Technical Drawing of parts and assemblies: introduction and preliminary

concepts

Prof. Pier Paolo Valentini

Outline of the lesson

Aim and Scope of Technical Drawings

Standardization

Preparation of part drawings

Dimensioning and tolerancing

Examples

Preparation of assembly drawings

From paper to CAD (Computer‐Aided Design)

Technical documents and drawings

The purpose of tecnical documents and drawings is to communicate and share ideas

A drawing has to fulfil the following requirements:

Technical correctness (no functional or practical errors, no representation errors)

Clarity (easy to be read and interpreted)

Completeness (not only geometrical information)

Uniqueness and no ambiguity (readable by different people in different countries)

Information in a technical drawing

Shape

Structure

Functionality

Dimensions

Precision

Materials

…

A technical drawing must include several pieces of information, not only the shape of a component

Standardization in a technical drawing

The information in a technical drawing must be presented according to national/international standards in order to:

Use the same representation rule (and the same language)

Reduce the cost of stock components (screws, nuts, pipe, etc.)

Avoid ambiguity of misrepresentation

Apply symbols and coded simplification

Standards are notlawsbut

Some laws are based on standards

Sheet dimensions and layout

Sheet dimensions and the layout of the drawings are standardized

Sheet dimensions Sheet layout

A4A1

A2

A3

A0

Common dimensions

A4 (210x297)

A3 (297x420)

A1 (594x841)

A0 (841x1189, 1 m2)

Location of the

drawing(s)

Title block and/or part list18.0 cm, variable height

Name of the company

Title and number of the drawing

The scale – standardized (1:1 is better)

Method of projection

List of parts (names, number, quantity,

material)

Frame

From real 3D world to 2D sheet space

Objects in the real works have three dimensions. Our perception is three 

dimensional too but the sheet in which we draw has only 2 dimensions

Need for moving from 3D to 2D without

loosing information of wrong interpretation of

the shapes

Need for looking at the object from

different points of view.

The best choice for technical drawing is to

use the orthogonal projections (no

distorsions)

The drawing is sketched by projecting from aninfinite distance (parallel rays), perpendicularly tothe projection plane (the sheet) all the points andedges of the object. It is better to align the objectto the main reference axes in order to have a moreclear representation.

object

Projection rays are perpendicular to the projection plane

projection plane

Orthogonal (orthographic) projection

Starting with the placement of the object in a convenient position (in order

to simplify and make the drawing clearer …

We choose the first projection (main projection)

as the representation which includes much of

information (geometrical and functional)

We continue with drawing other projections with

respect to orthogonal planes of view in order to

describe ALL the geometrical features of the

component.

All the visible edges are drawn with a continuous

bold line. All hidden edges are drawn with a

dashed line.

In many cases, there is no need to draw all the 6

views.

Layout of the views

After choosing the number and the typology of the views, they have to be located on the sheet 

plane. The symbol of the method has to be sketched in the block title. There are several 

methods, the most important ones are:

First angle method

(European method, or

Method E)

Third angle method

(American method or

Method A)

Symbols 

Section views

In many cases the views are not sufficient to describe all the geometrical features of an 

object. In these cases, section views can be added

Section views are drawn by indicating the cutting plane in one of the standard projection with a dash‐dot   line 

and two arrows

The cut of material is depicted with an uniform hatch with inclined lines. Touching parts are drawn with different hatch 

inclinations

The positioning of the section views follows the same rules of the standard views

Special cases in drawing section views

Rib features are drawn without the hatch

Axial‐symmetric connection parts (screws, pins, nails, shafts, etc.) are always represented without sectioning

YES

Small areas are filled

Example of projections and sections

A‐A

Dimensions

Dimensions have to be included explicitly (numerically) in the drawings and cannot be only measured on the drawings

Reference line

Dimension line (parallel to edge)

Ending arrow

Numerical value(in mm or deg)

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7

Special symbols can be used for diameters

and radiiReference lines 

should not intersect dimension lines 

Example of projections and dimensioning

Drawing of assemblies

An assembly drawing must include clear information about the functionality and the meting conditions among all the parts. 

