(2) Clamp the material in the form blocks as Shrinking shown in figure 5-36.
(3) Using a soft-faced stretching mallet, start hammering near the ends and work the flange down smoothly and gradually to prevent cracking and splitting. Planish the flange and angle as described in the previous procedure, and trim and smooth the edges, if necessary. Form blocks Curved Flanged Parts Curved flanged parts are usually hand formed. Of the types shown in figure 5-37, the one with relief holes is probably the simplest to form. It has a Hardwood concave flange (the inside flange) and a convex wedge block fiange (the outside flange).
The concave flange is formed by stretching, the convex flange by shrinking. Such parts may be formed with the aid of hardwood or metal forming blocks. These blocks are made in pairs similar to those used for straight angle bends and are identified in the same manner. They differ in that they are made specifically for the particular part to be formed, they fit each other exactly, and they con.
form to the actual dimensions and contour of the finished article.
The mating parts may be equipped with aligning pins to aid in lining up the blocks and holding the metal in place. The blocks may be held together by C-clamps or a vise. They also may be held together with bolts by drilling through both forms and the Stretching metal, provided the holes do not affect the strength of the finished part. The edges of the forming block are rounded to give the correct radius of bend to the part, and are undercut to allow for springback of the metal. The undercut is especially necessary if the material is hard or if the bend must be highly accurate.
Note the various types of forming represented in figure 5-37. In the plain nose rib, only one large convex flange is used; but, because of the great distance around the part and the likelihood of buckles in forming, it is rather difficult to form. The flange and the beaded portion of this rib provide sufficient strength to make this a very good type to use. In the type with relief holes, the concave flange gives difficulty in forming; however, the outside flange is broken up into smaller sections by relief holes (notches inserted to prevent strains in a FicURE 5-36. Forming a flanged angle.
Riveted Bead angles Flange Rib using crimps, relief holes, beads, and riveted angles Plain nose rib Relief holes Crimps Rib with relief holes Rib with crimps and beads FIGURE 5-37. Nose ribs.
bend). In the type with crimps and beads, note that crimps are inserted at equally spaced intervals. The crimps are placed to absorb material and cause curving, while also giving strength to the part.
In the other nose rib illustrated, note that a combination of the four common forming methods is applied. They are crimping, beading, putting in relief holes, and using a formed angle riveted on at each end. The beads and the formed angles supply strength to the part.
The major steps in forming a curved flange part are explained in the following paragraphs.
Cut the material to size (allowing for trim), locate and drill holes for alignment pins, and re- FICURE 5-38. Forming a concave curve.
move all burrs (jagged edges). Place the material between the wooden blocks. Clamp blocks tightly in a vise so that the material will not move or shift.
Clamp the work as closely as possible to the particu- soft mallet or wooden wedge block, start hammering lar area being hammered to prevent strain on the at a point a short distance away from the beginning of the concave bend and continue toward the center form blocks and to keep the metal from slipping (figure 5-38). of the bend. This procedure permits some of the excess metal along the tapered portion of the flange Bend the flange on the concave curve first. This to be worked into the curve where it will be needed.
practice may keep the flange from splitting open or Continue hammering until the metal is gradually cracking when the metal is stretched. (Should this worked down over the entire flange, flush with the occur, a new piece will have to be made.) Using a form block.
Starting at the center of the curve and working toward both ends, hammer the convex flange down over the form (figure 5-39). Strike the metal with glancing blows, at an angle of approximately 30° off perpendicular, and with a motion that will tend to pull the part away from the block.
FIcURE 5-39. Forming a convex curve.
Stretch the metal around the radius bend and remove the buckles gradually by hammering on a wedge block.
While working the metal down over the form, keep the edges of the flange as nearly perpendicular to the block as possible. The wedge block helps keep the edge of the metal perpendicular to the block, leasens the possibility of buckles and of splitting or cracking the metal, and aids in removing buckles.
Finally, trim the flanges of excess metal, planish, remove burrs, round the corners (if any), and check the part for accuracy.
Bumping Bumping on a form block or female die and bumping on a sandbag are the two common types practiced. In either method only one form is required, a wooden block, lead die, or sandbag. A good example of a part made by the block cr die type of bumping is the "blister" or streamlined cover plate. Wing fillets constitute a good example of parts that are usually formed by bumping on a sandbag.
