From: Geometrical Dimensioning and Tolerancing for Design, Manufacturing and Inspection (Third Edition), 2021. Flatness vs Surface Finishįlatness and Surface Finish are typically on quite different scales, with Surface Finish being a much finer scale. Datum axis: Table 13.11 gives the recommended number and location of radial sections (measuring sections) for the assessment of datum axes, according to TGL 43 043. The Flatness feature control frame may also rest on an extension line for a dimension of the surface. Simply callout the Flatness allowance in a feature control frame with an arrow leader designating the surface being controlled. Use of Flatness in a drawing is straightforward. The range of indicator travel must be less than the specified flatness tolerance. Place the surface to be measured face down on the surface plate and sweep it over the indicator tip. One way to achieve this measurement is to place a dial indicator such that its tip comes up through a hole in a surface plate. The sweep must be in a plane parallel to the desired flatness measurement plane. Measurement of Flatnessįlatness is typically measured by sweeping an indicator, probe, or other sensor over the surface. While Flatness is measured over a plane, Straightness is measured over a line.
Think of Straightness as the 2D equivalent of Flatness. A surface may be within flatness tolerance but not parallel to any other surface or datum. Parallelism uses a datum to control a surface but there are no datums associated with Flatness. These two are often confused, but in fact they’re quite different. Since Flatness refines a surface, it may be used in a Tolerance Stack. The distance between these two planes is the Flatness of the surface. The surface controlled must lie entirely within the space between the two planes. The other lies at the deepest trough of any irregularity of the surface. One lies at the peak of the tallest irregularity in the surface. The Tolerance Zone for a Flatness Tolerance is based on two parallel planes. Select the extension line of the dimension. It controls how much a surface on a part may deviate from an ideal plane. On the Datum Feature Symbol dialog, expand the Leader section and click. Flatness is a form control, because it is used to control the form or shape of a feature. It is not dependent on any other datums or features. When tagging these tolerances to a hole or pin, you usually need to include the diameter symbol before the number.
I’m thinking particularly of position and perpendicularity. Failure to include a diameter symbol in a feature control frame when needed. The GD&T Flatness symbol references how flat a surface is. But I want to point out a few of the more common mistakes that I encounter in my travels. GD&T Flatness Symbol Definition of Flatness
A leader line can also be used to connect the datum feature symbol to the view in some applications.Easy Guide to GD&T: Flatness When a surface is designated as a datum feature on a part, the datum feature symbol is placed on the edge view of the surface or on an extension line in the view where the surface appears as a line. Tangent plane: A plane that contacts the high points of the specified feature surface. The leader line can be shown as a dashed line if the datum feature is not on the visible surface. The datum feature symbol can also be placed on a leader line directed to the surface. The datum feature symbol is placed on the edge view or on an extension line in the view where the surface appears as a line. Click to select an edge of a part view and drag away from the edge to establish the datum line. Methods for placing datum feature symbols on datum feature surfaces. Use Datum to create and place associative datum symbols to the drawing view on a surface that appears as a linear or circular edge to identify datum planes in the part: Steps Copy link. Chapter 3 Datums 81 Angled Surface Datum feature symbol placed on edge view of surface or extension line from edge view Surface datum feature symbols must be clearly separated from dimension line arrowheads The Drawing The Meaning The Drawing The Meaning Example 3-3.