Sheetmetal is at the heart of automotive metalworking. From the most basic flat-sided race car to the most exacting restoration, if you’re working with cars you’ll soon be working with sheetmetal. Some of the exercises already discussed in this book have involved cutting and bending basic flat sheetmetal. In this chapter, we’ll look at shaping and bending sheetmetal into more useful parts.
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Basic Sheetmetal Work
Like many skills, working sheetmetal is easy to start and difficult to do well. Yet of all the skills in this book, sheetmetal fabrication is probably the easiest to master. Much of sheetmetal work is accomplished without welding or other heat-related processes. You can undertake a sheetmetal project with nothing more than some aircraft shears and a benchtop brake and get good results. Throw in a hardware store pop-riveter and a manual bead rolling wheel and you’re well on your way to professional output.
Sheetmetal generally comes in three forms—mild steel, aluminum, and stainless steel. Of these, aluminum and mild steel are by far the most commonly used for automotive projects.
Sheetmetal fabricators prefer aluminum for dashboards, bulkheads, and most other purposes because it is light, soft, and easy to work with. The downside to aluminum is that it tears easily and you really need a TIG or spool gun MIG setup to weld it. But this is a small tradeoff against the light weight and ease of use. Aluminum also polishes up nicely and doesn’t oxidize like steel. It can be anodized with color, painted, and even comes pre-coated if you like. The cutting projects from Chapter 4 were executed in sheet aluminum precoated with red paint and covered with a protective plastic sheet so that the blemishes created during the working process peeled right off with the plastic cover.
Whether you’re pounding a dented body panel or fabricating an entire custom interior for a race car, you’re bound to work with sheetmetal in the automotive world.
Sheetmetal comes in various materials and thicknesses, but the processes by which you turn a flat sheet into something useful are always more or less the same.
With a little work, a basic sheet becomes a precision part, like this Subaru rear differential skid plate from Primitive Racing.
For fenders, door skins, firewalls, and some critical structures, mild steel is preferred. While it is harder and heavier than aluminum, it’s also more forgiving of errors and doesn’t work-harden as much. So you’re less likely to rip or tear mild steel. Steel is also easier to weld.
Take a look at any project you have in mind and the right material for the job should be obvious. Less obvious is the thickness, or gauge, of sheetmetal to use. For example, sheetmetal of various materials comes in a variety of thicknesses.
Choosing the correct sheetmetal is a function of considering the project you’re undertaking and the strength and quality of various materials. Obviously, thin sheetmetal won’t do for a car floor that has to hold the weight of a driver in a seat, but in general, you should try to choose the lightest material that will serve your purpose.
For example, for a bulkhead, you generally want at least .063 aluminum, and that assumes that you are using your bead roller to put some shape into the sheet for strength. For a weight-bearing floor, use 14 or 12 gauge steel. Remember that you’re always trading off weight against strength when choosing a gauge of sheetmetal to use. Many racing organizations will specify the minimum acceptable gauge for sheet metal in critical applications such as firewalls and protective enclosures.
When you measure a sheetmetal project, remember that you can create an entire 6-sided box from a single piece of flat sheetmetal. It’s a matter of knowing which small pieces to snip out and how you will bend it up. For an easy example, disassemble a cardboard box and spread it out. If you separate only those seams that are stapled or glued together, you can see that it started as a flat sheet and was bent to shape with just a few seams joined up. Sheetmetal works the same way. The rule of thumb with sheetmetal is that you need to account for all sides, but because the bends tend to be sharp, you don’t have a large extra bend calculation as with tubing. You do have to measure enough, however, to make your attachments. So for an angled corner, it’s not enough to bring two raw edges together. Leave a little extra on one side to bend in and have a place to rivet, weld, braze, or otherwise fasten your seam.
This complex transmission tunnel cover is made of mild steel. The pieces were carefully cut, handhammered on a buck, and welded together for an exact fit.
Of all the products that automotive fabricators use, sheetmetal is the easiest to cut. You don’t need expensive tools, but they make your life easier. A large, free-standing sheetmetal shear is great for straight cuts in sheets up to 8 feet long. A smaller benchmounted shear is useful for most projects. But many fabricators get along just fine using only aircraft shears.
It’s convenient and fun to use every kind of sheet metal tool such as this full size jump shear, but you can get by nicely with just a set of basic hand shears.
Whatever tool you use, take your time cutting sheetmetal to get good results. Lay out your pattern in felttip pen on the metal and be sure to note the side on which you want to cut! You can always trim a little more, but if you cut too small, you have to start over. Sheetmetal never goes as far as it looks like it will when you’re handling the big sheet, so give yourself plenty of extra.
Getting nice crisp bends in sheetmetal is the mark of the true craftsman. The best and easiest way to do this is with a sheetmetal leaf box and pan brake.
