Thickness of Diameter of Rivet (Inches) Sheet (Inches) 1/16 3/32 1/8 5/32 3/16 1/4 5/16 3/8
0.014 71 107 143 179 215 287 358 430 .016 82 123 164 204 246 328 410 492 .018 92 138 184 230 276 369 461 553 .020 102 153 205 256 307 410 412 615 .025 128 192 256 320 284 512 640 768 .032 164 245 328 409 492 656 820 984 .036 184 276 369 461 553 738 922 1,107 .040 205 307 410 512 615 820 1,025 1,230 .045 230 345 461 576 691 922 1,153 1,383 .051 261 391 522 653 784 1,045 1,306 1,568 .064 492 656 820 984 1,312 1,640 1,968 .072 553 738 922 1,107 1,476 1,845 2,214 ,081 622 830 1,037 1,245 1,660 2,075 2,490 091 699 932 1,167 1,398 1,864 2,330 2,796 .102 784 1,046 1,307 1,569 2,092 2,615 3,138 .125 961 1,281 1,602 1,922 2,563 3,203 3,844 .156 1,198 1,598 1,997 2,397 3,196 3,995 4,794 .188 1,445 1,927 2,409 2,891 3,854 4,818 5,781 250 1,921 2,502 3,202 3,843 5,125 6,405 7,686 .313 2,405 3,208 4,009 4,811 6,417 7,568 9,623 .375 2,882 3,843 4,803 5,765 7,688 9,068 11,529 .500 3,842 5,124 6,404 7,686 10,250 12,090 15,372 FIGURE 5-2. Bearing strength chart (pounds).
Given : Maintaining Original Contour L = 2-1/4 (2.25) in. Form all repairs in such a manner that they will T = 0.040 in. fit the original contour perfectly. A smooth contour Size of rivet: 0.040 x 3 = 0.120, so rivet is especially desirable when making patches on the must be 1/8 in. or 0.125. smooth external skin of high-speed aircraft.
S = 331 (from the shear strength chart).
B = 410 (from the bearing strength chart).
(Use S to find number of rivets per side as it is smaller than B.)
Keeping Weight to a Minimum Keep the weight of all repairs to a minimum.
Make the size of the patches as small as practicable and use no more rivets than are necessary. In many Substituting in the formula: cases, repairs disturb the original balance of the structure. The addition of excessive weight in each
2.25 x 0.040 x 75,000 6,750 repair may unbalance the aircraft so much that it 331 331 will require adjustment of the trim-and-balance = 20.39 (or 21) tabs. In areas such as the spinner on the propeller, rivets/side. a repair will require application of balancing Since any fraction must be considered as a whole number, the actual number of rivets required would be 21 for each side, or 42 rivets for the entire repair.
patches so that a perfect balance of the propeller assembly can be maintained.
GENERAL STRUCTURAL REPAIR Aircraft structural members are designed to per- 129
form a specific function or to serve a definite purpose. The prime objective of aircraft repair is to restore damaged parts to their original condition.
Very often, replacement is the only way in which this can be done effectively. When repair of a damaged part is possible, first study the part carefully so that its purpose or function is fully understood.
Strength may be the principal requirement in the repair of certain structures, while others may need entirely different qualities. For example, fuel tanks and floats must be protected against leakage; but cowlings, fairings, and similar parts must have such properties as neat appearance, streamlined shape, and accessibility. The function of any damaged part must be carefully determined so that the repair will meet the requirements.
INSPECTION OF DAMAGE When visually inspecting damage, remember that there may be other kinds of damage than that caused by impact from foreign objects or collision.
A rough landing may overload one of the landing gear, causing it to become sprung; this would be classified as load damage. During inspection and "sizing up of the repair job," consider how far the damage caused by the sprung shock strut extends to supporting structural members.
A shock occurring at one end of a member will be transmitted throughout its length; therefore, inspect closely all rivets, bolts, and attaching structures along the complete member for any evidence of damage. Make a close examination for rivets that have partially failed and for holes which have been elongated.
