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Computer-Aided Design

Dr. Karen L. Wang

Table of Contents

Design Application: Dimensioning, Toleranceing and Drafting

Dimensioning 3
Tolerancing Parts and Assemblies 3
Two types of Tolerances 3
Conventional Tolerancing 4
Geometric Tolerancing 4
Tolerance Analysis 6
Creating Drawings with Computer-Aided Drafting 6
An Overview of Drawing and Detailing 6Procedures for Solid-Based Drafting 7
Geometric Dimensioning and Tolerancing 8
Screw Sizes and Threads per Inch 9
More websites---- 9

Design Application: Dimensioning, Toleranceing and Drafting

In this section we will discuss a couple of the applications of solid modeling: dimensioning, tolerancing and drafting.

Solid Model

Design ApplicationsMechanical Assembly
Mechanism Design
Sheet Metal
Computer-Aided Drafting
Tolerance Analysis
Finite Element Analysis
Rapid Prototyping

Before a design is released for manufacturing, it is necessary to perform the following:

1. detailing of the design, which includes the selection of standard components;
2. determination of dimensions and tolerances;
3.determination of special manufacturing notes, and
4. final drafting


A drawing is expected to convey a complete description of every detail of a part. However, dimensioning is as important as the geometric information. In manufacturing, a drawing without dimension is only worth as much as the paper on which it is drawn. Dimensions convey the requiredsize, whereas tolerances convey the required precision. Those are critical to the manufacturing of a part. This information can effect the choice of process (es) to be used, and fixture location, and machines required to produce a part.

According to the American National Standard Institute (ANSI) standards, the following are the basic rules that should be observed in dimensioning any drawing:Basic Rules in Dimensioning a Part

1. Show enough dimensions so that the intended sizes and shapes can be determined without calculating or assuming any distances.
2. State each dimension clearly, so that it can be interpreted in only one way.
3. Show the dimensions between points, lines, or surfaces that have a necessary and specific relation to each other or that control thelocation of other components or mating parts.
4. Select and arrange dimensions to avoid accumulations of tolerances that may permit various interpretations and cause unsatisfactory mating of parts and failure in use.
5. Show each dimension only once.
6. Where possible, dimension each feature in the view where it appears in profile and where its true shape appears.
7. Whereverpossible, specify dimensions to make use of readily available materials, parts, tools, and gauges.

Tolerancing Parts and Assemblies

Because it is impossible to produce the exact dimension specified, a tolerance is used to show the acceptable variation in a dimension. The higher the quality a produce has, the smaller the tolerance value specified. So tolerance is a measure of qualityin a manufactured product.

1 Two types of Tolerances

There are two types of tolerances: Conventional Tolerancing and Geometric Tolerancing.

1 Conventional Tolerancing


Conventional Tolerancing: bilateral, symmetric limit and no tolerance.

2 Geometric Tolerancing

Geometric Tolerancing specifies the tolerance of geometric characteristics. Basic geometriccharacteristics as defined by the ANSI Y14.5M 1985 standard include

Straightness Perpendicularity
Flatness Angularity
Roundness Concentricity
Cylindricity Roundout
Profile True Position

A standard way to convey the geometric tolerancing is with GD & T (Geometric Dimensioning and Tolerancing) symbols as shown below.

Geometric Characteristic Symbols...
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