But here again, these are large, expensive, free-standing tools. Their weight and size help make perfect, crisp bends. But you can get by and do good work with a simple bench-mount leaf brake. For boxes and more complex shapes, be sure to get a box and pan brake with adjustable fingers so that you can put a bend in a part that already has a crosswise bend. See the project to create a basic box later in this chapter for details.
Russ LaFontaine makes a race car body panel with a box and pan brake.
For smaller jobs, many fabricators get reasonable results with a machinist’s vise. You can make brake jaws from angle aluminum that will help you. Use aluminum, wood, or some other soft metal, however, to avoid damaging your work. Then the trick is just to bend slowly with as much pressure as possible right at the bend. That’s how a brake works—it applies even pressure right at the bend.
The critical tool that sets a sheetmetal fabricator apart is a bead roller. This tool puts a step or ridge into your sheet work. This allows you to put a little bit of depth into a piece of sheetmetal, increasing the cross-section of the sheetmetal without increasing its real thickness. That cross-section makes the sheetmetal shape more rigid. That’s why you see professional jobs that include a number of ribs or raised areas worked into the sheetmetal. This helps stop oil canning, where the body of the sheetmetal panel tends to wobble back and forth.
A bead roller is a simple tool a wheel or motor operates two matched dies. These dies can create a raised (or recessed) ridge or a singlesided step in sheetmetal when you squeeze the metal and roll it through the dies. The bead roller also includes an adjustment lever that controls the space between the dies. Just place your sheet between the dies, snug down the lever, and give the wheel a turn. The metal is pulled through the die and the shape of the die is transferred directly to your sheet. Once transferred, the shape is called a bead—presumably due to its resemblance to a welding bead.
Russ Nyberg demonstrates making a step in aluminum sheet metal with a bead roller. This inexpensive tool turns flat sheet into customfabricated panels.
Raising a step in the middle of this panel helps keep it firm, and it looks great, too.
You can also find dies that make a thin raised line in sheet for embossing, or dies that make an angle bend at the edge of a piece of sheetmetal. This function is different from a brake because a brake makes straight line bends, and the bead roller allows you to follow a curve as you move the sheet through the machine. This is handy if you need to make a lip around a curve. A manual bead roller with a selection of dies is an inexpensive addition to your shop, and gives your sheetmetal projects a professional look with just a little practice.
One of the most common requirements is to fold the edge of a piece of sheetmetal over onto itself to create a lip. This is called a hem in sheetmetal work, like making a hem in sewing. If you have a die to put an angle in the edge of a piece of sheet, you can start a hem in your bead roller, but to use a bead roller to make a hem is typically a three-step process with progressive dies. The advantage of this process is that you can make a lip on a curved piece. You have to finish that lip with a bit of hammer work, however. More often, fabricators make a nice sharp bend past 90 degrees in their brake, then open up the brake and fit the bent edge into the brake and use the closing action of the brake to complete the fold. This technique makes a good clean straight-line hem.
Copying Body Lines
One of the toughest challenges in sheetmetal forming is making a piece that follows a complex curve or cut line in a car’s bodywork. Automotive body panels are typically stamped using huge dies, and the whole piece is shaped in just a few steps. If you have to replace a piece of metal with a complex shape, it’s likely to require all the tools of the trade.
For making complex curves that is, curves in more than one direction—you really need an English wheel, or be prepared to spend a lot of time making a model (called a buck) of the shape and hammerforming the metal on the buck. By moving the sheet within the English wheel and rolling it back and forth, you can gently stretch the metal to create the desired shape.
Cut lines are more difficult and require hammer forming. This is where your bead roller comes into play. Find (or make) the correct dies to get the right angle of bend or raised bead in the metal, and then run your sheet through, following the curve of the cut line. This is much more art than science, and depending on the piece you are replicating, could take several attempts.
To make a hem in a straight edge, first bend the lip past 90 degrees, then open the brake wide enough to insert the bent lip.
All you have to do is close the brake again to fold over the hem. You get a nice, crisp hem with this technique.
Custom car door formed entirely from 16 gauge aluminum. Curt Oliver used an English wheel and hammer forming techniques to make this part. Notice how the cut line of the fender follows through the door. Copying lines like this takes real talent and refined skill.
Yet another tool (there are always more tools to buy!) used to make compound curves in concert with the shot bag is a slapper. This is a modified wooden or metal hammer that looks rather like a spatula. As the name implies, you slap the sheetmetal against the shot bag or buck that you’ve made. A slapper is usually wrapped in leather to soften the edges of the blow and get smooth results.
When you’ve got the basic shape, be prepared to do some more massage to get it integrated and welded into your car’s bodywork. This can involve hammer forming, hammer welding, and then traditional hammer and dolly bodywork to smooth it into place. The difficulty of recreating complex bodywork lines is why there’s a thriving market in replacement body panels made from the original dies, or dies made to match the original part!
