The best results are always obtained by using the correct tool for the task. Care and maintenance of all tools is very important, since damaged or inefficient tools can lead to injury of the user or damage to the components. A range of common hand tools is considered in this part of the course.
4.1.1 Engineer’s Rule
An engineer’s rule (refer to Fig. 1) is made from high-carbon steel and is graduated in Imperial and Metric units. Rules are classified by the length and width of their graduated portion, must be kept free from rust and should not be subjected to rough usage. The most common engineer’s rule has a length of 300mm (1ft) but rules can be obtained in lengths of up to 1,800mm (6ft).
The increment graduation marks are etched into the rule surface providing a grooved recess. These grooves enable dividers to be set to a greater accuracy, as the divider points can be felt to ‘drop in’ to the recess.
Engineer’s Rule 1 2 3 4 5 6 7 8 9 10 11 24 25 26 27 28 29 30 1 2 3 4 1 0 1 1 1 2 Metric Scale Edge View Grooves Imperial Scale
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4.1.2 ScriberA scriber (refer to Fig. 2) is used for marking lines on the surface of metals. Scribers are made from high-carbon steel and are classified by their length. One end of the scriber is usually bent at right angles to enable lines to be scribed in difficult places such as through a hole.
All scribed lines, on soft materials, must only be cutting (boundary) lines, and none must be left on the surface of the metal on completion, as they can cause cracks. Other lines, including bend lines and lines for the position of rivets must be marked with a sharp pencil.
Scriber points must be kept sharp and fine by careful ‘stoning’, with an oil stone, rather than an abrasive wheel (grindstone). Using a wheel is likely to generate too much heat, which will result in the temper being drawn from the steel and the point of the scriber becoming soft and useless.
When not in use (and as with other tools with sharp points), placing pieces of cork, plastic or similar material over their points will protect them.
4.1.3 Key-Seat Rule
Key-seat rules are used for marking-off lines, parallel to the axis, on the surface of tubes or round bars (refer to Fig. 2). Sometimes referred to as ‘Box Squares’, key-seat rules are usually graduated and are classified by their length.
Scriber and Key Seat Rule Fig. 2
Round Bar
Key SeatRule
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4.1.4 Fitter’s SquareThe fitter’s square is used for setting out lines at right angles to an edge or surface, and for checking right angular work for ‘truth’. Squares are made, to very fine limits, of high-carbon steel and are classified by the length of the blade.
The blade and the stock have their opposing edges ground truly parallel with the two limbs set at exactly 90° to each other.
To preserve its accuracy it is essential that it is handled carefully at all times and, when not in use, kept in a protective case or box.
When testing a square for accuracy, it may be checked for truth against an accurately machined right angular test piece such as a ‘V’ block or master square. If this is not possible, a test may be carried out (refer to Fig. 3) as follows: • Place the stock against the true edge of a flat surface and scribing a line on
the surface, using the outside edge of the blade
• Turn the square over and check the outside edge of the blade against the previously scribed line.
If the square is accurate, the blade edge and the scribed line will be in line. In a similar manner, the inside edge of the blade can be tested.
Error True Edge
Testing a Square Fig. 3
JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (MECHANICAL)
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4.1.5 Combination SetThe Combination Set (refer to Fig. 4), consists of a graduated steel rule, which has a machined groove running along the centre of its entire length. The rule can be slid into three different ‘heads’ and secured, by a locking screw device, so that the combination of rule and ’head’ will enable certain tasks to be accomplished. The Centre Head is used, with the rule, to locate the centre line of bars or round tubes.
The Square Head has one working surface at 90° and another at 45° to the locked rule. This allows the tool to be used, either in a similar manner to the Fitter’s Square (to check the squareness of work), or it may be used for the marking out of mitre joints and bevels.
A spirit level and scriber are, sometimes, accommodated in the base of the Square Head, to permit a check to be done on the horizontal or vertical accuracy of workpieces.
