Originally Posted by jboggs
Can you get your hands on a Machinery's Handbook? If not I would STRONGLY recommend you buy one. It would be well worth the investment. Look up the chapter on "Allowances and Tolerances". You will see that there are two standard ANSI systems of fits and tolerances (inch and metric). The metric ANSI system is based on the ISO system. Both of them describe a range of fits for mating shafts and holes.
In the inch system, each standard fit is given a name like RC (for Running or Sliding Clearance Fit) or LN (for Locational Interference Fit), based on the "tightness" of the fit to serve a specific function. Pages of data show you the specific tolerances in each of those fits for certain size ranges of holes or shafts. You will notice that some of them do indeed show a double plus or a double minus for the actual tolerance based on some standard dimension.
In the metric pages you will see a similar approach. But you will also see something I first saw on German drawings done to DIN standards. Individual tolerance ranges are referenced by number and letter, like "H8" or "f7". (Cap letters for holes; small letters for shafts.) There are charts that describe very clearly what the intended fit is for each of these ranges. This allows the designer to simply select the shaft/hole size he wants and the type fit he wants. Then he just calls out a shaft diameter as "45 g6" for example. The machinist can then go to the tables to identify exactly what the tolerance is for a 45mm shaft with a g6 tolerance. Frankly I really like that system but it is so unknown here in the US it is pointless to try to use it. It will also give you double pluses and double minuses.
So, bottom line: whoever told you that double pluses or minuses are wrong was "misinformed". Unless the designer in misusing them, they are in fact based on ANSI and ISO standards for fits and tolerances.