A machine builder's schedule is unforgiving: a bracket that arrives late or out of tolerance stops an assembly line, and no price saving makes up for that.
Aluminium for light structures, steel and stainless for strength and corrosion resistance, engineering plastics where weight, insulation or chemical resistance matter. Milling, turning and Swiss-type machining cover the part families a machine builder typically needs, from frames and plates to shafts, bushings and small fittings.
The most useful thing a supplier can do is arrive early in the design. A DFM review during prototyping removes cost and risk while changes are still free, and the same process then carries the part into series so nothing is re-learned between stages.