Another kind of damage to watch for is that caused by weathering or corrosion. This is known as corrosion damage. Corrosion damage of aluminum material is usually detected by the white crys.
talline deposits that form around loose rivets, scratches, or any portion of the structure that may be a natural spot for moisture to settle.
Burnishing-Polishing of one surface by sliding contact with a smooth, harder surface. Usually no displacement nor removal of metal.
Burr-A small, thin section of metal extending beyond a regular surface, usually located at a corner or on the edge of a bore or hole.
Corrosion--Loss of metal from the surface by chemical or electrochemical action. The corrosion products generally are easily removed by mechanical means. Iron rust is an example of corrosion.
Crack-A physical separation of two adjacent portions of metal, evidenced by a fine or thin line across the surface, caused by excessive stress at that point. It may extend inward from the surface from a few thousandths inch to completely through the section thickness.
Cut-Loss of metal, usually to an appreciable depth over a relatively long and narrow area, by mechanical means, as would occur with the use of a saw blade, chisel or sharp-edged stone striking a glancing blow.
Dent-Indentation in a metal surface produced by an object striking with force. The surface surrounding the indentation will usually be slightly upset.
Erosion-Loss of metal from the surface by mechanical action of foreign objects, such as grit or fine sand. The eroded area will be rough and may be lined in the direction in which the foreign material moved relative to the surface.
Chattering Breakdown or deterioration of metal surface by vibratory or "chattering" action.
Usually no loss of metal or cracking of surface but generally showing similar appearance.
Definition of Defects Types of damage and defects which may be observed on parts of this assembly are defined as follows:
Galling-Breakdown (or build-up) of metal sur:
faces due to excessive friction between two parts having relative motion. Particles of the softer metal are torn loose and "welded" to the harder.
Brinelling--Occurrence of shallow, spherical depressions in a surface, usually produced by a part having a small radius in contact with the surface under high load.
Gouge-Grooves in, or breakdown of, a metal surface from contact with foreign material under heavy pressure. Usually indicates metal loss but may be largely displacement of material.
Inclusion-Presence of foreign or extraneous material wholly within a portion of metal.
material is introduced during the manufacture of rod, bar or tubing by rolling or forging.
Nick-Local break or notch on edge. Usually displacement of metal rather than loss.
Pitting-Sharp, localized breakdown (small, deep cavity) of metal surface, usually with defined edges.
Damage Repairable by Patching Damage repairable by patching is any damage exceeding negligible damage limits which can be repaired by bridging the damaged area of a component with a material splice. The splice or patch material used in internal riveted and bolted repairs is normally the same type of material as the damaged part, but one gage heavier. In a patch repair, filler plates of the same gage and type of material as that in the damaged component may be used for bearing purposes or to return the damaged part to its original contour.
Scratch-Slight tear or break in metal surface from light, momentary contact by foreign material.
Score-Deeper (than scratch) tear or break in metal surface from contact under pressure. May show discoloration from temperature produced by friction.
Stain-A change in color, locally causing a noticeably different appearance from the surrounding Upsetting--A displacement of material beyond the normal contour or surface (a local bulge or bump). Usually indicates no metal loss.
CLASSIFICATION OF DAMAGE Damages may be grouped into four general classes. In many cases, the availability or lack of repair materials and time are the most important factors in determining whether a part should be repaired or replaced.
Negligible Damage Damage which does not affect the structural integrity of the member involved, or damage which can be corrected by a simple procedure without placing flight restrictions on the aircraft, is classified as negligible damage. Small dents, scratches, cracks, or holes that can be repaired by smoothing, sanding, stop drilling, or hammering out, or otherwise repaired without the use of additional materials, fall in this classification.
Damage Repairable by Insertion Damage which can be repaired by cutting away the damaged section and replacing it with a like section, then securing the insertion with splices at each end is classified as damage repairable by inser- Damage Necessitating Replacement of Parts Replacement of an entire part is considered when one or more of the following conditions exist:
(1) When a complicated part has been extensively damaged.
(2) When surrounding structure or inaccessibility makes repair impractical.
(3) When damaged part is relatively easy to replace.
(4) When forged or cast fittings are damaged beyond the negligible limits.
