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Improving Manufacturability by Properly Applying GD&T as an Effective Design Tool

Most modern manufacturers, in all industries, now have Design for Manufacture (DFM) as part of their design processes. Efforts range from simple requests for larger tolerances, to thoughtful and thorough review of design intent by collaborative teams of design, manufacturing and quality disciplines. The DFM process is extremely important to the success of a manufacturing project and should be given a high priority. Without careful attention to the manufacturability of a part or assembly, it’s unlikely that the project will achieve optimum performance or profit results.

This article will discuss use of Geometric Dimension and Tolerance (GD&T) as a tool in the effort to create a part or assembly that meets design intent and can be manufactured at a profit. GD&T, as defined in ASME Y14.5M 2009, is the national standard, creating a concise language used on engineering documentation to provide one clear definition of mechanical parts. In other words, it is the language used by the designer to convey intent to the manufacturing engineers and technicians responsible for creating the product, and to the quality engineers and technicians tasked with verifying the compliance of the product to design criteria. When properly applied, the designer can communicate complex shapes and relationships to the manufacturing team, leaving no room for interpretation.

Proper application of GD&T maximizes tolerances without sacrificing product quality. As the tolerance increases, the cost of the product decreases. Prior to the advent of GD&T, datums as absolute locations did not exist. Parts were dimensioned as a set of distances from one feature to another, with each distance having a tolerance. The result was a “stack up” of tolerances and questions as to which feature held precedence over another. The relationship of features was open to interpretation. The proper use of datums allows the designer to establish primary features based on function, and then relate secondary features to those datums. This “functional” approach to selecting datums will usually result in the least variation in the part or assembly at the lowest cost. Tolerances on features relating to datums can be maximized. An important consideration is that datums must be designed as sufficient size. This is in accordance with the Y14.5M-2009 standard, and helps to assure reproducible inspection of parts. It is critical that the design be manufacturable and measureable. If you cannot measure the feature, you cannot assure compliance.

Defining the size and location of a radius is a frequent problem. Without directly opposed points (180+ degrees of arc), you cannot reproducibly find the center of a radius. When toleranced directly, the person measuring the finished part must determine where the center is out in space in order to decide if the dimension is in tolerance. If the part is measured on a CMM (co-ordinate measuring machine) or other system that locates measured data points on the surface and calculates the radius and location, a different set of issues arises. These systems typically rely on software to calculate the radius size and location, most often by using the sum of least squares algorithm. Even a slight variation in the location of a measured point, often within the measurement uncertainty of the system, will result in a large difference in the size and location of the radius.

A method to dimension radii, especially when there is less than 180 degrees of arc, is to define the surface as a profile. This creates unilateral or equilateral parallel boundaries that define the location and size of the radius. With this method, it is possible to establish compliance without relying on the sum of least squares by using conventional measuring techniques such as surface plates, fixtures and indicators. When designing a feature that becomes a collection of arcs, lines, surfaces, etc., use profile tolerance as opposed to individual entities, which must each be verified.