FITTING2.1. INTRODUCTION 2.1. INTRODUCTION
Machine tools are capable of producing work at a faster rate, but, there are Machine tools are capable of producing work at a faster rate, but, there are occasions when components are processed at the bench. Sometimes, it becomes occasions when components are processed at the bench. Sometimes, it becomes necessary to replace or repair a component which must fit accurately with another necessary to replace or repair a component which must fit accurately with another component on re-assembly. This involves certain amount of hand fitting. The component on re-assembly. This involves certain amount of hand fitting. The accuracy of work done depends upon the experience and skill of the fitter.
accuracy of work done depends upon the experience and skill of the fitter.
The term,” Bench work” refers to the production of components by hand on The term,” Bench work” refers to the production of components by hand on the bench, whereas fitting deals with the assembly of mating parts, through removal the bench, whereas fitting deals with the assembly of mating parts, through removal of metal, to obtain the required fit.
of metal, to obtain the required fit.
Both the bench work and fitting requires the use of number of simple hand Both the bench work and fitting requires the use of number of simple hand tools and considerable manual effort. The operations in the above works consist of tools and considerable manual effort. The operations in the above works consist of filing, chipping, scrapping, sawing, drilling, tapping, etc.
filing, chipping, scrapping, sawing, drilling, tapping, etc. 2.2. Holding tools
2.2. Holding tools 2.2.1. Bench vice: 2.2.1. Bench vice:
The bench vice is a work-holding device (Fig.2.1).It is the most commonly The bench vice is a work-holding device (Fig.2.1).It is the most commonly used vice in a fitting shop. It is fixed to the bench with bolts and nuts. The vice body used vice in a fitting shop. It is fixed to the bench with bolts and nuts. The vice body consists of two main parts, fixed jaw and a movable jaw. When the vice handle is consists of two main parts, fixed jaw and a movable jaw. When the vice handle is turned in a clockwise direction, the sliding jaw forces the work against the fixed jaw. turned in a clockwise direction, the sliding jaw forces the work against the fixed jaw. The greater the pressure applied to the handle, the tighter is work held. Jaws caps The greater the pressure applied to the handle, the tighter is work held. Jaws caps made of soft material are used to protect finished surface, gripped in the vice. The made of soft material are used to protect finished surface, gripped in the vice. The size of the vice is specified by the length of the jaws. The vice body is made of cast size of the vice is specified by the length of the jaws. The vice body is made of cast iron which is strong in compression, weak in tension and so fractures under shocks iron which is strong in compression, weak in tension and so fractures under shocks and therefore should never be hammered.
and therefore should never be hammered.
Fig.2. 1 Bench vice Fig.2. 1 Bench vice
2.2.2. V -block with clamp:
The v-block is a rectangular or square block with V-groove on one or both sides, opposite to each other. The angle of the ‘V’ is usually 900.V-block with a clamp is used to hold cylindrical work securely, during layout of measurements, for measuring operation or for drilling (Fig.2.2.).For this the bar is placed longitudinally in the V-groove and the screw of V-clamp is tightened. This grips the rod firmly, with its axis parallel to the axis of the V-groove.
Fig.2. 2 V-Block with clamp Fig.2. 3 C-Clamp 2.2.3. C-clamp:
This is used to hold work against an angle plate or V-block or any other surface, when gripping is required(Fig.2.3).Its fixed jaw is shaped like English alphabet,”C” and the movable jaw is round in shape and directly fitted to the threaded screw at the end.
The working principle of this clamp is the same as that of the bench vice. 2.3. Marking and Measuring Tools
2.3.1. Surface Plate:
The surface plate (Fig.2.4) is machined to fine limits and is used for testing the flatness of the work piece .It is also used for marking out small works and is more precise than the marking table. The degree of fineness of the finish depends upon whether it is designed for bench work in a fitting shop or for using in an inspection room. The surface plate is made of cast iron, hardened steel or granite stone. It is specified by length’ width’ height’ grade. Handles are provided on two opposite sides, to carry it while shifting from one place to another.
2.3.2. Angle plate:
The angle plate is made of cast iron. It has two surfaces, machined at right angle to each other (Fig.2.5).Plates and components, which are to be marked out, may be held against the upright face of the angle plate, to facilitate the marking. Slots are provided on the angle plate to clamp the work in position.
