^chnique

MAKING A NOSE BOWL BY RON COVELL

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n the past two installments of this series, we looked at both hand and power tools used for working with sheet metal. In the next few installments, we'll go through the process of making a rather complex part, a nose bowl, using very basic tools. The subject for our demonstration is a 1928 Curtiss Robin, owned by Alan Holloway of Holloway E n g i n e e r i n g in Quincy,

California. Al is a great enthusiast, flying a Stearman he restored himself, and his business specializes in reb u i l d i n g radial engines. (See his website at www.radialengine.com.) He is currently restoring the Curtiss Robin, and the nose bowl was missing. He was fortunate to meet another Curtiss Robin owner, Dick Fischer, and Dick graciously offered to let Al borrow his nose bowl for a pattern.

These aircraft used the OX-5 V-8 engine, and the nose bowl has a unique shape to match the frontal outline of the engine. The first step is to examine the part closely, and decide on the approach to take — generally this will include making decisions about how many pieces to divide the part into, and where the joints will be placed. The tools used for shaping the sheet metal

The author Ron Covell holds an original Curtiss-Robin nose bowl, graciously lent to us by Dick Fischer to use as a pattern for creating a copy for Alan Holloway's restoration project. 46 AUGUST 1999

We're using 1/4" masking tape here to plot where the joints between panels will be placed. The process used for shaping the metal has a lot to do with where the joints are positioned [see text).

have a lot to do with the decisions made at this point. For example, if the shaping will be done using hand tools, it is much easier to do stretching than shrinking. On the other hand, if the metalworker has equipment that can do powerful shrinking deep into the panel, this is sometimes a faster means of creating shapes. The joints might be made in different locations, depending on how the individual panels will be shaped. Sometimes fewer joints are required if powerful equipment is available which can shape larger panels. For our example, we will do primarily mallet and sandbag shaping, using a little assistance from an inexpensive shrinker to help pull the edges down. Working in this way, it is easiest to place the welded joints in the middle of the areas with greatest curvature. The approach we are taking breaks the nose bowl up into 6 major pieces, making each piece relatively simple to shape. We started by using 1 /4" masking tape to lay out the location of the joints. The next step in any metal shaping job of this magnitude is to make a wooden buck. The purpose of a buck

This photo gives you a general idea of the strategy we're using for breaking the large, complex nose bowl up into simpler pieces.

is often misunderstood. Generally, a buck acts as a template which defines the contours of the panels being shaped, and is used for checking the contours of sheet metal panels shaped off the buck. Occasionally a buck will be reinforced in certain areas, so that

some metal shaping can be done by hammering on the buck. This type of buck is known as a 'working buck', and we chose to create this type of buck for reasons that will become apparent shortly. The process of making a buck is SPORT AVIATION 47

The perimeter of the nose bowl is traced onto a sheet of plywood. The piece cut to this line becomes the base for the buck.

3/4" masking tape is used to mark the locations of the stations of the buck. 48 AUGUST 1999

grcaily simplified if an original part is available. We traced the perimeter of the nose bowl onto a sheet of 3/4" plywood, and cut a piece out to form the base of our buck. Decisions must be made about where the 'stations' or contoured elements of the buck will be placed. In general, stations are spaced closely in areas where the contours change rapidly, and can be spaced farther apart in areas where the contours change more slowly. We laid out the locations of the stations on the original nose bowl using 3/4" masking tape. Next, a contour gauge is used to measure the curvature at each tape line. This contour gauge is used to lay out cut lines on a sheet of 3/4" plywood, the part is cut slightly oversize with a bandsaw (a saber saw would work as well), and the edge is carefully sanded to the trim line with a disk sander. Once all the stations are cut and sanded to size, it's time to assemble them on the base of the buck. We used drywall screws and glue to hold the pieces together. Minor adjustments of the edges of the stations are made at this point, to ensure the face angle will match the sheet metal panels, and that the contours defined by all the stations are smooth and continuous. You'll notice in the photographs that our buck has lots of holes in it. These holes are there for several reasons. The smaller holes allow us to use small C-clamps to hold the edges of the panels we are shaping tight against the buck. Some of the larger holes allow a better view of the sheet metal panel from the backside. The holes around the openings for the radiator air inlet and propeller shaft allow us to form flanges in these areas, as a step toward creating the wired edges that reinforce these openings. An added bonus of all these holes is that they make the buck lighter, and easier to move around the shop. You'll notice that the front surface of the buck appears to be made of solid wood. It is actually made of MDF (medium density fiberboard) which is an ideal material to use as a base for hammering against. MDF

