CAD Craft

A resource to CAD Craft specialists, and a showcase of my work — in both material and virtual worlds

CAD Tips: Nuances of concentric design and GD&T

The use & application of concentric or coaxial features in mechanical design is a common component of the CAD skill set, though it is often misunderstood. As mentioned in an earlier post, GD&T(geometric dimensioning & tolerancing— ASME Y14.5) is a baseline tool and industry standard. Many of the principles of GD&T come into use in the design of coaxial features & parts. Further, one can gain a more direct understanding of their applicability through corresponding aspects within the CAD package itself. In SolidWorks, this is most evident in Relations and Mates. Parenthetical references to sections below refer to the text of the ASME Y14.5 standard.  

(In this discussion,
there is the presumption that most readers
have a certain basic grasp of fundamental geometric terms & principles.
Links have been provided in certain instances to augment this.)

Relations are exactly what the word suggests — the establishment of constraints on how two or more elements of a sketch or feature relate to one another. These include such things as concentricity, parallelism, coincidence, tangency, perpendicularity, etc. Relations are the geometrical laws which enforce constraints on a design, thereby controlling certain variables so that your model ends up being fully defined. It is not always necessary for a sketch to be fully defined to move forward with the creation of a feature, but for newbies, it is the safest path.

In GD&T, these principles appear in essentially two contexts:  Limits(2.4) & Geometrical characteristics(3.3.1). Limits are familiar to us in many contexts, most commonly through the use of dimensioning, which establishes limits on size and relative position. Geometrical characterics are represented via symbols which are a language independent means of conveying this information, which was one of the key bases for establishing this standard.

In CAD tools, relations embody & implement these principles of limits & geometrical characteristics. However, the variations in how we apply these principles will have a significant impact on the stability & simplicity of our model, and overall design. For instance, when establishing concentricity in a sketch, what you are doing is essentially saying that the two circular sketch elements share a common center. At the assembly level, when establishing Mates(assembly relations), concentricity is reflective of the real world, where it can mean either coaxiality(common axis), or concentricity of surfaces. This may seem a trivial difference at first, until we understand how limits — particularly limits of size — and other geometrical characteristics, which are used to determine the acceptability of a part, come into play. Limits are implicitly linked to tolerances and tolerancing, which will be addressed later.

For example, when discussing limits of size in the context of Y14.5, there are three aspects of concern: MMC, LMC & RFS. MMC = Maximum Material Condition. This is often confused in its application to holes vs shafts, though it is simply common sense, if we don’t over think it. When dealing with holes, the MMC  is when the hole is smallest, when there is the most material remaining. With shafts, or shperoid features, MMC is when the diameter is at its maximum, when the greatest amount of material is present. Conversely, LMC = Least Material Condition, the reverse of MMC. In this sense, a binary relationship established — MMC is the +, LMC is the -. RFS = Regardless of Feature Size.

Each of these relates to the application of tolerances — the range of acceptable variation of a particular dimension. As the acronym GD&T suggests, there are tolerances of position, and tolerances of size, as well as tolerances of form. The first two are familiar to most, but the impact of the latter can have a critical impact as well. Tolerances of form come into play most significantly in relations to the acceptability of a part at the point of inspection — the extent to which the manufactured form adheres to the specified parameters of the design. These factors directly impact such things as fit(ease of assembly), and the subsequent reliability of the device in question. In GD&T, these are represented largely by a part’s geometric characteristics, such as straightness, flatness, angularity, circularity or cylindricity(out of round), etc.

In CAD, at the part level, such factors are implicitly controlled by the application through relations within a sketch, or between sketches. The model represents the ideal goal. In the real world of machined parts, tolerances must be taken into consideration to allow for the inherent variances in the manufacturing process. Through the wise implementaion of tolerances, one can strongly influence the level of difficulty in the machining process, and therefore the rate of rejected parts, which is directly related to the cost of a particular design. This makes clear that the application of one’s tolerancing philosophy must be a balance between the needs of the part/assembly, in terms of its efficient functionality, and those of the actualities of the machining process. This is why I often stress(no pun intended) how crucial it is to be familiar with the machining capabilities of a given organization or vendor prior to initiating a design so that these aspects can be factored in effectively.

Clearly, the straightness, flatness, angularity or out-of-roundness of a part will impact its fit into other mating parts. For this reason, the use of tolerances actually create what is called a tolerance zone. For instance, a hollow cylindrical tolerance zone is created by a combination of the positional tolerance of a hole and its size tolerance, as you see in Fig. 1 below.

An illustration of a hole's tolerance zone

Figure 1: An illustration of a hole's tolerance zone

In Fig. 2, we see in the top view how the phantom cylinder  overlaps the actual hole on both sides.

An illustration of a hole's tolerance zone, top view

Figure 1: An illustration of a hole's tolerance zone, top view

In a part composed of multiple coaxial features, it is easy to see how such factors are important, as shown in Fig. 3 below.

A part composed of coaxial features

Figure 1: A part composed of coaxial features

Below are images of an assembly which further demonstrate these principles.

A simple gear drive assembly

Figure 4: A simple gear drive assembly


A simple gear drive assembly - top view

Figure 5: A simple gear drive assembly - front view


A simple gear drive assembly - top view

Figure 6: A simple gear drive assembly - top view


A simple gear drive assembly - section view

Figure 7: A simple gear drive assembly - section view


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