2.3.3. Universal scribing block:
This is used for scribing lines for layout work and checking parallel surfaces (Fig.2.6).Referring the figure, it may be noted that its spindle can be quickly adjusted to any angle, by an adjusting screw. In some designs, the base of the block will have a “V” shaped groove, to enable the block to rest on round bars if require, to set-off the dimensions from the bar to the surface of the components.
Fig.2. 6.Universal scribing block Fig.2. 7 Try-square 2.3.4. Try-square:
It is a measuring and marking tool for 90 degrees angle. In practice, it is used for checking the square ness of many types of small works, when extreme accuracy is not required (Fig.2.7).The blade of the try-square is made of hardened steel and the beam, of cast iron or steel. The size of the try-square is specified by the length of the blade.
2.3.5. Combination Set:
It is a combination of measuring tools used for measuring linear dimensions, angular dimensions and for checking flatness of surfaces. It consists of a rule, square head, centre head, protractor and spirit level (Fig.2.8).This may be used as a rule, a square, a depth gauge, for marking meters (450), for locating the centre on the end of a round bar and for measuring and marking angles. The rule is made of tempered steel with grooves. The combination set is specified by the length of its rule.
A scriber is a slender steel tool, used to scribe or mark lines on metal work pieces (Fig.2.9).It is made of hardened and tempered high carbon steel. The tip of the scriber is generally ground at 120 to 150. It is generally available in lengths, ranging from 125mm to 250mm.It has two pointed ends. The bent end is used for marking lines where the straight end can not reach.
2.3.7. Odd-leg caliper:
This is also called ‘Jenny Caliper’ or Hermaphrodite’. This is used for marking parallel lines from a edge and also for locating the centre of round bars (Fig.2.10.).As shown in figure, it has one leg pointed like a divider and the other leg bent like a caliper. It is specified by the length of the leg up to the hinge point.
Fig.2. 10 Odd-leg caliper Fig.2. 11 Divid 2.3.8. Divider:
It is basically similar to the calipers except that its legs are kept straight and pointed at the measuring edge. This is used for marking circles, arcs, laying out perpendicular lines, bisecting lines, etc.(Fig.2.11).It is made of case hardened mild steel or hardened and tempered low carbon steel. Its size is specified by the length of the legs.
These are used for making indentations on the scribed lines, to make them visible clearly. These are made of high carbon steel’s punch is specified by its length and diameter, say as 150,12.5mm.It consists of a cylindrical knurled body, which is plain for some length at the top of it. At the other end, it is ground to a point the tapered point of the punch is hardened over a length of 20 to 30 mm.
This is used to lightly indent along the layout lines, to locate centre of holes and to provide a small centre mark for divider point, etc.For this purpose, the punch is ground to a conical point having 60 degrees included angle (Fig.2.12a.).
This is similar to the dot punch, except that it is ground to a conical point having 90 degrees included angle (Fig.2.12b).It is used to mark the location of the holes to be drilled.
2.4.1 .Cutting tools: 2.4.1 .Hacksaw:
The hacksaw is used for cutting metal by hand. it consists of a frame ,which holds a thin blade, firmly in position .Hacksaw blade is specified by the number of teeth per centimeter. Hack saw blades have a number of teeth ranging from 5 to 15 per centimeter .blades having lesser number of teeth per cm are used for cutting soft materials like aluminium, brass and bronze. Blades having large no of teeth per centimeter are used for cutting hard materials like steel and cast iron.
Figure 2.13 shows two types of hacksaw frames and a blade.
Hacksaw blades are classified as: (i) all hard and (ii) Flexible types. The all hard blades are made of H.S.S, hardened and tempered throughout to retain their cutting edges longer .these are used to cut hard metals .these blades are hard and brittle and can break easily by twisting and forcing them into the work while sawing .flexible blades are made of H.S.S or low alloy steel but only the teeth are hardened and the rest of the blade is soft and flexible. These are suitable for use by un-skilled or semi –skilled persons.
Fig.2. 13 hacksaw frames with blades
The teeth of the hacksaw blade are staggered , as shown in fig.2.14 and are known as a ‘set of teeth ‘.these are slots wider than the blade thickness ,preventing the blade from jamming.
