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Engineering CAD software is a constantly evolving tool that allows engineers to design more rapidly and efficiently than ever before. However, one basic element that has been consistently required over time is a detailed drawing. Drawings are the key piece of communication between Engineers, Machinists, Fabricators, Vendors and Customers.
An important and critical way an Engineer conveys the design intent within the drawing is with the use of GD&T (Geometric Dimensioning & Tolerancing). This isolates the functional and non-functional features, and creates a relationship from one feature to the next. GD&T provides a lot of information regarding how a part or assembly will be manufactured or assembled. Most importantly, it allows for repeatability for parts and assemblies in order to ensure that consistent quality is achieved. Below please find seven tips on using GD&T and things to consider when adding GD&T to your drawing:
- Get Familiar. There are many symbols and callouts that are used with GD&T. Each one defines a requirement within the part to achieve the functionality and repeatability that is required. Taking classes, using textbooks or industrial handbooks and on the job experience are all ways to develop the necessary GD&T skills.
- Use Company and/or Industry Standards. Use the GD&T standards that a company or an industry already have in place. Examples of these are ASME (American Society of Mechanical Engineers) and ISO (International Organization for Standardization). In some cases, companies will create “best practice” standards which have been developed over time as a company gains experience within the industry they serve. These standards are the foundation on how to approach designing and detailing.
- Know the Intent of the Design. Every part designed serves a specific purpose within a family of parts. To convey the intent, Engineers use GD&T to isolate the features that are critical to the performance. For example, a part using dowel pins to establish position relative to another feature or part. On the other hand, GD&T also identifies the non-functional areas that are not critical to performance, allowing the fabricator to focus on the functional areas. Properly using GD&T ensures that parts will work together according to the overall design intent of the Engineer.
- Watch Out for Increased Costs. Using GD&T can drive costs up or down. Adding a lot of GD&T and the use of tight tolerancing can add significant costs. In most cases, it creates extra steps in the fabrication process to achieve the tolerance being called out. Additionally, it creates the need for inspection to ensure the tolerance specified is meet. Therefore, avoid adding GD&T to parts if it is not required.
- Understand Fabrication Tools and Inspection Instruments. When designing, understanding the tooling and the fabrication process for how your part will be made is important. When calling out geometrical features or using tight tolerancing, keep in mind how it can be achieved. For example, calling out perpendicularity or parallelism for a surface or surfaces may require additional machining and a grinding process. After fabrication, inspection may be required. If a part is simple, this inspection could be performed with the use of calipers. If a part is complex, the use of a CMM (Coordinate Measuring Machine) may be required. Becoming familiar with fabrication tools and inspection instruments is a valuable tool when applying GD&T.
- Consider the Material Selected. Material specification can determine what kind of tolerancing can be held. Materials such as plastics, steels and aluminums behave differently depending on their destined environment. Materials can expand or contract depending on the temperature. The fabrication process can also cause the material to behave differently. These factors need to be taken into consideration when adding GD&T to the details of the drawing.
- Know Your Manufacturing Skill Set. It is easy for Engineers to forget the skill sets required to fabricate the parts and implement the callouts of the design. Design and detailing in CAD is only a portion of the process before becoming a completed part. When it comes to designing and the use of GD&T, working with machinists, welders, fabricators and other professionals will provide better insight regarding the overall picture.
There are a lot of factors to take into consideration when designing a part and using GD&T. The use of GD&T will always be a way of conveying the Engineer’s intention when detailing a part in either a drawing or a CAD model. There are many resources to take advantage of in order to better understand the use of GD&T. Correctly applying GD&T will keep costs of parts to a minimum, ensure better communication between your fabricators and repeatability from one part to the next.