The lead die or the wooden block designed for bumping must have the same dimensions and contour as the outside of the blister. To provide sufhcient bumping weight, and to give sufficient bearing surface for fastening the metal, the block or die should be at least 1 in. larger in all dimensions than the form requires.
When forming the wooden block, hollow it out with saws, chisels, gouges, files, and rasps. Smooth and finish it with sandpaper. Make the inside of the form as smooth as possible, because any slight irregularity will show up on the finished part. Prepare several templates (patterns of the cross section), such as those shown with the form block for the blister in figure 5-40, so that the form can be checked for accuracy.
Shape the contour of the form at points 2, 3, and
4. Shape the areas between the template check points to conform to the remaining contour and to template 4. Shaping of the form block requires particular care because the more nearly accurate it is, the less time it will take to produce a smooth, finished part.
Correct clamping of the material to the form block is an important part of the block-forming operation. Several methods are possible. For parts such as the blister, one of the best means of clamping the material is to use a full metal cutout or steel holddown plate as shown in figure 5-10.
In this process, place the holddown plate directly over the material to be formed and clamp it in position with bolts or C-clamps. Tighten the C.
clamps or bolts just tight enough to hold the material flat against the face of the form block, but not so tight that the metal cannot be drawn into the form. If the material is not held flat against the face of the form, it will bend up or buckle away from the block. If it is not permitted to slip into the concave depression a little, the blister portion will become very thin in places.
Holddown plates should be of heavy steel, 1/8 in.
for small forms and 1/4 in. or heavier for large forms.
If the material for making an all-metal holddown plate is not available, use a hardwood cutout. Make the cutout and use it in the same manner as the 160
Holddown plate Templates Form block FicURE 5-40. Form blocks and templates.
steel plate, but take greater precautions to make sure that the material is held as desired.
Pieced form clamps can be used if an all-metal holddown plate or hardwood cutout is not available or if a full cutout cannot be used. Be careful to clamp them properly and locate them so that they align with the edge of the form. If they are not aligned accurately, the material will bulge.
After preparing and checking the form, perform the bumping process according to the following general steps:
(1) Cut a metal blank to size, allowing an extra 1/2 to 1 in. to permit "drawing."
(2) Apply a thin coat of light oil to the block and to the aluminum to prevent galling (scraping on rough spots).
(3) Clamp the material between the block and steel plate, as previously described, so that it will be firmly supported yet able to slip a little toward the inside of the form.
(4) Clamp the bumping block in a bench vise.
With a soft-faced mallet or with a hardwood drive block and suitable mallet, start the bumping near the edges of the form.
(5) With light blows of the mallet, work the material down gradually from the edges.
Remember that the object of the bumping process is to work the material into shape by stretching it, rather than by forcing it into the form with heavy blows. Always start bumping near the edge of the form;
never start near the center of the blister.
(6) Smooth the work as much as possible before removing it from the form. This can be done by rubbing the work with the rounded end of a maple block or with the round end of a stretching mallet.
(7) Remove the blister from the bumping block and trim it, leaving a 1/2-in. flange.
(8) Finally, drill the rivet holes, chamfer the edges 45°, and clean and polish the part.
Bumping on a sandbag is one of the most difficult types of sheet-metal hand forming, because there is no exact form block to serve as a guide. In this type of forming operation, a depression is made into a sandbag to take the shape of the hammered portion of the metal. The depression or pit has a tendency to shift from the hammering. This necessitates readjusting from time to time during the bumping process. The degree of shifting depends largely on the contour or shape of the piece being formed, and whether glancing blows must be struck to stretch, draw, or shrink the metal.
