Two important points need to be understood before you understand the bonus tolerance in GD&T.
GD&T provides functional tolerances
Manufacturing tolerances are different from functional tolerances. It is derived from functional tolerance.
Another concept you need to understand is:
Material condition - It is extreme allowed conditions of any feature based on dimension and dimensional tolerance. For example a shaft or with dimension 10+/-1 mm can vary between 9 mm to 11 mm. These extreme sizes are written as maximum material condition (MMC) and Least Material Condition (LMC). When shaft is largest (11 mm) then it will have maximum material and the material condition will be MMC. For smallest shaft (9mm), the material condition will be (LMC). It is exactly reverse for holes. When hole is largest (11 mm) it will remove maximum material to make largest hole and least material will remain after making of hole. Therefore, material condition for hole of largest size will be LMC and material condition for hole of smallest size will be MMC. Remember, manufacture is expected to make these components between 9 to 11 mm and they can manufacture any size between them and it has to be accepted by designer. Actual size can be 9 mm, 10 mm, 10.5 mm, etc.
With above background, you can say,
Bonus tolerance = Difference between actual size and size at given material condition
Suppose given material condition for shaft is MMC (11 mm) and actual size produced is 10.5 mm then bonus tolerance will be 11–10.5=0.5 mm
Consider another example of hole. Given material condition is MMC (9 mm) and actual size is 10 mm then bonus tolerance will be 10–9=1 mm.
Alternatively, you can say, to simplify and remember:
Bonus tolerance is deviation of size from given material condition.
We assumed basic understanding of GD&T and related terms. If anyone do not have the background then it will be difficult to understand above answer and it is okay as the GD&T topic is little complicated.
To learn the basics and create a foundation you may like to go through best selling nook on Amazon by name Simplified GD&T which is being popular due to its numerous color picture explanations and systematic approach to start from basics and to lead to comprehensive topics. It also include design steps numerical exercises. It will help. You may connect to us through our website
All the best and happy learning!
- Azuko Technical Institute