Bonus tolerance is a benefit of geometric tolerancing and dimensioning. In this article we will see how you can tolerance a fit while using the benefit of bonus tolerance to reduce manufacturing cost.
There are five types of geometric tolerances that can have bonus tolerance:
- Straightness
- Angularity
- Perpendicularity
- Parallelism
- Position
Straightness
Bonus tolerance is only available if maximum material condition (MMC) is specified. It is used for axis tolerance and diameter deviating from MMC. There is no datum reference.
Two types of straightness:
- Surface straightness
- Axis straightness
Bonus tolerance is available for axis straightness when MMC is specified.
Go/No Go Gauge Diameter = 1.005 + .002 = 1.007
Max Bonus Tol = 1.005 – .998 = .007
Angularity
Perpendicularity and parallelism are special cases of angularity where the angle is 90o and 0o respectively. Angularity has a basic dimension, the angle. The angle is referenced to a datum.
Two types of angularity:
- Surface angularity
- Axis angularity
Bonus tolerance is available for axis angularity when MMC is specified.
Go/No Go Gauge Diameter = .998 - .003 = .995
Max Bonus Tol = 1.005 – .998 = .007
Perpendicularity
Perpendicularity is a special case of angularity where the angle is 90o. There is no basic dimension since the angle is always 90o. Perpendicularity is referenced to a datum.
Two types of perpendicularity:
- Surface perpendicularity
- Axis perpendicularity
Bonus tolerance is available for axis perpendicularity when MMC is specified.
Go/No Go Gauge Diameter = .998 - .003 = .995
Max Bonus Tol = 1.005 – .998 = .007
Parallelism
Parallelism is a special case of angularity where the angle is 0o. There is no basic dimension since the angle is always 0o. Parallelism is referenced to a datum.
Two types of parallelism:
- Surface parallelism
- Axis parallelism
Bonus tolerance is available for axis parallelism when MMC is specified.
Go/No Go Gauge Diameter = .998 - .003 = .995
Max Bonus Tol = 1.005 – .998 = .007
Position
Commonly referred to as true position.
The position deviation is calculated by:
Note that when the diameter symbol is applied to the position tolerance, the tolerance zone is cylindrical. If not used, error is limited to the x and y directions (a square).
RFS
In a regardless of feature size (RFS) material condition, the part is out of tolerance since .009 > .008 position tolerance.
MMC
For maximum material condition (MMC), as the hole gets larger and approaches LMC there is a bonus tolerance available. MMC is used to apply bonus tolerance when fitting a mating hole/shaft.
The bonus tolerance is:
Bonus tol = 1.001 – .998 = .003
Adding bonus tol to position tol, the new position tolerance is:
Total position tol = .008 + .004 = .011
For MMC, the part is within tolerance since .009 < .011.
LMC
For least material condition (LMC), as the hole gets smaller and approaches MMC there is a bonus tolerance. LMC is used differently than MMC. LMC is used to maintain a minimum wall thickness, for example. When fitting parts together is important, MMC must be used.
The bonus tolerance is:
Bonus tol = 1.005 - 1.001 = .004
Adding bonus tol to position tol, the new position tolerance is:
Total position tol = .008 + .004 = .012
For LMC, the part is within tolerance since .009 < .012.
Note that if a pin was required to fit in the hole, using LMC is not going to allow more tolerance to fit. LMC is used to make sure that a minimum wall thickness of the hole is maintained.
MEboost
MEboost has a bonus tolerance calculator that will guide you through these calcuations effortlessly.