A special purpose machine is justified when one operation repeats at high enough volume, for long enough, that a dedicated machine costs less over its life than doing the work on general-purpose equipment. The decision turns on the stability of the requirement as much as on the volume.
7 min read · Last reviewed 21 September 2026 · All resources
An SPM is usually proposed to solve a capacity problem, a consistency problem or a labour problem. Each has cheaper alternatives worth ruling out first: a fixture and a better tool path for capacity, a poka-yoke for consistency, a semi-automatic aid for labour.
The case for a dedicated machine gets stronger as the operation gets simpler, more repetitive and more stable. It gets weaker as the part mix widens or the design is still changing.
Full automation attracts attention, but a semi-automatic machine — the operator loads and unloads, the machine performs the operation — is frequently the better economics. It removes the variable part of the cycle while avoiding the cost and complexity of part handling.
Handling is usually the expensive part of automation and the most common source of downtime. Where an operator is already present, automating the load rarely pays for itself.
Acceptance criteria written after a machine is built become a negotiation. Written before, they are a specification. They should state what the machine must produce, to what quality, at what rate, and how that will be demonstrated.
It is worth being explicit about the conditions: with which material, on which shift, with how many consecutive parts, and who witnesses it.
The most common disappointment is not a machine that does not work. It is a machine that works exactly as specified, for a part that stopped being made in that quantity eighteen months later.
That risk is commercial rather than technical, and it is the buyer's to judge. A supplier can tell you whether the machine will do the job; only you can say how long the job will last.
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A general-purpose machine keeps its flexibility and its resale value; a dedicated machine usually wins on cycle time and consistency for the one operation. If the requirement might change, flexibility is worth paying for.
The current process and its output, the part drawing, the target rate, the quality characteristics that matter, and the level of automation you have in mind. Photographs or a video of the operation as it runs today are extremely useful.
Whatever the contract says, which is why acceptance criteria belong in it. Agreeing the measurement method up front prevents most disputes about whether a target was met.
Sometimes, and it is worth asking before commissioning something new. A retrofit or modification can address a specific limitation at a fraction of the cost of a dedicated machine.
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