Where accuracy actually matters
Most structural steel has generous tolerances — a beam a millimetre long is nobody’s problem. Some components do not have that luxury.
Curtain wall brackets. They position the glass. An error at the bracket becomes a visible misalignment in the elevation, and there is nowhere to hide it.
Spider fittings. For point-fixed structural glazing, where bolts pass through holes in the glass. The fitting and the hole have to correspond precisely, and glass cannot be adjusted afterwards.
Repeating base plates and connections. Where the same component appears across a grid, small variations accumulate. By the twentieth bay, hand-marked work has drifted enough to be visible.
Bearing faces. A bolted connection that has to bear evenly across a face rather than on a high spot.
Repeatability is the real product
A single accurate bracket is achievable by hand with care. Fifty identical brackets are not — hand work drifts, and on a repeating façade the drift accumulates in one direction and shows.
Machining from a file produces the fiftieth component to the same tolerance as the first. That is what keeps a grid true across a building.
Complex geometry
Profiles that would be slow and uncertain to cut by hand are cut directly from the drawing. An awkward shape becomes a setup cost rather than a test of hand skill, and it can be reproduced exactly.
This matters most on architectural metalwork where the geometry is the design.
Cost
Machining costs more per component than conventional fabrication, mostly in setup. Two things change the calculation:
Quantity. Setup is amortised, so unit cost falls as numbers rise.
Site time. Components that locate correctly first time remove the packing, easing and adjustment that otherwise happens on site — which on a façade package is frequently the larger cost.
For a one-off bracket, conventional fabrication is usually the better spend, and we will say so.
Materials
Steel, aluminium and stainless all machine well with appropriate tooling and speeds.
Stainless needs particular care: it work-hardens if fed too slowly, and any contact with carbon steel tooling embeds particles that rust later. Dedicated tooling for stainless is not fussiness — it is what stops a stainless component developing rust spots six months after installation.
Where it sits in the process
Machining is preparation. Components are then welded into assemblies or bolted into place. The joining method has to be chosen and sequenced so that welding distortion does not undo the accuracy the machining established — see structural fabrication for the whole sequence.