STRESSES IN STRUCTURAL MEMBERS Forces acting on an aircraft, whether it is on the ground or in flight, cause pulling, pushing, or twisting within the various members of the aircraft structure. While the aircraft is on the ground, the weight of the wings, fuselage, engines, and empennage causes forces to act downward on the wing and stabilizer tips, along the spars and stringers, and on the bulkheads and formers. These forces are passed on from member to member causing bending, twisting, pulling, compression, and shearing.
As the aircraft takes off, most of the forces in the fuselage continue to act in the same direction; but because of the motion of the aircraft, they increase 131
in intensity. The forces on the wingtips and the wing surfaces, however, reverse direction and instead of being downward forces of weight, they become upward forces of lift. The forces of lift are exerted first against the skin and stringers, then are passed on to the ribs, and finally are transmitted through the spars to be distributed through the fuselage.
The wings bend upward at their ends and may flutter slightly during flight. This wing bending cannot be ignored by the manufacturer in the origi nal design and construction, and cannot be ignored during maintenance. It is surprising how an aircraft structure composed of structural members and skin rigidly riveted or bolted together, such as a wing, can bend or act so much like a leaf spring.
The five types of stresses (figure 5-3) in an aircraft are described as tension, compression, shear, bending, and torsion (or twisting). The first three are commonly called basic stresses, the last two, combination stresses. Stresses usually act in combinations rather than singly.
Tension Tension (or tensile stress) is the force per unit area tending to stretch a structural member. The strength of a member in tension is determined on the basis of its gross area (or total area), but calculations involving tension must take into consideration the net area of the member. Net area is defined as the gross area minus that removed by drilling holes or by making other changes in the section. Placing rivets or bolts in holes makes no appreciable difference in added strength, as the rivets or bolts will not transfer tensional loads across holes in which they are inserted.
Compression Compression (or compressive stress) is the force per unit area which tends to shorten (or compress) a structural member at any cross section. Under a compressive load, an undrilled member will be stronger than an identical member with holes drilled through it. However, if a plug of equivalent or stronger material is fitted tightly in a drilled member, it will transfer compressive loads across the hole, and the member will carry approximately as large a load as if the hole were not there. Thus, for compressive loads, the gross or total area may be used in determining the stress in a member if all holes are tightly plugged with equivalent or stronger material.
Tension B.
Compression Torsion Shear E Tension Compression Bending FICURE 5-3. Five stresses acting on an aircraft.
Shear Shear is the force per unit area which causes adjacent particles of material to slide past each other. The term "shear" is used because it is a sideways stress of the type that is put on a piece of paper or a sheet of metal when it is cut with a pair of shears. Shear stress concerns the aviation mechanic chiefly from the standpoint of rivet and bolt applications, particularly when attaching sheet stock, because if a rivet used in a shear application gives way, the riveted or bolted parts are pushed sideways.
Bending Bending (or beam stress) is actually a combination of two forces acting upon a structural member 132
at one or more points. In figure 5-3 note that the bending stress causes a tensile stress to act on the upper half of the beam and a compreasive stress on CREASING the lower half. These stresses act oppositely on the BEAKHORN STAKE STAKE WITH HORN two sides of the center line of the member, which is called the neutral axis. Since these forces acting in Ť APRON opposite directions are next to each other at the neutral axis, the greatest shear stress occurs along COPPERSMITH SQUARE STAKE BLOWHORN STAKE this line, and none exists at the extreme upper or lower surfaces of the beam. T SHANK Torsion COMMON HATCHET STAKE
SQUARE STAKE Torsion (or twisting streas) is the force which tends to twist a structural member. The stresses arising from this action are shear stresses caused by BEVEL EDGE the rotation of adjacent planes past each other BOTTOM STAKE SQUARE STAKE around a common reference axis at right angles to these planes. This action may be illustrated by a rod fixed solidly at one end and twisted by a weight HORN placed on a lever arm at the other, producing the CANDLEMOLD STAKE CONDUCTOR STAKE on the rod at some distance from each other. A equivalent of two equal and opposite forces acting SHANK shearing action is set up all along the rod, with the center line of the rod representing the neutral axis. NEEDLECASE STAKE SOUD MANDREL STAKE
SPECIAL TOOLS AND DEVICES FOR SHEET METAL The airframe mechanic does a lot of work with special tools and devices that have been developed to make his work faster, simpler, and better. These special tools and devices include dollies and stakes and various types of blocks and sandbags used as support in the bumping process.