front panel. With the buck completed, the first

panel. This pattern was transferred to a sheet of a l u m i n u m , which was panel we'll deal with is the largest — trimmed to the proper size. The bends the one comprising the top and face on the top of the panel were created section of the nose bowl. We're using by freehand bending. When these .050" 3003 H-14 aluminum for this contours matched the buck pretty project, slightly thicker than the .040" well, 1/8" locating holes were drilled original. The thicker metal is more through the aluminum sheet and into forgiving to work with, and the slight the buck. A piece of 3/4" plywood a d d i t i o n a l weight is offset by its was cut to sandwich the face of the greater durability. A pattern was made panel against the buck, to hold it flat from chip board, allowing about 1/2" as the edges were contoured by hamextra material around the edges of the mering and shrinking.

j ***eesn't have voids in it like inexpen-

si»Je plywood does, and it is much • v easier to cut, shape, and sand than particleboard. The edges of the MDF face were radiused to act as a form block to ease the shaping of the

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A contour gauge is used to measure the curvature needed for cutting each station out of a 3/4" plywood sheet.

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Here's our completed buck. The small holes in the stations are to allow the use of Cclamps to hold the sheet metal panels temporarily in place while fitting to the buck.

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50 AUGUST 1999

The front section of the buck is faced with MOF [medium density fiberboardJ. The large cutouts allow the forming of a fknge, needed to start the wired edges used to strengthen these openings. The perimeter of this section is rounded to create a form block, used to help shape the edge of the aluminim front pane/.

The first sheet metal panel is freehand bent to match the shape of the buck, located with alignment pins, and clamped into place on the buck.

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To ease the shaping of the metal in

the tightest radius of the corner, a small 'V shaped piece of metal was trimmed out, and the edges that required shaping were annealed. The annealing process is fairly simple — a light coating of soot from a pure acetylene flame is applied to the

metal, then a neutral flame is used to

heat the metal carefully to the point where the soot film just burns off. A

rosebud welding tip helps with this

process, but it is important not to over-heat the metal, since the annealing temperature is close to the melting temperature of aluminum. It is better not to anneal the whole panel

in this case, since the flat section

With everything clamped tight, the metal is tapped down to meet the MDF form.

would most likely warp from heating, and the un-annealed portions of the panel will have more stiffness if they are left in the half-hard condition.

The metal is placed on the buck,

positioned with pins placed through the alignment holes, and the clamping board is squeezed against the

buck with clamps. Next, the edge of

the aluminum panel is tapped gently down against the MDF portion of the buck. The entire p e r i m e t e r of the panel is shaped in this manner, but there is a small amount of spring

back, leaving a gap between the alu-

minum and the MDF. The easiest way

to bring the metal down tight against

the buck is to work it with a mechanical shrinker. After a few dozen

carefully spaced shrinks and a little

Here's a close-up shot of the corner — the area where the greatest shrinking was required. Notice that a small relief was cut to ease the shaping in this area.

more tapping on the buck, our front panel is nicely shaped! In the October issue, we'll continue with shaping the side panels for

the nose bowl, and start gas welding

some panels together and metal finishing them. +

About the Author

A shrinker was used around the perimeter of the panel, to help bring the edges down tight against the buck. In the next installment, we'll continue by shaping the side and bottom panels for the nose bowl. 52 AUGUST 1999

Ron Covell has been a professional metalworker for over 35 years. He operates a business named "Covell Creative Metalworking ", offering a complete line of metalworking tools, books, and videotapes, as well as offering a series of metalworking workshops nationwide. You can reach him at 106 Airport Boulevard, #201, Freedom, CA 95019 831 768-0705, or you can send E-mail to covell@cruzio. com.

