When creating a Datum Feature Symbol, the system automatically names it the next available name in the standard sequence, starting with A, then B, C, and so on. See Using Text Styles and Formatting Text. Datum feature symbols placed within geometric tolerances imply a datum in addition to the callout for the geometric tolerance. You can also apply text styles or Text menu options to format individual markers. To control the text characteristics (font size and style) globally, assign a text style to the class. The datum reference letter (which corresponds to the Feature Control Frame reference letter) automatically increments each time you insert a symbol. Edit the marker from the Attributes palette (see Marker Attributes). To add a shoulder to the leader line, click the Selection tool from the Basic palette and drag the control points on the object.

The properties can be edited from the Object Info palette. Set the default properties, and click OK. The first time you use the tool in a file, a properties dialog box opens. Most Surface Finish measurements are averages of some kind, while Flatness is a worst case.Ĭlick to visit our reference page for more information about Surface Finish.A datum feature symbol consists of a marker, leader, and datum reference letter.Ĭlick the Datum Feature Symbol tool from the Dims/Notes tool set.Ĭlick to place the object in the drawing, and click again to set the rotation. Flatness vs Surface Finishįlatness and Surface Finish are typically on quite different scales, with Surface Finish being a much finer scale. The Flatness feature control frame may also rest on an extension line for a dimension of the surface. If necessary, datum references and modifiers applicable to the feature or the datums are also contained in the box. Simply callout the Flatness allowance in a feature control frame with an arrow leader designating the surface being controlled. Feature Control Frame - is a rectangular box containing the geometric characteristics symbol, and the form, runout or location tolerance. 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. 'Datum Identification frame (Symbol) - The datum identification frame (symbol) contains the datum reference letter in a rectangular box, usually preceded and followed by a dash (any letter except I, O, or Q).' To my knowledge, ASME Y14.5M-1994 says that a datum feature symbol is A with a 60deg triangle for a 'arrowhead'. 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. It controls how much a surface on a part may deviate from an ideal plane. 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. The GD&T Flatness symbol references how flat a surface is. GD&T Flatness Symbol Definition of Flatness