Feature Control Frame
The feature control frame provides a speci- fied control for single or multiple features. The rectangular frame is constructed, as required by the designer, to control one specific feature or group of features. A feature control frame must contain at least a geometric characteristic symbol and a tolerance value. Feature control frames are read from left to right and line by line in the case
of composite control frames. A feature control frame may contain the following symbols and tol- erance:
•
Geometric characteristics•
Diameter symbol•
Tolerance•
Tolerance modifier•
Datum reference letter(s)•
Datum modifierFigure 4-1 shows an example of a feature control frame that was specified to control the position of a feature or group of features. The first symbol in a feature control frame is the geometric characteristic symbol.
Figure 4-1 This feature control frame controls the position of a feature.
Attachment
Introduction
Feature control frames may be attached to features in various ways. The method of attach- ment picked by the designer determines the effect of the control specified for that feature or group of features. Feature control frames may be at-
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FEATURE CONTROL FRAMES
4
tached to a surface, axis, or center line. With each method of attachment, the feature control is lim- ited to only that portion of the part or feature to which the frame is attached. For example, if a fea- ture control frame is attached to a surface exten- sion line, then only that surface is controlled. Surface
Feature control frames that control surfaces either control the entire surface or just the surface indicated. For example, round features are con- trolled all around. The control applies all around because it is usually too difficult to orient the part for control on one side. Generally, a designer will want the same control to apply all around the part. In Figure 4-2, an example of how a feature con- trol frame is attached to the surface of a circular part is shown.
Caption: Figure 4-2 Feature control frame applied to the surface of a circular part.
Axis or Center Line
Feature control frames associated with round or width-type features are attached to extension lines of that feature. The interpretation means the axis or center line of the feature is controlled with no regard for the feature surface. The designer is specifying the required control so that fasteners will pass through parts or so that parts will mate with each other. The specification of such callouts means the axis or center line orientation—not the sides of the feature—is the critical concern. Ex-
amples of how an axis or center line is controlled are shown in Figure 4-3. (Note: Chapter 6 has many examples of feature control frame attach- ment.)
Figure 4-3 How an axis or center line is controlled.
The feature control frame is usually attached to the controlled feature with one of four meth- ods. These methods are as follows, with an exam- ple in Figure 4-4:
a) The feature control frame is placed below a dimension pertaining to a feature. The leader is from the dimension.
b) A leader from the feature control frame runs to the controlled feature.
c) A side or end of the feature control frame is attached to an extension line from the feature. The feature surface must be a plane. .500±.005 .003 .003 .500±.005 .003 .003 .500±.005 .250±.005 .001 .001 A B .002 .002 A .002 .002M B
Figure 4-4 Attaching feature control frames to the controlled feature.
d) A side or end of the feature control frame is attached to a feature-of-size dimension extension line.
Content
Geometric Characteristic Symbol The feature control frame consists of many compartments that contain information specified by the designer. The first compartment of the frame always contains the geometric characteris- tic symbol for Form, Profile, Orientation, Loca- tion, or Runout. Figure 4-5 provides an example of a Feature Control Frame. The design in Figure 4-4 includes Feature Control Frames for Form, Orientation, and Runout.
Figure 4-5 Feature Control Frame.
Tolerance
The next compartment always contains a tol- erance (Figure 4-6). The tolerance is either a di- ameter or a width. If the tolerance is cylindrical, the diameter symbol will precede the specified tolerance. The tolerance is always a total toler-
ance and not a plus/minus tolerance, as with the coordinate method of dimensioning.
Datum Reference Letters
When required, the datum reference letter or letters follow the stated tolerance (see Figure 4- 7). These letters are not always required, and the number of letters may vary from one to three, de- pending on the datum reference frame required. The alphabetical order is insignificant; the order from left to right is what establishes the order of precedence for the datum reference frame. The first letter identifies the primary datum plane, the second letter identifies the secondary datum plane, and the third letter identifies the tertiary datum plane.
Figure 4-7 Datum reference letters.
Modifiers
Modifiers are required in certain situations, or they may be specified in other cases depending on design requirements. Modifier specification is explained in detail in Chapters 5 and 6. The mod- ifier symbols appear in feature control frames, as shown in Figure 4-8. Refer to Chapter 2 for defi- nitions and the remaining text for applications.
Figure 4-8 Modifier symbols.
GEOMETRIC CHARACTERISTIC GEOMETRIC CHARACTERISTIC
SYMBOL SYMBOL 1.5 M D EM P
TOLERANCE ZONE SIZE TOLERANCE ZONE SIZE 1.5 M
Figure 4-6 Compartment containing a tolerance.
DATUM REFERENCE LETTERS DATUM REFERENCE LETTERS
1.5 M D EM P
MODIFIERS MODIFIERS