Geometric Dimensioning and Tolerancing for Mechanical Design Part 4 ppsx

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Chapter

4

Datums

Datums are the reference surfaces or the starting points for the location and

orientation of features. They are essential for appropriate and complete toler￾ancing of a part. Datum geometries can become very complicated when they

are features of size, compound datums, or features of an unusual shape. Ge￾ometric dimensioning and tolerancing provides the framework necessary for

dealing with these complex datums. The simpler plus or minus tolerancing sys￾tem ignores these complexities, which means that plus or minus toleranced

drawings cannot adequately tolerance size features. As a result, many plus or

minus toleranced drawings are subject to more than one interpretation.

Chapter Objectives

After completing this chapter, you will be able to

Define a datum

Explain how a part is immobilized

Demonstrate how datum features apply

Select datum features

Demonstrate the proper application of datum feature symbols

Demonstrate how to specify an inclined datum feature

Explain how datum planes are established on a cylindrical part

Explain how datums are established

Explain the application of multiple datum features

Demonstrate the application of partial datum features

Explain the use of datum targets

47

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48 Chapter Four

Figure 4-1 The three mutually perpendicular intersecting planes of a datum refer￾ence frame.

Definition

Datums are theoretically perfect points, lines, and planes. They establish the

origin from which the location or geometric characteristics of features of a part

are established. These points, lines, and planes exist within a structure of three

mutually perpendicular intersecting planes known as a datum reference frame

as shown in Fig 4-1.

Immobilization of a Part

Parts are thought to have six degrees of freedom, three degrees of translational

freedom, and three degrees of rotational freedom. A part can move back and

forth in the X direction, in and out in the Y direction, and up and down in the

Z direction. It can also rotate around the X-axis, the Y-axis, and the Z-axis.

A part is oriented and immobilized relative to the three mutually perpendic￾ular planes of the datum reference frame (as shown in Fig. 4-2) in a selected

order of precedence. The datum reference frame is not absolutely perfect, but it

is made sufficiently accurate with respect to the part to consider it to be perfect.

Parts are relatively imperfect. In order to properly place an imperfect, rectan￾gular part in a datum reference frame, the primary datum feature sits flat on

one of the planes of the datum reference frame with a minimum of three points

of contact that are not in a straight line. The secondary datum feature is pushed

up against a second plane of the datum reference frame with a minimum of two

points of contact. Finally, the part is slid along the first two planes of the da￾tum reference frame until the third datum feature contacts the third plane of

the datum reference frame with a minimum of one point of contact. The pri￾mary datum plane on the part contacting the datum reference frame eliminates

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Datums