GD&T reduces the ambiguity of design intent.
You’ve just designed a gear, tinkering with tolerances and tweaking leader lines to ensure your blueprint is properly called out. Then you get the prototype and realize that the keyway is in the wrong orientation.
But why? 15 degrees is 15 degrees, you think. How could they get it wrong?
You then realize that you forgot to call out datums – 15 degree leader line was just interpreted as an orientation callout for a bolt-circle pattern of holes, instead of orienting the gear teeth.
And why did the manufacturer interpret it that way? Because the blueprint says they can, even though you intended differently.
Utilizing GD&T makes design intent more clear without having to use additional lines or taking up blueprint space with notation. Plan out your design with the right symbols and basic dimensions, and you’ll have an easier time down the road.
QC folks know which inspection method to use.
When a designer uses GD&T for their prints, they also have more control in the inspection process. Here, on the QC side, interpreting an ASME Y14.5 compliant print informs our inspection plan. For example, we know to report a hole’s cylindricity if it is called out.
And this level of control can be granular. Positional callouts involve datuming, basic dimensions, and bonus tolerances. Informed uses of these specs will lead to relaxed (and, therefore, more passable) dimensions for the less critical features, while highlighting those features that require special attention.
The other side of the coin...
The benefits of using GD&T for your prints can only be reaped if they are fully compliant. It’s not enough to slap on a profile call-out (especially one without datums in the feature control frame!) and call it a day. You run the risk of adding more ambiguity, as both manufacturers and quality inspectors down the line will scramble to figure out the mixed messages of your prints.
We’ve seen a lot of messy prints in our 24-plus years in the industry. Oftentimes, we’re even tasked to interpret a bad print for clients who were “gifted” with a previous engineer’s work. They rely on our expertise and experience to determine what the true critical features are, and we work with them to develop an inspection plan that they can take back to their team. We’re often rewarded with the offer to execute that plan.
Do you have a part that needs to be inspected and need some GD&T advice? Were you "gifted" with a messy print? Contact us!
We’d love to have you stop by our office to talk shop or, if you prefer, email us a blueprint and we’ll respond along with a quote.