A person picking up a container does not care whether it is three millimetres wider than the last one. A gripper, a slot or a scanner does. That difference – between a tolerance that is invisible to a person and one that stops a machine – is the single biggest adjustment required when a container moves from a manual process to an automated one, and it is usually discovered rather than specified.
Where manual tolerance comes from
Manual processes tolerate variation because the person compensates: they adjust their grip, they nudge the box, they read the label from a different angle. That compensation is free and invisible, and over years it allows container tolerances to widen without anyone noticing. A fleet bought to a loose specification, or one that has drifted in service, works perfectly well by hand and is unusable automatically.
What the machine cannot compensate for
A gripper programmed to a width either closes on the box or does not. A slot either accepts the load or rejects it. A scanner either sees the symbol in its field or does not. Each of these is binary at some threshold, and the threshold is set by the tolerance. Where a fleet straddles the threshold, some containers work and some do not, and the failures look random.
Typical numbers, and why they vary so much
A conveyor with guides might tolerate five millimetres of width variation. A parallel gripper might tolerate two. An ASRS slot might tolerate one, because the clearance budget is shared with the machine’s own positioning accuracy. There is no general answer – the number comes from the equipment – and the mistake is assuming a container that works on one automated process will work on another.
Which dimensions matter most
Footprint matters more than height on a conveyor, because guides work on width. Height matters more than footprint in a high-bay slot, because the machine detects height. Flatness matters more than either for vacuum or for reading. Knowing which dimension the equipment is sensitive to lets the specification be tight where it needs to be and loose where it can be – which is cheaper than tightening everything.
Holding a tolerance in production
A supplier can hold a tighter tolerance with better tooling, more inspection and a higher rejection rate, and all three cost money. The question is how much tighter is worth buying. Where the equipment needs one millimetre and the standard process gives three, the extra two are worth paying for. Where the equipment needs three and the process gives five, changing the equipment setting is usually cheaper.
Drift, and specifying for year three
A tolerance specified at delivery is not a tolerance held in service. Containers creep under load, move with temperature and take a set from washing. Where a specification is written, it should say what tolerance is held after a defined period in defined conditions, not just at delivery. A container that meets tolerance new and misses it after a year is the usual cause of an automated system that degrades. The audit that catches this is described in the pre-automation box audit.
Writing the requirement
State the dimension, the tolerance, the condition it is measured under, and when it has to hold. Four parts, and the third and fourth are the ones usually missing. A supplier given all four can quote accurately; one given the first two will quote a drawing and the difference will surface in year two.