Chisels are used for removing surplus metal or for cutting thin sheets (fig.2.15). These tools are made from 0.9% to 1.0% carbon steel of octagonal or hexagonal section. Chisels are annealed, hardened and tempered to produce a tough shank and a hard cutting edge. Annealing relieves the internal stresses in the metal .the cutting angle of the chisel for general purpose is about 600.
Fig.2. 15 flat chisel Fig.2. 16 Combination plier 2.4.3. Combination plier:
This is made of high carbon steel by proper hardened and tempering and is used for cutting as well as for gripping the work (Fig.2.16).It has small cutting edges in both the jaws, which make it able to cut small diameter wires. The serrations in the jaws offer the facility for gripping, it is named as combination plier.Its handles are well insulated, which makes it suitable for electrical working.
2.4.4. Nose plier:
It is similar to combination plier but its gripping jaws have extra length in tapered form which makes it suitable for gripping small objects and also gripping in narrow spaces (Fig.2.17).
Fig.2. 17 Nose plier 2.4.5. Bench Drilling machine:
Holes are drilled for fastening parts with rivets, bolts or for producing internal threads. Bench drilling machine is the most versatile machine used in a fitting shop for the purpose (fig.2.18).twist drills, made of tool or high speed steel are used with the drilling machine for drilling holes.
Tirumala Engineering College 7
Fig.2. 18 Bench Drilling machine Fig.2. 19 Portable drill 2.4.6 .Portable electric drill:
It is the most useful of all pieces of drilling equipment .it is readily portable and convenient for use. Generally, it is used for drilling holes in small pieces held in a vice (fig. 2.19).the capacity of the portable drill is designated by the maximum diameter of the hole it can drill in steel.
2.5 .Finishing tools 2.5.1. Files:
Filing is one of the methods of removing small amounts of material from the surface of a metal part. A file is a hardened steel tool, having slant parallel rows of cutting edges or teeth on its surfaces. On the faces, the teeth are usually diagonal to the edge. On end of the file is shaped to fit into a wooden handle. Fig 2.20 shows the parts of a hand file. The hand file is parallel in width and tampering slightly in thickness, towards the tip. It is provided with double cut teeth on the faces, single cut on one edge and no teeth on the other edge, which is known as the safe edge.
Fig.2. 20 Hand file 2.5.2. Types of files:
Files are classified according to their shape, cutting teeth and pitch or grade of the teeth. Figure 2.21 shows the various types of files based on their shape. Types of files and their description and uses are given below. Further, single cut and double cut files are shown in fig.2.22.single cut files are rows of teeth running in one direction, across their faces and double cut files have a second row of teeth, cut diagonally to the first row.
Fig.2. 21 Types of files Fig.2. 22 Single and double cut files
Rectangular in section, tapered in thickness but parallel in width. The faces have double cut teeth and one of the edges, single cut. The other edge does not have any teeth and hence called as safe edge file. It is used for filing a surface, at right angle to an already finished surface.
Rectangular in section and tapered for 1/3rd length in width and thickness. The faces have double cut teeth and the edges, single cut. Used for general purpose filing.
Square in section and tapered for 1/3rd length on all faces. All the faces have double cut teeth. Used for filing corners and slots and also o cut keyways.
Equilateral triangular in section and tapered for 1/3rd length on all faces .All the faces have double cut teeth. Used for filing internal corners.
Half round file:
It has one flat face, connected by a curved face and tapered for 1/3rd length. The curved face is not exactly semi-circular but only a part of circle. The flat face has double cut teeth and the curved face, single cut. Used for filing concave surfaces and internal corners.
Circular cross-section and tapered for 1/3rd length. It has double cut teeth. Used for filing concave surfaces and circular openings.
Swiss or needle file:
It is normally 150mm long, with cut teeth. Used for filing corners, grooves, narrow slots, etc., in intricate work.
It is a metal brush, used for cleaning the files, to free them from filings, clogged in-between the teeth (Fig.2.23).
Fig.2. 23 File card 2.6. Miscellaneous Tools:
2.6.1. Ball-peen hammer:
Hammers are named, depending upon their shape and material and specified by their weight. A ball-peen hammer has a flat face, which is used for general work and a ball end, particularily used for riveting (Fig.2.24).
Fig.2. 24 Ball-peen hammer 2.6.2. Cross-peen hammer:
It is similar to ball-peen hammer, except the shape of the peen (Fig.2.25).This is used for chipping, riveting, bending and stretching metals and hammering inside the curves and shoulders.
