UNIT 9 TOOLS FOR BASIC LAYOUT

Transcription

1 UNIT 9 TOOLS FOR BASIC LAYOUT Tools for Basic Structure 9.1 Introduction Objectives 9.2 Tools for Scribing 9.3 Accessories 9.4 Summary 9.5 Key Words 9.1 INTRODUCTION The process of making reference mark on the workpiece is called layout. Laying out includes marking the straight lines, center point, circles, arcs on the surface of the workpiece to indicate the position of hole to be drilled and the amount of material to be removed by shaping or milling machine. These marks indicate shape, size and features of the workpiece. marks are often indicated on the surface where machining will take place. Operator uses these marks as a guideline for machining during checking of workpiece by actual measurement. Workpiece may cut according to layout mark. The layout should be made from the base line or the machined edge to ensure the accuracy of the layout and correct position of dimensions with respect to each other. The accuracy of a finished job is generally determined by the amount of care taken while laying out. The layout is very important for cutting the stock, filing and offhand grinding, drilling, milling etc. One of the jobs of the operator is to measure and layout at the place, where the stock will be cut. This kind of layout may be marked by pencil or chalk. marked by pencil or chalk, is called as semi-precision layout. Semi-precision layout is generally done by steel ruler, which gives tolerance up to 0.5 mm. Precision layout can be complex and requires sophisticated tools for operation and generally done by tools that give tolerance up to mm or finer. The reference points, reference plane, reference lines are indicated by laser and optical instruments in aircraft and ship building industries. Objectives After completing this unit, you should be able to understand various types of tools required for layout, the necessity for accurate layout, and the purpose of layout tools such as scriber, dividers, squares etc. 9.2 TOOLS FOR SCRIBING LAYOUT Surface Plate It is very important tool for layout operation. It provides an accurate reference plane. The reference plane is that plane from which measurements may be made for both layout and inspection. A surface plate may be used for large amount of work. The surface plate used for large work is called as layout table. Surface plate should be covered when it is not in use and kept clean when it is in use. Further, it should never be blown by hammer. This will affect the accuracy of the reference surface. Surface plate may be made of cast iron or granite. Surface plate of cast iron is shown in Figure 9.1. Cast iron based surface plate is made from good quality casting. When the working surface of the casting has been finished to desired degree of flatness, the distortion will be reduced by the age hardening. 5

2 Design for Tool Engineering Then casting has been allowed to age hardened to relieve internal stresses. The ribs are provided on underside of the surface plate for rigidity. The large cast iron plate is mounted on a heavy stand or legs with provision for levelling. The plate is levelled periodically to ensure that its working surface remains flat. Figure 9.1 : Surface Plate Granite is superior to cast iron as granite is harder, denser and also gives resistance to water. The surrounding flat surface will not get affected, if it is chipped. Due to such excellent properties, granite is often used now-a-days. Also granite is aged in the earth for long time which results in relieving of internal stresses in earth itself. The temperature withstanding capacity of granite is also more than other material. Dyes Scriber dyes are used to make the layout marks visible on the surface of the workpiece. dyes may be blue, white or red in color. Different dye colors may be used depending upon the surface color of the workpiece. The Scriber is a tool used for making layout lines on metal surfaces. The scribers are made of tool steel with hardened and tempered points. The point of scriber should be sharp in order to mark fine, clear layout lines. The pocket scriber has removable tip that can be stored in the handle. This allows the scriber to be carried safely in the pocket. The pocket scriber is shown in Figure 9.2. Divider Figure 9.2 : Scriber The divider is a tool with hardened steel points used for transferring measurements, comparing distances and marking arcs and circles. Spring divider is commonly used everywhere for the same purpose. Figure 9.3 depicts a spring divider. Dividers are adjustable and are classified according to size, i.e. by maximum opening between the two points. To set the divider to a size, place one point on 1 cm of rule and adjust the other point until it splits the graduation line at correct distance. The graduation lines on a ruler are V-shaped and a divider may be set more exactly by feeling that the point is on the graduation line than by seeing it visually. 6

3 Tools for Basic Hermaphrodite Caliper Figure 9.3 : Divider It is a caliper having one straight leg and other bent leg. The tip is adjustable for length. A straight leg has a point so that it can be used to scribe lines parallel to an edge or shoulder. It can also used to locate the centers of round workpieces. The hermaphrodite caliper is shown in Figure 9.4. To set this tool, place bent leg on the end of a ruler and adjust until the pointed leg is at the desired graduation. The bent leg is placed against the round workpiece and arc is marked at the end of workpiece. The tangent marks can be laid out by adjusting the leg spacing. And the centre of workpiece can be located by marking four arcs at 90 o. Square Figure 9.4 : Hermaphrodite Caliper It is used to layout lines at right angles or parallel to machined edge. It is also used to check accuracy of surface which must be at right angle to each other and to set up workpiece for machining. Square is shown in Figure 9.5. Square should be handled with extreme care to preserve their accuracy. Figure 9.5 : Square 7

