If the tolerance zone is completely above the basic size line, how is it indicated as per BIS?
By using only positive deviation values
On a limits-and-fits diagram the basic (nominal) size is drawn as a horizontal reference line called the zero line. All deviations of the tolerance limits are measured as distances from this zero line.
By BIS convention a distance measured above the zero line is a positive deviation, while a distance measured below it is a negative deviation.
A tolerance zone is bounded by an upper deviation and a lower deviation. If the whole zone lies completely above the basic size line, then both of these boundaries sit above the zero line.
Since both the upper and the lower limits are above zero, both deviations must be positive quantities.
The tolerance is therefore written using only positive deviation values marked above the basic size.
Using only negative values (option 1) would describe a zone lying entirely below the line, and combining positive and negative values (option 3) would describe a zone straddling the zero line, so neither fits a zone that is wholly above.
A zero deviation (option 2) would mean a limit sitting on the basic size line, not above it.
Hence, the answer is By using only positive deviation values.
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