Dollies and Stakes Sheet metal is often formed or finished (planished) over variously shaped anvils called dollies and stakes. These are used for forming small, odd.
shaped parts, or for putting on finishing touches for which a large machine may not be suited. Dollies are meant to be held in the hand, whereas stakes are designed to be supported by a flat cast iron bench plate fastened to the workbench (figure 5-4).
Most stakes have machined, polished surfaces which have been hardened. Do not use stakes to back up material when chiseling, or when using any similar cutting tool because this will deface the surface of the stake and make it useless for finish work.
V-Blocks V-blocks made of hardwood are widely used in airframe metalwork for shrinking and stretching metal, particularly angles and flanges. The size of
HOLLOW MANDREL STAKE DOUBLE-SEAMING STAKE : Bench plate. * alt de li FIGURE 5-4, Bench plate and stakea the block depends on the work being done and on personal preference. Although any type of hardwood is suitable, maple and ash are recommended for best results when working with aluminum alloys.
Hardwood Form Blocks.
Hardwood form blocks can be constructed to duplicate practically any aircraft structural or nonstructural part. The wooden block or form is shaped to the exact dimensions and contour of the part to be formed.
Shrinking Blocks Machinist's A shrinking block consists of two metal blocks vise and some device for clamping them together. One block forms the base, and the other is cut away to provide space where the crimped material can be hammered. The legs of the upper jaw clamp the material to the base block on each side of the crimp so that the material will not creep away but will remain stationary while the crimp is hammered flat (being shrunk). This type of crimping block is designed to be held in a bench vise.
Shrinking blocks can be made to fit any specific need. The basic form and principle remain the same, even though the blocks may vary considerably in size and shape.
Sandbags A sandbag is generally used as a support during Utility the bumping process. A serviceable bag can be bench vise made by sewing heavy canvas or soft leather to FIGURE 5-5. Vises.
form a bag of the desired size, and filling it with sand which has been sifted through a fine mesh screen.
Before filling canvas bags with sand, use a brush to coat the inside of it with softened paraffin or beeswax, which forms a sealing layer and prevents the sand from working through the pores of the canvas.
Holding Devices Vises and clamps are tools used for holding materials of various kinds on which some type of operation is being performed. The type of operation being performed and the type of metal being used determine the holding device to be used.
The most commonly used vises are shown in figure 5-5; the machinist's vise has flat jaws and usually a swivel base, whereas the utility bench vise has scored, removable jaws and an anvil-faced back jaw. This vise will hold heavier material than the machinist's vise and will also grip pipe or rod firmly. The back jaw can be used for an anvil if the work being done is light.
The carriage clamp, or C-clamp, as it is commonly called, is shaped like a large C and has three main parts: (1) The threaded screw, (2) the jaw, and (3) the swivel head. The swivel plate, which is at the bottom of the screw, prevents the end from turning directly against the material being clamped.
Although C-clamps vary in size from 2 in, upward, their function is always that of clamping or holding FIGURE 5-6. Cleco fastener.
The shape of the C-clamp allows it to span obstructions near the edge of a piece of work. The greatest limitation in the use of the carriage clamp 134
is its tendency to spring out of shape. It should never be tightened more than hand-tight.
The most commonly used sheet-metal holder is the Cleco fastener (figure 5-6). It is used to keep drilled parts made from sheet stock pressed tightly together. Unless parts are held tightly together they will separate while being riveted.
This type of fastener is available in six different sizes: 3/32-, 1/8-, 5/32-, 3/16-, 1/4-, and 3/8-in.
Cross head The size is stamped on the fastener. Special pliers are used to insert the fastener in a drilled hole. One pair of pliers will fit the six different sizes.
