Manufacturing processes

Why it matters

A part's cost, strength, accuracy and lead time depend as much on how it is made as on its shape. The same bracket might cost a few dollars as a stamped steel part in a run of 100,000, or many times that machined from solid aluminium as a one-off. Designers who understand processes choose sensible ones early and avoid designing parts that are expensive or impossible to make.

The main process families

Machining (subtractive). Material is cut away from a solid block or bar: milling, turning (lathe), drilling, grinding. CNC machines follow programs generated by CAM software.

• Strengths: tight tolerances (around ±0.05 mm is routine on many features, and much tighter is possible at a cost), good surface finish, almost any metal or engineering plastic, no tooling cost.
• Limits: material waste, cost per part stays fairly high, internal shapes limited by tool access (cutters are round, so inside corners have a radius).

Casting (formative, liquid). Molten metal is poured or injected into a mould.

• Sand casting: cheap moulds, large parts, rough surface and loose tolerances (often around ±1 mm or more on bigger dimensions), usually machined afterwards on critical faces.
• Investment casting: fine detail and good finish, moderate tooling.
• Die casting (aluminium, zinc, magnesium): steel dies are expensive, but parts come out fast and accurate. Suits high volumes.

Forming (solid). Forging hammers or presses hot or cold metal into shape, giving excellent strength because the grain flow follows the shape (crankshafts, connecting rods, hand tools). Sheet metal work (cutting, punching, bending, stamping) makes enclosures, brackets and panels cheaply, especially with progressive dies at high volume.

Injection moulding (plastics). Molten plastic is injected into a steel or aluminium mould. Moulds are costly (from a few thousand to well over a hundred thousand dollars) and take weeks to make, but each part costs very little and cycles take seconds. Designs need uniform walls, draft angles and no undercuts unless the mould has side actions.

Additive manufacturing (3D printing). Parts are built layer by layer: FDM (plastic filament), SLA (resin), SLS/MJF (nylon powder), metal powder-bed fusion. No tooling, complex internal shapes are possible, but parts are slower to make, cost more per part at volume, and properties can differ by build direction. A later lesson covers it.

Joining. Welding, brazing, adhesives and fasteners turn several simple parts into an assembly. Sometimes a welded assembly of plate is cheaper than one complex casting.

Volume and tooling: the break-even idea

Processes trade fixed cost (tooling, setup, programming) against cost per part. A simple model:

total cost = tooling + (cost per part * quantity)
break-even quantity = (tooling_B - tooling_A) / (per_part_A - per_part_B)

Example: a plastic housing can be CNC machined for $15.00 each with no tooling, or injection moulded for $0.50 each after a $20,000 mould.

break-even = (20,000 - 0) / (15.00 - 0.50) = 1,379 parts

Below about 1,400 parts, machining is cheaper; above it, moulding wins, and it wins by more as volume grows. Real quotes add lead time, mould maintenance, material and finishing, but the shape of the decision is the same.

How to choose

Ask these questions in roughly this order:

1. Quantity. One, a hundred, ten thousand or a million? This narrows the field fastest.
2. Material. Does the function need steel, aluminium, a specific plastic? Some processes only work with some materials.
3. Geometry and size. Thin walls, internal channels, large flat plates, round parts (turning) or prismatic parts (milling)?
4. Tolerance and finish. Tight fits usually mean machining, at least on the critical features.
5. Strength and loading. Forgings and machined wrought metal usually beat castings for fatigue-critical parts.
6. Lead time and budget. Tooling takes weeks; machining and printing can take days.

Often the answer is a combination: cast or forge near shape, then machine the bearing bores and mating faces.

A note on shop safety

Machine tools, presses, foundries and moulding machines can cause severe injuries: entanglement in rotating spindles and chucks, flying chips, crushing, burns and fumes. This lesson is awareness only. Never operate shop equipment without the training, supervision and guarding your school, makerspace or employer requires.

Common mistakes

• Designing for machining, then moulding the part. Sharp inside corners and uneven walls that are fine for CNC cause sink marks and warping in moulding.
• Ignoring the break-even. Ordering an injection mould for 200 parts, or machining 50,000 of a part that could be stamped.
• Tight tolerances everywhere. Only the functional features need them.
• Forgetting secondary operations. Castings usually need machining; most parts need deburring, finishing or coating.

Recap

• Machining: accurate, no tooling, higher cost per part.
• Casting, forging, stamping and moulding: tooling up front, cheap parts at volume.
• Additive: no tooling, complex shapes, best at low volume.
• Choose by quantity, material, geometry, tolerance, strength and lead time; break-even analysis compares options.

Try it

The Do tab has a quiz on matching parts to processes and working out a break-even quantity.