Drawing reading and measurements

Drawing reading and
Measurements

Agenda:
Introduction
Fundamentals of blue print drawing
Scales
Lines
Dimensioning
Cutting planes
Projections
Basics of welding symbols
Limits, fits and tolerance
Exercise
Conclusion

Engineering Drawing
Drawings are also necessary for engineering industries since they are required and
are being used at various stages of development of an engineering product.
In an industry, these drawings help both the technical as well as commercial staffs at various stages
like:
◦ Conceptual stage
◦ Design stage
◦ Modification stage
◦ Prototype development stage
◦ Process and production planning
◦ Production
◦ Inspection
◦ Marketing
◦ Servicing and maintenance, etc.

A perfect engineering drawing should have the following information:
◦ Shape of an object
◦ Exact Sizes and tolerances of various parts of the object
◦ The finish of the product
◦ The details of materials
◦ The company’s name
◦ Catalogue no of the product
◦ Date on which the drawing was made
◦ The person who made the drawing

Types of drawings
Civil
Architectural
Structural
Mechanical
Plumbing
Piping
Pneumatic/Hydraulic
Electrical
A general engineering drawing can be divided into
the following five major areas or parts.
1. Title block
2. Grid system
3. Revision block
4. Notes and legends
5. Engineering drawing (graphic portion)

Size and Layout of drawing sheets
Sizing calculation:-
x:y=1:√2

Title block
An important feature on every drawing sheet. This is located at the bottom right hand
corner of every sheet and provides the technical and administrative details of the
drawing. The title box is divided into two zones.
Identification zone :
1. Title of the drawing,
2. Sheet number,
3. Scale (s),
4. Symbol, denoting the method of projection,
5. Name of the firm, and
6. Initials of the staff designed, drawn, checked and approved.

Additional information zone :
1.Job order number,
2.Surface treatment, roughness, etc.,
3. Key to machining and other symbols,
4. A general note on tolerance on dimensions, not individually toleranced,
5. Reference to tools, gauges, jigs and fixtures,
6. Parts list, and
7. Alternations and revisions.

Item reference
The item references should be assigned in sequential order to each component
part shown in an assembly and/or each detailed item on the drawing.

Planning of assembly drawings
 Where a number of drawings are
required to detail a complete design,
an assembly drawing is necessary.
 Such a drawing will show the design
to a convenient scale, and the
drawing or part numbers which are
the constituents of the particular
assembly are listed in a tabular form

Detailed view of main assembly:

Folding of drawing prints
The folding procedure that present here conforms to the DIN 824 standard, based an "A"
paper sizes. All large prints of sizes higher than A4 are folded to A4 sizes;
The bottom right corner shall be outermost visible section and shall have a width not less than
210 mm.
Advantages:
 A folded drawing is easier to archive and takes less space
 A set of folded drawings is conveniently organized in a ring binder or file folder, easy to slip in a briefcase to
bring along to meetings
 A set of folded drawings in a binder/folder can be paged through without unfolding, because the title block is
always visible in the lower right corner
 Once you have flipped through the drawings and found the one you wish to view (or present), a folded
drawing is easily unfolded to its full size without first needing to remove it from the binder/folder
 A folded drawing, put in an envelope, is cheaper to sent (by post) than a rolled-up drawing in a cardboard tube

Drawing reading and measurements

Scales
A scale is defined as the ratio of the linear dimensions of the object
as represented in a drawing to the actual dimensions of the same.
Drawings drawn with the same size as the objects are called full sized drawing.
It is not convenient, always, to draw drawings of the object to its actual size.
e.g. Buildings, Heavy machines, Bridges, Watches, Electronic devices etc.
Hence scales are used to prepare drawing at
◦ Full size
◦ Reduced size
◦ Enlarged size

The scales are represented by the size of the object in the drawing to actual size,
both expressed in same units and it is called as Representative factor.
RF=Drawing size/Actual size
Say 1:50 ,where 1 represent the size of the object in the drawing and 50
represent the actual size of the object both in same unit.

Types of Scale
Engineers Scale : The relation between the dimension on the drawing and the
actual dimension of the object is mentioned numerically (like 10 mm = 15 m)
Graphical Scale: Scale is drawn on the drawing itself. This takes care of the
shrinkage of the engineer’s scale when the drawing becomes old.
Types of Scale :-
 Plain Scale
 Diagonal Scale
 Vernier Scale
 Comparative scale
 Scale of chords

Primitive geometric forms
The shapes of objects are formed from primitive geometric forms . These are
Point
Line
Plane
Solid
Doubly curved surface and
Warped surface

Lines
•Lines is one important aspect of technical drawing. Lines are always used to
construct meaningful drawings.
•Various types of lines are used to construct drawing, each line used in some
specific sense. Lines are drawn following standard conventions mentioned in
BIS (SP46:2003).
•A line may be curved, straight, continuous, segmented. It may be drawn as thin
or thick
•The ratio of thick to thin line shall not be less than 2:1

A typical use of various lines in an
engineering drawing

Dimensioning
The size and other details of the object essential for its construction and function. Using lines,
numerals, symbols, notes, etc.,are required to be indicated in a drawing by proper
dimensioning.
IS 10718:1983 speaks for the principles, standards in dimensioning and method of
dimensioning an object.

