Insights into what can go wrong and imposing constraints.
Last month’s column dealt with the perfect imaginary world of geometric dimensioning and tolerancing (GD&T). Here we address the imperfect real world of physical parts, where the main questions are: 1) what can go wrong and 2) how can we limit what can go wrong to guarantee function.
Determining what can go wrong and imposing unambiguous, functional limits of imperfection, are the essence of the GD&T “encoding” process. An important skill in this regard is the skill of exaggeration, namely the ability to deform geometric features to such an extent in our imaginations that we can make those potential imperfections obvious in order to then research their effects on assembly and operation.
The Imperfect Real World
Mechanical devices (an automobile, for example) consist of subassemblies (a diesel engine for example), which, in turn, consist of component parts such as pistons, piston rings and bearings. Parts, in turn, are collections of features-for example, cylindrical, planar, slab-like, spherical-each of which has degrees of freedom in terms of their form, size, orientation and location, as well as components such as entire surfaces, point collections, surface lines, axes and median lines, to name just a few, on which we impose controls.
For example, pistons consist of collections of internal and external coaxial, cylindrical features, bounded by collections of conical features (chamfers), bounded, in turn, by collections of planar features, and more, some of which are shown in Figure 1. Each feature of a part must serve at least one function, and it is our duty in the design group to research and understand those functions so we can impose constraints on its size, form, orientation and location to guarantee assembly and operation.
Machine Part Features and Components
After we have selected a part and the particular feature we need to control, say the cross bore in the piston, and determined what could go wrong-in this case everything, namely size, form, orientation and location-It is time to select the components of the feature we’ll need to address in each case, and then the appropriate tool with which to achieve our goals. Let’s start by viewing the five fundamental components of a cylindrical feature and the GD&T tools which apply to each of them as shown in Figure 2.
Clearly we must use the entire surface of the bore to control its size for which we will probably select the diameter tool. Even though (due to the Envelope Rule) the diameter tool controls both size and form, we might want to refine the control of form by adding the cylindricity tool, which again applies to the entire surface.
Turning to control of the orientation and location of the bore, the most appropriate component is clearly the axis, for which the only tool is the Position tool. These are the tools applied to the cross bore in Figure 1.
Geometry Control Tools and Feature Components
To get a substantial grip on them, here are tables illustrating the four groups of Geometry Control Tools defined in the ASME Y14.5 2009 standard, together with the feature components they address and the shapes of the tolerance zones they define.
Please also note the group of tools referred to as discouraged, namely tools for which we can easily substitute other more capable tools, as shown in the table.
As we have seen, the imperfect real world of physical parts consists of imperfect features and their imperfect components on which we impose constraints using tolerance zones defined by various GD&T tools. These ideas are fundamental to understanding GD&T and will be referred to again and again as we go forward. In our next column we will talk about the modifiers defined in the ASME Y14.5 2009 standard and how they expand our ability to encode and manage feature functions.
Note: We continue to encourage readers to submit questions, and look forward to providing succinct answers to as many as possible.