Replacement parts from a sample

Replacement parts from a sample are parts made from an existing physical part, usually a worn, broken or sole surviving one, when no drawings exist or the original can no longer be bought. Also described as making parts to a sample or reproducing obsolete spare parts, it is the most common practical application of reverse engineering.

It starts with a functional analysis: where the part sits, what loads it, what it mates with and why it failed. Then the geometry is captured: simple turned and prismatic parts with calipers, a micrometer and a height gauge, freeform parts by 3D scanning, with precise functional dimensions verified on a CMM. A broken part is pieced together from its fragments or reconstructed from the mating part, the symmetrical side or neighboring parts.

Standard features need special care. Threads must be identified including the thread system and pitch: an M8 and a 5/16 UNC thread, for example, have almost the same diameter but a different pitch. For gears, the module (or diametral pitch on inch gears), number of teeth, pressure angle, profile shift and, on helical gears, the helix angle must be determined. Bearing seats and keyways take their dimensions from standards, not from measured values.

The material is identified from machine documentation, spectral analysis of its composition and hardness testing, which also hints at the heat treatment. For plastics, lab analysis (such as infrared spectroscopy) helps, though a proven engineering plastic with similar properties is often chosen instead. The output is a CAD model and a technical drawing with tolerances; the part is then made by CNC machining, 3D printing, casting or another process to suit quantity and loading.

This route makes sense when the machine builder no longer exists or no longer supplies the part, when the original is unreasonably expensive or has a long lead time while the machine sits idle, or when the part keeps failing and is worth improving along the way, for example with a better material, a stronger cross section or a different heat treatment. Typical examples are gears, shafts, bushings, pulleys, levers, brackets, plastic covers and parts of older machines and vehicles.

It also pays off preventively: for critical machine parts without documentation, the model and drawing can be prepared before the part breaks. After a failure, the part then only needs to be made, and downtime is shorter.

The most expensive mistake is faithfully copying a worn or deformed part. Functional surfaces must be restored to nominal dimensions based on the mating part and the function, and tolerances set anew. Send the mating part with the sample, or at least the dimensions of adjoining parts, and describe the operating conditions: load, speed, temperature, environment and lubrication.

A copy of the shape in a different material or without the right heat treatment can fail much sooner than the original. For safety-critical parts (lifting and pressure equipment, brakes, load-bearing members), copying the shape is not enough: the material and load capacity must be verified, and it must be clear who is liable for the part.

Also decide whether you want one part or documentation for repeat production too, and who will own it. If the original part is still protected by a patent, a utility model (in countries that have them) or a registered design (a design patent in the US), check that nothing prevents you from copying it.