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A county that has always made things fly

Somerset has been an engineering county for generations. The workshops around Taunton, Wellington and Chard, the toolrooms tucked away on industrial estates off the A38, the machine shops that grew up alongside the aircraft and helicopter work in the south of the county — they all share a particular way of working. Quiet, methodical, and slightly obsessive about measurement. That heritage still shapes how aerospace parts are made here today, because the habits passed down through the trade are exactly the habits that modern aircraft work demands.

When you hold a machined bracket that weighs less than a bag of sugar, it is easy to forget what it is being asked to do. It will sit behind a panel, hold a pipe or a sensor or a hinge, and be expected to survive thousands of flight cycles, vibration, temperature swings from −50°C at altitude to baking heat on a sun-soaked apron, and decades of service. The difference between a part that does all that and one that fails is often measured in microns.

What precision actually means on the shop floor

Drawings rarely say “make it accurate”. They say something like Ø12.000 +0.013/−0.000, or a flatness callout of 0.01 mm across a mating face. Talk to a machinist and they will translate that into practical terms: the bore has thirteen microns of room, and the flat face must not bow by more than the thickness of a decent coat of paint. General machining tolerances of ±0.1 mm are considered comfortable. Aerospace work regularly calls for ±0.05 mm, and critical features can demand five microns or tighter.

It is not only size that matters. Geometric tolerances govern how a part behaves:

• Concentricity on a bearing bore, so a shaft spins without whipping.
• Flatness on a sealing face, so a gasket is not asked to do the impossible.
• Perpendicularity on a mounting flange, so bolt holes line up without forcing the assembly.
• Surface finish on a fatigue-critical fillet, because a scratch can become a crack.

Measurement is the real craft

Nobody machines to five microns by eye. The work depends on a chain of measurement that starts with calibrated gauge blocks and runs through micrometers, bore gauges, height gauges, surface roughness testers and, for the complex work, coordinate measuring machines. Every instrument has a calibration schedule, and every reading is only as good as the temperature it was taken at.

Most inspection here happens at a controlled 20°C, and for good reason. Aluminium expands by roughly 23 microns per metre for every degree Celsius. A 300 mm aluminium part that warms by 5°C during machining grows by nearly 35 microns — several times the tolerance on a tight feature. So machinists use coolant generously, let parts stabilise before final measurement, and think carefully about where heat is going. In a county where the weather swings from damp Atlantic air to warm summer afternoons, that discipline is not theoretical.

Tooling, setups and small decisions that add up

Sharp tooling is not a luxury. A dull cutter rubs rather than shears, generating heat that distorts the workpiece and burrs the edge. Carbide inserts with the right geometry, changed on a schedule rather than when they finally fail, keep dimensions stable through a batch. For abrasive composites and hard alloys, diamond-coated tooling earns its keep.

Feeds and speeds matter just as much. Too aggressive and the part chatters; too timid and the tool wears by rubbing. Thin-walled sections need support, so the fixture often does as much work as the cutter. Soft jaws bored in situ, vacuum chucks, purpose-made fixtures and sacrificial backing plates all hold a part steady without crushing it. A spring pass at the end, taking a few microns with a fresh edge, can be the difference between a surface that passes inspection and one that does not.

Checks, traceability and the nerve to scrap a part

Aerospace quality control is layered. A first-off inspection confirms the setup before production runs. In-process checks catch drift while there is still time to correct it. Final inspection signs the part off against the drawing. Statistical process control watches the trend rather than the individual reading, warning when a machine is slowly wandering.

Then there is non-destructive testing: dye penetrant for surface-breaking defects, magnetic particle inspection on ferrous parts, radiography where internal soundness really matters. Every component carries paperwork — material certificates, heat lot numbers, serial numbers, route cards, certificates of conformity. If a part cannot be traced, it cannot fly.

The hardest lesson for any machinist is knowing when to scrap. A part that is out of tolerance by three microns is scrap, however many hours went into it. Arguing the point wastes more time than making a replacement, and it puts a question mark over everything else in the batch.

Passing the skills on

Somerset’s aerospace supply chain survives on people who can read a drawing properly, set a machine confidently and admit when something is not right. Apprenticeships, college courses and old-fashioned mentoring in small machine shops keep those skills alive. The technology has changed — digital readouts, CAM programming and probing systems are now standard — but the underlying values have not. Measure twice, cut once, and never trust a number you have not verified.

That is the quiet importance of precision machining in aircraft parts. It is not glamorous, and it rarely makes headlines. But every safe landing owes something to a machinist in Taunton or somewhere nearby, standing at a machine, checking a micrometer, and deciding that good enough is not quite good enough.