The legal implications of tolerancing

The importance of correct tolerancing can be seen by the following example in which incorrect tolerancing resulted in a massive financial penalty for a company. A company produced a design drawing for a particular part which they sent out to a subcontractor for manufacture. The part was manufactured according to the drawings and returned to the contractor. Unfortunately, when the part was assembled into the main unit, it didn't fit. Some mating features did not align correctly and assembly was impossible. The contractor insisted the subcontractor had not made the part to the drawing and of course the subcontractor insisted they had! The case went to court and an expert witness was appointed. This expert witness was one of my predecessors in design teaching, hence I know about the case. The problem was that the designer in the contracting company used chain tolerancing when he should have used running tolerancing for a particular feature. He neglected to take into account the effect of tolerance build-up and the result was that the part did not fit in the assembly. Unfortunately, what he had in his mind he didn't put down on the drawing - back to communication 'noise' again (described in Chapter 1). The subcontractor made the part correctly within the chain tolerancing stated on the drawing so it wasn't their fault that the part didn't fit. The outcome of the case was that the court found in favour of the subcontractor and the contractor had to bear the costs. Such court and legal costs can be very high and indeed crippling. For example, in another case known by the author involving a design dispute, the court ruling and resulting damages were such that a subcontractor was bankrupted.

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