When forming by this method, prepare a contour template or some sort of a pattern to serve as a working guide and to ensure accuracy of the fin- 161
the exposed portions. After completely covering the similar paper, folding it over the part to be dupli- would have to be stretched to fit, and attach addi- part, trim the pattern to exact size. ished part. Make the pattern from ordinary kraft or tional pieces of paper with masking tape to cover cated. Cut the paper cover at the points where it (6) At frequent intervals during the bumping they become too large. shape by applying the template. If wrin- kles are formed, work them out before inward until the desired shape is obtained. process, check the part for accuracy of Shape the entire part as a unit.
from which the part is to be formed. Although the idea of the approximate shape of the metal to be pattern will not lie flat, it will give a fairly accurate Open the pattern and spread it out on the metal (7) Finally, with a suitable stake and planish- ing hammer, or with a hand dolly and planishing hammer, remove small dents and hammer marks.
been placed on the material, outline the part and the portions to be stretched using a pencil. cut, and the pieced-in sections will indicate where the metal is to be stretched. When the pattern has (8) With a scribe, mark around the outside it is smooth. Clean and polish the part. of the object. Trim the edge and file until Add at least 1 in. of excess metal when cutting the Joggling material to size. Trim off the excess metal after A joggle is an offset formed on an angle strip to bumping the part into shape. allow clearance for a sheet or an extrusion. Joggles it will be fairly easy to shape since a simple contour If the part to be formed is radially symmetrical, formers. One of these members, usually the former, are often found at the intersection of stringers and template can be used as a working guide, making a has the flange joggled to fit flush over the flange of pattern to indicate the portions of unequal stretch- the stringer. The amount of offset is usually small;
ing unnecessary. However, the procedure for bump- therefore, the depth of the joggle is generally speciing sheet metal parts on a sandbag follows certain fied in thousandths of an inch. The thickness of the basic rules which can be applied to any part, re- material to be cleared governs the depth of the gardless of its contour or shape. joggle. In determining the necessary length of the
(1) Lay out and cut the contour template. joggle, it is common practice to allow an extra 1/16
(2) Determine the amount of metal needed, (3) Place a sandbag on a solid foundation heavy cardboard, or thin plywood. make a pit in the bag. Analyze the part to capable of supporting heavy blows and, lay it out, and cut it to size, allowing at least 1/2 in. excess. with the aid of a smooth-faced mallet, determine the correct radius of the pit for This can be made of sheet metal, medium- position in the brake. flange down while the metal is still in the same metal of the correct thickness to give the desired up along the line of the joggle. Hold a piece of form it on a cornice brake by inserting and bending offset under the bent-up portion, and pound the between the joggled, overlapped part. which joggles can be formed. If the joggle is to be made on a straight flange or flat piece of metal, in to give enough added clearance to assure a fit There are a number of different methods by
(4) Select a soft round-faced or bell-shaped ing blows, about 1/2 to 1 in. from the edge. in the left hand, place the portion to be sandbag. Strike the metal with light glanc- bumped near the edge of the pit on the metal part. Holding one edge of the metal the forming operation. The pit will change be re-adjusted occasionally. than the contour desired on the sheet- mallet having a contour slightly smaller with the hammering it receives and must ciently soft and resilient to permit forming alumi- alloy dies. Dies of aluminum alloy are preferred since they are easier to fabricate than those of steel and scratches are easily removed from their sur- num alloy parts on them without marring, and nicks it is easily worked. If a number of similar joggles used only a few times, hardwood is satisfactory as are to be produced, then use steel or aluminum and will wear about as long. These dies are suffi- forming blocks or dies made of hardwood, steel, or aluminum alloy may be used. If the die is to be Where a joggle is necessary on a curved flange,
(5) Continue bumping toward the center, re- faces.
volving the metal and working gradually When using joggling dies for the first time, test 162
them for accuracy on a piece of waste stock. In this way you will avoid the possibility of ruining already fabricated parts. Always keep the surfaces of the blocks free from dirt, filings, and the like, so that the work will not be marred.
Working Stainless Steel When working with stainless steel, make sure that the metal does not become unduly scratched or marred. Also take special precautions when shearing, punching, or drilling this metal. It takes about twice as much pressure to shear or punch stainless steel as it does mild steel. Keep the shear or punch and die adjusted very closely. Too much clearance will permit the metal to be drawn over the edge of the die and cause it to become work-hardened, resulting in excessive strain on the machine.
