MARKING & MARKING TOOLS
Marking out
Marking out means the transferring of forms and dimensions on to the work piece from a drawing
1.Purposes of marking
1. To define the required shape and size of work piece
2. To indicate the exact positions of holes, points etc.
3. To prevent the wastage of material.
4. To provide the guidance for machining
2 Systems of marking
In engineering practice mainly two systems of marking are there
1. Rectangular co-ordinate system.
2. Polar co-ordinate system
2.1 Rectangular co- ordinate system
The position of points in this system is dimension relative to two axes made at right angle to each other. By this method the possibility of error is less and the distance between the points can easily be calculated and checked.
2.2 Polar co-ordinate system
In this system distances are measured or located along radial lines from a common center. The accuracy with which the points are positioned depends upon the correct setting of the angle and the distance from the common center. As the distance from the common center increases, a slight angular error would cause a great displacement of the point
3 Marking Tools
3.1 Scriber:
It is used for scribing lines on work pieces. Its point angle is 150. A scriber, which is used on a marking stand, has a point angle of 60 degree.
3.2 Divider:
It is used for drawing arcs and circles and also for transferring dimensions.
3.3 Dot punch:
It is used for marking the intersecting points of two lines and also to make the marked line more visible. Its tip is hardened and tempered. Point is ground to an angle of 600
3.4 Center punch:
It is similar to a dot punch except that the point is ground to an angle of 900. It is used to make deep marks to guide the drill during the commencement of drilling operation
3.5 Double dot punch:
It is used to mark equally spaced dots on a work. Equally spaced dots are mainly necessary for chain drilling operations. It consists of two punches one fixed and other movable.
3.6 Marking stand:
It consists of a base, a pillar scriber and a movable clamp. The movable clamp carries a scriber and it can be fixed in any desired position by a locking screw. The scriber is positioned with the aid of a steel rule. It is used mainly to mark lines parallel to the reference surface.
Surface Plate:
It is mainly used to support jobs for marking. It also acts as base from which actual measurements are taken. It can also be used for testing surface for flatness and is specified by its length and breadth. It is made of grey cast iron and is finished by the operation of scraping
3.8 V block:
It is used for locating round work pieces. It is made of cast iron or case hardened mild steel. Angle of V is 90 degree. It is specified by the maximum diameter of the work that can be held between two V-blocks, when one is kept inverted over the other
3.9 Angle Plate:
It has two planes perfectly right angle to each other. It is made of grey cast iron. It is used for supporting work right angle to the surface plate while marking
3.10 Trammels :
Trammels are used for scribing circles and arcs of large radii and transferring of dimensions from one place to another. This tool is having two perpendicular point holders in which different points like divider points, pencil point, ballpoint etc. can be held
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