CAD Craft

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

Archive for GD&T

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.

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CAD: Process & Technique

Some users and others are coming to CAD fairly recently, and may be unfamiliar with its origins, how it got to be what it is today. Having been in the design field since well before its introduction, I thought I might be able to share some insights. This is pertinent because in many instances, the actual process and mindset one needs to bring to the CAD process is ensconced within the process of its evolution.

For an iteration of the technological/corporate developments is CAD, I have found The History of CAD to be an exceptional resource. One of the keys I found intriguing was in his section on 1970-1980, where Bozdoc asserts:

“MCS was founded in 1971 by Dr. Patrick J. Hanratty. Since the day it was founded in 1971, MCS has enjoyed an enviable reputation for technological leadership in mechanical CADD/CAM software. In addition to selling products under its own name, in its early years MCS also supplied the CADD/CAM software used by such companies as McDonnell Douglas (Unigraphics), Computervision (CADDS), AUTOTROL (AD380), and Control Data (CD-2000) as the core of their own products. In fact, industry analysts have estimated that 70% of all the 3-D mechanical CADD/CAM systems available today trace their roots back to MCS’s original code.”

I consider this to be important because, as many CAD polyglots will tell you, there are certain challenges which seem largely identical across certain types of CAD apps. This is also pertinent in that it further underscores the veracity of the premise that what you learn — in terms of technique/process — on one system is generally mappable to others.

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