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.
Tool rest.
FIGURE 5-15. Bench grinder.
This type grinder is generally equipped with one medium-grain and one fine-grain abrasive wheel.
The medium-grain wheel is usually used for rough grinding where a considerable quantity of material is to be removed or where a smooth finish is unimportant. The fine-grain wheel is usually used for sharpening tools and grinding to close limits because it removes metal more slowly, gives the work a smooth finish, and does not generate enough heat to anneal the edges of cutting tools. When it is necessary to make a deep cut on work or to remove a large amount of metal, it is usually good practice to grind with the medium-grain wheel first and then finish up with the fine-grain wheel.
The grinding wheels are removable, and the grinders are usually designed so that wire brushes, polishing wheels, or buffing wheels can be substituted for the abrasive wheels.
As a rule, it is not good practice to grind work on the side of an abrasive wheel. When an abrasive wheel becomes worn, its cutting efficiency is reduced because of a decrease in surface speed. When a wheel becomes worn in this manner, it should be discarded and a new one installed.
Before using a bench grinder, make sure the abrasive wheels are firmly held on the spindles by the flange nuts. If an abrasive wheel should come off or become loose, it could seriously injure the operator in addition to ruining the grinder.
Another hazard is loose tool rests. A loose tool rest could cause the tool or piece of work to be "grabbed" by the abrasive wheel and cause the operator's hand to come in contact with the wheel.
If this should happen, severe wounds may result.
Always wear goggles when using a grinder, even if eyeshields are attached to the grinder. Goggles should fit firmly against your face and nose.
This is the only way to protect your eyes from the fine pieces of steel. Goggles that do not fit properly should be exchanged for ones that do fit.
Be sure to check the abrasive wheel for cracks before using the grinder. A cracked abrasive wheel is likely to fly apart when turning at high speeds.
Never use a grinder unless it is equipped with wheel guards.
FORMING MACHINES Forming machines can be either hand operated or power driven. Small machines are usually hand operated, whereas the larger ones are power driven.
Straight line machines include such equipment as the bar folder, cornice brake, and box and pan brake. Rotary machines include the slip roll former and combination machine. Power-driven machines are those that require a motor of some description for power. These include such equipment as the power-driven slip roll former, and power flanging machine.
Bar Folder The bar folder (figure 5-16) is designed for use in making bends or folds along edges of sheets.
This machine is best suited for folding small hems, flanges, seams, and edges to be wired. Most bar folders have a capacity for metal up to 22 gage in thickness and 42 inches in length.
Before using the bar folder, several adjustments must be made for thickness of material, width of fold, sharpness of fold, and angle of fold.
Stop Operating handle Adjustable collar Wing Folding blade 45° and 90° stops Frame Gage adjusting screw Locking screw FICURE 5-16. Bar folder.
(R) Top nose bar Clamping handles Top leaf Bending leaf (E) Bending leaf Bending adjustment bolt leaf handle
(2) Bending leaf Bending tension bolt leaf handle
FRONT VIEW Clamping handles (Q) Balance weight (V) Bed adj (M) Clamping handles (G) -Bed adjustment screw bolt -(A) Slot casting REAR VIEW adjustment screw (H) Slot casting (O) Top shaft adjustment screw (J) Link adj block (B) Slot casting (Y) Bending lock screw leat hinge (D) hinge adj screw (S) Bed end- Bending leaf housing "(L) Link adj *(K) Link set screw screw (I) guide Stop gage *(N) Adjustable stop (F) Bed end adj screw (U) Stop gage rod (P) Bending leaf handle •(C) Legs
RIGHT HAND END VIEW FIGURE 5-17. Cornice Brake, 142
The adjustment for thickness of material is made by adjusting the screws at each end of the folder.
As this adjustment is made, place a piece of metal of the desired thickness in the folder and raise the operating handle until the small roller rests on the cam. Hold the folding blade in this position and adjust the setscrews so that the metal is clamped securely and evenly the full length of the folding blade. After the folder has been adjusted, test each end of the machine separately with a small piece of metal by actually folding it.
There are two positive stops on the folder, one for 45° folds or bends and the other for 90° folds or bends. An additional feature (a collar) is provided and can be adjusted to any degree of bend within the capacity of the machine.