Aligned system
In the aligned system the dimensions are placed perpendicular to the dimension line
in such a way that it may be read from bottom edge or right hand edge of the
drawing sheet.
The horizontal and inclined dimension can be read from the bottom where as all the
vertical dimensions can be read from the right hand side of the drawing sheet.

Unidirectional system
In the unidirectional system, the dimensions are so oriented such that they can
be read from the bottom of the drawing.

Series dimensioning
Chain dimensioning (Continuous dimensioning).
All the dimensions are aligned in such a way that an arrowhead of one
dimension touches tip-to-tip the arrowhead of the adjacent dimension. The
overall dimension is placed outside the other smaller dimensions.

Parallel dimensioning:
Parallel dimensioning (Progressive dimensioning) All the dimensions are shown
from a common reference line. Obviously, all these dimensions share a common
extension line. This method is adopted when dimensions have to be established
from a particular datum surface

Combined dimensioning:
Combined dimensioning. When both the methods, i.e., chain dimensioning and
parallel dimensioning are used on the same drawing, the method of
dimensioning is called combined dimensioning.

Rules to be followed for
dimensioning
•Each feature is dimensioned and positioned
only once.
•Each feature is dimensioned and positioned
where its shape shows.
•Size dimensions – give the size of the
component.
•Every solid has three dimensions, each of the
geometric shapes making up the object must
have its height, width, and depth indicated in
the dimensioning.

Functional dimension (F) – A dimension that is
essential to the function of the part.
 Non functional dimension (NF) – A dimension that
is not essential to the function of the part.
 Auxiliary dimension (AUX) – A dimension given for
information purpose only. It does not govern the
production or inspection operations and is derived
from other values shown on the drawing.
 An auxiliary dimension is enclosed in parenthesis and
no tolerance applies to it.
Classification of Dimensioning:

The specification of dimension lines

Notes
• Notes should always be written horizontally
in capital letters and begin above the leader
line and may end below also.
• Notes should be brief and clear &the
wording should be standard in form.

Cutting planes
The cutting plane(s) should be indicated by means of line. The cutting plane
should be identified by capital letters and the direction of viewing should be
indicated by arrows.
In principle, ribs, fasteners, shafts, spokes of wheels and the like are not cut in
longitudinal sections and therefore should not be hatched

Sectional views
A sectional view is obtained by imagining the object, as if cut by a cutting plane
and the portion between the observer and the section plane being removed.
Full section

Half section
A half sectional view is preferred for symmetrical objects. The cutting plane
removes only one quarter of an object. For a symmetrical object, a half sectional
view is used to indicate both interior and exterior details in the same view.

General principals of presentation

Projection
In engineering, 3-dimensonal objects and structures are represented graphically on a 2-
dimensional media. The act of obtaining the image of an object is termed
“projection”. The image obtained by projection is known as a “view”
Working sketch ? The design engineer uses orthographic or pictorial views to record his ideas, free
hand. These are called working sketches.

Two projection methods used are:
Perspective Parallel

Orthographic Projection
Orthographic projection is a parallel projection technique in which the plane of
projection is perpendicular to the parallel line of sight.
Orthographic projection technique can produce either pictorial drawings that show
all three dimensions of an object in one view or multi-views that show only two
dimensions of an object in a single view.

Orientation of views from projection planes
Multi-view drawings gives the complete description of an object. For conveying the complete
information, all the three views, i.e., the Front view, Top view and side view of the object is
required.
To obtain all the technical information, at least two out of the three views are required.
 It is also necessary to position the three views in a particular order. Top view is always
positioned and aligned with the front view, and side view is always positioned to the side of
the Front view and aligned with the front view.
 Depending on whether 1st angle or 3rd angle projection techniques are used, the top view
and Front view will be interchanged. Also the position of the side view will be either towards
the Right or left of the Front view.

Conventional view placement
The three-view multi-view drawing is the standard used in engineering and technology.
Because the other three principal views are mirror images and do not add to the knowledge
about the object.