The Protractor Head also has a spirit level, which rotates with the head, and allows the head to be used, singly, as a clinometer or, in conjunction with the rule, it may be used to mark out and check angles on workpieces.
Combination Set Fig. 4 Centre Head Square Head Scriber Spirit Level Groove Protractor Head
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4.1.6 Surface Plates and Tables
Surface plates (and surface tables which are larger), while not actually being classed as marking out or measuring tools, are simply blocks of grey cast iron with finely machined faces which can be used as a standard of flatness. They may also be used to provide a true surface, from which marking out, measuring and testing can be done.
Surface plates are usually mounted on a bench and, normally, only have three supports, or feet, to ensure steadiness, if the surface of the bench were to be slightly uneven.
Surface tables are free standing, on the workshop floor, and their sheer weight provides the required steadiness.
The standard of the surface finish varies. The better grades are scraped and the cheaper ones are merely planed. The accuracy of a planed table depends upon the accuracy of the machine producing it.
Surfaces of grade ‘A’ standard would only be used in Standards Rooms, grade ‘B’ surfaces are for inspection work while grade ‘C’ surface plates and tables would be found in typical workshops.
Surface plates and tables can be used to test for flatness, providing the standards required are not too high. The surface of the plate is lightly smeared with a mixture of engineer’s blue and a few drops of oil. The piece to be tested has to be rubbed lightly on to the surface plate and any high spots will show up as blue spots on the test piece. These spots will be filed or scraped until the whole surface shows blue.
After use, a light film of oil should be applied to the working surface of the surface plates and tables. They should, then, be protected with a wooden cover, to prevent the onset of corrosion.
4.1.7 V Blocks
V Blocks are accurately machined, six-sided, rectangular blocks (generally made of cast iron), which may be used, on surface plates and tables, to hold a round bar, which can then be marked in a variety of ways, to give centres and lines parallel to its side. V blocks are classified by the maximum diameter of the work, which they can hold.
All opposite sides of the blocks are parallel and all adjacent faces are square to each other. A 90° groove (in the shape of a V) is machined in two (longer) opposite faces, but the grooves are cut at different depths, to cater for bars of different diameters.
JAR 66 CATEGORY B1 MODULE 7 MAINTENANCE PRACTICES (MECHANICAL)
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The V-cut grooves have a small, square-cut, clearance groove in the bottom of the V. This ensures that any oil, or dirt runs off the sides of the V and does not clog the bottom of the V, causing an imperfect seating of any bar which were to be placed in the blocks.
V blocks are made in (identified) matching pairs, which must always be used together, so that a block of one pair should not be used with one of another pair. Some V blocks also have grooves machined along the other two longer, parallel, sides, to locate specially designed clamps, which may be used to securely hold work while it is being accurately marked out or drilled.
4.1.8 Surface Gauge (Scribing Block)
A Surface Gauge, or Scribing Block (refer to Fig. 5), is another marking out tool, used, on a surface plate or table, in conjunction with a scriber (and, occasionally, with V blocks), for the marking of lines, which are parallel to a true surface.
The scriber is clamped to a spindle, which can be accurately pivoted, by means of a fine adjustment screw, on the heavy base. The base, which is generally made from cast iron (or hardened steel) is machined to be as flat as the surface plate on which it slides, but it is also grooved (in a similar manner to the V block) so that it can be used on round stock when required.
Two friction-fit pins, in the base, may be pushed down, to assist in drawing lines parallel to a true edge.
Scribing Block with V Blocks and Surface Plate Fig. 5
Scribing Block V Blocks
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4.1.9 DividersDividers are used to set out distances and to scribe arcs and circles. The legs are made of high-carbon steel, the spring made of spring steel and the adjusting mechanism of mild steel.
Dividers are classified by the length of their legs. The points should be kept sharp and of equal length by stoning only the outside of the legs. If grinding is used to sharpen the points, it must be done very carefully, as the temper of the points can be drawn, leaving them soft.