Sheet-metal screws are sometimes used as temporary holders. The metal sheets must be held tightly together before installing these screws, since the self-tapping action of the threads tends to force the sheets apart. Washers placed under the heads of the screws keep them from marring or scratching the metal.
Cutting blade Front gage Bevel gage Extension arms Housing
- Foot treadle FICURE 5-7. Squaring ahears.
METALWORKING MACHINES Without metalworking machines a job would be more difficult and tiresome, and the time required to finish a task would be much longer. Some of the machines used are discussed here; these include the powered and nonpowered metal-cutting machines, such as the various types of saws, powered and nonpowered shears, and nibblers. Also included is the forming equipment (both power driven and nonpowered), such as brakes and forming rolls, the bar folder, and shrinking and stretching machines.
Metal Cutting Manually Operated Tools-Lever Type Squaring shears provide a convenient means of cutting and squaring metal. These shears consist of a stationary lower blade attached to a bed and a movable upper blade attached to a crosshead (figure 5-7). To make the cut, the upper blade is moved down by placing the foot on the treadle and pushing downward.
The shears are equipped with a spring which raises the blade and treadle when the foot is removed. A scale, graduated in fractions of an inch, is scribed on the bed. Two squaring fences, consisting of thick strips of metal and used for squaring metal sheets, are placed on the bed, one on the right side and one on the left. Each is placed so that it forms a 90° angle with the blades.
Three distinctly different operations can be performed on the squaring shears: (1) Cutting to a line, (2) squaring, and (3) multiple cutting to a specific size. When cutting to a line, the sheet is placed on the bed of the shears in front of the cutting blade with the cutting line directly even with the cutting edge of the bed. The sheet is cut by stepping on the treadle while the sheet is held securely in place by the holddown clamp.
Squaring requires several steps. First, one end of the sheet is squared with an edge (the squaring fence is usually used on the edge). Then the remaining edges are squared by holding one squared end of the sheet against the squaring fence and making the cut, one edge at a time, until all edges have been squared.
When several pieces must be cut to the same dimensions, use the gage which is on most squaring shears. The supporting rods are graduated in fractions of an inch, and the gage bar may be set at any point on the rods. Set the gage at the desired distance from the cutting blade of the shears and push each piece to be cut against the gage bar. All the pieces can then be cut to the same dimensions without measuring and marking each one separately.
Scroll shears (figure 5-8) are used for cutting irregular lines on the inside of a sheet without cutting through to the edge. The upper cutting blade is stationary while the lower blade is movable. The machine is operated by a handle connected to the lower blade.
/. зі ∞ FIGURE 5-8. Scroll shears.
Throatless shears (figure 5-9) are best used to cut 10-gage mild carbon sheet metal and 12-gage stainless steel. The shear gets its name from its construction; it actually has no throat. There are no obstructions during cutting since the frame is throatless. A sheet of any length can be cut, and the metal can be turned in any direction to allow for cutting irregular shapes. The cutting blade (top blade) is operated by a hand lever.
The rotary punch (figure 5-10) is used in the airframe repair shop to punch holes in metal parts.
This machine can be used for cutting radii in corners, for making washers, and for many other jobs where holes are required. The machine is composed of two cylindrical turrets; one mounted over the other and supported by the frame. Both turrets are synchronized so that they rotate together, and index pins assure correct alignment at all times. The index pins may be released from their locking position by 136
FICURE 5-10. Rotary punch.
FICURE 5-9. Throatless shears.
rotating a lever on the right side of the machine.
This action withdraws the index pins from the tapered holes and allows an operator to turn the turrets to any size punch desired.
When rotating the turret to change punches, release the index lever when the desired die is within I in. of the ram, and continue to rotate the turret slowly until the top of the punch holder slides into the grooved end of the ram. The tapered index locking pins will then seat themselves in the holes provided and, at the same time, release the mechanical locking device, which prevents punching until the turrets are aligned.
To operate the machine, place the metal to be worked between the die and punch. Pull the lever on the top side of the machine toward you. This will actuate the pinion shaft, gear segment, toggle link, and the ram, forcing the punch through the metal. When the lever is returned to its original position, the metal is removed from the punch.
