Flat bottom holes have two important areas which must be clearly defined on a print. These areas are shown as A and B in Drawing #28.

If a flat bottom is required in the blind hole, this should be specified by noting the tolerance limits and surface finish if these are important. If only a portion of the bottom of the hole is important, as shown and a drill point can be allowed, this too should be noted on the drawing.

If a “sharp” corner is required at A, it should be dimensioned as discussed under Sharp Corners.

An additional problem with flat bottom holes is measuring the relationship of the bottom to the axis if flat bottom is critical. A full indicator movement (FIM) is required to establish the relationship of the flat bottom to the axis of the hole.