GD&T - Part 6; Profile

Line vs Surface Profile

Profile tolerances are can be applied in two ways; a line or a surface profile. Both are usually applied to irregular shapes where other tolerances are difficult communicate or cannot be used. 

A line profile will typically apply to a specified cross section(s) in a component at which the boundary is controlled by a 2D tolerance zone with relation to the nominal boundary.

Unlike a line profile tolerance, a surface profile tolerance is applied to the whole feature and is controlled by a 3D tolerance zone typically bound by two offset surfaces to the nominal.

Datums are the key to good profile tolerances

In both cases, profile tolerances only control the form of a feature. To control the feature's orientation or location, datum features must be called out in the tolerance frame. In the examples below, all four examples would pass inspection!

All round, all over and within limits

Profile tolerance zones can be applied in three ways:

All round - symbolised with a single circle and stated with a leader line from a tolerance frame to an edge or surface, an all round tolerance zone can be applied either across the full length of the component (surface) or the full boundary at a specified cross section (line).

All over - symbolised with a double circle and stated with a leader line from a tolerance frame to an edge or surface, and all over tolerance zone is applied over the full component. Note, an all over tolerance zone can only be used with a surface profile tolerance.

Within limits - greater control over both line and surface profile tolerances can be applied to specific areas of a component. When using profile tolerance in this manor, no symbol is shown on the leader line, instead boundary conditions are stated above the tolerance frame as shown. 

The leader line must land within the boundary you wish to control.

Bi-lateral, Unilateral or Unequal Tolerance Zones

Unlike all the other geometric tolerances we have spoken about profile tolerances can be applied as a nominal tolerance zone (bi-lateral, equal both sides), a biased tolerance zone (unilateral) or a variable tolerance zone (unequal offset to nominal).

To specify a unilateral or unequal tolerance zone a ‘UZ’ symbol followed by the required offset value can be stated within the tolerance frame. The specified amount of offset can be in both the positive (material on) and negative (material off) directions.