When drilling stainless steel, use a high-speed drill ground to an included angle of 140°. Some special drills have an offset point, whereas others have a chip curler in the flutes. When using an ordinary twist drill, grind its point to a stubbier angle than the standard drill point. Keep the drill speed about one-half that required for drilling mild steel, but never exceed 750 r.p.m. Keep a uniform pressure on the drill so the feed is constant at all times. Drill the material on a backing plate, such as cast iron, which is hard enough to permit the drill to cut all the way through the stock without pushing the metal away from the drill point. Spot the drill before turning on the power and also make sure that when the power is turned on, pressure is being exerted.
To avoid overheating, dip the drill in water after drilling each hole. When it is necessary to drill several deep holes in stainless steel, use a liquid coolant. A compound made up of 1 Ib. of sulfur added to 1 gal. of lard oil will serve the purpose.
Apply the coolant to the material immediately upon starting the drill. High-speed portable hand drills have a tendency to burn the drill points and excessively work-harden the material at the point of contact; thus high-speed portable hand drills should not be used because of the temperatures developed.
A drill press adjustable to speeds under 750 r.p.m.
is recommended.
Working Magnesium Magnesium, in the pure state, does not have sufficient strength to be used for structural purposes;
but, as an alloy, it has a high strength-to-weight ratio. Its strength is not affected by subzero temperatures, and this increases its adaptability to aircraft use. The nonmagnetic property of magnesium alloys makes it valuable for instrument cases and parts.
While magnesium alloys can usually be fabri cated by methods similar to those used on other metals, it must be remembered. that many of the details of shop practice cannot be applied. Magnesium alloys are difficult to fabricate at room temperature; therefore, operations other than the most simple ones must be performed at high temperatures. This requires preheating of the metal, or dies, or both.
Magnesium alloy sheets may be cut by blade shears, blanking dies, routers, or saws. Hand or circular saws are usually used for cutting extrusions to length. Conventional shears and nibblers should never be used for cutting magnesium alloy sheet because they produce a rough, cracked edge.
Shearing and blanking of magnesium alloys require close tool tolerances. A maximum clearance of from 3 to 5% of the sheet thickness is recommended. The top blade of the shears should be ground with an included angle of from 45° to 60°.
The shear angle on a punch should be from 2° to 3°, with a 1° clearance angle on the die. For blanking, the shear angle on the die should be from 2° to 3° with a 1° clearance angle on the punch. Holddown pressures should be used when possible. Cold shearing should not be accomplished on hard-rolled sheet thicker than 0.064 in. or annealed sheet thicker than 1/8 in. Shaving is used to smooth the rough, flaky edges of magnesium sheet which has been sheared. This operation consists of removing approximately 1/32 in. by a second shearing.
Hot shearing is sometimes used to obtain an improved sheared edge. This is necessary for heavy sheet and plate stock. Annealed sheet may be heated to 600° F., but hard-rolled sheet must be held under 400° F., depending on the alloy used. Thermal expansion makes it necessary to allow for shrinkage after cooling, which entails adding a small amount of material to the cold metal dimensions before fabrication.
Sawing is the only method used in cutting plate stock more than 1/2 in.-thick. Bandsaw raker-set blades of 4- to 6-tooth pitch are recommended for cutting plate stock or heavy extrusions. Small and medium extrusions are more easily cut on a circular cutoff saw having six teeth per inch. Sheet stock can be cut on bandsaws having raker-set or straight-set teeth with an 8-tooth pitch. Bandsaws should be equipped with nonsparking blade guides 163
to eliminate the danger of sparks igniting the magnesium alloy filings.
Cold-working most magnesium alloys at room temperature is very limited because they workharden very rapidly and do not lend themselves to any severe cold-forming. Some simple bending operations may be performed on sheet material, but the radius of bend must be at least seven times the thickness of the sheet for soft material and 12 times the thickness of the sheet for hard material. A radius of two or three times the thickness of the sheet can be used if the material is heated for the forming operation.
Wrought magnesium alloys tend to crack after they are cold-worked. Therefore, the best results are obtained if the metal is heated to 450° F. before any forming operations are attempted. Parts formed at the lower temperature range are stronger because the higher temperature range has an annealing effect on the metal.
There are some disadvantages to hot-working.
First, heating the dies and the material is expensive and troublesome. Second, there are problems in lubricating and handling materials at these temperatures. However, there are some advantages to hotworking magnesium in that it is more easily formed when hot than are other metals and springback is reduced, resulting in greater dimensional accuracy.
