When specifying parts to a machine shop, it is important to include all necessary tolerances. Although CNC machines are very precise, there are still some slight differences between repeats of the same part, usually about + or - 0.005 inches (0.127 mm), which is about twice the width of a human hair. To save cost, buyers should only specify tolerances for areas of the part that need to be particularly precise because those areas will come into contact with other parts. While there are standard tolerances for different levels of machining (as shown in the table below), not all tolerances are created equal. For example, if a part must never be larger than a measurement, it might have a specified tolerance of +0.0/-0.5 to indicate that it can be slightly smaller, but not larger in that area.
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