Production systems

Craft, mechanised, automated, assembly line, hybrid, and computer integrated manufacturing — in order from person to machine.

In a mechanised system a person operates the machine for every part. In an automated one the machine runs itself and a person supervises.

Ask how many are made, how identical they are, and how much judgement each needs. Many identical units with no judgement means an assembly line.

Real products have repetitive stages and judgement stages. Machines take the repetition; people take inspection, awkward assembly and final set-up.

It buys consistent quality, continuous running and a collapsing labour cost per part. It costs high capital, slow expensive changes, and bad parts made in bulk before a fault is noticed.

Repetitive demotivating work, and the whole line stopping if any one station does. It is also inflexible once laid out.

Machines take the repetitive work and people take the judgement, so it copes with variation full automation cannot. The cost is two systems to balance.

It links design, production, stock and quality into one system, so a design change flows straight to the machines and every part is traceable — and the plant depends entirely on that software.

One-off, batch, mass, mass customisation and continuous.

Tooling is a fixed cost divided by the number of parts. A £20,000 tool adds £2,000 to each of ten parts and 2p to each of a million.

The set-up between batches: tools changed, machines reset, first parts scrapped. Fifty made ten times costs far more per unit than 500 made once.

Restarting a float glass line or a chemical plant costs more than running it, because the process must be brought back to temperature and the first output is scrap.

The type of part, the material, the scale of production, the production system available, cost constraints, and environmental considerations.

Scale, because it decides whether the tooling can be paid for — though a shape or a material the process cannot handle rules it out whatever the volume.

Start with a cheaper process at lower volume, prove the market, then tool up — and write down the volume that triggers the change.

A factory uses what it has and what its people know, so it may machine a part a moulder would mould, or buy the moulded part in.

Its material, the process that made it, how it joins to the next part, and why each was chosen.

It is the best single predictor of assembly time and cost. Fewer parts means fewer stations, less stock, fewer mistakes and usually lower impact.

Grouping work so each station takes about the same time. One station at 90 seconds among others at 30 sets the whole line to 90 seconds per unit.

Where the fault is cheapest to fix. Testing a board before it is sealed inside an enclosure costs minutes; afterwards it costs the whole product.

Draft angles so the part releases, uniform wall thickness so it cools without sinking, radii instead of sharp internal corners, and no undercuts.

The line where the two halves of the tool meet, which cannot be avoided. A designer chooses where it runs so it looks deliberate rather than like a fault.

Parts are designed so they fit only one way round and the product assembles from one direction — which is why so many products have all their screws on one face.

Colour changes mean a machine changeover and wasted material, so colours are run in batches from a limited range.