WHAT ARE AUXILIARY REFERENCES? WHAT ARE THEIR USES?

For Sheet Metal Parts;

In my previous blog post, I mentioned the 3-2-1 rule in referencing. However, for large and bulky pieces, it may not be enough to pass a plane from 3 points, a straight line from 2 points, and an origin from 1 point. In this case, we use auxiliary references.

Reference points are actually the points where the tolerances of the part are closest to zero in theory. You hold the part from these points and measure the tolerances of the other points.

As the part grows, it becomes more difficult to provide tolerances. Therefore, the manufacturer is relieved by holding more from certain parts of the part. However, the thing to be careful about is that you cannot assign auxiliary references to the functional or aesthetic surface points of the part. If you assign auxiliary references to these points of the part, you will be misled because you will see these points close to zero in the measurement. You will move the tolerance to an area that is not important.

Auxiliary references can be assigned by the designer, or after a request from the manufacturer. While the designer should assign auxiliary references for large parts before a request is received from the manufacturer, the manufacturer should not request auxiliary references from many points in order to relax himself.

Auxiliary reference symbols can be displayed in different ways.

In the past, main references and supporting references were shown as follows;

If there is a filled triangle, it is shown as the main reference (symbol R1), if there is only a hexagon (symbol R14), it is shown as the auxiliary reference.

With the spread of ASME, main references began to be named A1-A2-A3, B1-B2 and C1. Auxiliary references continue as A4-A5 ...... D.

For CNC Parts;

There is no auxiliary reference definition for CNC parts. Every reference you assign to critical areas of the part can be considered as a main reference. The difference with sheet metal parts is that sheet metal part reference points are not tolerated, while CNC parts references are tolerated.

Each reference should be identified by a different letter of the alphabet (except I, O, or Q). When the alphabet is exhausted, double letters (AA through AZ, BA through BZ, etc.) are used. These letters have no meaning other than to distinguish the reference features. Although the letters do not have to be assigned consecutively or to start with A, there are advantages and disadvantages to doing both. In a complex assembly, it may be desirable to coordinate letters among the various drawings so that the same feature is not B in the detail part drawing and C in the assembly drawing. It would be more appropriate to use letters that are distant from B in the assembly part drawing (such as KL). It can be confusing if two different parts in an assembly both have a reference feature G and these features do not match. On the other hand, a reader of drawings that do not go consecutively may be frustrated when looking for missing data where letters are omitted. (Example: In references labeled DEK, the reader may need to look for references such as FE.) However, such letter choices are usually left to company policy and may be based on the typical complexity of the company's drawings.

Examples: