(4) The bend allowance for the second bend is Bend tangent lines the same as that for the first bend Flat layout Sight lines
(0.331). Mark off this distance. (See of a channel figure 5-31.)
(5) The third flat, C, is equal to the overall dimension less the setback. Lay off this distance. (See figure 5-31.)
Flat C = 1.250 - 0.239 .761' 7/051" = 1.011 in.
(6) Adding the measurements of flats A, B, and C, and both bend allowances, (0.761 + 0.331 + 1.522 + 0.331 + 1.011), the sum is 3.956, or approximately 4.00 inches. Totaling the three flats, A, B, and C, 1 in., 2 in., and 1-1/4 in., respectively, the sum is 4.250 in. of material length.
This illustrates how setback and bend allowance affect material lengths in forming straight line bends. In this case, the reduction is approximately 1/4 in.
After all measurements are calculated, cut the material and mark off the brake or sight lines as shown in figure 5-31.
Duplication of Pattern When it is necessary to duplicate an aircraft part and blueprints are not available, take measurements directly from the original or from a duplicate part.
In studying the following steps for laying out a part to be duplicated, refer to the illustrations in figure
5-32.
Draw a reference (datum) line, AB, on the sample part and a corresponding line on the template material (example 1, figure 5-32).
Next, with point A on the sample part as a center, draw an arc having a radius of approximately 1/2 in. and extending to the flanges (example 2, figure 5-32).
Draw similar arcs each with a radius 1/2 in.
greater than the previous one until the entire part is marked. In case there is an extremely sharp curve in the object, decrease the distance between the arcs to increase the number of arcs. This procedure will increase the accuracy of the layout. An arc must Folded channel / 3/16R 90° FIGURE 5-31. Flat layout of a channel.
pass through every corner of the part; one arc may pass through more than one corner (example 3, figure 5-32).
Locate the coordinate point on the layout by measuring on the part with dividers. Always measure the distance from the reference point to the beginning of the bend line on the flange of the part.
After locating all points, draw a line through them, using a French curve to ensure a smooth pattern (example 4, figure 5-32).
Allow for additional material for forming the flange and locate the inside bend tangent line by measuring, inside the sight line, a distance equal to the radius of bend of the part.
Using the intersection of the lines as a center, locate the required relief holes. Then cut out and form as necessary.
Relief Holes Wherever two bends intersect, material must be removed to make room for the material contained in the flanges. Holes are therefore drilled at the intersection. These holes, called relief holes, prevent strains from being set up at the intersection of the 153
FIGURE 5-32. Duplicating a pattern.
inside bend tangent lines which would cause the metal to crack. Relief holes also provide a neatly trimmed corner from which excess material may be trimmed.
The size of relief holes varies with thickness of the material. They should be not less than 1/8 in. in diameter for aluminum alloy sheet stock up to and including 0.064-in. thick, or 3/16 in. for stock ranging from 0.072 in. to 0.128 in. in thickness.
The most common method of determining the diameter of a relief hole is to use the radius of bend for this dimension, provided it is not less than the minimum allowance (1/8 in.).
Relief holes must touch the intersection of the inside bend tangent lines. To allow for possible error in bending, make the relief holes so they will extend 1/32 to 1/16 in. behind the inside bend tangent lines. It is good practice to use the intersection of these lines as the center for the holes (figure 5-33). The line on the inside of the curve is cut at an angle toward the relief holes to allow for the stretching of the inside flange.
Lightening Holes Lightening holes are cut in rib sections, fuselage frames, and other structural parts to decrease weight. To keep from weakening the member by Inside bend tangent lines Relief holes FIGURE 5-33. Locating relief holes.
removal of the material, flanges are often pressed around the holes to strengthen the area from which the material was removed.
Lightening holes should never be cut in any structural part unless authorized. The size of the lightening hole and the width of the fange formed around the hole are determined by design specifications. Margins of safety are considered in the specifications so that the weight of the part can be decreased and still retain the necessary strength.
Lightening holes may be cut by any one of the following methods:
(1) Punching out, if the correct size punch die is available.
(2) Cutting out with a fly cutter mounted on a drill.
(3) Scribing the circumference of a hole with dividers and drilling around the entire circumference with a small drill, allowing enough clearance to file smooth.
(4) Scribing the circumference of the hole with dividers, drilling the hole inside the circumference large enough to insert aviation snips, cutting out excess metal, and filing smooth.