When heating magnesium and its alloys, watch the temperature carefully as ths metal is easily burned. Overheating also causes small molten pools to form within the metal. In either case, the metal is ruined. To prevent burning, magnesium must be protected with a sulfur dioxide atmosphere while being heated.
Proper bending around a short radius requires the removal of sharp corners and burrs near the bend line. Layouts should be made with a carpenter's soft pencil because any marring of the surface may result in fatigue cracks.
It is permissible to heat small pieces of magnesium with a blowtorch, provided proper precautions are exercised. It must be remembered that magnesium will ignite when it is heated to a temperature near its boiling point in the presence of oxygen.
Press or leaf brakes can be used for making bends with short radii. Die and rubber methods should be used where bends are to be made at right angles, which complicate the use of a brake. Roll forming may be accomplished cold on equipment designed for forming aluminum. The most common method of forming and shallow drawing magnesium is an operation in which a rubber pad is used as the female die. This rubber pad is held in an inverted steel pan which is lowered by a hydraulic press ram. The press exerts pressure on the metal and bends it to the shape of the male die.
The machining characteristics of magnesium alloys are excellent, making possible the use of maximum speeds of the machine tools with heavy cuts and high feed rates. Power requirements for machining magnesium alloys are about one-sixth of those for mild steel.
Filings, shavings, and chips from machining operations should be kept in covered metal containers because of the danger of combustion. To repeat a previous reminder, in case of a magnesium fire, do not try to extinguish it with water. The oxygen in the water supports the combustion and increases the intensity of the fire. Dry powder (sodium bicarbonate) is the recommended extinguishing agent for
RIVET LAYOUT Rivet layout consists of determining (1) the number of rivets required; (2) the size and style of rivet to use; (3) its material, temper condition, and strength; (4) the size of the rivet holes; (5) dis.
tance of the rivet holes and rivets from the edges of the patch; and (6) the spacing of the rivets throughout the repair. Since distances are measured in terms of rivet diameters, application of the measurements is simple once the correct rivet diameter is determined.
Single-row, two-row, and three-row layouts designed for small repair jobs are discussed in this section. More complicated layouts for large repairs, which require the application of rivet formulas, are The type of head, size, and strength required in a rivet are governed by such factors as the kind of forces present at the point riveted, the kind and thickness of the material to be riveted, and location of the riveted part on the aircraft.
The type of head required for a particular job is determined by its installation location. Where a smooth aerodynamic surface is required, countersunk head rivets should be used. Universal head rivets may be used in most other locations. If extra strength is required and clearance permits, roundhead rivets may be used; if the necessary clearance is not available, flathead rivets may be used.
The size (or diameter) of the selected rivet shank should correspond in general to the thickness of the 164 ........
material being riveted. If too large a rivet is used in a thin material, the force necessary to drive the rivet properly will cause an undesirable bulging around the rivet head. On the other hand, if too small a rivet diameter is selected for thick material the shear strength of the rivet will not be great enough to carry the load of the joint. As a general rule, the rivet diameter should be not less than three times the thickness of the thicker sheet. Rivets most commonly chosen in the assembly and repair of aircraft range from 3/32-in. to 3/8-in. diameter.
Ordinarily, rivets smaller than 3/32-in. diameter are never used on any structural parts which carry stresses.
When rivets are to pass completely through tubular members, select a rivet diameter equivalent to at least one-eighth the outside diameter of the tube. If one tube "sleeves" or fits over another, take the outside diameter of the outside tube and use oneeighth of that distance as the minimum rivet diameter. A good practice is to calculate the minimum rivet diameter and then use the next larger size When determining the total length of a rivet for installation, the combined thickness of the materials to be joined must be known. This measurement is known as grip length (B of figure 5-41). The total length of the rivet (A of figure 5-41) should be equal to grip length plus the amount of rivet shank necessary to form a proper shop head. The length of rivet required to form a shop head is 1-1/2 times the diameter of the rivet shank (C of figure 5-41).
A - Total rivet length B - Grip length C - Amount of rivet length needed for proper shop head (1½ X rivet dia.)
D - Installed rivets FIcURE 5-41. Determining length of rivet.