For forming angles of 45° or 90°, the correct stop is moved into place. This will allow the handle to be moved forward to the correct angle. For forming other angles, the adjustable collar shown in the inset view of figure 5-16 is used. This is accomplished by loosening the setscrew and setting the stop at the desired angle. After setting the stop, tighten the setscrew and complete the bend.
To make the fold, adjust the machine correctly and then insert the metal. The metal goes between the folding blade and the jaw. Hold the metal firmly against the gage and pull the operating handle toward the body. As the handle is brought forward, the jaw automatically raises and holds the metal until the desired fold is made. When the handle is returned to its original position, the jaw and blade will return to their original positions and release the metal.
Comice Brake The cornice brake (fig. 5-17) has a much greater range of usefulness than the bar folder. Any bend formed on a bar folder can be made on the cornice brake. The bar folder can form a bend or edge only as wide as the depth of the jaws. In comparison, the cornice brake allows the sheet that is to be folded or formed to pass through the jaws from front to rear without obstruction.
In making ordinary bends with the cornice brake, the sheet is placed on the bed with the sight line (mark indicating line of bend) directly under the edge of the clamping bar. The clamping bar is then brought down to hold the sheet firmly in place.
The stop at the right side of the brake is set for the proper angle or amount of bend, and the bending leaf is raised until it strikes the stop. If other bends are to be made, the clamping bar is lifted and the sheet is moved to the correct position for bending.
The bending capacity of a cornice brake is determined by the manufacturer. Standard capacities of this machine are from 12- to 22-gage sheet metal, and bending lengths are from 3 to 12 ft. The bending capacity of the brake is determined by the bending edge thickness of the various bending leaf bars.
Most metals have a tendency to return to their normal shape-a characteristic known as springback. If the cornice brake is set for a 90° bend, the metal bent will probably form an angle of about 87° to 88°. Therefore, if a bend of 90° is desired, set the cornice brake to bend an angle of about 93° to allow for springback.
Slip Roll Former The slip roll former (figure 5-18) is manually operated and consists of three rolls, two housings, a base, and a handle. The handle turns the two front rolls through a system of gears enclosed in the housing.
FICURE 5-18. Slip roll former.
The front rolls serve as feeding or gripping rolls.
The rear roll gives the proper curvature to the work. The front rolls are adjusted by two front adjusting screws on each end of the machine. The rear roll is adjusted by two screws at the rear of each housing. The front and rear rolls are grooved to permit forming of objects with wired edges. The upper roll is equipped with a release which permits easy removal of the metal after it has been formed.
When using the slip roll former, the lower front roll must be raised or lowered so that the sheet of metal can be inserted. If the object has a folded edge, there must be enough clearance between the rolls to prevent flattening the fold. If a metal requiring special care (such as aluminum) is being 143
formed, the rolls must be clean and free of imperfections.
The rear roll must be adjusted to give the proper curvature to the part being formed. There are no gages that indicate settings for a specific diameter;
therefore, trial-and-error settings must be used to obtain the desired curvature.
The metal should be inserted between the rolls from the front of the machine. Start the metal between the rolls by rotating the operating handle in a clockwise direction.
A starting edge is formed by holding the operating handle firmly with the right hand and raising the metal with the left hand. The bend of the starting edge is determined by the diameter of the part being formed. If the edge of the part is to be flat or nearly flat, a starting edge should not be formed.
Be sure that fingers or loose clothing are clear of the rolls before the actual forming operation is started. Rotate the operating handle until the metal is partly through the rolls and change the left hand from the front edge of the sheet to the upper edge of the sheet. Then roll the remainder of the sheet through the machine.
If the desired curvature is not obtained, return the metal to its starting position by rotating the handle counterclockwise. Raise or lower the rear roll and roll the metal through the rolls again. Repeat this procedure until the desired curvature is obtained, then release the upper roll and remove the metal.
If the part to be formed has a tapered shape, the rear roll should be set so that the rolls are closer together on one end than on the opposite end. The amount of this adjustment will have to be deter- If the job being formed has a wired edge, the distance between the upper and lower rolls and the distance between the lower front roll and the rear roll should be slightly greater at the wired end than Forming Processes Before a part is attached to the aircraft during either manufacture or repair, it has to be shaped to fit into place. This shaping process is called forming. Forming may be a very simple process, such as making one or two holes for attaching, or it may be exceedingly complex, requiring shapes with complex curvatures.
