Tolerances: the conversation that saves the most money
Most part cost is decided by what you ask the factory to guarantee. How to decide which dimensions matter and let the rest go.
A drawing with a general tolerance of plus or minus 0.05 millimetres on every dimension is a drawing that asks the factory to measure everything and reject a lot of it. The factory prices that in. The product is not better for it. It is only more expensive.
The tolerance conversation is the one where we save clients the most money per hour, and it is the one most teams skip.
What a tolerance is for
A tolerance is a promise about a dimension, and promises cost money. Tighter promises need better tools, slower cycles, more inspection and more scrap. So the question is never "how precise can this be?" but "what happens if it is not?".
For most dimensions on most parts, the answer is: nothing. A cosmetic rib that is 0.3 millimetres taller than drawn will not be noticed. Those dimensions get the general tolerance of the process, and the general tolerance of injection moulding is far looser than most CAD users assume.
The dimensions that matter
Typically a handful per part:
- Mating surfaces between parts that have to seal, click or slide.
- Locations of features that align with something the user sees: a button in a hole, a lens in a bezel.
- Anything that carries a bearing, a shaft or a PCB.
- Dimensions that determine a safety clearance.
These get a specific tolerance, a datum, and a reason written next to them on the drawing. The reason matters: a factory that knows why a dimension is tight will protect it. A factory that sees a wall of tight numbers will protect none of them.
Stack-ups
A gap between two parts is never one dimension. It is the sum of several, each with its own variation. Before tightening any tolerance, add up the stack. Often the fix is not a tighter tolerance but a different design: a feature that locates two parts directly instead of through a third, or a gap made deliberately visible so variation reads as intent.
On the Xenolidar G6 housing, the sealing surface between the two cast halves is the one place where the stack had to be controlled. Everything else on the casting carries the general tolerance of the process, which is what keeps a cast part affordable.
Process first
Tolerances belong to processes. A machined part, a cast part, a moulded part and a sheet-metal part have different normal variations, and a tolerance that is free on one is impossible on another. Decide the process before you decide the tolerances, and if the tolerance a design needs is not available from the process the budget allows, change the design.
Talk to the factory
The best tolerance drawing is one the factory has seen before it is final. A moulder will tell you which dimensions they can hold for free, which ones cost a slower cycle, and which ones they will refuse. That conversation takes an hour and changes the drawing more than any internal review.
A rule of thumb
If you cannot say what fails when a dimension is out of tolerance, the tolerance is too tight. Loosen it, write down the reason you kept the ones you kept, and let the factory do what it is good at.