Engineering drawing is the graphical representation of the product to be manufactured or designed. It must contain all the information required to understand its physical appearance. Drawing instruments are those instruments which are used to prepare drawings easily and accurately.
Below is the list of some drawing instruments:
T-Square - T-square is made up of hard-quality wood which is used for making horizontal lines & Testing of the set squares.
SET-SQUARES - Set-square is a drawing instrument triangular in shape with one of the angles as 90°.
Two most commonly used set squares are 30°-60° set-square & 45° set square.
Drawing instrument box contains the following:
(i) Compass - The compass is used for drawing circles and arcs of circles.
(ii) Scale - Scale is a tool generally made up of wood, steel, celluloid or plastic or card board & used to measure true or relative dimensions of an object to the drawing.
(iii)Protractor - Protractor is a drawing tool to measure the angles which can not be measured by set square.
The most common type of protractor is semi-circular having 100 mm diameter having the least count of 1°, is numbered at every 10° interval as shown in figure.
(iv)French curves- French curves are drafting instruments which are used for drawing smooth curves or to connect the given points in smoothly.
(v)Drawing Paper - The standard sizes of drawing papers recommended by the Bureau of India Standards (B.I.S.). are given in the table:
(vi) Drawing Pencils: The pencils used for drawing is little bit different from the ordinary pencils.
The hardness is shown by H, its hardness can is described by putting some numeric values depending upon the hardness i.e. 2H, 3H, 4H etc.
Similarly, the grade becomes softer according to the figure placed in front of the letter B, i.e. 2B, 38, 48 etc.
(vii) Drawing Pins or Clips: These are used to fix the drawing paper on the drawing board.
(viii) Drafter : It is an instrument which can fullfill all the task performed by using T-square, set-squares, scales and the protractor.
Lines: The thicknesses of lines are varied according to the drawing and are finalized either by ink or by pencil.
Single-stroke Vertical Capital letters:
Single-stroke Vertical small letters
Types of dimensions: Two types of dimensions needed on a drawing are:
(i) size or functional dimensions,
(ii) location or datum dimensions (shown by letters F and L respectively)
The length of the arrowhead should be about three times its maximum width is drawn freehand with two strokes made in the direction of its pointed end.
Placing of Dimensions:
The two systems of placing dimensions are:
(i) Aligned system: In the aligned system, the dimension of any size is placed perpendicular to the dimensioning line in such a way that it may be read from the bottom edge or the right-hand edge of the drawing sheet.
The dimensions should be placed in the middle and above of the arrow.
(ii) Unidirectional system: In unidirectional system all dimensions can be read from the bottom edge of the drawing sheet (horizontally placed).
The dimension lines are cut from the center & dimensions are placed in that place.
A scale is defined as the ratio of the linear dimensions of element of the object as represented in a drawing to the actual dimensions of the same element of the object itself.
For Reducing Scale drawing, Scale < 1.
For Enlarging Scale drawing, Scale > 1.
Representative fraction: The ratio of the length of the object represented on drawing to the actual length of the object represented is called the Representative Fraction (i.e. R.F.).
It is an unitless quantity.
Length of the scale = R.F. × maximum length required to be measured
Types of scales: To measure any dimension of an object, various scales can be used. The scales used in practice are classified as under:
(i) Plain scales
(ii) Diagonal scales
(iii) Comparative scales
(iv) Vernier scales
(v) Scale of chords.
(i) Plain scales: A plain scale consists of a line divided into suitable number of equal parts or units, and it is further sub-divided into smaller parts.
Plain scales represent either two units or a unit and its sub-division.
(ii)Diagonal scales: Whenever to show any distance, 3 divisions are required then plain scale will not be able to show them accurately then a new scale is introduced is known as diagonal scale which can be measure very minute distance.
In this scale measurements are required in three units; for example, dm, cm and mm, or km2, m2, cm2
(iii) Comparative scales: Scales having same representative fraction but graduated to read different units are called comparative scales.
A drawing drawn with a scale reading inch unit can be read in metric units by means of a metric comparative scale, constructed with the same representative fraction.
(iv) Vernier scales: Vernier scales, like diagonal scales, are used to read to a very small unit with great accuracy.
A Vernier scale consists of two parts, a primary scale and a Vernier scale.
Least count of a vernier: It is the difference of 1 primary scale division and 1 vernier scale division.
Least count = 1 primary scale division – 1 vernier scale division.
(v) Scale of chords: The scale of chords is used to set or measure angles when a protractor is not available.
Note – To divide quarter circle into 3 equal parts, take radius of circle as length & draw an arc from both the end points.
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