The factory and interchangeable parts
Make many identical things, and make them repairable.
Everything for this step
- Field guide
- Engineering → 7 sections
- Builds
- None yet.
- Reference sheets
- None.
- Sciences it rests on
- Engineering
- Track
- Production, level 1 of 1
Why this matters
Interchangeability is a bigger idea than any single machine. When parts are made to a tolerance rather than fitted individually, a broken machine can be repaired with a spare instead of by a craftsman — and that changes maintenance from an art into a supply problem.
The shape of it
- Design to a tolerance and gauge to it. Every dimension gets a permitted range; every part is checked against a go/no-go gauge rather than against its mating part. This is the entire concept.
- Build jigs and fixtures so the machine, not the operator's skill, determines the dimension. This is what lets a moderately trained worker produce accurate parts consistently.
- Work from drawings, distributed, identical and versioned. Verbal specification does not scale beyond one shop.
- Lay out for flow. Material should move in one direction through the building with minimum backtracking. Most factory inefficiency is transport, not machining.
- Power it with line shafting from one engine, or later with individual motors once you reach electricity.
- Then confront what you have built. The factory system produced enormous output and, historically, appalling conditions: child labour, sixteen-hour days, maiming machinery, and towns built with no sanitation. None of that was technically necessary — it was a choice about how the surplus was distributed. Guard the machinery, limit the hours, and school the children. See Patterns.
- Keep the craft skills alive alongside. A society that can only mass-produce cannot make the first of anything, and cannot repair what it has not standardised.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Draw every part with its tolerances, and control the versions
Verbal specification does not scale beyond one shop. Every part gets a numbered drawing showing every dimension with its permitted range — for example 25.00 mm +0.02/−0.00 — the material and the finish.
Version control: each drawing carries a number, a revision letter and a date. One master set is held by a named person; copies are printed from it — see printing — and superseded copies are withdrawn and destroyed. A shop working from an old revision makes parts that do not fit.
2 · Gauge to the tolerance, not to the mating part
This is the entire concept. Every critical dimension gets a go/no-go gauge: the go end must fit, the no-go end must not. A part is checked against the gauge, never against the part it will mate with.
Gauges are calibrated against the settlement's standards — see one metre, one kilogram — numbered, and re-checked on a schedule, because gauges wear. Fitting parts individually and calling it interchangeable manufacture is the named failure.
3 · Build jigs and fixtures that locate on datums
A jig or fixture makes the machine, not the operator's skill, determine the dimension.
Locate every part by the 3-2-1 principle: three points define a plane under the part, two points a line along one side, one point the end — six contact points fix all six ways a part can move, with no more. Locate from the same datum surfaces the drawing measures from. Clamp so the clamping force pushes the part onto its locators, never away from them.
4 · Lay out for flow
Most factory inefficiency is transport, not machining. Draw the route each part takes through the building, step by step, and measure the distance it travels.
Arrange machines in the order of the operations, so material moves in one direction from raw stock to finished store, with no backtracking. Power it with line shafting from one engine, and later with individual motors once there is electricity.
5 · Inspect first articles, sample the run, and stop on defects
- First-article inspection: the first part from every new set-up is fully measured against the drawing before the run starts.
- Sample inspection: pull parts at fixed intervals through the run and gauge them.
- Defect record: every rejected part is logged with its fault.
- Stop on repetition: a defect found twice stops the machine until its cause is found — worn tool, loose fixture, bad stock.
A scrapped part at the machine costs little; a bad part in a customer's machine costs the reputation of every part.
6 · Guard the machinery, limit the hours, and school the children
The factory system produced enormous output and, historically, appalling conditions: child labour, sixteen-hour days, maiming machinery. None of that was technically necessary — it was a choice about how the surplus was distributed.
- Guard every belt, gear, shaft and nip point, and lock off power before anyone reaches in — see water power for lock-off.
- Limit working hours in published law.
- No children at machines; children are in school.
- Keep an injury register, investigate every injury, and publish the totals.
7 · Keep a toolroom and the craft skills alive
A society that can only mass-produce cannot make the first of anything, and cannot repair what it has not standardised.
Keep a toolroom of skilled makers who build the jigs, fixtures, gauges and one-off parts, with apprentices. Hold a stock of spare parts for every standard assembly. The test of interchangeability is a real repair: a machine repaired with a spare part made months earlier by a different worker, with no fitting.