PM

A place marker, or PM, is a hardened steel pin or dowel inserted into a fixture, die, or assembly jig to establish and maintain the precise position of a workpiece or component during machining, stamping, forming, or assembly. It serves as a mechanical reference point that prevents drift and ensures repeatability across multiple production runs. PMs are typically cylindrical, ranging from 3 mm to 25 mm in diameter, with a finished length protruding just enough to engage a corresponding hole in the part itself.

The mechanism is simple but critical. A part is pressed or seated against one or more PMs, which slide into pre-drilled or pressed holes in that part. This contact locates the workpiece in two or three axes simultaneously, depending on how many PMs are used and how they are arranged. Because both the PM and the receiving hole are held to tight tolerances, typically ±0.005 inches or better, the repeatability is excellent. Once located, the part is held in place by clamps or pressure while the actual work, cutting, forming, welding, or fastening, takes place around it.

Common variants include straight PMs (simple pins), shoulder PMs (with a step to prevent over-insertion), and headed PMs (with a cap for easier handling). The material is usually hardened steel, AISI D2 or similar, to resist wear from repeated insertion and removal. In high-speed or high-volume operations, PMs may be spring-loaded or mounted in quick-change carriers to speed up setup and tear-down without sacrificing accuracy.

The critical failure mode is wear of the PM surface or the receiving hole. Fretting corrosion can occur where the PM meets the part, especially in humid shops or when ferrous parts contact bare steel PMs without lubrication. Burring at the hole edge, caused by thousands of part insertions, gradually damages both the pin and the hole, increasing runout and eventually rejecting good parts. Regular inspection and replacement of worn PMs is essential; replacement intervals depend on production volume and material hardness.

PMs are foundational to fixture design across stamping, machining, and assembly. They are cheaper and more durable than magnetic or vacuum locating systems and require no external power. In modular fixture systems and quick-change pallets, PMs are the primary locating method, making them invisible but indispensable to high-mix manufacturing. Their success depends entirely on hole quality and cleanliness; a burr, scale, or chip left in a PM hole degrades accuracy and can damage the pin on the next cycle.