The points of dividers should be protected, when not in use, in a similar manner to those of scribers and such tools.
4.1.10 Callipers
Callipers (refer to Fig. 6) are a type of measuring device, typically used to measure diameters and distances or for comparing sizes. The three basic types of calliper are:
• Outside Callipers: Used to measure the outside diameter of an object and have legs that point inwards
• Inside Callipers: Used to measure the inside of a hole and have legs that point outwards
• Odd-Leg Callipers (Hermaphrodite or ‘Jenny’ Callipers): This tool is really half callipers and half dividers. It may be used for scribing arcs on metal surfaces from an edge, for scribing lines parallel to an edge or surface, (provided accuracy is not of great importance), and for finding the centre of a round bar.
Outside Inside Oddleg
Callipers Fig. 6
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4.1.11 HammersHammers (refer to Fig. 7) are classified by their weight and type of head. Steel heads are forged and manufactured from high-carbon steel. Most shafts are made from straight-grained Ash or Hickory and are secured to the head by wedging.
As can be seen from Fig. 7, the main types of engineering hammers are the: • Ball Pein: The flat surface is used for most general-purpose work whilst the
ball pein is used primarily for riveting-type operations
• Straight Pein: Used for general work, the narrow, straight pein being particularly suitable for use where access to the work is limited
Types of Hammers Fig. 7
Ball Pein Straight Pein Cross Pein
Hide/Copper Face
Plastic Face
Hide Face
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• Cross Pein: As for the straight pein, but the axis of the pein is at 90° to that of the shaft
• Hide/Copper Face: The rawhide facing enables heavy blows to be delivered without damaging the surface of the work, while the copper face may be used for heavier types of work than hide faced hammers
• Rubber Head and Plastic Face: More modern versions of the Hide Face hammer. Can often have one of each type of face on each end of the head • Claw Hammer (not shown): More commonly used for woodworking. The face
is used for hammering nails whilst the claw is used for removing nails • Body Hammer (not shown): Little used in aircraft work, as they are primarily
used to remove dents and blemishes from sheet metal. They are also known as planishing hammers.
The weight of hammer required can be found with experience. Before use, it must be ensured that the head is secure on the shaft. The shaft should be gripped close to the end opposite the head, as proper control is not possible if it is held close to the head.
4.1.12 Punches
Although punches are not ‘pounding tools’, they do allow the force from a hammer blow to be concentrated in the immediate area of the punch tip. This in turn means that the pressure at the end of the punch is increased compared to a hammer blow without a punch.
Over a period of time, the hammered shank end of a punch, tends to deform into the shape of a mushroom. To reduce the chance of a metal chip flying off and causing injury, during punching operations, the deformation should be removed and the shank end returned to its original shape by the use of a bench grinder. Eye or face protection should always be used when using punches of any type. The types of punches, more commonly found in an engineer’s toolkit, include: • Centre Punches
• Pin Punches • Hollow Punches • Drifts
The first three punches are, usually, constructed from hexagonal (or knurled, round) rods of tempered, cast steel with a length of approximately 127 mm (5 in), a gripping diameter of approximately 3.175 mm (0.125 in) and a smaller, driving end of the appropriate size.
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Centre Punches are relatively sharp-pointed tools, used to make an indentation in metal. The indentation aids in locating the centre of a hole and for starting a drill bit when drilling the hole. The points may be ground at angles between 60° to 90°, depending on the hardness of the metal on which the punch is being used. The softer the metal, then the larger will be the angle of the punch’s point.
When using a centre punch, it must be struck hard enough to give an indentation large enough for a drill bit to start, but not so hard as to distort the metal.
