GD&T - STRAIGHTNESS OF A CYLINDRICAL SURFACE 'CIRCLE 1'

INTRODUCTION

Consider the surfaces of the cylinders shown. Which cylindrical surfaces are straight? In reality, no cylindrical surface is perfectly straight. How would we know if the cylindrical surface on the bottom is straight enough?

Straightness intro

STRAIGHTNESS CONTROL - CYLINDRICAL SURFACE

The straightness tolerance may be used to control the straightness of a planar surface, a cylindrical surface, a center plane or a centerline. When controlling a cylindrical surface, the straightness control (u) defines how much each surface line element may deviate from a perfectly straight line.

Straightness Control: Perfect straightness occurs when all points of an element lie on the same line. Straightness is a form control. The straightness control (u) defines how much each surface line element in a particular direction on a real part may vary from an ideal straight line.

Tolerance Zone: The straightness tolerance zone is the area between two parallel lines. The distance between the parallel lines is the value of the straightness control tolerance. Each line element of the surface must, independently, lie within the area defined by the tolerance zone. The straightness tolerance does not control how cylindrical the part is.

Straightness Tolerance Zone

Straightness Tolerance Zone

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

Feature Control Frame

To apply a straightness control to a surface, the FCF is placed in the view where the controlled surface elements appear as lines. The FCF may point to the surface. The FCF’s shown below apply a straightness tolerance to the entire surface. The surface elements must, independently, lie between two parallel lines that 0.1 mm apart.

Feature Control Frame Placement

INSPECTION

The straightness of a cylindrical surface may be inspected as follows:

  1. The cylindrical part is placed on a surface plate. The surface plate represents one of the lines that define the tolerance zone.
  2. A gage wire, the same diameter as the straightness control tolerance values, is used to inspect the straightness of each line element of the surface.
  3. If the wire fits between the part and the surface plate, the straightness error is too large and the part is not within specifications.

Inspecting Straightness