Using figure 5-41 and the above information, the formula A = B + C was developed. (A, total rivet length; B, grip length; C, material needed to form a shop head.)
Properly installed rivets are shown in D of figure
5-41. Note carefully the method used to measure total rivet lengths for countersunk rivets and the other types of heads.
Whenever possible, select rivets of the same alloy number as the material being riveted. For example, use 1100 and 3003 rivets on parts fabricated from 1100 and 3003 alloys, and 2117-T and 2017-T rivets on parts fabricated from 2017 and 2024 alloys.
The 2117-T rivet is usually used for general repair work, since it requires no heat treatment, is fairly soft and strong, and is highly corrosion resistant when used with most types of alloys. The 2024-T rivet is the strongest of the aluminum alloy rivets and is used in highly stressed parts. However, it must be soft when driven. Never replace 2024-T rivets with 2117-T rivets.
The type of rivet head to select for a particular repair job can be determined by referring to the type used within the surrounding area by the manufacturer. A general rule to follow on a flush-riveted aircraft is to apply flush rivets on the upper surface of the wing and stabilizers, on the lower leading edge back to the spar, and on the fuselage back to the high point of the wing. Use universal head rivets in all other surface areas.
In general, try to make the spacing of the rivets on a repair conform to that used by the manufacturer in the area surrounding the damage. Aside from this fundamental rule, there is no specific set of rules which governs spacing of rivets in all cases.
However, there are certain minimum requirements which must be observed.
The edge distance, or distance from the center of the first rivet to the edge of the sheet, should be not less than two rivet diameters nor more than four.
The recommended edge distance is about two and one-half rivet diameters. If rivets are placed too close to the edge of the sheet, the sheet is likely to crack or pull away from the rivets; and if they are spaced too far from the edge, the sheet is apt to turn up at the edges.
Rivet pitch is the distance between the centers of adjacent rivets in the same row. The smallest allowable rivet pitch is three rivet diameters. The average rivet pitch usually ranges from six to eight rivet diameters, although rivet pitch may range from four to 10 rivet diameters. Transverse pitch is the perpendicular distance between rivet rows; it is usually equal to 75% of the rivet pitch. The small- 165
est allowable transverse pitch is two and one-half rivet diameters.
When splicing a damaged tube and the rivets pass completely through the tube, space the rivets four to seven rivet diameters apart if adjacent rivets are at right angles to each other, and space them five to seven rivet diameters apart if the rivets are in line (parallel to each other). The first rivet on each side of the joint should be not less than two and one-half rivet diameters from the end of the sleeve.
The general rules of rivet spacing, as applied to straight-row layout, are quite simple. In a singlerow layout, first determine the edge distance at each end of the row then lay off the rivet pitch (distance between rivets) as shown in figure 5-42. In the two-row layout, lay off the first row as just described, place the second row a distance equal to the transverse pitch from the first row, and then lay off rivet spots in the second row so that they fall midway between those in the first row. In the three-row layout, first lay off the first and third rows, then determine the second row rivet spots by using a straightedge. (See figure 5-42.)
RIVET INSTALLATION The various tools needed in the normal course of driving and upsetting rivets include drills, reamers, rivet cutters or nippers, bucking bars, riveting hammers, draw sets, dimpling dies or other types of countersinking equipment, rivet guns, and squeeze riveters. Self-tapping screws, C-clamps, and fasteners are riveting accessories commonly used to hold sheets together when riveting.
Several of these tools were discussed earlier in this chapter. Other tools and equipment needed in the installation of rivets are discussed in the following paragraphs.
Hole Duplicators When sections of skin are replaced with new sections, the holes in the replacement sheet or in the patch must be drilled to match existing holes in the structure. These holes can be located with a hole duplicator. The peg on the bottom leg of the duplicator fits into the existing rivet hole. The hole in the new part is made by drilling through the bushing on the top leg. If the duplicator is properly made, holes drilled in this manner will be in perfect alignment. A separate duplicator must be used for each diameter of rivet.
Rivet pitch (6 to 8 dia.)
Edge distance - (2 to 2½ dia) Single-row layout Transverse pitch - (75% of rivet pitch) Two-row layout Three-row layout FICURE 5-42. Rivet spacing.
