How does a design go from the computer screen to something you hold in your hand? Not being able to fully answer this question is a huge risk in manufacturing because . One of the important tools engineers use to ensure success is Geometric Dimensioning and Tolerancing (GD&T).
A good technical drawing is essential for communicating your mechanical part designs to a manufacturer. Drafting, as a professional discipline, is all about creating technical drawings that are as unambiguous as possible, and that means defining features explicitly. The most basic implementation of that concept is dimensioning, where you state the distance or angle between features. A proper technical drawing will also include tolerances for those dimensions, and I recently explained how to avoid the pitfall of stacking those tolerances.
Dimensions and tolerances alone, however, don’t tell the complete story. On their own, they don’t specify how closely the geometric form of the manufactured part needs to adhere to your perfect, nominal representation. That’s what we’re going to dig into today with GD&T.
Having the Perfect Part Fabricated
Imagine you’re drafting the simple cylindrical part on the left. You would, at the very least, specify the length and diameter of the cylinder. If you’re thorough—and you should be—you would also include the acceptable tolerances for each of those.
The problem is that the cylinder may not end up perfectly round, and the flat end surfaces may not be perfectly perpendicular to the axis of the cylinder. The finished part could match your drawing, but still be ruined by an oblong shape or an angled faces. Even with tight tolerances, your part could end up looking like the part on the right (which I’ve dramatically exaggerated for illustration purposes).
The reason for the potential discrepancy is that manufacturing is imperfect. The part may have been turned on a poorly maintained lathe, which caused the oblong profile. A long cylinder could have been divided up into many pieces with your specified length using a bandsaw, but maybe the blade was worn and it drifted during the cut. For those reasons, it can be important to explicitly state what the geometric form needs to be, in addition to the basic dimensions.
Defining Your Three-Dimensional Form
Like everything else related to drafting, the specifics of Geometric Dimensioning and Tolerancing vary between different sets of standards. But, the general concepts are universal. GD&T gives you the ability to define the three-dimensional form of your parts with various “controls.” Really, they’re just geometric tolerances, but I’m going to refer to them as controls in this article in order to avoid confusion with the standard dimensional tolerances you’re probably used to.
The Circularity (or Roundness) control, for example, would let you tell the manufacturer how close to perfect the circular profile of your cylinder needs to be. If you said it needed a Circularity of 0.01 mm, that means a dial indicator run around the cylinder won’t show a difference of more than 0.01 mm between the highest and lowest points.
As you can imagine, GD&T is …read more
Source:: Hackaday