The answer is cost. Tight tolerances require grinding, lapping, and CMM inspection. If a 100mm long cosmetic cover plate is dimensioned at 100mm ±0.01mm because the drafter forgot to set defaults, the part will cost 500% more than necessary.
The following table provides an excerpt from the ISO 2768-MH tolerance chart for linear dimensions:
For example, suppose we have a linear dimension with a nominal size of 100 mm. We want to apply a tolerance class M. According to the ISO 2768-MH tolerance chart, the tolerance value for a linear dimension with a nominal size between 80 mm and 120 mm is ±0.5 mm.
The answer is cost. Tight tolerances require grinding, lapping, and CMM inspection. If a 100mm long cosmetic cover plate is dimensioned at 100mm ±0.01mm because the drafter forgot to set defaults, the part will cost 500% more than necessary.
The following table provides an excerpt from the ISO 2768-MH tolerance chart for linear dimensions: iso 2768-mh tolerance chart
For example, suppose we have a linear dimension with a nominal size of 100 mm. We want to apply a tolerance class M. According to the ISO 2768-MH tolerance chart, the tolerance value for a linear dimension with a nominal size between 80 mm and 120 mm is ±0.5 mm. The answer is cost
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