GD&T - PERPENDICULARITY OF A SURFACE AND CENTER PLANE 'CIRCLE 1'

INTRODUCTION

Consider the two parts shown. Which part has sides that are perfectly perpendicular to each other? In reality, no surface is perfectly perpendicular to another. How would we know if the part on the right has surfaces that are perpendicular enough?

PERPENDICULARITY CONTROL - SURFACE

The perpendicularity tolerance may be used to control the perpendicularity of a planar surface, a cylindrical surface, a center plane or a centerline. The perpendicularity control as applied to a surface or a center plane is very similar. Differences arise with the application of the feature control frame and inspection. When differences occur they will be explicitly explained.

When controlling a surface, the perpendicularity control (b) defines how much a surface may deviate from a right angle (90^o) with respect to an identified datum (a perfectly flat plane).

Perpendicularity Control: Perfect perpendcularity occurs when a surface is exactly at a right angle (90^o) to a datum. Perpendicularity is an orientation control. The perpendicularity control (b) defines how much a surface on a real part may vary from being perpendicular to a specified datum.

Tolerance Zone: The perpendicularity tolerance zone (when controlling a surface) is the volume between two parallel planes that are perpendicular to the datum plane. The distance between the parallel planes is the value of the perpendicularity control tolerance. The surface being controlled must lie within the volume defined by the tolerance zone.

Feature Control Frame: To control the perpendicularity of a surface, a feature control frame (FCF) is used to apply the tolerance to the desired surface.

To apply a perpendicularity control to a surface, the FCF may point to the surface or be attached to the extension line that extends from the surface. The FCF’s shown below apply a perpendicularity tolerance to the entire surface. The entire surface must lie between two parallel planes that are 0.1 mm apart. The planes are oriented 90^o from datum A. The datum feature used to establish the datum is identified with a datum feature symbol.

INSPECTION - PLANAR SURFACE

The perpendicularity control may be inspected as follows:

1. Place the datum feature surface specified in the FCF on a surface plate. The surface plate is used to simulate the datum and is termed the simulated datum.
2. A gage block that has nearly perpendicular sides is placed in contact with the controlled surface.
3. The gap between the square and the surface is the perpendicularity error. The error is verified with a gage wire that has the same diameter as the perpendicularity control tolerance stated in the FCF. If the wire fits between the surface and the square the error is too large and the part is bad.

The perpendicularity control may also be inspected as follows:

1. A gage block that has nearly perpendicular sides is placed on a surface plate.
2. The datum feature surface specified in the FCF is placed in contact with the gage block and clamped. The gage block is used to simulate the datum and is termed the simulated datum.
3. Run a dial indicator along the controlled surface at random.
4. 4. If the full indicator moment (FIM) of the dial indicator exceeds the perpendicularity control tolerance value, the surface did not meet specifications.