Another form of Centre Punch is the ‘Dot’ or ‘Prick’ Punch (also ‘Pricker’), which has a finer point and is used to make indentations along a drawn line when the line is, otherwise, difficult to see. The indentations may also be used, when sawing down to a line, as ‘witness’ points, to show that the cutting is accurate. Centre punches should not be used to drive out pins or rivets from their holes. Pin Punches, as their name implies, are the tools to be used for the removal of pins and rivets from their respective holes. The driving end of a Pin Punch is cut flat, and its diameter ground to match that of the pin or rivet which is being driven from its hole. Pin Punches may be found with parallel or tapered driving ends. Hollow Punches are used to punch out bolt (or stud) holes in soft, thin sheets, such as shimming or gasket materials, which are difficult to cut with drills. The material being cut, should be supported by a wooden block, to avoid damaging the cutting end of the Hollow Punch.
Drifts may be fashioned from aluminium alloy, copper or steel bars (or tubes), and are used for driving out bearings, bushes or shafts from their respective cages or housings.
Only steel drifts should be used on bearings, due to the possibility of small metal chips, from the softer metals, breaking off and fouling the bearing assemblies. 4.1.13 Metal-Cutting Chisels
Metal-cutting chisels (also called Cold Chisels) are used in conjunction with steel hammers. Chisels are forged, usually using short lengths of hexagonal-sectioned, high-carbon steel bars, with the cutting edge hardened and tempered.
To prevent flying particles when hammering, the striking end is not hardened and is, therefore, comparatively softer. Periodically, the burr, that forms at the striking end of the chisel, should (in a similar manner to punches), be removed by filing or grinding.
Alternatively, the chisels may be made of nickel-alloy steel, specially heat-treated, to produce a long-lasting cutting edge.
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Chisels are classified by their shape, overall length, cross-section of shank and width of cut. There are four principal shapes of chisels (refer to Fig. 8), in general use. They are the:
• Flat
• Cross-Cut • Diamond-Point • Half-Round.
Flat chisels are used for general chipping work, such as parting sheet metal or cutting flat surfaces, preparatory to filing. The cutting edge is formed slightly convex.
Cross-Cut (or Cape) chisels are used to cut narrow, flat-bottomed, grooves, such as keyways in shafts or where it is not practical to use a flat chisel. These chisels are also used to remove the heads of round-headed rivets during repairs.
Diamond-Point chisels are particularly useful for cutting in corners, cutting small oil grooves and for rectifying an incorrect start when drilling.
Half-Round (and may, also, be called Round) chisels are general-purpose, grooving chisels, which are suitable for cutting half-round, bottomed, grooves. They are also suitable for rectifying an incorrect start when drilling.
When selecting a chisel for a specific task, consideration must be given both to the nature of the work and to the material that is to be cut. The nature of the work governs the choice of shape, whilst the angle formed by the cutting edge is influenced by the hardness of the metal.
Half-Round Diamond-Point Cross-Cut Flat Chisel Types Fig. 8
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In general, it may be assumed that the softer the metal the more acute should be the cutting angle. Table 5 shows some suggested cutting angles for use on typical metals, found in aircraft engineering workshops.
Table 1
SUGGESTED CHISEL CUTTING ANGLES
Hard Steels 70°
Mild Steels 60°
Soft Metals 40°
High-carbon, steel chisels, should be sharpened by grinding on an abrasive wheel, but nickel-alloy, steel chisels are sharpened by filing. The cutting edge of the chisel must be kept cool, during grinding, by frequent immersion in water, which will prevent the temper being drawn from the metal.
4.1.14 Bench Vice
The bench vice (refer to Fig. 9) is used to firmly grip the material or item upon which work is being done in a workshop. The body of the vice is provided with detachable steel jaws. The screw is made with a square or with a buttress thread. Most types of bench vice have a quick-release mechanism, operated by a small lever. The jaws can then be slid either open or closed until the correct position is reached. The lever disengages the half nut from the thread to permit the sliding action and it is driven back into engagement by a strong spring. Bench vices are classified by the length of their jaws.
The height of the top of the vice above the ground is important, and should ideally, be level with the technician’s elbow when standing adjacent to the vice.