalised in the Cambridge Engineering Selector (CES) software — decomposes material selection into four sequential steps.
Express what the material must do in measurable terms. Performance requirements fall into several categories:
Write these as constraints (must satisfy) and objectives (optimise where possible). Constraints eliminate candidates; objectives rank the survivors.
Apply the hard constraints first. If a component must operate at 200°C, eliminate all materials with maximum service temperature below 200°C. If it requires welding, eliminate materials with poor weldability. If corrosion resistance is required without coating, eliminate unprotected carbon steels.
This step narrows a field of thousands of candidate materials to a manageable shortlist of ten to thirty. The screening does not need to be precise — a factor of two safety margin on mechanical limits is appropriate at this stage.
Within the surviving shortlist, rank candidates by a combined metric of performance per unit cost. Ashby's method uses performance indices — ratios of material properties that correspond to specific loading geometries and objectives. The most commonly used are:
These indices are derived from the physics of the loading case. They are not empirical rules of thumb — a material with a higher index genuinely outperforms one with a lower index for that specific objective.
The top two or three candidates from the ranking step are verified against the complete specification. This includes:
Do Not Over-Specify
Over-constrained material specifications are common and expensive. A tolerance that requires hardened tool steel when annealed grade performs adequately, or a corrosion resistance rating that requires a high-alloy stainless when 304 is sufficient, increases cost and lead time on every unit produced. Each specification requirement should be traceable to a specific performance need.
Performance indices make the trade-off between material properties visible and quantitative. Consider a structural bracket loaded in bending. The objective is minimum mass at fixed stiffness. The relevant index is:
M = E^(1/2) / ρ
where E is Young's modulus (GPa) and ρ is density (kg/m³). A material with a higher value of M gives more stiffness per unit mass.
| Material | E (GPa) | ρ (kg/m³) | M = E^(1/2)/ρ × 10³ |
|---|---|---|---|
| Mild steel (1020) | 200 | 7,850 | 1.80 |
| 6061-T6 aluminium | 69 | 2,700 | 3.08 |
| Titanium (Ti-6Al-4V) | 114 | 4,430 | 2.41 |
| CFRP (unidirectional) | 135 | 1,600 | 7.27 |
| Polycarbonate | 2.4 | 1,200 | 1.29 |
Aluminium outperforms steel on this index by 71%. For a stiffness-limited bracket, switching from steel to aluminium produces a lighter component even though aluminium requires more cross-sectional area to achieve the same stiffness — the density reduction more than compensates.
CFRP dominates on this index, which explains its prevalence in aerospace. But CFRP's raw material cost is fifteen to thirty times that of aluminium per kilogram, and processing cost is high. The index tells you the best physical performance; cost per unit of performance determines what makes economic sense at a given volume.
A UAV structural bracket must carry 400 N at a 75 mm moment arm. The design requirement is maximum stiffness-to-weight ratio. The budget constraint is under $15 per unit at 200 units per year. Three candidates emerge from screening:
6061-T6 Aluminium
4130 Chromoly Steel
Ti-6Al-4V
Decision: 6061-T6 aluminium. Lightest option, well within budget, fastest machining, lowest supply risk. Chromoly steel saves $0.16 in material but costs $2.42 more per unit after machining and produces a heavier part. Titanium exceeds the cost constraint and offers no functional advantage for this application.
Include Machining Cost in the Comparison
Raw material cost per kilogram is a poor proxy for part cost. Difficult-to-machine materials (titanium, Inconel, hardened steels) consume more machine time and tool wear than the material price differential suggests. Always estimate machining time before comparing total unit cost.
A contract manufacturer was producing a pneumatic manifold block from 316L stainless steel. The specification had been inherited from a previous product in a corrosive marine environment. The current application was an indoor automation panel with dry instrument air and no corrosive exposure.
Material review:
The corrosion resistance requirement did not apply to the current installation. Screening against the actual environment (dry indoor air) allowed aluminium to pass the corrosion constraint. Ranking by cost:
At 320 units per year, the specification change saved $13,376 annually. The engineering change required redrawing one part, updating the material specification, and qualifying a second supplier. Total engineering cost: $2,400. Payback period: 68 days.
The lesson is not that stainless is always wrong — it is that specifications inherited from a different application may carry requirements that no longer apply. Always validate the requirement before retaining the material.
A material that performs well and costs less is still a poor choice if it is difficult to source. Availability has several dimensions:
Form and size: Is the material available in the stock form your process requires (sheet, rod, tube, extrusion, casting)? Custom extrusions or non-standard bar stock often carry significant MOQ and lead time penalties.
Supplier diversity: Can you source from at least two independent suppliers? Single-source materials carry the same supply risk as single-source components. Commodity materials (1020 steel, 6061 aluminium, 304 stainless, ABS, polycarbonate) are available from many distributors. Specialty alloys, proprietary grades, or materials with limited production capacity are higher risk.
Lead time: Long-lead materials force higher safety stock. A 12-week lead time material requires carrying 12+ weeks of inventory at each point in the supply chain. The carrying cost of that inventory is a real cost that should be included in the material comparison.
Price stability: Commodity metals (steel, aluminium, copper) track published indices and are relatively predictable. Rare earth materials, specialty polymers, and proprietary alloys can have volatile pricing and supply constraints that make long-term cost modelling unreliable.
Specify to Available Stock Sizes
Wherever possible, design your geometry around standard stock sizes rather than requiring custom dimensions. A bracket designed around 3 × 25 mm flat bar uses commodity stock with immediate availability from any metal distributor. A bracket requiring 3.5 × 22 mm flat bar may need to be sliced from plate, increasing cost and lead time.
Document the selection rationale. Record the performance requirements, the candidates considered, and the reason the selected material was preferred. When the product is revised, the team will know whether the original logic still applies or whether conditions have changed enough to reconsider.
Validate inherited specifications. When modifying an existing design, check that each material requirement is still relevant to the current application. Requirements migrate between products and persist long after their original context has changed.
Use commodity materials by default. The engineering case for a specialty material must be made explicitly. Commodity materials (6061 aluminium, 1020/1045 steel, 304 stainless, ABS/polycarbonate/nylon) are available globally, have well-understood properties, and can be sourced from multiple suppliers. Specialty materials impose cost, lead time, and supply risk that must be justified.
Consider the full processing cost. Material cost per kilogram is a starting point, not a conclusion. Machinability, formability, and weldability directly affect processing time and tool consumption. A material that costs 40% more per kilogram but machines three times faster may be cheaper per finished part.
Qualify materials before they are on the critical path. If a novel material is under consideration for a new product, obtain samples and run processing trials before committing it to the design. Material qualification that happens in parallel with tooling design is much less disruptive than qualification forced by a supply problem at launch.
Material selection determines what a component is made from. The next decision is how to balance the competing pressures every project faces — speed, quality, and cost. Trade-off analysis gives you a structured way to make those decisions explicitly, document them, and communicate them to stakeholders rather than arriving at them by accident.
