Practical Handbook for Machining Material Selection

A practical decision framework for material selection

1. Function first: define hard requirements such as strength, rigidity, wear resistance, corrosion resistance, insulation, or thermal conductivity first, and avoid sacrificing key performance to save money.
2. Precision and surface: high fit precision, thin walls, deep cavities, or mirror-finish requirements will significantly affect material selection and the post-processing path.
3. Batch size and lead time: material availability, tool wear, and fixturing methods differ greatly between single-piece prototyping and mass production.
4. Cost accounting: do not look only at the material unit price; include machining hours, tool consumption, scrap rate, and post-processing as well.

The logic of combining materials and post-processing

Frequently Asked Questions (FAQ)

Where exactly does material cost go

• Material unit price: the blank cost of aluminum alloys and engineering plastics is usually lower than that of stainless steel and titanium alloys, but thin-walled or large parts require more allowance, so actual material usage will be higher than the net weight.
• Machining hours: materials with high hardness and high toughness require conservative cutting parameters, lengthening both roughing and finishing time, which is the main source of quotation differences.
• Tools and losses: difficult-to-machine materials wear tools faster, and in small batches the tool cost allocated per piece is more noticeable.
• Post-processing and logistics: processes such as anodizing, passivation, sandblasting, and heat treatment add extra cycle time and costs, and cross-factory collaboration also brings transportation and waiting time.
• Scrap and rework: once problems such as thin-wall deformation, poor chip evacuation in deep cavities, or mirror-surface scratches occur, rework costs often exceed the price difference of the material itself.

Common combinations of materials and post-processing

• 6061 aluminum alloy + anodizing: balances lightweighting, corrosion resistance, and appearance, suitable for parts such as enclosures, brackets, and panels.
• 7075 aluminum alloy + hard anodizing or spraying: higher strength, suitable for load-bearing structural parts, but corrosion resistance is relatively weaker, so surface treatment is more worth investing in.
• Stainless steel + passivation or sandblasting: suitable for humid, clean, or food-contact scenarios; passivation helps restore the passive film damaged by cutting.
• Engineering plastics + deburring or annealing: plastic parts usually do not receive metal-style surface treatment; the focus is on controlling burrs, internal stress, and dimensional stability.
• Parts requiring electrical conductivity or heat dissipation: avoid insulating coatings and prioritize metal substrates with conductive treatment.

The most common pitfalls in material selection

• Selecting material only by strength: ignoring toughness, corrosion resistance, and machinability, resulting in parts that are made but cannot be assembled or have a short service life.
• Treating the grade as the only standard: the same grade in different conditions (such as different heat treatment or supply conditions) can differ significantly in performance, so the condition requirements should be clearly stated on the drawing.
• Underestimating the difficulty of thin walls and deep cavities: these structures demand high material rigidity and cutting stability, so deformation risk must be considered when selecting materials.
• Reversed post-processing sequence: finishing before heat treatment may cause dimensional out-of-tolerance due to deformation, so the process sequence needs to be fixed during the process review stage.
• Ignoring availability: niche materials or special specifications may have long lead times, so supply availability must be confirmed during the prototyping stage.

Checklist for self-review before prototyping

• Are the key functional indicators clearly defined (strength, corrosion resistance, insulation, thermal conductivity, wear resistance, etc.)?
• Does the drawing specify the material grade and temper, rather than just "aluminum alloy" or "stainless steel"?
• Are there features sensitive to material and process, such as thin walls, deep cavities, or mirror finishes?
• Are the post-processing requirements compatible with the material, and is the process sequence reasonable?
• Will batch size and lead time affect material availability and processing methods?

Visual reference for material selection