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GD&T – Part 2 of 3 – When to use Fancy Symbols

GD&T – Part 2 of 3 – When to use Fancy Symbols

When to use Fancy Symbols

Geometric Dimensioning and Tolerancing (GD&T) is super complicated – it takes 40hrs to cover the introductory class at ASME. Honestly, as I mentioned in the previous blog, you can get away with using several symbols to cover 80% of the situations.

You should be aware that GD&T is often unnecessary. Here are a few times that this is the case:

  • Low precision requirements: If the title block tolerances are sufficient, or even much tighter than you require, there is a pretty good chance that adding GD&T won’t improve the performance of the part. In this case, it’s best to skip GD&T.    
  • Shapes or features that don’t interact with other parts: It’s a pretty good rule of thumb that if your feature or surface doesn’t interact with another part surface or dynamic fluid, it won’t need any tighter dimensions, so GD&T won’t be needed. This goes for simple and complex shapes.  
  • Cost considerations: You would hope that GD&T wouldn’t add more cost but guaranteed the machine or fab shop will add cost for this. Training additional personnel to interpret the GD&T, the additional QC required, and time to machine parts to tighter tolerances.   
  • Time Constraints: When a product needs to be made quickly, you may not have time to add GD&T and to account for it in manufacturing. This can be the case for the prototype build when you are using a fail-fast, fail-forward technique. However, if your product requires tight tolerances to achieve the desired performance, GD&T is often the correct decision.

When it comes to communicating design specifications and tolerances, GD&T is the tool to use. However, one should use it thoughtfully based on the needs of each project at hand.

GD&T Could Reduce Costs
  • Improved Communication: GD&T is not easy to do correctly. But when you do it right, then the manufacturer can focus on what matters most. Such as when the relationship between multiple features is critical. When multiple parts fit together, not doing proper tolerance stack-up and GD&T can cause you to scrap an entire order. The machine shop will tell you – “They are manufactured within tolerance.” Then any rework or changes goes towards the development process. 
  • Reduced Inspection Costs: GD&T reduces the time to inspect as it provides a clear and concise way of specifying tolerances. This can reduce the time and cost, and help the manufacturer focus on the features that matter.  
    • Pro tip: When doing dimensioning, make the dimensions as loose as possible. A feature with the potential to be saw cut should not have a super tight tolerance. It’s easier to tighten up some dimensions where needed than to loosen everything up. It’s also more relaxing to do it this way, haha.
  • Reduced Assembly Time: Using GD&T coupled with Tolerance Stack Up Analysis can simplify the assembly process. It does this by ensuring the parts fit together with minimal adjustment and no additional rework. This is how properly designed parts fit together in an assembly. 
  • Reduced Design Changes: GD&T helps prevent design changes and iterations because it forces you to think more about your design. Spend the time to determine where the critical dimensions need to be located. This will help you understand the design better, and often cause you to simplify the design. Simple designs, or designs with more effort or thought put towards them, can have fewer parts (think DFMA), and fewer parts, and more thought means fewer design changes later.

GD&T can be costly to implement, but its benefits surely exceed. I hope this helped to showcase that and more.

About the Author: Willem Mast is the owner and operator of WMD Squared.  He loves mechanical design of all types, optimizing engineering systems, and bicycles.