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.

9 Comments

  1. 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.

  2. Is there anything that governs whether a material specification takes precedence over a critical dimension? To provide some more detail: the material call out is ASTM A513 type 5. The dimension in question is the OD of sink drawn tubing. The print calls out 22.00 mm. When referencing the global tolerances in the title block, it would allow 22.00 +/- 0.25mm. However, the material standard previously referenced allows +/- 0.005mm. Should this tolerance be noted on the call-out for 22.00 mm ? Or, is it standard that the material specification takes precedence over any dimensions on the print? Hope this is not confusing, I’ll gladly provide any additional information.

    • It is generally understood that a dimension/tolerance given in the field of a drawing outweighs any other tolerance such as the title block tolerances or general notes.
      Although an ASTM spec such as yours isn’t a direct “tolerance,” I still think that it would be the one to follow for that item. Title block tolerances are pretty much the lowest level of hierarchy, used when nothing else on the drawing addresses a tolerance.
      That said, I don’t know of a specific paragraph within ASME Y14.5 that directly speaks to your question.

  3. Do welding symbols on a print follow the tolerance callout in the title block?

    • According to ASME Y14.5, a general tolerance block covers “all dimensions on a drawing for which tolerances are not otherwise specified” (paragraph 2.1.1(e)).

      And paragraph 1.3.22 defines a dimension as a number “used to define the form, size, orientation or location, of a part or feature.”

      So I suppose yes, welding could appeal to the title block tolerance callout only insofar as it pertains to size, form, orientation, or location. (Not to weld strength/integrity or other parameters.) That said, however, I really wonder if a designer intends for that. But I’m just interpreting what the legal standard says.

      • The reason I ask is because of the following:

        A welder pointed out on the print he is working from wants a 5/16″ weld on a 1/4″ self/revel. Obviously, the only way to achieve this is by having the weld 1/16″ under size. But the fabricating tolerance is +/- 1/16″….I am still not convinced that weld sizes on a print fall under the fabricating tolerances. One reason is the weld symbols on our prints are in fractions, however, the tolerances in the title block only specify decimal tolerances. I realize a guy can convert fractions to decimals, but would think it should not work that way.

      • Welding symbols call out the spec such as fillet size, length of weld, pitching etc have their own welding tolerance, you can refer to AWS welding code book. Drawing tolerance do not apply to welding, you wont meet the tolerance even +-1.0mm.

  4. Is it best practice to allow designer/engineers to modify the tolerance block for each drawing to make it more applicable to the design’s need or to enforce the defaults that are set on the template (that were already agree upon)? I see the value in modifying per the design/drawing needs but I fear allowing them to drift from the defaults.

  5. Yes, I think it is indeed a best practice to allow that. The goal is to design the best part — to be functional, with ample tolerances for manufacturing, with the highest quality but low cost.

    By blinding following the same title block tolerances for every drawing, you get commonality but at the price of tying the hands of the designer and manufacturer.

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