MAKING APART NOSE BOWL II BYRONCOVELL n the last installment of this series (see Sport Aviation August 1999), we looked at the beginning of a fairly complex project, forming a new nose bowl for a 1928 Curtiss Robin, using only basic hand tools. In that article, we created a wooden buck to define the contours of the nose bowl, and shaped the first piece of metal to fit the buck. Now we're ready to continue with the process. Using a flexible paper material called 'chip board,' we'll make a pattern for the next piece to fit the buck. The first metal piece we made fit the top and front of the buck, so we'll make the upper side pieces next. When shaping metal with hand tools, it is generally easiest to design your panels so the welded seams are in the areas of greatest curvature. Most often this will mean the central area of each panel will have the lowest crown, making it easier to shape by hand. The chip board pattern is made about 1/2" oversize, so after the part is shaped there will still be enough metal to slightly overlap the first piece. The same pattern can be used for both sides of the nose bowl, since it is symmetrical. When the pattern is finished, it is transferred onto a sheet of 3003 H-14 aluminum, .050" thick, and two parts are cut out.

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The aluminum we've chosen, a manganese alloy in the half-hard condition, is a little too stiff to easily shape with a mallet and sandbag. To ease the shaping process, we'll anneal the metal before working it. A very light coating of soot from a pure acetylene flame is applied to the parts, then a neutral flame is used to gently heat the panels until the soot film just burns off. The panels are allowed to cool naturally, which leaves them in the fully annealed or dead soft condition. The part is rough-shaped by working with a mallet and sandbag. We're using a urethane headed mallet here, and working into a 12" diameter bag filled with coarse sand. The hammering pattern starts in the center of the panel, and then spirals out toward the edges. It is important to leave a border untouched about one inch wide around all the edges. Hammering in this zone would stretch the metal, and be counter-productive to our goal of achieving a domed shape. The fit of the panel is constantly checked against the buck, and typically several repetitions of hammering and adjusting are required to get a good fit. Once the center of the panel has the right contour, a hand-operated shrinking machine is used on the edges to help pull

We're using chip board as a material to make the pattern for the upper side portion of the nose bowl, allowing a 1/2" overlap. 94 OCTOBER 1999

them down, since these areas have a higher-crown shape than the center. When the overall shape is fairly well established with the mallet, sandbag and shrinker, the next step is to smooth out all the lumps these tools leave. The process we use is called planishing, which means to make smooth by hammering. We've chosen a dolly with a low-crown face, and propped it on a sandbag so it's supported in a working position. We'll use a slap hammer that has a large, lowcrown face to do the hammering. The entire surface of the panel is worked between the hammer and dolly, and then the fit is re-checked against the buck. Typically a little adjusting will be required at this point. Sometimes just twisting the edges of the panel with your hands is sufficient, but other times you'll find some additional work is required with a mallet and sandbag or shrinker to get a good fit on the buck. The more experience you have doing this work, the more streamlined the process gets. A beginner generally goes through many repetitions of reshaping, tweaking and adjusting, and a seasoned professional might go through only one or two.

The pattern is transferred onto a piece of .050" aluminum sheet with a felt tip marker.

The lower side pieces are the next to

be formed. We'll make a pattern on one side of the buck using chip board, transfer it onto an aluminum sheet, and cut the pieces out. Since this panel does not

have a compound curve in the center, it doesn't require annealing. We simply form the contour in the panel by freehand bending against the workbench. Once the center of the panel matches the contour of the buck, all that is left is

shaping the edges. We're using a home-

made post dolly for this operation. The post dolly is clamped to the edge of our workbench, and the edge of the panel is curled over using a slap hammer. This part of the process goes very quickly, and after a couple of checks against the buck and a little 'tune-up' work, these panels are properly shaped. The last major piece to shape is the lower center panel. This is the most

After the panel is cut out, it is annealed with an oxy-acetylene torch.

With the middle of the panel rough-shaped, the edges are drawn down with a hand-operated shrinking machine.

