Geometric Tolerances In Product Design

Geometric tolerances-Geometric dimensioning and tolerancing (GD&T) is a system for defining and communicating engineering tolerances. It uses a symbolic language on engineering drawings and computer-generated three-dimensional solid models that explicitly describes nominal geometry and its allowable variation. It tells the manufacturing staff and machines what degree of accuracy and precision is needed on each controlled feature of the part. GD&T is used to define the nominal (theoretically perfect) geometry of parts and assemblies, to define the allowable variation in form and possible size of individual features, and to define the allowable variation between features.

All Over Specification [ASME Y14.5-2009 Section 8.3.1.6] - In addition to a general profile of a surface tolerance there is the option of specifying that the tolerance applies all over on the field of the drawing. It is important to realize that this specification, whether in a general note or on the field of the drawing, applies UNLESS OTHERWISE SPECIFIED.

All Around This Side of Parting Line - To apply a requirement to all features all around one side of a parting line, the graphical symbol for all around this side of parting line is indicated on the leader line.

All Over This Side of Parting Line  - To apply a requirement to all features all over one side of a parting line, the graphical symbol for all over this side of parting line is indicated on the leader line.

Angularity - is the condition of a surface, axis, or centerplane, which is at a specified angle from a datum plane or axis.

Arc Length - indicating that a dimension is an arc length measured on a curved outline. The symbol is placed above the dimension.

Basic Dimension - used to describe the exact size, profile, orientation or location of a feature. A basic dimension is always associated with a feature control frame or datum target. (Theoretically exact dimension in ISO)

Limits and Tolerences for Design

Design and Manufacturing

A machine element, after design, requires to be manufactured to give it a shape of a product. Therefore, in addition to standard design practices like, selection of proper material, ensuring proper strength and dimension to guard against failure, a designer should have knowledge of basic manufacturing aspects.

In this lesson, we will discuss briefly about some of the basic manufacturing requirements and processes.

First and foremost is assigning proper size to a machine element from manufacturing view point. As for example, a shaft may be designed to diameter of, say, 40 mm. This means, the nominal diameter of the shaft is 40 mm, but the actual size will be slightly different, because it is impossible to manufacture a shaft of exactly 40 mm diameter, no matter what machine is used. In case the machine element is a mating part with another one, then dimensions of both the parts become important, because they dictate the nature of assembly. The allowable variation in size for the mating parts is called limits and the nature of assembly due to such variation in size is known as fits.

Limits

Fig. 1 explains the terminologies used in defining tolerance and limit. The zero line, shown in the figure, is the basic size or the nominal size. The definition of the terminologies is given below. For the convenience, shaft and hole are chosen to be two mating components.

Fundamentals Of Machine Design

Introduction

Design is essentially a decision-making process. If we have a problem, we need to design a solution. In other words, to design is to formulate a plan to satisfy a particular need and to create something with a physical reality. Consider for an example, design of a chair. A number of factors need be considered first:

(a) The purpose for which the chair is to be designed such as whether it is to be used as an easy chair, an office chair or to accompany a dining table.

(b) Whether the chair is to be designed for a grown up person or a child. (c) Material for the chair, its strength and cost need to be determined.

(d) Finally, the aesthetics of the designed chair.

Almost everyone is involved in design, in one way or the other, in our daily lives because problems are posed and they need to be solved.

 concept of machine design

Decision making comes in every stage of design. Consider two cars of different makes. They may both be reasonable cars and serve the same purpose but the designs  are  different.  The  designers  consider  different  factors  and  come  to certain conclusions leading to an optimum design. Market survey gives an indication of what people want. Existing norms play an important role. Once a critical decision is made, the rest of the design features follow. For example,once we decide the engine capacity, the shape and size, then the subsequent course of the design would follow. A bad decision leads to a bad design and a bad product.

Design may be for different products and with the present specialization and knowledge bank, we have a long list of  design disciplines e.g. ship design, building design, process design, bridge design, clothing or fashion design and so on.

Types of design

There may be several types of design such as

Adaptive design

This is based on existing design, for example, standard products or systems adopted for a new application. Conveyor belts, control system of machines and mechanisms  or  haulage  systems  are  some  of  the  examples  where  existing design systems are adapted for a particular use.

Developmental design

Here we start with an existing design but finally a modified design is obtained. A

new model of a car is a typical example of a developmental design  .

Examples of Sections Views

Examples of Sections Views

SECTIONS OF SOLIDS

Full Section View
A full section view is made by passing the imaginary cutting plane completely through the object. As shown in figure 1, all the hidden features intersected by the cutting plane are represented by visible lines in the section view. Surfaces touched by the cutting plane have section lines drawn at a 45-degree angle to the horizontal.  Hidden lines are omitted in all section views unless they must be used to provide a clear understanding of the object. The top view of the section drawing shows the cutting plane line, with arrows pointing in the direction of line of sight to view the sectioned half of the object. In a multi-view drawing, a full-sectioned view is placed in the same position that an unsectioned view would normally occupy, I.e., a front section view would replace the traditional front view.

Figure 1 shows a full section view of an object.

Half Section view
Half sections are created by passing an imaginary cutting plane only halfway through an object. Hidden lines are omitted on both halves of the section view. Hidden lines may be added to the un-sectioned half, for dimensioning or for clarity. External features of the part are drawn on the un-sectioned half of the view. A center line, not an object line, is used to separate the sectioned half from the un-sectioned half of the view. The cutting plane line shown in the top view. The cutting plane line in the top view is bent at 90° and one arrow is drawn to represent the line of sight needed to create the front view in section. Half section views are used most often on parts that are symmetrical, such as cylinders. Also, half sections are commonly used in assembly drawings when external features are also to be shown. figure 2 shows a half section view of an object.

Figure 2 shows the cutting plane passing halfway through an object and one quarter of the object being removed