Geometric Dimensioning and Tolerancing for Mechanical Design Part 6 ppt

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Chapter

6

Orientation

Orientation is the general term used to describe the angular relationship be￾tween features. Orientation controls include parallelism, perpendicularity, an￾gularity, and, in some cases, profile. All orientation controls must have datums.

It makes no sense to specify a pin, for instance, to be perpendicular. The pin

must be perpendicular to some other feature. The other feature is the datum.

Chapter Objectives

After completing this chapter, you will be able to

Specify tolerances that will control flat surfaces parallel, perpendicular, and

at some basic angle to datum features

Specify tolerances that will control axes parallel, perpendicular, and at some

basic angle to datum features

The orientation of a plane surface controlled by two parallel planes and an

axis controlled by a cylindrical tolerance zone will be discussed in this chapter.

When a plane surface is controlled with a tolerance zone of two parallel planes,

the entire surface must fall between the two planes. Since parallelism, perpen￾dicularity, angularity, and profile control the orientation of a plane surface with

a tolerance zone of two parallel planes, they also control flatness if a flatness

tolerance is not specified. When it is desirable to control only the orientation

of individual line elements of a surface, a note, such as EACH ELEMENT or

EACH RADIAL ELEMENT, is placed beneath the feature control frame.

When an axis is controlled by a cylindrical tolerance zone, the entire axis must

fall inside the tolerance zone. Although axes and center planes of size features

may be oriented using two parallel planes, in most cases, they will be controlled

by other controls, such as a position control, and will not be discussed in this

chapter. The position control is a composite control, which controls location

87

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Source: Geometric Dimensioning and Tolerancing for Mechanical Design

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MHBD031-06 MHBD031-Cogorno-v5.cls April 11, 2006 17:55

88 Chapter Six

and orientation at the same time. Parallelism, perpendicularity, and angularity

are often used to refine the orientation of other controls such as the position

control.

Parallelism

Definition

Parallelism is the condition of a surface or center plane, equidistant at all points

from a datum plane; also, parallelism is the condition of an axis, equidistant

along its length from one or more datum planes or a datum axis.

Specifying parallelism of a flat surface

In a view where the surface to be controlled appears as a line, a feature control

frame is attached to the surface with a leader or extension line, as shown in

Fig. 6-1. The feature control frame contains a parallelism symbol, a numerical

tolerance, and at least one datum. The datum surface is identified with a datum

feature symbol. Parallelism tolerance of a flat surface is a refinement of the size

tolerance, Rule #1, and must be less than the size tolerance. The size feature

may not exceed the maximum material condition (MMC) boundary, and the

thickness at each actual local size must fall within the limits of size.

Interpretation. The surface being controlled in Fig. 6-1 must lie between two

parallel planes separated by the parallelism tolerance of .005 specified in the

feature control frame. The tolerance zone must also be parallel to the datum

plane. In addition, the surface must fall within the size tolerance, the two par￾allel planes .020 apart. The entire part in Fig. 6-1 must fit between two parallel

planes 1.020 apart. The controlled surface may not exceed the boundary of

.005 .005 A

3.00

1.00

2.00

7.00

.XX = ± .01 A

ANGLES = ± 1°

.020

The .005 parallelism

tolerance zone must be

parallel to datum A.

Figure 6-1 Specifying a plane surface parallel to a plane surface.

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Orientation