Design Basics of Geometric Dimensioning and Tolerancing

World War I and World War II were two of the biggest tragedies of human history in terms of lives lost, infrastructure destroyed, and economies collapsed. 

One of the few industries which naturally thrived in before and during the wars was the arms and ammunition manufacturing industry. Swathes of the populace were involved in manufacturing large amounts of weaponry.

The problem with such manufacturing was the lack of expertise, design, and planning that went into it. The different assembly parts were manufactured without any modularity or interchangeability. A single malfunction could lead to the weapon becoming misfiring, or worse - not firing. 

While a large volume of arms was manufactured, there was significant wastage as well. The silver lining of all this was the standardization of the manufacturing process, including the very vital design field of Geometric Dimensioning and Tolerancing or GD&T.

At its very essence, GD&T is built based on two pre-war systems: the system of positive-negative dimensioning and positive-negative tolerance. For example, if a tire whose thickness is 10 millimetres, is manufactured with an error tolerance of 0.5-1 mm, then its relevant parts (mates) also have to be designed and built based on the same error tolerance range.

 

 

Vital Concepts of Geometric Dimensioning and Tolerancing

The concepts of GD&T were introduced after the world wars to lend a certain level of standardization to the manufacturing process around the world.

 

1. Nominal 

An error variation in the geometry of parts has to be allowed. Therefore, a baseline has to be defined that is theoretically perfect in terms of accuracy and precision. This baseline is known as the nominal value. It is a number that helps determine the geometry of a part in the ideal scenario of no error made.

 

2. Variation

In an ideal world, the manufacturing of all components would be accurate to the degree of a picometre. Unfortunately, the manufacturing world does not function on being ideal. Getting such a level of accuracy and precision will lead to high manufacturing costs, thereby endangering the entire sustainability of the manufacturing business. In simple words, 100% accuracy and precision are too costly.

The tolerancing aspect of the GD&T helps manufacturers retain the interchangeability of parts. The high accuracy levels of the ideal world are not required. Variation is the allowable range for the error in the dimensions of the part or individual features of the part. For example, a solid shaft with a diameter of 10mm and a tolerance variation of +-0.1 mm would fit with a hollow shaft of appropriate dimension and the same variation.

 

 

Current Trends Associated with Geometric Dimensioning and Tolerancing

 

1. Leading Software Solutions

VisVSA

VisVSA helps in the statistical analysis of the dimension-based variability of parts and assemblies while using the Monte Carlo simulation as its underlying base. Parameters specific to tolerance are dynamically extracted from computer-aided design models. 

Based on a defined tolerance range, the attributes of features are varied to generate sensitivity analysis, understand the contributing attributes, and the weightage of each attribute to the overall dimensional variability.

3DCS

3DCS is an end-to-end dimensional engineering software solution including modeling services, tolerance analysis, and SPC quality management. This solution put to use by companies in varied industries such as automotive, aerospace, medical device, high-tech electronics, and industrial machinery. 

Major companies that use 3DCS include: 

• Airbus
• Boeing
• Fiat Chrysler Automobiles
• General Motors
• LG
• Nissan
• Philips Medical
• Samsung
• Tesla
• Volkswagen

CETOL

In addition to VisVSA and 3DCS, another leading software solution is CETOL 6-sigma, which uses a combination of geometric, mathematical, and kinematics-based modeling along with automated variation analysis and GD&T simulations to help users get a detailed understanding of the performance of the assembly. 

CETOL 6-sigma is commonly used by Tier-1 and Tier-2 suppliers for predicting tolerance. One constraint of CETOL is that, while VisVSA and 3DCS can be used for complex and flexible parts, CETOL can only be used for solid parts such as engine and rigid casting.

 

2. GD&T Utilization by OEMs and Suppliers

Among the most common aspects of the automotive industry is horizontal integration. In simple terms, across the entire automotive supply chain, OEMs or original equipment manufacturers will manufacture some parts of the product, while outsourcing the others to Tier 1, Tier 2, Tier 3 suppliers to reduce the manufacturing costs on their side.

Since all the parts are not manufactured at the same location, Geometric Dimensioning and Tolerancing, GD&T, play a crucial role in ensuring that the parts fit to complete the assembly process. 

Consider the simple example of a BIW tooling process. Different aspects of BIW, like the front end, underbody, body sight, and framing will be handled by different players in the supply chain. However, for final assembly, a standard GD&T framework has to be followed to ensure commonly agreed on nominal dimensions and tolerance variations.

 

3. Global Importance of GD&T

People in different countries often speak different languages. However, manufacturing in the automotive industry is often a globalization-heavy exercise. For all parts manufactured, the expectation is of a baseline standard of quality. 

For ensuring the expected quality of manufacturing, there is a need for a commonly-understood language that provides a clear understanding of the accuracy and precision required in each part. That common language or globally accepted standard is Geometric Dimensioning and Tolerancing or GD&T.

In the 21st century, major OEMs such as Renault, Ford, Volkswagen, Fiat, etc. are trying to iron out tolerancing issues in the design phase itself. The global automotive supply chains dependent on each other. Even a small discrepancy in the manufacturing phase can lead to long delays in assembly. 

 

Conclusion

GD&T may have originated out of necessity after the Second World War, but it has a huge role to play in maintaining the peace of mind of people working in the automotive world.

If you want to learn more about the concepts associated with Geometric Dimensioning and Tolerancing, you can visit us at Skill-Lync.