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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)
10
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