How GD&T Makes Better Medical Products Possible
Geometric dimensioning and tolerancing (GD&T) is intimidating even for trained engineers to pick up. GD&T has a set of vocabulary that is initially impenetrable and throws out a lot of traditional drawing practices. Once you get past all the new terms and how they interact with one another, you end up with a superior drawing with better tolerance zones and clear communication of design. With peoples health in the balance, this practice is critical in the medical device field.
Tolerance Zone Improvement
In traditional limit or coordinate tolerancing, the zones defined often fail to capture the actual geometry of the parts or their functional requirements. A hole is normally defined by an x- and y-value with a plus/minus limit. This defines a box for the center of the hole to reside. A square zone and a round hole allow greater offset at the corners of this box than the defined limit in each direction.
Locating the hole with a Position Tolerance defines a limit in any given direction: a round zone with a round hole. This leads to a 57% increase in the tolerance zone with no sacrifice in quality. Less parts rejected during production without risking fit or performance.
This also reflects real-world gauging of parts. A typical gauge will have a pin the size of the tolerance zone. The inspection team can read the value directly off the drawing and be assured it will work. While the drawing now clearly dictates the size of the gauge pin, it does not dictate how to apply it.
GD&T also provides controls for Form and Orientation. By establishing reference Datums and geometric tolerances to these Datums, it is simple to control surface variations, angularity to other surfaces and alignment of parts using tools like Flatness, Perpendicularity and Runnout.
Clear Communication of Design Intent
Frequently, a tight tolerance on a feature only tells you that it will be hard to make. Getting these tolerances can involve pushing a supplier to use a specific fabrication method that may require them to retool or train. This can all happen only because it is on the drawing.
Why is it so tight? Often assembly demands are so strict with all the stack up of so many parts and processes that the only tool available to the designer is a tight tolerance to fix their stack-up. Manufacturing is then stuck with an expensive part even if their skills could simplify production. That is only possible if they understand the why of a design, and drawings with limit tolerancing rarely accomplish this.
GD&T is strictly concerned with functionality and design intent of the features. The tolerance zones match the shape of the feature and may also reflect the shape of the mating feature. A tool like a Projected Tolerance zone shows the production team what the designer needs from that part: alignment to another part. In contrast, Straightness communicates that something will align to or along that given feature. Instead of a tight tolerance, the part has the shape it needs with any process that supplies it.
Some features are simply underdefined by the capabilities of traditional dimensioning. Complex surfaces use Profile tolerances to define boundaries for the surface rather than a complex row of dimensions calling out arbitrary points. Rotating bodies use Total Runnout to ensure center of mass stays in place instead of dictating angular or dimensional limits. For something as valuable and potentially dangerous as an MRI machine, these sort of tolerances cannot be left up to chance.