Introduction of Engineering Drawing Instruments like drawing board, drawing sheet, protector, pencils, setsquares etc. for iti Engineering Drawing 1st year CBT Examination.
इंजीनियरिंग ड्राइंग एवं ड्राइंग इंस्ट्रूमेंट का परिचय टॉपिक का अध्ययन यहाँ से करे। आईटीआई प्रथम वर्ष की इंजीनियरिंग ड्राइंग विषय की ऑनलाइन परीक्षा की तैयारी के लिए उपयोगी पाठ सामग्री निचे उपलब्ध करवाई जा रही है।
Engineering Drawing Instruments
1. Drawing Board
Drawing board is one of the main equipment of Engineering Drawing
It is used for supporting the drawing paper/tracing paper for making drawings.
It is made of well seasoned wood strips of about 25 mm thick or masonite, free from knots and warping.
It should be softer enough to allow insertion and removal of drawing pins.
Two battens are fastened to the board by screws, in slotted joints.
The working edge (ebony) must be straight.
Standard drawing boards are designated as follows as per IS:1444-1989
2. ‘T’ Square
‘T’ Square has two parts., head and blade One of the edges of the blade is the working edge. The blade is screwed to this head such that the working edge is at right angle to head.
The ‘T’ square is used with its head against the ebony edge of the drawing board to draw horizontal lines, parallel lines and to guide/hold the setsquares, stencils etc. ‘T’ square should never be used as a hammer or as guide for trimming papers
The standard ‘T’ square are designated as follows with dimensions shown in mm; as per IS:1360-1989
3. Set squares
Set square Transparent celluloid /Plastic set squares are preferred and are commonly used rather than ebonite ones.
They are two in number, each having one corner with 90°. The set square with 60°-30° of 250 mm long and 45° of 200mm long is convenient for use.
Set Squares sometimes lose their accuracy due to internal strains. So they should be tested periodically.
Sometimes set squares have french curves. Set squares are used to draw all straight lines except horizontal lines.
30°- 60° set squares, angular lines in the multiples of 15°, Parallel lines to a given inclined line and perpendicular to can be drawn.
4. Protractor
Protractor is an instrument for measuring angles. It is semi-circular or circular in shapes and is made of flat celluloid sheet.
The angles can be set or measured from both sides, aligning the reference line and point ‘0’ with the corner point of the angle.
5. French curves
These are made in many different shapes, normally come in sets of 6,12,16 etc. French curves are best suited to draw smooth curves/ arcs (which cannot be drawn by a compass) with ease. To draw a smooth curve using a french curve first set it by trial against a part of the line to be drawn, then shift it to the next portion.
Each new portion should fit at least three points on the curve just drawn. It should be seen that the curve (radius) is increasing or decreasing smoothly and no corner should be
formed on the curve.
6. Large compass
Large Compass has a knee joint in one leg that permits the insertion of pen or pencil point or attaching a lengthening bar with pen or pencil point attached to it. It is used for drawing large circles/arcs and also for taking large measurements.
The pin on the other leg can be swivelled to vertical position when drawing large circles, while drawing the circles of arcs it should be held in such a way that the needle point leg and pencil point leg should be bent so as to make it perpendicular to the paper.
7. Large divider
Large divider is used to transfer dimensions and dividing lines into a number of equal parts. Divider with adjustable joints is preferable rather than plain legs.
8. Bow instruments
Bow instruments may have adjusting wheels and nuts. To draw circles, it is better to mark the required distance separately and set the instruments and check. Then only the circles or arcs should be drawn on the drawing.
9.Pencils, Grade and Selection
In the drawing, standard pencils (lead encased in wood) and semi-automatic pencils are made. Pencil leads are made of graphite with kaolin (clay) of varying proportion to get the desired grades. More the kaolin, higher the hardness.
Grades of pencils
Pencils are graded according to the hardness or softness of the lead.
- Hardest pencil is 9H grade and softest pencil is 7B grade. Selection of grade of pencils depends on the type of line work required and paper on which it is used.
- Softer lead pencils are used to produce thicker and darker line work, but they wear out quickly. Medium grades of H, 2H are used for general line work as well as for lettering.
- Harder grade leads produce lighter and thinner lines. Most construction line work is done with 4H, 5H and 6H pencil leads, producing thin but also sufficiently dark by exerting pressure.
Selection of pencils
Pencil grades vary from one brand to another brand. Select the grades of the pencil depending upon the type of line work. For construction lines, you can choose 2H or 3H, for lettering and object lines grade H pencils. In general H, HB and 2H are used.
Medium hard - H
Medium soft - HB
Hard - 2H
Now-a-days automatic (Mechanical) pencils or clutch pencils are available in different sizes (lead dia 0.3, 0.5, 0.7 or 0.9 mm). They are easy to handle as there is no reduction of holding length pencil leads can be replaced, as per required grade of hardness. They produce lines of uniform width without sharpening.
10. Drawing Sheet
Drawing paper: These are of two types:- Hand-made paper
- Mill-made paper
Size and Layout of drawing sheets
While working or handling, the papers are liable to tear on the edges. So slightly large size (untrimmed) sheets are preferred. They are trimmed afterwards. IS:10811:1983 lays down such as designation of preferred trimmed and untrimmed sizes.The basic principle involved in arriving at the sizes of the drawing paper is as under. The area of the biggest size (A0) is 1m and its breadth and length are in the ratio 1:√2.
The drawing sheets are designated by symbols such as A0, A1, A2, A3, A4 and A5. A0 being the largest. Table 1 below gives the length and breadth of the above sizes of sheets. (Trimmed and untrimmed)
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