4 Design for Tool Engineering Bevel Protractor The bevel protractor is used to layout and check the angle. The protractor can be adjusted from 0 to 180 o. The scale may be graduated from 0 to 90 o from both right and left side. The accuracy of the bevel protractor is ( ± 5 ). Centre Head The head forms a center square when clamped to the rule. It can be used for locating centers of round, square and octagonal stock. The centre head should be used only on workpiece which is true in shape. Square Head The square head and rule is used to layout lines parallel and at right angles to an edge. It may be used as a depth gauge or for checking 45 o and 90 o angles. The square head can be moved along the ruler to any position. Steel Rule Steel rule is used as a straight edge or for measuring. It may be fitted to the center head, bevel protractor or to the square head. Metric combination steel rulers are generally graduated in 1 mm and 1/2 mm while imperial combination set rules are generally graduated in inch, i.e. 8 th or 16 th on one side and 32 nd and 64 th on other side. Combination Set It is one of the most useful and versatile tools in the machine shop. The set consist of steel ruler, bevel protractor, centre head and square head. Figure 9.6 shows combination set. Protractor Head Steel Rule Centre Head Square Head Scriber Figure 9.6 : Combination Set Prick Punch A prick punch also known as layout punch is made of tool steel in which both ends are hardened and tempered. It has an included point angle of 30 o. The prick punch is shown in Figure 9.7. Included Angle of 30 o 8 Figure 9.7 : Prick Punch The slim point provides easy location of the punch on a scribe line. It is usually used to layout mark along layout lines, to mark centers for drilled holes and also to mark center for divider point. The prick punch is the punch which is used only to preserve the location of a layout mark while creating minimum damage to the workpiece. The punch mark may affect the surface finish of the workpiece. Center Punch

5 Center punch is also layout instrument made of tool steel in which both ends are hardened and tempered. It is same as that of prick punch but only difference being that it has an included point angle of 90 o. It is used to mark the workpiece before drilling. It is generally used to enlarge the mark made by prick punch so that drill may be started easily and accurately. Following points should be remembered while using prick or center punch : (a) (b) (c) (d) Surface Gauge SAQ 1 Hold the punch at an angle of 45 0 and place the point on layout line. Insure that the point of punch should be sharp. Examine the position of the punch mark and correct it if necessary. Bring the punch to a vertical position and tap it gently with a light hammer. The surface gauge is the instrument which is used on a surface plate or any flat surface for scribing lines in layout work. It consists of heavy base and an upright spindle to which a scriber is clamped. The base of the surface gauge has a V-groove which allows it to be used on the cylindrical work as well as on the flat surface. Surface gauge has pins in the base which may be pushed down so that the surface gauge can be used against the edge of a surface plate or a slot. A surface gauge can also be used as a height gauge and for leveling work in a vise. A surface gauge may be set to a size or dimension by using combination square which is most accurate for most of the layout work. (a) (b) (c) (d) What is the purpose of laying out the workpiece? Why should all layouts be made from a base line or machined surface? Explain prick and centre punch? Explain layout tools with neat sketch. Tools for Basic 9.3 LAYOUT ACCESSORIES Angle Plate Parallels Clamps An angle plate is a precision tool having L-shape. It is made of cast iron or hardened steel, machined to an accurate angle of 90 o with all working surfaces and edges ground. Angle plate is used to hold the workpiece parallel and at right angle to the surface. C-clamp is generally used to fasten the workpiece with an angle plate. Slots and tapered holes are provided with some angle plate for clamping the workpiece. Parallels are used in layout operation for raising the workpiece to a suitable height. It provides a solid seat for workpiece. The work surface is held parallel to the top of the surface plate by using the parallel. C-clamps are used to fasten the workpiece to an angle plate or drill press table. It is also used for holding two or more pieces of metal together. C-clamp is widely used for this purpose. C-clamp is shown in Figure 9.8. The shape of C-clamp is of letter C. 9

6 Design for Tool Engineering V-block Figure 9.8 : C-clamp V-block is made of cast iron or heat-treated steel in a wide range of sizes. They are machined with a 90 o V-shaped slot on the top and bottom. V-block is shown in Figure 9.9. Some V-blocks are provided with U-shaped clamps to hold the workpiece securely. They are used for laying out round work and to hold it for machining. SAQ 2 (a) (b) (c) Figure 9.9 : V-block List various layout accessories with uses. What is V-block? Explain the purpose of V-block? Explain angle plate, parallel with figures. 9.4 SUMMARY 10 Laying out includes marking the straight lines, center point, circles, arcs on surface of workpiece to indicate the position of hole to be drilled or the amount of material to be removed by shaping or milling machine. These marks specify shape, size and features of workpiece. Scriber, divider, surface plate, squares, calipers, prick punch, center punch etc. are used for layout purpose. Surface plate provides accurate reference plane. Hermaphrodite caliper is used to scribe lines parallel to an edge and to locate the centers of round workpieces. The bevel protractor is used to layout and check the angle. Combination set is used for laying out straight line. Center head can be used for locating centers of round, square and octagonal stock. Angle plate is used to hold the workpiece parallel and at right angle to a surface. C Clamps are used to tie the workpiece to an angle plate or drill press table. V-blocks are used to hold the workpiece firmly.

7 9.5 KEY WORDS Tools for Basic Prick Punch Center Punch Surface Gauge : A prick punch is used to layout mark along layout lines, for drilled holes and to mark center for divider point. : A center punch is used to enlarge the mark made by prick punch so that drill may be started easily and accurately. : Surface gauge is used on a surface plate or any flat surface to scribe lines in layout work and to level work in vise. 11