Here’s an interesting tolerance question: Have you ever seen a plus/plus tolerance? (or minus/minus?) The ASME standard does not mention this, so a purist would say that it is illegal (see paragraphs 2.2 and 2.3 of the Y14.5 standard). But I’ve seen examples such as a hole dimensioned with:
The real problem is that, with a casual glance, you might not realize that a part made at the “nominal” size of 1.25 is a bad part! (The size limits are 1.252 – 1.255.) So the obvious question is: Why did the engineer (or designer) choose this odd method of expressing a tolerance?
The most likely answer is because there is some desired fit with a mating feature. Depending on the assembly, they may want a press fit, transition fit, or clearance fit. So if the example given above is a hole, then a pin in the mating part may be dimensioned with a minus/minus tolerance. In this way we can say that the pin and hole each have a nominal size of 1.250 inches, but the respective tolerances will ensure that there is always a little looseness (clearance fit).
In the metric system, an entire method of coding different limits and fits has been developed. A nominal size would be given, along with a letter and number. For example, 16D8 corresponds to a hole with a nominal size of 16 mm, but with actual limits of 16.050 to 16.077 mm (I had to look this up in a table!). The “16D8” is usually not understood in American design usage, so if a design is being converted from another country to an American program, the engineer usually translates it into an odd-looking “plus/plus” tolerance:
(Similarly, external features use a lowercase letter, so a corresponding hole might be 16d8, which corresponds to 15.923 to 15.950 mm.)
For more information on the code letters and numbers for this ISO usage, consult a Machinery’s Handbook or the ANSI standard for “Preferred Metric Limits and Fits,” ANSI B4.2-1978.