It is prepared with the same guidelines for sketching views and sections with some little differences

Specific guidelines for assemblies

Dimensions should be avoided, except those expressing the maximum size. Each component will be drawn in a specific 

document with its dimensions

The choice of the views/sections should be made in order to have a better 

understanding of the mounting and the functionality

In sections, all mating parts has to be represented using hatches with different 

patterns

All the parts have to be numbered and their list has to be included just above the title 

block

Tolerances

Dimensions in drawings are referred to ideal features (no error) but perfect shapes

cannot be manufactured.

Actual parts have dimensions which differ from those prescribed in the drawings

There is the need to control and specify a range in which a real dimension may be

comprised in order to be considered acceptable (the parts preserve functionality, resistance

and other important properties).

Real dimensions should be comprised between two values (limits) which define the

TOLERANCE of the dimension

Only functional dimensions are subjected to tolerance specifications

10

tolerance

Ideal part (drawing) Real parts

Including dim. tolerances in drawings

There are two ways for adding a tolerance range to a dimension

writing min and max deviation as an apex of the dimension

use a symbolic representation with letter and number (coded by international

standards)

Geometrical tolerances

In many cases, it can be useful to control the deviation from the ideal shape  geometrical tolerance

Geometrical tolerances control:

The shape (straightness, planarity, circularity, cylindricity, profile, surface)

The orientation (parallelism, perpendicularity inclination)

The location (position, symmetry, concentricity)

The rounout (local or global)

Allocation of the geometrical tolerances is more complex than the allocation of the dimensional ones

The geometrical tolerances are included with symbols over the relative features

Examples of geometrical tolerances

Straightness tolerance controls 

that the axis of the pin is within a 

cylinder of 0,1 mm of diameter

Cylindricity tolerance controls 

that the surface of the pin is 

within two cylinders with 

different diameters with a 

maximum difference of 0,03 mm

Example of location tolerances

The location tolerance imposes that the centers of the holes are within circles 

of 0,28 mm of diameter located as stated by the inspection dimensions 

(dimension in squares) with respect to A B and C datum features

Roughness specification

In some cases, it is important to include specifications about the roughness of one

or more surfaces for both aesthetic and functional (mating) purposes.

These specifications also influence the choice of the manufacturing process.

10

1 1ml n

iim

Ra y x dx yl n

Arithmetic mean roughness

Surface

MANUFACTURING PROCESS

ROUGHNESS SPECIFIC. (IN MICRONS)

GROOVE ORIENTATION

ALLOWANCE

Examples of roughness specification

MeaningGRINDING

0.4Ra The surface has to have an arithmetic 

mean roughness of 0,4 μm with parallel

grooves achieved by grinding, without

the removal of material layer

(modification of the dimension)

Different symbols

generic

with removal of material

without removalof material

all the surfaces

From paper to CAD

Computer‐aided design applications are widely used in all 

technical offices because they can help many design activities 

and preparing drawing (actually it is a marginal contribution)

Advantages in using CAD Disadvantages in using CAD

Modelling 3D shape before

drawing (checking of geometrical

feasibility)

Giude in assembly and mating

relationships among parts

Help in designing complex shapes

with a lot of intersecting, hidden and

curve edges

Possibility of association shape‐

drawing for easily updating the

drawing

Electronic version of drawings

Produced drawings have to be

corrected in many cases

The shape is translated in a very

precise way, but the standards

specifications are not always satisfied

(many standards are country‐

dependent)

Deep knowledge of drawing rules

and standards is required in order to

produce correct documents (that is the

reason we study drawing by hand!)

Summary of the lesson

The knowledge of the rules and methodologies for preparing drawing is essential for a correct production of technical material and clear and not ambiguous communication

International and National Standards are a guide towards a correct and complete representation

The representation of the shape is not sufficient for industrial purposes. Other information (dimensions, tolerances, roughness, etc.) need to be included.

Computer‐aided applications can be useful for designing and optimizing the shape, but the preparation of technical drawings requires a specific effort