For planishing the shape smooth, we're using a slap hammer and dolly with a low crown face. Notice how we've supported the

dolly with a sandbag, so we have a free hand to hold the panel.

difficult panel, since it has a mediumcrown shape over most of its surface, and it's a fairly large part to shape by hand. Again, a pattern is made form chip board, allowing at least a 1/2" margin around the edges, and a piece is cut from aluminum sheet to match the pattern. We'll anneal the whole part, since every square inch needs shaping. Once cooled, the panel is bent by hand to take on the profile of the

The rough shaping is done with a mallet and sandbag.

The rough-shaped panel is test-fitted against the buck.

In this shot, you'll see that about 1/4 of the panel is planished. This took about one minute. SPORT AVIATION 95

The panel is test-fitted to the buck to test its shape as the plan-

ishing progresses.

The planishing nears completion. It took about five minutes to

planish this panel smooth.

This is a shop-made post dolly, created by welding a dolly onto a We're using the post dolly and slap hammer to round over the shop made support. edges of the panel.

You can see how well the contours of the panels match each othec

After a few adjustments, it fits the buck very well. 96 OCTOBER 1999

The lower side piece is shaped freehand by holding it flat against the workbench, and gently lifting one edge.

The edge of this panel is rounded over using the slap hammer and post dolly.

Another test fitting on the buck, paying attention to how the contours of the panels match.

buck. Next, the shrinker is used on the edges to start pulling them down. If you look closely at the photos, you'll see that the shrinking process alone has put the majority of the shape in this panel, before any hammering was done! Once the edges are shaped, the center portion of the panel needs to be bulged out, and this is done with the mallet and sandbag. Several test-fittings and minor readjustments are required to get the contour just right. The next step is planishing, to smooth out all the bumps left by the mallet. It takes an experienced person about 20 minutes to planish a panel this size by hand. Using a pneumatic planishing hammer or an English Wheel would certainly speed this part of the process, but our goal in this article is to show how you can make seemingly complicated panels using only hand tools! With all the panels shaped to match the buck, the next step is to scribe and trim the mating edges, and then weld them together. For convenience, we're using a TIG machine for the tack welding, but it could be oxy-fuel tack welded as well. The advantages of the TIG process are that it can be done right on the buck, it's fast, and there is no flux to mix or clean up. If you choose to do the tacking with oxy-fuel, after the panels are fitted to the buck and the edges trimmed, you'll need to put witness marks along the seam at convenient intervals. Next, the panels will be clamped together off the buck, making sure the witness marks are aligned and the tack welds can be made without danger of charring the buck. Tack welds are placed along the seams 3/4" to 1" apart. If you are oxyfuel tack welding, be sure to wash the flux off thoroughly at this point. Next, the joint and the tack welds are worked carefully with a hammer and dolly to

Using chip board to make a pattern for the bottom piece. Note the use of push pins to anchor the board to the buck.

ensure the edges fit flush, and to close any gaps between the panels as tightly as possible. Once the seams are properly aligned, they are finish welded. We chose to oxy-acetylene weld these seams, since this is the equipment readers are most likely to have, and it leaves a very strong, yet very workable weld. Oxy-hydrogen or TIG welding would also be good processes to use. A special flux is needed for gas welding aluminum. It comes as a powder, and is mixed with water (or alcohol) to form a thin paste. (Note:

Sometimes trace amounts of impurities are found in water, and this can greatly diminish the effectiveness of the flux. If you are having problems with your welding, try using distilled water for the flux.) Mix only as much flux as you plan to use that day, to ensure it stays fresh. The paste is brushed onto the seam, both inside and out, using a small acid brush. Aluminum oxidizes very quickly at elevated temperatures, and the flux is necessary to exclude atmospheric oxygen from the weld area, so

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Comparing the freshly-minted nose bowl with its 70-year-old cousin, you can see that they match very well!

So there you have it! A f a i r l y complicated part made to precise tolerances, using only the simplest of tools: a m a l l e t and sandbag, hammer and dolly, a shrinker, an oxy-acetylene welding outfit, and a l i t t l e ingenuity! Although this project is probably a little too advanced for a complete novice, a person with a couple of successful metal shaping projects under his or her belt and who feels some confidence in t h e i r a b i l i t y can probably handle a job like this. *

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