Rivet Cutters In cases where rivets of the required length are unavailable, rivet cutters can be used to cut rivets to the desired length. When using the rotary rivet cutter, insert the rivet in the correct hole, place the required number of shims under the rivet head, and squeeze as though it were a pair of pliers. Rotation of the disks will cut the rivet to give the right length, which is determined by the number of shims inserted under the head. When using a large rivet cutter, place it in a vise, insert the rivet in the proper hole, and cut by pulling the handle, thus sbearing off the rivet. If regular rivet cutters are not available, diagonal cutting pliers can be used as a substitute cutter.
Bucking Bars A bucking bar is a tool which is held against the shank end of a rivet while the shop head is being formed. Most bucking bars are made of alloy bar stock, but those made of better grades of steel last longer and require less reconditioning. Bucking bars are made in a number of different shapes and sizes to facilitate rivet bucking in all places where rivets are used. Some of the various bucking bars are shown in figure 5-43.
FIGURE 5-43. Bucking bars.
The bars must be kept clean, smooth, and well polished. Their edges should be slightly rounded to prevent marring the material surrounding the riveting operation.
Hand Rivet and Draw Sets A hand rivet set is a tool equipped with a die for driving a particular type rivet. Rivet sets are available to fit every size and shape of rivet head. The ordinary set is made of 1/2-in. carbon tool steel about 6 in. long and is knurled to prevent slipping in the hand. Only the face of the set is hardened and polished.
Sets for round and brazier head rivets are recessed (or cupped) to fit the rivet head. In selecting the correct set, be sure that it will provide the proper clearance between the set and the sides of the rivet head and between the surfaces of the metal and the set. Flush or flat sets are used for countersunk and flathead rivets. To seat flush rivets prop:
erly, be sure that the flush sets are at least 1 in. in diameter.
Special draw sets are used to "draw up" the sheets to eliminate any opening between them before the rivet is bucked. Each draw set has a hole 1/32 in. larger than the diameter of the rivet shank for which it is made. Occasionally, the draw set and rivet header are incorporated into one tool. The header part consists of a hole sufficiently shallow so that the set will expand the rivet and head it when struck with a hammer.
Countersinks The countersink is a tool which cuts a coneshaped depression around the rivet hole to allow the rivet to set flush with the surface of the skin. Countersinks are made with various angles to correspond to the various angles of the countersunk rivet heads.
Special stop countersinks are available. Stop countersinks are adjustable to any desired depth, and the cutters are interchangeable so that holes of various countersunk angles can be made. Some stop countersinks have a micrometer set arrangement, in increments of 0.001 in., for adjusting the cutting depths.
Dimpling Dies The process of making an indentation or a dimple around a rivet hole so that the top of the head of a countersunk rivet will be flush with the surface of the metal is called dimpling. Dimpling is done with a male and female die, or forms, often called punch and die set. The male die has a guide the size of the rivet hole and is beveled to correspond to the degree of countersink of the rivet head. The female die has a hole into which the male guide fits, and is beveled to a corresponding degree of countersink.
When dimpling, rest the female die on a solid surface then place the material to be dimpled on the female die. Insert the male die in the hole to be dimpled and with a hammer strike the male die until the dimple is formed. Two or three solid hammer blows should be sufficient. A separate set of dies is necessary for each size of rivet and shape of rivet head.
An alternate method is to use a countersunk head rivet instead of the regular male punch die, and a draw set instead of the female die, and hammer the rivet until the dimple is formed.
Dimpling dies for light work can be used in portable pneumatic or hand squeezers. If the dies are used with a squeezer, they must, of course, be adjusted accurately to the thickness of the sheet being dimpled.
Pneumatic Rivet Guns The most common upsetting tool used in airframe repair work is the slow-hitting pneumatic hammer called a rivet gun. Pneumatic guns are available in various sizes and shapes (figure 5-44). The capacity of each gun, as recommended by the manufacturer, is usually stamped on the barrel; pneumatic guns operate on air pressures of from 90 to 100 pai Slow hitting (long stroke) riveting hammers Offset handle Pushbutton Pistol grip Fast-hittin ers Pushbutton Pistol grip Offset handle FICURE 5-44. Types of rivet guns.
