Coordinate tolerancing system

plus/minus tolerancing

For about one hundred fifty years, a tolerancing approach called "coordinate tolerancing" was the predominant tolerancing system used on engineering drawings. Coordinate tolerancing is a dimensioning system where a part feature is located (or defined) by means of rectangular dimensions with given tolerances. An example of coordinate tolerancing is shown in Figure 1-7.

Shortcomings of coordinate tolerancing

Coordinate tolerancing was very successful when companies were small, because it was easy to talk to the machinist to explain what the drawing intent was. Over the years, as companies. grew in size, parts were J obtained from many sources. The ability for the designer and machinist to communicate directly had diminished, and the shortcomings of the coordinate tolerancing system became evident. Coordinate tolerancing simply does not have the completeness to precisely communicate the part requirements.

Coordinate tolerancing contains three major shortcomings. They are:

  1. Square or rectangular tolerance zones
  2. Fixed-size tolerance zones
  3. Ambiguous instructions for inspection

Square tolerance zones

Let's look at the coordinate tolerancing shortcomings in more depth. First, let's examine the tolerance zone for the 8.0-8.4 dia. hole locations from the part in Figure 1-7. The hole location tolerance zone is formed by the max. and min. of the vertical and horizontal location dimensions.

Figure 1-8 shows that a 0.5 square tolerance zone would be, formed. The illogical aspect of a square tolerance zone is that the hole can be off its nominal location in the diagonal directions a greater distance than in the vertical and horizontal directions. A :more logical and functional approach is to allow the same tolerance for a hole location in all directions, creating a cylindrical tolerance zone.

Fixed-size tolerance zones

Next, let's discuss how coordinate tolerancing uses fixed-size tolerance zones, The print specification requires the center of the hole to be within a 0.5 square tolerance zone, whether the hole is at its smallest size limit or itslargest size limit. When the important function of the holes is assembly, the hole location is most critical when the. hole is at its minimum limit of size. If the actual hole size is larger than its minimum size limit, its location tolerance can be correspondingly larger without affecting the part function.

Square and fixed-size tolerance zones can cause functional parts to be scrapped. Since coordinate tolerancing does not allow for cylindrical tolerance zones or tolerance zones that increase with the hole size, lengthy notes would have to be added to a drawing to allow for these conditions.

Ambiguous instructions for inspection

A third major shortcoming of coordinate tolerancing is that it has ambiguous instructions for inspection. Figure 1-9 shows two logical methods an inspector could use to set up the part from Figure 1-7 for inspecting the
holes. The inspector could rest the part on the face first, long side second and the short side third, or the inspector could rest the part on the face first, the short side second and the long side third.

Because there are different ways to hold the part for inspection, two inspectors could get different measurements from the same part. This can result in two problems: good parts may be rejected Of, worse yet, bad parts could be accepted as good parts.

The problem is that the drawing does not communicate to the inspector which surfaces should touch the gaging equipment first, second, and third. When using coordinate tolerancing, additional notes would be required to communicate this important information to the inspector.

As you can see, coordinate tolerancing has some very significant shortcomings. That's why its use is rapidly diminishing in industry. However, coordinate tolerancing is not totally obsolete; it does have some useful applications on engineering drawings. The chart in Figure 1-10 shows appropriate uses for coordinate tolerances on engineering drawings.

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