The diameter of the punch is stamped on the 8120 front of each die holder. Each punch has a point in its center which is placed in the centerpunch mark to punch the hole in the correct location.
Metal-Cutting Power-Operated Tools The electrically operated portable circular-cutting Ketts saw (figure 5-11) uses blades of various diameters. The head of this saw can be turned to any desired angle, and is very handy for removing damaged sections on a stringer. Advantages of a Ketts saw are:
(1) The ability to cut metal up to 3/16 in.
thick.
(2) No starting hole is required.
(3) A cut can be started anywhere on a sheet of metal.
(4) The capability of cutting an inside or outside radius.
To prevent grabbing, keep a firm grip on the saw handle at all times. Before installing a blade, it should be checked carefully for cracks. A cracked blade can fly apart and perhaps result in serious injury.
FIGURE 5-11. Ketts saw.
The portable, air powered reciprocating saw (figure 5-12) hạs a gun-type shape for balancing and ease of handling and operates most effectively at an air pressure of from 85 to 100 p.s.i. The reciprocating saw uses a standard hacksaw blade and can cut a 360° circle or a square or rectangular hole. This saw is easy to handle and safe to use.
FIGURE 5-12. Reciprocating saw.
A reciprocating saw should be used in such a way that at least two teeth of the saw blade are cutting at all times. Avoid applying too much downward pressure on the saw handle because the blade may break.
Nibblers Stationary and portable nibblers are used to cut metal by a high-speed blanking action. The cutting or blanking action is caused by the lower die moving up and down and meeting the upper stationary die. The shape of the lower die permits small pieces of metal approximately. 1/16-in. wide to be cut out.
The cutting speed of the nibbler is controlled by the thickness of the metal being cut. Sheets of metal with a maximum thickness of 1/16 in, can be cut satisfactorily. Too much force applied to the metal during the cutting operation will clog the dies, causing the die to fail or the motor to overheat.
The spring-loaded screw on the base of the lower die should be adjusted to allow the metal to move freely between the dies. This adjustment must be sufficient to hold the material firmly enough to prevent irregular cuts. The dies may be shimmed for special cutting operations.
Portable Power Drills One of the most common operations in airframe metalwork is that of drilling holes for rivets and bolts. This operation is not difficult, especially on light metal. Once the fundamentals of drills and their uses are learned, a small portable power drill is usually the most practical machine to use. However, there will be times when a drill press may prove to be the better machine for the job.
Some portable power drills will be encountered which are operated by electricity and others which are operated by compressed air. Some of the electrically operated drills work on either alternating or direct current, whereas others will operate on only one kind of current.
Portable power drills are available in various shapes and sizes to satisfy almost any requirement (figure 5-13). Pneumatic drills are recommended for use on projects around flammable materials where sparks from an electric drill might become a fire hazard.
When access to a place where a hole is to be drilled is difficult or impossible with a straight drill, various types of drill extensions and adapters are used. A straight extension can be made from an ordinary piece of drill rod. The twist drill is attached to the drill rod by shrink fit, brazing, or silver soldering. Angle adapters can be attached to either an electric or pneumatic drill when the location of the hole is inaccessible to a straight drill.
Angle adapters have an extended shank fastened to the chuck of the drill. In use, the drill is held in one hand and the adapter in the other to prevent the adapter from spinning around the drill chuck.
A flexible extension can be used for drilling in places which are inaccessible to ordinary drills. Its flexibility permits drilling around obstructions with a minimum of effort.
Straight electric Right-angle electric Straight air Right-angle air 360° air FICURE 5-13. Portable power drills.
When using the portable power drill, hold it firmly with both hands. Before drilling, be sure to place a backup block of wood under the hole to be drilled to add support to the metal.
The twist drill should be inserted in the chuck and tested for trueness or vibration. This may be visibly checked by running the motor freely. A drill that wobbles or is slightly bent should not be used since such a condition will cause enlarged holes.
The drill should always be held at right angles to the work regardless of the position or curvatures.
Tilting the drill at any time wher drilling into or withdrawing from the material may cause elonga.
tion (egg shape) of the hole.
Always wear safety goggles while drilling.
