Title Block Tolerances and GD&T
In the geometric tolerancing system, basic dimensions are used to override general tolerances (sometimes called title block tolerances). But let’s investigate these general tolerances a little more closely. A sample tolerance block is shown below, as taken from a drawing using metric (millimeters). First, notice that the tolerance allowed depends on the number of digits used after the decimal. This is common practice; at other times the tolerances may be divided based on the size of the dimension (1 to 10 mm, then from 10 to 50 mm, etc.). In our example, a separate tolerance is given for angles.
Some companies are trying to move away from these title block tolerances. It may be because they want to define everything with GD&T or other direct methods. While that might be OK to some extent, I would be hesitant to eliminate the entire idea of general tolerances, for one specific reason: the angular tolerance. Recall the old drafting rule that 90 degree angles are implied; they do not need to be dimensioned. But if the general tolerance block is removed, these 90 degree angles — unless they have GD&T applied — will have no tolerance!So in your efforts to improve drawings and streamline your designs, don’t go overboard. Title block tolerances are just fine, as long as you don’t get too lazy and let everything fall back on those numbers.
August 14th, 2011 at 11:24 am
Not only are tolerance standards in the title block a good thing, it should be expanded to include basic GD&T features as well. When there is an understanding of expectations, like with the notation “U.O.S.”, tolerances and other feature descriptions do not need to be repeated as often.
It may require a little time to develop some basic rules for a company’s internal standards. It may also require some time to communicate the geometric tolerances in a format that can be applied to the entire document. For example, perpendicularity may need to be represented as an angle in order to be applied to all thicknesses.
Following this advice will save time, produce a cleaner drawing and can, in some cases, reduce the probability of errors.
Let’s not always assume that “old school” is automatically obsolete. Best practices should be based on results and not a presumed expiration date.