Fig.2. 25 Cross-peen hammer Fig.2. 26 Straight –peen hammer 2.6.3. Straight-peen hammer:
This is similar to cross-peen hammer, but its peen is in –line with the hammer handle (Fig.2.26).It is used for swaging, riveting in restricted places and stretching metals.
2.6.4. Screw driver:
A screw driver is designed to turn screws (Fig.2.27a).The blade is bade of steel and is available in different lengths and diameters. The grinding of the tip to the correct shape is important .A Phillips (star) screw driver is specially designed to fit the head of Phillips screws. The end of the blade is fluted instead of flattened (Fig.2.27b).The size of a screw driver is specified by the length of the metal part from handle to the tip.
Fig.2. 27 Screw Drivers 2.7. Fitting Operations:
Riveting the metal with a chisel is called and is normally used where machining is not possible (Fig.2.28).While chipping, safety goggles must be put on, to protect eyes from the flying chips. To ensure safety of others, a chip guard is placed in position. Care should be taken to see that the chisel is free from mushroom head.
For better results, the angle of chipping must be the same throughout the operation.
There are several methods of filing, each with a specific purpose (Fig.2.29).With reference to the figure, the following may be noted:
1. Holding the file For heavy work and to remove more metal, a higher pressure is used. For light and fine work, a light pressure is applied.
2. Filing internal curves A part of half round file only makes contact as shown, during filing operation. Movement of the file is indicated by arrows.
3. Cross filing Cross filing is carried out across two diagonals, to produce medium surface finish.
NOTE The possibility of the surface becoming curved is drastically reduced due to continuous changing of directions.
4. Draw filing A smooth file with a flat face is used for this purpose and to produce fine grained structure.
NOTE: Draw filing is preferred for the edges of long and narrow work pieces. For this, the file is placed at right angle to the work and held in both the hands across its body, as close as possible. During work, the file is pushed backward and farward, along the edge being filed.
2.7.3. Pinning of files this is caused by soft metals, clogging the file teeth and scratching the surface of the work. The pins are removed with a file card. Pinning may be prevented by rubbing chalk into the teeth before filing.
2.7.4. Checking flatness and square ness To check flatness, the try-square is placed as shown in Fig.2.30a.No light should be seen between the bottom edge of the square and the top surface of the work piece, when both are held against light.Similarly, the flatness across thickness of plate is tested as shown. The square ness of one edge with respect to another reference edge is checked as shown in Fig.2.30b.
Fig.2. 29 Filling Fig.2. 30 Use of Try square 2.7.5. Scraping:
It is an operation by which high spots left on the surface of a job after filing or other machining operations, are removed by using a tool known as scraper .scraping is aimed at removing only a little amount of metal, to get perfect flat surface. Scraping operation is carried out by holding the scraper in the both hands and then removing it too and fro (fig.2.31)
Fig.2. 31 Scraping 2.8. Marking and measuring:
Accurate marking is the first step and the methods and instruments used are common in all fitting works. Measurements are taken either from a finished edge or from the center line. Scriber lines on non ferrous materials and oxide coated steels are readily visible but bright steel needs coating with copper sulphate solution or engineers glue (Prussian blue), for the visibility of the line.
Measuring and testing are continuous processes through out the manufacturing, wheather working with hand tools or machines. Degrees of accuracy are specified on the drawings. The following are the measuring methods in the order of increasing accuracy.
Fig.2. 32 Methods of measuring and marking
a. Direct measurement from a rule b. Calipers set to a rule
c. Calipers set to a plug gauge d. Vernier calipers
e. Micrometer f. Dial indicator
2.9. Principles of sawing
Hack saw blades are specified by length and number of teeth in a standard length along the cutting edge. The correct choice of pitch should ensure that at least 3 teeth are in contact with the section to be sawn (fig.2.33).
Fig.2. 33 Selection of blade pitch
Hacksaw blade should be inserted in the frame, with the teeth pointing forward, as the saw cuts on forward stroke only. The blade should be tightening such that the blade produces a musical twang when struck lightly with the finger. Only little downward pressure is needed in sawing, as teeth are designed to pull themselves into the work. About 40 strokes per minute is the correct sawing speed.