Pneumatic guns are used in conjunction with interchangeable rivet sets. Each set is designed to fit the type of rivet and location of the work. The shank of the set is designed to fit into the rivet gun.
Force to buck the rivet is supplied by an air-driven hammer inside the barrel of the gun (figure 5-45).
The sets are made of high-grade carbon tool steel and are heat treated to give them strength and wear resistance.
Some precautions to be observed when using a rivet gun are:
(1) Never point a rivet gun at anyone at any time. A rivet gun should be used for one purpose only to drive or install rivets.
(2) Never depress the trigger mechanism unless the set is held tightly against a block of wood or a rivet.
(3) Always disconnect the air hose from the rivet gun when it wil not be in use for any appreciable length of time.
Squaaze Rivators The squeeze method of riveting is limited since it can be used only over the edges of sheets or assemblies where conditions permit, and where the reach of the squeeze riveter is deep enough. There are three types of rivet squeezers- hand, pneumatic, and pneudraulic. They are basically alike except that in the hand rivet squeezer, compression is supplied by hand pressure; in the pneumatic rivet squeezer, by air pressure; and in the pneudraulic, by a combination of air and hydraulic pressure.
One jaw is stationary and serves as a bucking bar, the other jaw is movable and does the upsetting.
Riveting with a squeezer is a quick method and requires only one operator.
Squeeze riveters are usually equipped with either a C-yoke or an alligator yoke. Yokes are available in various sizes to accommodate any size of rivet.
The working capacity of a yoke is measured by its gap and its reach. The gap is the distance between the movable jaw and the stationary jaw; the reach is the inside length of the throat measured from the center of the end sets..
End sets for squeeze riveters serve the same purpose as rivet sets for pneumatic rivet guns and are available with the same type heads. They are interchangeable to suit any type of rivet head. One part of each set is inserted in the stationary jaw, while the other part is placed in the movable jaws. The manufactured head end set is placed on the stationary jaw whenever possible. However, during some operations, it may be necessary to reverse the end sets, placing the manufactured head end set on the movable jaw.
PREPARATION OF RIVET HOLES It is very important that the rivet hole be of the correct size and shape and free from burrs. If the hole is too small, the protective coating will be scratched from the rivet when the rivet is driven 168
Sliding valve Piston Set Blank rivet sleeve set Set retainer Exhaust Cylinder Throttle deflector Throttle lever Throttle valve Throttle tube Movement of air during forward stroke Bushing Movement of air during rearward stroke hegulator adjustment screw Air path FIGURE 5-45. Rivet gun nomenciature.
through the hole. If the hole is too large, the rivet will not fill the hole completely. When it is bucked, the joint will not develop its full strength, and structural failure may occur at that spot.
If countersinking is required, consider the thickalloy are drilled with greater accuracy by a drill having an included angle of 118° because the large angle of the drill has less tendency to tear or elongate the hole.
Center punch locations for rivet holes before beness of the metal and adopt the countersinking ginning the actual drilling. The center punch mark method recommended for that thickness. If dimpling is required, keep hammer blows or dimpling pressures to a minimum so that no undue work-hardening occurs in the surrounding area. Diameter Rivet Pilot Size Ream Size Drilling 3/32 3/32 (.0937)° 40 (.098) To make a rivet hole of the correct size, first drill 1/8 1/8 (.125) 30 (.1285) a hole slightly undersize. This is known as predrill- 5/32 5/32 (.1562) 21 (.159) ing, and the hole is called a pilot hole. Ream the 3/16 1/4 3/16 (.1875) 1/4 (.250) 11 (.191) F (.257) pilot hole with a twist drill of the correct size to get 5/16 5/16 (.3125) • (.316) the required dimension. Pilot and reaming drill 3/8 3/8 (.375) V (.377) sizes are shown in figure 5-46. The recommended clearance for rivet holes is from 0.002 to 0.004 in. "Note that ream size exceeds the maximum tolerance of .004 inch. This is permissible only if the next When drilling hard metals the twist drill should larger drill size happens to be so much larger than have an included angle of 118° and should be oper- the tolerance of .004 inch.
ated at low speeds; but for soft metals, use a twist drill with an included angle of 90° and it should be operated at higher speeds. Thin sheets of aluminum FIGURE 5-46. Pilot and reaming twist drill sizes.