Parts are formed at the factory on large presses or by drop hammers equipped with dies of the correct shape. Every part is planned by factory engineers, who set up specifications for the materials to be used so that the finished part will have the correct temper when it leaves the machines. A layout for each part is prepared by factory draftsmen.
Forming processes used on the flight line and those practiced in the maintenance or repair shop are almost directly opposite in the method of procedure. They have much in common, however, and many of the facts and techniques learned in the one process can be applied to the other.
Forming is of major concern to the airframe mechanic and requires the best of his knowledge and skill. This is especially true since forming usually involves the use of extremely light-gage alloys of a delicate nature which can be readily made useless by coarse and careless workmanship. A formed part may seem outwardly perfect, yet a wrong step in the forming procedure may leave the part in a strained condition. Such a defect may hasten fatigue or may cause sudden structural failure.
Of all the aircraft metals, pure aluminum is the most easily formed. In aluminum alloys, ease of forming varies with the temper condition. Since modern aircraft are constructed chiefly of aluminum and aluminum alloys, this section will deal with the procedures for forming aluminum or aluminum alloy parts.
Most parts can be formed without annealing the metal, but if extensive forming operations, such as deep draws (large folds) or complex curves are planned, the metal should be in the dead soft or annealed condition. During the forming of some complex parts, operations may have to be stopped and the metal annealed before the process can be continued or completed. Alloy 2024 in the "O" condition can be formed into almost any shape by the common forming operations, but it must be heattreated afterward.
When forming, use hammers and mallets as sparingly as practicable, and make straight bends on bar folders or cornice brakes. Use rotary machines whenever possible, If a part fits poorly or not at all, do not straighten a bend or a curve and try to re-form it, discard the piece of metal and start with a new one.
When making layouts, be careful not to scratch aluminum or aluminum alloys. A pencil, if kept sharp, will be satisfactory for marking purposes.
Scribers make scratches which induce fatigue failure; but they may be used if the marking lines fall 144
outside the finished part, that is, if the scribed line will be part of the waste material. Keep bench tops covered with material hard enough to prevent chips and other foreign material from becoming imbed.
ded in them. Be sure also to keep bench tops clean and free from chips, filings, and the like. For the protection of the metals being worked, keep vise jaws covered with soft metal jaw caps.
Stainless steel can be formed by any of the usual methods but requires considerably more skill than is required for forming aluminum or aluminum alloys. Since stainless steel work-hardens very readily, it requires frequent annealing during the forming operations. Always try to press out stainless steel parts in one operation. Use dies, if possible.
FORMING OPERATIONS AND TERMS The methods used in forming operations include such sheetmetal work processes as shrinking, stretching, bumping, crimping, and folding.
Bumping Shaping or forming malleable metal by hammering or pounding is called bumping. During this process, the metal is supported by a dolly, a sandbag, or a die. Each contains a depression into which hammered portions of the metal can sink.
Bumping can be done by hand or by machine.
Crimping Folding, pleating, or corrugating a piece of sheet metal in a way that shortens it is called crimping.
Crimping is often used to make one end of a piece of stovepipe slightly smaller so that one section may be slipped into another. Turning down a flange on
• a team is also called crimping. Crimping one side of a straight piece of angle iron with crimping pliers will cause it to curve, as shown in figure 5-19.
Stretching Hammering a flat piece of metal in an area such as that indicated in figure 5-19 will cause the material in that area to become thinner. However, since the amount of metal will not have been decreased, it will cover a greater area because the metal will have been stretched.
Stretching one portion of a piece of metal affects the surrounding material, especially in the case of formed and extruded angles. For example, hammering the metal in the horizontal flange of the angle strip over a metal block, as shown in figure 5-19, would cause its length to be increased (stretched);
therefore, that section would become longer than the section near the bend. To allow for this difference in length, the vertical flange, which tends to keep the material near the bend from stretching, would be forced to curve away from the greater length.
Shrinking During the shrinking process, material is forced or compressed into a smaller area. The shrinking process is used when the length of a piece of metal, especially on the inside of a bend, is to be reduced.
Sheet metal can be shrunk in two ways: (1) By hammering on a V-block (figure 5-20), or (2) by crimping and then shrinking on a shrinking block To curve the formed angle by the V-block method, place the angle on the V-block and gently B.
FIGURE 5-19. Crimping and stretching.