- MozGlobal Engineering
- Fabrication
- 5 min read
The manufacturer discontinued it, the agent has none, and the machine still has years of service left. Copying the worn part is not as simple as it sounds.
Sooner or later, equipment outlives its spare parts supply. The manufacturer discontinued the model, the local agent has none, the lead time from abroad is measured in months — and the machine has years of useful service left in it. Fabricating a replacement is often the right answer. It is also less straightforward than "copy the old one".
The worn part is not the drawing
The component you are holding has been in service. It is worn, possibly deformed, possibly already repaired once by somebody with a welder and good intentions. Measuring it and reproducing those measurements faithfully reproduces the wear along with the geometry.
The work is to reconstruct the original dimensions from the worn part: measuring surfaces that have not been in contact, working back from the mating components, and using the running clearances the assembly requires. A shaft measured at its worn journal will be undersized. Measured at an unworn section and checked against the bearing bore, it will not.
Material matters more than the shape
Geometry is the visible part of the problem and usually the easier half. Whether the original was hardened, what grade of steel it was, whether a surface treatment was doing work that is not obvious by eye — these decide whether the replacement lasts as long as the original or fails in a fraction of the time.
- Hardness of the original, measured rather than assumed
- The duty: load, speed, temperature, and whether the service is abrasive or corrosive
- Surface treatments, which are easy to miss and often load-bearing
- Whether the part was designed to fail first, protecting something more expensive behind it
That last point deserves attention. Some components are deliberately the weakest element in an assembly. Reproducing one in a stronger material moves the failure somewhere far more expensive.
Before making a part stronger than the original, find out whether the original was weak on purpose.
Improve only where you understand the reason
It is tempting to correct what looks like a design shortcoming while the part is being made anyway. Sometimes that is right — a component that has failed three times in the same place is telling you something. Often the feature that looks like a shortcoming is doing a job that is not visible from the part alone.
The reasonable position: change what you can explain, and reproduce what you cannot.
Fit is proven on the machine
A part that measures correctly on the bench and does not fit on the machine is a common and frustrating outcome, usually because the assembly it goes into has its own accumulated wear and deviation. Whoever fabricates should be present, or close, when it is fitted. The adjustment that a real installation demands is quick to make when the fabricator is there and turns into a dispute when they are not.
Record what you made
The dimensions, material and treatment used should be written down and kept. The part will wear out again. The second time should not require repeating the whole reverse-engineering exercise from another worn component.
