When a plastic housing needs a threaded insert, the fastener spec on the drawing looks deceptively simple. In production, that spec is where most rework and rejection claims begin.
plastic assembly fasteners what tolerance to call out
The thread on a brass insert nut for a sensor housing is where buyers under-specify most. A 6.3 mm internal thread called to ±0.1 mm looks fine on paper, but engagement into a 1.2 mm wall changes clamp load dramatically. State the thread class and minimum engagement depth, not just the nominal diameter.
For CNC turned parts going into a molding run, the knurl on the outer body holds the insert in the melt. A 0.4 mm knurl height is typical, but "knurled" without depth and pitch leaves the machinist guessing. Specify both values on the drawing.
plastic assembly fasteners inspection you should request
A thread gauge check per lot is non-negotiable for threaded inserts. Ask for a go/no-go gauge report on every batch, not just the first article. For stainless steel bushings in electronics enclosures, call out Ra 0.8 µm or better to prevent fretting on the mating plastic.
Burr control at the thread exit matters more than most drawings show. A 0.1 mm burr on a 3 mm pin cracks a polypropylene housing during press-fit. Request deburr verification or a thread-exit photo in the inspection package.
plastic assembly fasteners what to put in your RFQ
Weeda Precision runs drawing-based CNC turning for brass, copper, and stainless fasteners in prototype and batch volumes. Send the current drawing revision number, annual volume by season, and packaging needs: bagged, taped, or strip for auto-assembly. A 50-piece first-article sample before the production lot saves two weeks of revisions.
Name the material grade explicitly: C360 brass for general use, CuAl10Fe for conductivity, 303 stainless for corrosion. If the insert sits through a molding cycle, state the peak temperature so the supplier confirms the alloy will not anneal under load.
Before releasing the PO, cross-check the spec against your last three rejected lots. The tolerance that failed last time is the one to tighten next.