zerogage - Sweep Gages Sweep Gages
Hole Location Inspection
Inspection of parts having holes that have a position tolerance are most commonly checked by using relation gages, sometimes called receiver or functional, that employ hard plugs. Figure 1 shows the geometry employed in determining the plug size. The plug diameter is equal to the minimum hole size minus two times the position tolerance (2r).

Figure 1

This functional checking method will provide assurance of part interchangeability. However, quite often the actual position of the hole is required for a positive quality control analysis as well as an aid in making proper machine adjustments. One method of determining the actual position is by making a lay-out inspection of the part.

By using soft plugs, a position check can be made faster and easier than by the lay-out method. Soft plugs are most commonly referred to as sweep gages. The sweep gage has a probe that contacts the periphery of the hole and transfers its' movement to an attached stationary indicator dial.

Figures 1 & 2 show the same hole condition. Figure 2 illustrates the operation of the sweep gage.

In fig. 2

  • P- radius of hole in permissible position
  • G- radius of probe
  • r- radius of tolerance zone
  • T- 2r (total indicator reading, TI R)
  • T /2- actual position

The pilot, or body, of the sweep gage is contained within a fixture on true center, the same as a hard plug would be located.

The sweep gage is inserted into the hole and the probe is rotated independently of the indicator until the lowest indicator reading is observed.

Figure 2

The indicator dial face can be set at "zero", or the reading can simply be noted by the operator. (The sweep gage has a position indicator to establish the direction of the lowest indicator reading).

A complete 360 degree sweep of the hole will show the "total indicator reading" (as most often specified), or the "actual position" can be determined by dividing the total indicator reading by two.

The tolerance on the hole diameter does not affect this check since the reading is the difference between the maximum and minimum readings as seen in figure 2.

Figure 3 shows the actual position in the second quadrant and illustrates how the actual position can be found in any quadrant.

Figure 3