Learn how to apply bonus tolerance for true position to complex or irregular features, such as slots, splines, and profiles, with GD&T tips and examples.

MMC defines the worst case condition of a part that will still guarantee, because it is still within the prescribed tolerances, the assembly between pin(s) and hole(s). When a hole is at its smallest (MMC) and a pin is at its largest condition (also MMC), we can be sure that we will still be able to assemble that part. Thus, MMC is widely used in cases where clearance fits are common

Last updated on Nov 26, 2024

How do you deal with complex or irregular features when using bonus tolerance for true position?

Bonus tolerance for true position is a useful concept in geometric dimensioning and tolerancing (GD&T) that allows you to increase the size of the tolerance zone for a feature based on its actual size. This can help you achieve better functional fit and reduce manufacturing costs. But how do you apply bonus tolerance for true position when the feature is complex or irregular, such as a slot, a spline, or a profile? In this article, we will explore some tips and examples of how to deal with these scenarios.

1 Identify the datum reference frame

The first step is to identify the datum reference frame (DRF) that defines the orientation and location of the feature. The DRF consists of one or more datums, which are features or planes that act as reference points for measurement. The DRF is usually specified in the feature control frame (FCF), which is a rectangular box that contains the geometric symbol, the tolerance value, and the datum references. For example, if the FCF says |TP|0.5|A|B|C|, it means that the feature must be located within a cylindrical tolerance zone of 0.5 mm diameter, referenced to datums A, B, and C.

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2 Determine the feature size

The next step is to determine the feature size, which is the actual size of the feature measured across the two opposed points or elements that are farthest apart. The feature size can be either the maximum material condition (MMC) or the least material condition (LMC), depending on whether the feature is a hole or a pin. The MMC is the condition where the feature has the most material, such as the smallest hole or the largest pin. The LMC is the opposite, where the feature has the least material, such as the largest hole or the smallest pin. The feature size affects the amount of bonus tolerance that can be applied.

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3 Calculate the bonus tolerance

The bonus tolerance is the difference between the MMC and the actual feature size. For example, if the MMC of a hole is 10 mm and the actual feature size is 9.8 mm, the bonus tolerance is 0.2 mm. This means that the tolerance zone can be increased by 0.2 mm, resulting in a total tolerance of 0.7 mm (0.5 + 0.2). The bonus tolerance is only valid when the feature is controlled at MMC or LMC, which is indicated by a modifier symbol (M or L) in the FCF. For example, |TP|0.5|M|A|B|C| means that the feature is controlled at MMC and can receive bonus tolerance.

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Sai GOLI (AMRAeS PCQI)

Quality Engineer - Development/Programmes @ Safran | Master's in Automotive Engineering | Quality Management
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MMC defines the worst case condition of a part that will still guarantee, because it is still within the prescribed tolerances, the assembly between pin(s) and hole(s). When a hole is at its smallest (MMC) and a pin is at its largest condition (also MMC), we can be sure that we will still be able to assemble that part. Thus, MMC is widely used in cases where clearance fits are common

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4 Apply the bonus tolerance to the feature

The final step is to apply the bonus tolerance to the feature and check if it meets the true position requirement. Depending on the shape and complexity of the feature, different methods and tools may be used to measure and verify its position. For example, for a slot, you can use a pair of gage pins or a slot gage to measure its width and length, and then use a coordinate measuring machine (CMM) or a height gage to measure its position relative to the DRF. For a spline, you can use a spline gage to measure its pitch and profile, and then use a CMM or a dial indicator to measure its position relative to the DRF. For a profile, you can use a profile projector or a CMM to measure its shape and position relative to the DRF. In all cases, you need to ensure that the feature is within the tolerance zone that includes the bonus tolerance.

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5 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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Wahyudin P. Syam

Technical project manager in global navigation satellite system
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From geometrical measurement (tolerance verification), it is indeed a challenge to verify GD&T on irregular surfaces, such as, parts produces by additive manufacturing. Because, the nature of GD&T verification is that the minimum zone calculations are only affected by the extreme points (the highest or deepest feature). If the surface is irregular, hence, it will significantly increase the geometrical deviation (the minimum zone). One way to address this is by using a large probe tip in a CMM machine so that this large probe tip can perform morphological filter and will remove high frequency irregularity to make the minimum zone calculation becomes reasonable. Maybe, we can select the tip probe diameter to be 2x of the small feature size.

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How do you deal with complex or irregular features when using bonus tolerance for true position?

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1 Identify the datum reference frame

2 Determine the feature size

3 Calculate the bonus tolerance

4 Apply the bonus tolerance to the feature

5 Here’s what else to consider

Geometric Dimensioning & Tolerancing

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