When drilling through sheet metal, small burrs are formed around the edge of the hole. Burrs must be removed to allow rivets or bolts to fit snugly and to prevent scratching. Burrs may be removed with a bearing scraper, a countersink, or a twist drill larger than the hole. If a drill or countersink is used, it should be rotated by hand.
Drill Press The drill press is a precision machine used for drilling holes that require a high degree of accuracy. It serves as an accurate means of locating and maintaining the direction of a hole that is to be drilled and provides the operator with a feed lever that makes the task of feeding the drill into the work an easy one.
A variety of drill presses are available; the most common type is the upright drill press (figure 5-14).
When using a drill press, the height of the drill press table is adjusted to accommodate the height of the part to be drilled. When the height of the part is greater than the distance between the drill and the table, the table is lowered. When the height of the part is less than the distance between the drill and the table, the table is raised.
After the table is properly adjusted, the part is placed on the table and the drill is brought down to aid in positioning the metal so that the hole to be driled is directly beneath the point of the drill. The part is then clamped to the drill press table to prevent it from slipping during the drilling operation. Parts not properly clamped may bind on the drill and start spinning, causing the loss of fingers or hands or serious cuts on the operator's arms or body. Always make sure the part to be drilled is properly clamped to the drill press table before starting the drilling operation.
The degree of accuracy that it is possible to attain when using the drill press will depend to a certain extent on the condition of the spindle hole, sleeves, and drill shank. Therefore, special care must be exercised to keep these parts clean and free from nicks, dents, or warpage. Always be sure that the sleeve is securely pressed into the spindle hole.
Never insert a broken drill in a sleeve or spindle hole. Be careful never to use the sleeve-clamping 139
FIGURE 5-14. Drill press.
vise to remove a drill since this may cause the sleeve to warp.
Grinders The term grinder applies to all forms of grinding machines. To be specific, it is a machine having an abrasive wheel which removes excess material while producing a suitable surface. There are many kinds of grinding machines, but only those which are helpful to the airframe mechanic will be discussed here.
Grinding Wheels A grinding wheel is a cutting tool with a large number of cutting edges arranged so that when they become dull they break off and new cutting edges take their place.
Silicon carbide and aluminum oxide are the kinds of abrasives used in most grinding wheels. Silicon carbide is the cutting agent for grinding hard, britte material, such as cast iron. It is also used in grinding aluminum, brass, bronze, and copper. Aluminum oxide is the cutting agent for grinding steel and other metals of high tensile strength.
The size of the abrasive particles used in grinding wheels is indicated by a number which corresponds to the number of meshes per linear inch in the screen through which the particles will pass. As an example, a number 30 abrasive will pass through a screen having 30 holes per linear inch, but will be retained by a smaller screen having more than 30 holes per linear inch.
The bond is the material which holds the abrasive particles together in forming the wheel. The kind and amount of bond used determines the hardness or softness of the wheel. The commonly used bonds are vitrified, silicate, resinoid, rubber, and shellac. Vitrified and silicate are the bonds used most frequently, vitrified bond being used in approximately three-fourths of all grinding wheels made. This bonding material forms a very uniform wheel and is not affected by oils, acids, water, heat, or cold. The silicate bond, however, is best suited for grinding edged tools.
Resinoid bonded wheels are better for heavy-duty grinding. Rubber bonded wheels are used where a high polish is required. Shellac bonded wheels are used for grinding materials where a buffed or burnished surface is needed.
A pedestal or floor type grinder usually has a grinding wheel on each end of a shaft which runs through an electric motor or a pulley operated by a belt. This grinder is used for sharpening tools and other general grinding jobs.
The wet grinder, although similar to the pedestal grinder, differs from it in that the wet grinder has a pump to supply a flow of water on a single grinding wheel. The water reduces the heat produced by material being ground against the wheel. It also washes away any bits of metal or abrasive removed during the grinding operation. The water returns to a tank and can be re-used.
A common type bench grinder found in most metalworking shops is shown in figure 5-15. This grinder can be used to dress mushroomed heads on chisels, and points on chisels, screwdrivers, and drills. It can be used for removing excess metal from work and smoothing metal surfaces.