Form the flange by using a flanging die, or hardwood or metal form blocks. Flanging dies consist of two matching parts, a female and a male die. For flanging soft metal, dies can be of hardwood, such as maple. For hard metal or for more permanent use, they should be made of steel. The pilot guide should be the same size as the hole to be flanged, and the shoulder should be the same width and angle as the desired flange.
When flanging lightening holes, place the material between the mating parts of the die and form it by hammering or squeezing the dies together in a vise or in an arbor press. The dies will work more smoothly if they are coated with light machine oil.
Note that in the two form blocks shown on the left side of figure 5-34, the hole in the upper block is the same size as the hole to be flanged and is chamfered to the width of the flange and the angle desired, whereas in the lower block, the hole is the same diameter as that of the flange. Either type may be used. When using the upper block, center the material to be flanged and hammer it with a stretching mallet, around and around, until the flange conforms to the chamfer. When using the lower block, center the lightening hole over the hole in the block, then stretch the edges, hammering the material into the hole, around and around, until the desired flange is obtained. Occasionally, the chamfer is formed with a cone-shaped male die used in conjunction with the form block with which the part was formed.
HAND FORMING All forming revolves around the process of shrinking and stretching, and hand forming processes are no exception. If a formed or extruded angle is to be curved, either stretch one leg or shrink the other, whichever will make the part fit.
In bumping, the material is stretched in the bulge to make it "balloon," and in joggling, the material is stretched between the joggles. Material in the edge of lightening holes is often stretched to form a beveled reinforcing ridge around them.
Straight Line Bends Chamfered flanging block The cornice brake and bar folder are ordinarily used to make straight bends. Whenever such machines are not available, comparatively short sections can be bent by hand with the aid of wooden Cone-shaped or metal bending blocks by proceeding as explained male die in the following paragraphs.
After laying out and cutting a blank to size, clamp it rigidly along the bending line between two Forming block wooden blocks held in a vise. The wooden forming block should have one edge rounded for the desired radius of bend. It should also be curved slightly beyond the 90° point to allow for springback.
By tapping lightly with a rubber, plastic, or rawhide mallet, bend the metal protruding beyond the Plain flanging block bending blocks to the desired angle. Start tapping at one end and work back and forth along the edge FIGURE 5-34. Flanging form blocks. to make a gradual and even bend.
Continue this process until the protruding metal is forced down to the desired angle against the forming block. Allow for springback by driving the material slightly farther than the actual bend. If a large amount of metal extends beyond the bending blocks, maintain hand pressure against the protruding sheet to prevent "bouncing."
Remove any irregularities by holding a straight block of hardwood edgewise against the bend and striking it with heavy blows of a mallet or hammer.
If the amount of metal protruding beyond the bending blocks is small, make the entire bend by using the hardwood block and hammer.
Formed or Extruded Angles Both formed and extruded types of angles can be curved (not bent sharply) by stretching or shrinking either of the flanges. Curving by stretching the one flange is usually preferred since this process requires only a V-block and a mallet and is easily accomplished.
In the stretching process, place the flange to be stretched in the groove of the V-block. Using a stretching mallet, strike the flange directly over the V portion with light, even blows while gradually forcing it downward into the V. Too heavy a blow will buckle the angle strip. Keep moving the angle strip across the V-block, but always strike the spot directly above the V. Form the curve gradually and evenly by moving the strip slowly back and forth, distributing the hammer blows at equal spaces on the flange.
Lay out a full-sized, accurate pattern on a sheet of paper or plywood and periodically check the accuracy of the curve. Comparing the angle with the pattern will determine exactly how the curve is progressing and just where it needs to be increased or decreased. It is better to get the curve to conform roughly to the desired shape before attempting to finish any one portion, because the finishing or smoothing of the angle may cause some other portion of the angle to change shape. If any part of the angle strip is curved too much, reduce the curve by reversing the angle strip on the V-block, placing the bottom flange up, and striking it with light blows of the mallet.
Try to form the curve with a minimum amount of hammering, for excessive hammering will workharden the metal. Work-hardening can be recognized by a lack of bending response or by springiness in the metal. It can be recognized very readily by an experienced worker. In some cases, the part may have to be annealed during the curving operation. If so, be sure to heat treat the part again before installing it on the aircraft.
