GD&T bonus tolerance
Mufaddal Rasheed 
What is the meaning of Bonus tolerance in GD&T ? when is it applicable ?
Bonus tolerance is a term very commonly used when GD&T callout is used with material modifiers.
Take this example.
The feature control frame says that a positional deviation of 0.1 is allowed when the feature of size is manufactured at Maximum Material condition- largest shaft (MMC) which is 20+0.2 = 20.2.
It also means that as the size of manufactured part moves away from MMC i.e. from 20.2 towards 20.0 (nominal) the allowable positional deviation of the shaft with respect to datum (A,B,C) will increase by the same amount as how much the size is away from the MMC.
Lets consider a part which is manufactured as per this drawing and the size of the shaft measured is 19.9 mm .
The positional deviation allowed is sum of
- 0.1 (from feature control frame )
- 0.3 (20.2(MMC) -19.9 (actual part size)
The second part is called as the Bonus positional tolerance which is “bonus” because the size of the feature is deviating from MMC (as M modifier is used) .
So, this means that as per GD&T if the manufacturer makes the shaft at smaller sizes, the manufacturer has higher liberty to allow variation of shaft with respect to its true position with respect to the datums A,B,C whereas if the size is close to largest allowable the positional deviation is tighter .
This table is for a Shaft (or external feature of size) and with MMC modifier attached to the geometric tolerance in the feature control frame.
If the feature of size is a hole (internal) then the MMC and LMC sizes would be interchanged because smallest hole is MMC and largest hole is LMC.
Here are more examples with Bonus tolerance - Free Bonus tolerance Guide- click here
Categories: : GD & Tolerancing