Projection methods:1st angle and 3rd angle

Symbol of projection
• Either first angle projection(Europe & Major countries) or third angle projection(USA
and Australia) are used for engineering drawing.
• Second angle projection and fourth angle projections are not used since the drawing
becomes complicated

Conventional representation of machine
components

Symbolic representation of welds on
technical drawings

The manufacture of interchangeable parts require precision.
Precision is the degree of accuracy to ensure the functioning of a
part as intended
Tolerance: The permissible variation of a size is called tolerance. It is the difference between the maximum and
minimum permissible limits of the given size. If the variation is provided on one side of the basic size, it is
termed as unilateral tolerance. Similarly, if the variation is provided on both sides of the basic size, it is known as
bilateral tolerance.
Limits: The two extreme permissible sizes between which the actual size is contained are called limits. The
maximum size is called the upper limit and the minimum size is called the lower limit.
Deviation: It is the algebraic difference between a size (actual, maximum, etc.) and the corresponding basic size.
Actual deviation: It is the algebraic difference between the actual size and the corresponding basic size.
Upper : It is the algebraic difference between the maximum limit of the size and the corresponding basic size.

Lower: It is the algebraic difference between the minimum limit of the size and the
corresponding basic size.
Allowance: It is the dimensional difference between the maximum material limits of the mating
parts, intentionally provided to obtain the desired class of fit. If the allowance is positive, it will
result in minimum clearance between the mating parts and if the allowance is negative, it will
result in maximum interference.
Basic size: It is determined solely from design calculations. If the strength and stiffness
requirements need a 50mm diameter shaft, then 50mm is the basic shaft size. If it has to fit
into a hole, then 50 mm is the basic size of the hole. Here, the two limit dimensions of the shaft
are deviating in the negative direction with respect to the basic size and those of the hole in the
positive direction. The line corresponding to the basic size is called the zero line or line of zero
deviation

Fundamental of Tolerance
There are 18 grades of tolerances, designated as IT 01, IT 0 to IT 1 to IT 16,
known as “Fundamental tolerances”

Method of placing limit dimension
Method 1
Capital letters indicate the internal features and small letters indicate the
external features.

Method 2 :
In this method, the basic size and the tolerance values are indicated above the dimension line;
the tolerance values being in a size smaller than that of the basic size and the lower deviation
value being indicated in line with the basic size.

Method 3
In this method, the maximum and minimum sizes are directly indicated above
the dimension line

Datum feature
A datum feature is a feature of a part, such as an edge, surface, or a hole, which
forms the basis for a datum or is used to establish its location

Feature control frame
Reads as: The position of the feature must be within a .003
diametrical tolerance zone at maximum material condition
relative to datums A, B, and C.

MMC (Maximum Material Condition) is when a feature or part is at the limit of
size, which results in its containing the maximum amount of material.
LMC (Least Material Condition) refers to the size of a feature that results in the
part containing the minimum amount of material. Thus it is the minimum limit
of size for an external feature.
RFS (Regardless of feature size) indicates that a geometric tolerance applies to
any size of a feature that lies within its size tolerance.

Parallelism:
The condition of a surface or center plane equidistant at all points from a datum
plane, or an axis.
The distance between the parallel lines, or surfaces, is specified by the
geometric tolerance
±0.01

Standards followed by industries

Machining symbols
•Basic symbol
•Without material removal
•Material removal
•Special characteristics

Symbols specifying the directions of lay

Blue print reading
practice
1. What is the overall size of the housing?
2. What is the gasket size required for the top surface?
3. There are 4 tapped holes on the top. What is the size
of the tap?
4. Specify the location of the holes on the front face
5. Explain the note—4 HOLES, M10 DEEP 25.
6. What is the size of the opening at the top?
7. What is the diameter of the gasket required for the
front cover?
8. What is the corner radius of the top flange?
9. Locate the centre point of tapped holes M 6.
10. Find the dimensions A, B, C and D.

1. What is the overall size of the housing?
—178mm × 152mm × 102mm
2. What is the gasket size required for the top surface?
—134mm × 95mm
3. There are 4 tapped holes on the top. What is the size of the tap?
—M6
4. Specify the location of the holes on the front face.
—2 HOLES, M10 DEEP 25 AT 120 PCD
5. Explain the note—4 HOLES, M10 DEEP 25.
—There are two holes in the front and two more holes similarly placed at the back, each having a thread of nominal diameter of
10mm and depth of 25mm.
6. What is the size of the opening at the top?
—89mm × 50mm
7. What is the diameter of the gasket required for the front cover?
—144 mm
8. What is the corner radius of the top flange?
—R 10
9. Locate the centre point of tapped holes M 6.
—10mm × 10mm from the flange edges at each corner
10. Find the dimensions A, B, C and D.
—A = 9.5mm, B = 114mm, C = 9.5mm and D = 20mm

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