In specialist subcontracting, the buyer approaches a subcontractor because it masters a technology, process or piece of equipment the buyer does not have, and would have neither the interest nor the means to acquire. The reason is not lack of time, as in capacity subcontracting, but lack of skill or tooling: the supplier brings know-how.
Examples abound: quench hardening and heat treatments, anodising and other surface treatments, EDM, casting, forging, plastic injection moulding, non-destructive testing, special welding processes. A machinist who has parts painted, a machine builder who has shafts treated before assembly: all become buyers with respect to these specialists. In aerospace, special processes (heat treatment, surface treatment, non-destructive testing, welding) are often governed by specific approvals, such as the Nadcap programme.
The relationship is often longer-lasting and more technical than for capacity. The specialist advises: choice of the grade suited to the treatment, feasibility of a shape, size limits (size of a bath or furnace), a neighbouring process that is better suited. Involve them early in design, because some rules are decided on the drawing: vents, allowances, datums, areas to mask. The downsides are dependence on a small number of suppliers, batch turnaround times and the logistics of shuttling between workshops.
In a quote request
Describe the result expected (hardness, thickness, appearance, resistance) rather than just the name of a process, cite the applicable standard or specification and the material of the part, and specify the areas to treat or protect. Give quantities per batch, as many specialists charge a minimum, and the documents required (treatment certificate, inspection report). See the guide surface treatment of metals, and to match the right supplier to the need, choosing an industrial subcontractor.