Internal recesses and surfaces often are incorrectly dimensioned. The use of the common axis as a datum line is the key to correct dimensioning.

Figures 1 and 2 show a recessed groove as it is sometimes dimensioned. In Figure 1 dimensions A and B use a common axis, but dimension C uses the O.D. as a reference dimension. The recess diameter would include the tolerance allowed for dimension A, plus the tolerance on dimension C. This dimension would prove very difficult to measure.

In Figure 2 dimension D is an extension of diameter B. The recess diameter would include tolerance for D added to the tolerance of B. This also would result in a difficult to measure dimension.

The preferred dimensioning is shown in Figure 3. All diameters use the same axis and can be measured properly. A note stating “Diameters B and E, or A, B and E to be concentric with .xxx FIM” will eliminate any questions about concentricity. The location of the recess with respect to a datum plane is shown as dimension F and width as dimension G. Anything left to interpolation or interpretation is subject to error.

If finish is important, a small angle on the walls on the recess is recommended. If the recess groove must be burr free, it should be shown in a note.

Chamfers or radii on corners of a recess groove, shown in Figure 1, should be specified, if required. A chamfer may affect the use of a snap ring.