Curving an extruded or formed angle strip by shrinking may be accomplished by either of two methods, the V-block method or the shrinking block method. Of the two, the V-block is, in general, more satisfactory because it is faster, easier, and affects the metal less. However, very good results can be obtained by the shrinking block method.
In the V-block method, place one flange of the angle strip flat on the V-block with the other flange extending upward, as shown in figure 5-35. Hold it firmly so that it does not bounce when hammered, and strike the edge of the upper flange with light blows of a round, soft-faced mallet. Begin at one end of the angle strip and, working back and forth, strike light blows directly over the V-portion of the block. Strike the edge of the flange at a slight angle as this tends to keep the vertical flange from bending outward.
FIGURE 5-35. V-blocks.
Occasionally, check the curve for accuracy with the pattern. If a sharp curve is made, the angle (cross section of the formed angle) will close slightly. To avoid such closing of the angle, clamp the angle strip to a hardwood board with the hammered flange facing upward using small C-clamps.
The jaws of the C-clamps should be covered with masking tape. If the angle has already closed, bring the flange back to the correct angle with a few blows of a mallet or with the aid of a small hard- 156
wood block. If any portion of the angle strip is curved too much, reduce it by reversing the angle on the V-block and hammering with a suitable mallet, as explained in the previous paragraph on stretching. After obtaining the proper curve, smooth the entire angle by planishing with a soft-faced mallet.
If the curve in a formed angle is to be quite sharp or if the flanges of the angle are rather broad, the shrinking block method is generally used. In this process, crimp the flange which is to form the inside of the curve.
When making a crimp, hold the crimping pliers so that the jaws are about 1/8 in. apart. By rotating the wrist back and forth, bring the upper jaw of the pliers into contact with the flange, first on one side and then on the other side of the lower jaw. Complete the crimp by working a raised portion into the flange, gradually increasing the twisting motion of the pliers. Do not make the crimp too large because it will be difficult to work out. The size of the crimp depends upon the thickness and softness of the material, but usually about 1/4 in. is sufficient. Place several crimps spaced evenly along the desired curve with enough space left between each crimp so that jaws of the shrinking block can easily be attached.
After completing the crimping, place the crimped hange in the shrinking block so that one crimp at a time is located between the jaws. Flatten each crimp with light blows of a soft-faced mallet, starting at the apex (the closed end) of the crimp and gradually working toward the edge of the flange. Check the curve of the angle with the pattern periodically during the forming process and again after all the crimps have been worked out. If it is necessary to increase the curve, add more crimps and repeat the process. Space the additional crimps between the original ones so that the metal will not become unduly work-hardened at any one point. If the curve needs to be increased or decreased slightly at any point, use the V-block.
After obtaining the desired curve, planish the angle strip over a stake or a wooden form.
Flanged Angles The forming process for the following two flanged angles is slightly more complicated than that just discussed in that the bend is shorter (not gradually curved) and necessitates shrinking or stretching in a small or concentrated area. If the fange is to point toward the inside of the bend, the material must be shrunk. If it is to point toward the outside, it must be stretched.
In forming a flanged angle by shrinking, use wooden forming blocks similar to those shown in figure 5-36 and proceed as follows:
(1) Cut the metal to size, allowing for trimnanter to aning. Determine the ben bend and round th edge of the forming block accordingly.
(2) Clamp the material in the form blocks as shown in figure 5-36, and bend the exposed flange against the block. After bending, tap the blocks slightly. This induces a setting process in the bend.
(3) Using a soft-faced shrinking mallet, start hammering near the center and work the flange down gradually toward both ends.
The flange will tend to buckle at the bend because the material is made to occupy less space. Work the material into several small buckles instead of one large one and work each buckle out gradually by hammering lightly and gradually compressing the material in each buckle. The use of a small hardwood wedge block, as shown in figure 5-36, will aid in working out the buckles.
(4) Planish the flange after it is flattened against the form block and remove small irregularities. If the form blocks are made of hardwood, use a metal planishing hammer. If the forms are made of metal, use a soft-faced mallet. Trim the excess material away and file and polish.
Forming by Stretching To form a flanged angle by stretching, use the same forming blocks, wooden wedge block, and mallet as in the shrinking process and proceed as follows:
(1) Cut the material to size (allowing for trim), determine bend allowance for a 90° bend, and round off the edge of the block to conform to the desired radius of bend.
(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 ........