Bonus Tolerance in Geometric Dimensioning and Tolerancing (GDT): Understanding Its Role and Application

In Geometric Dimensioning and Tolerancing (GDT), bonus tolerance refers to an additional tolerance allowance granted when a feature's actual condition surpasses its specified basic size. This concept primarily applies to features that are dimension-controlled, such as holes and shafts. Ideal for enhancing manufacturing flexibility and cost efficiency, bonus tolerance is a vital aspect of GDT application.

Key Points about Bonus Tolerance

Bonus tolerance is applicable when a feature is defined by a size dimension, such as a hole's diameter or a shaft's size. It is used to determine the acceptable deviation from the ideal position, especially when the actual size of the feature exceeds the specified basic size.

Consider a hole with a position tolerance of ±0.2 mm. If the hole is actually larger than its minimum size (e.g., 10.2 mm instead of 10.0 mm), the position tolerance can be increased by the amount that the hole exceeds the basic size. This means the actual tolerance zone for the position can be expanded, leading to a more flexible manufacturing process.

Mathematically, the total tolerance is represented as:

Total Tolerance Specified Tolerance(basic size) Bonus Tolerance(difference between actual size and basic size)

The bonus tolerance is calculated as Bonus Tolerance (actual size) - (basic size).

Bonus tolerance offers significant benefits by allowing for greater manufacturing flexibility and potential cost savings. Parts manufactured with better-than-minimum sizes can meet dimensional requirements without additional precision machining, making the process more efficient and cost-effective.

The concept of bonus tolerance is particularly useful in assemblies where the fit and function of parts are more critical than their individual tolerances. This leads to easier assembly and better overall performance of the product.

A shaft of Maximum Material Condition (MMC) with a maximum dimension of 11 mm and Least Material Condition (LMC) with a minimum dimension of 9 mm is given a positional tolerance of 0.1 mm modifier M. The modifier means that a bonus positional shift of 0.1 mm is allowed when the shaft is at MMC. However, if the shaft is manufactured at LMC, the bonus tolerance is effectively (11 - 9) 2 mm, allowing for a larger positional shift. This flexibility is crucial for ensuring that the shaft can pass through a corresponding hole with ease.

Understanding bonus tolerance is essential for effective GDT application. It helps in optimizing manufacturing processes and ensuring proper fit and function in assembled products, leading to improved product performance and cost-efficiency.

For more details, watch the related video.