Another top challenge for sheetmetal workers is welding sheet. Because the material is so thin, your welder is likely to melt holes straight through the material. Low power is key here—use the lowest power setting available on your welder. If it’s not enough to start an arc or penetrate the material, raise the power in the smallest possible increments until you get a good weld. Be sure to test your welder on similar materials before you weld on your project!
TIG welding is popular with sheetmetal because the foot pedal control on a TIG welder allows you very fine power control and the sharp TIG tip allows you to keep the arc very small. At the other end of the spectrum is an AC stick welder you can try to weld sheet with one of these, but it’s nearly impossible to get good results.
This body mount brace was made from 1/16-inch mild steel sheet, hammer formed, and MIG welded together. The holes remove weight but do not substantially reduce its strength.
This racing seat was TIG welded from 3/16-inch aluminum sheet. Note the simple bends made with a brake to fit the side plate.
In between are the oxy-acetylene, wire-feed, and MIG welders. Of these, MIG is the best, because of its smooth arc and adjustable power. Gas welding sheetmetal takes a light touch, but it can be done and many fabricators like gas welds because they’re easier to smooth out and shape into the metal. Inexpensive wire-feed welders tend to make holes in sheetmetal because of their limited adjustability. If you plan to do a lot of sheetmetal construction, it may be worth your investment to purchase a spot welder.
Rivets are a common fastener solution for semi-permanent installations. Most rivets used now are blind pop rivets. They are called blind because you don’t need to have access to the back side of the work to install such a rivet. Just drill a hole, insert the rivet, and use your pop riveter to pull on the shaft until it breaks off.
Traditional rivets required access to the back side of the work, and the rivet was spread by upsetting the rivet shaft between a hammer and dolly. Pop rivets are usually made of aluminum, and occasionally mild steel. They work well for low-stress applications.
If you want to keep something in place forever, weld it. If you want to take it off now and again, use screws or bolts. If you want to take it off at a moment’s notice, use Dzus fasteners. But if you want to keep something in place a long time, but make it easy to remove with a little work, rivets are a great choice.
Rivets come in a variety of head and shaft sizes. You can get pop rivets down to 1/16-inch shaft size up to 3/16 inch at your local hardware store. You can also choose a depth for the rivet if you’re just joining two sheets of metal, a shallow depth works best. If you’re riveting a thick sheet to a piece of angle iron, you need a deeper rivet. If you’re not working blind that is, you have access to the back side, you can also get washers designed to add extra grip to your rivets. Put the washer on before you squeeze the rivet and the washer will be held fast on the back side of your work.
To remove a rivet, you simply drill out the center. The back side generally falls away, so if you are using the rivets blind for example, in a piece of box tube you’ll accumulate a bunch of old rivet bits in your structure. But that’s OK they don’t weigh much.
One derivative of rivets that auto fabricators find handy is the riv-nut. These go by a number of trade names, but the design is that when the rivet is set, it provides a threaded hole suitable for a machine screw. A riv-nut requires a special tool for installation, but they’re a godsend for making removable panels that must be more solidly affixed than Dzus fasteners allow.
The following projects take you through the basic skills of sheetmetal fabrication. It’s assumed that you have a brake, bead roller, and some means of cutting the sheetmetal. You can get by with aircraft shears, but larger tools make your job easier.
Making a Box
Our first project is to make a basic box. This involves measuring, marking, cutting, and bending a piece of aluminum sheet. We used 22 gauge aluminum that has red paint on one side and white on the other. The red side is also covered with a protective plastic skin that you remove when you’re done with your work. It helps keep the workpiece free from scratches.
1: Consider the drawing on this page. This is the plan for a box. It’s drawn as 6 x 8 inches, but you can scale this up or down as needed. But scaling this project down would be hard to do. We scaled it up to 12 x 16 inches for our example photos, and it was still about as small as our big brake could handle. The point is, the drawing works for any 3:4 ratio measurement, such as 15 x 20 or 18 x 24.
A riv-nut tool allows you to set a rivet that leaves a flush and threaded hole. It’s perfect for mounting things onto sheetmetal bulkheads, dashboards, and consoles.
This switch plate is mounted to its console using riv-nuts. The installation is clean and professional-looking. Riv-nut tools are not expensive, and should be in your inventory.
2: We used a basic computer drawing program to lay out our bends and cuts. This allowed us to think through what we wanted to make and how we planned to make it. The bends are noted with dashed lines, and the cuts with solid lines. The shaded areas are to be removed before we bend up the box. Then we printed a copy to test our idea. We cut out the shape and folded the paper to make sure that our idea would work.
We printed out our design and cut it out as we planned for the metal, then folded it up to make sure our plan would work in the brake.
Here’s the paper box all folded up. We’re confident the sheet metal will fold in the same way.
3: Next, copy your drawing onto the piece of aluminum you’re going to use. Use your carpenter’s square and tape measures to get all your measurements and lines right. You can draw it on both sides or just the back. It’s convenient to be able to see the plan from both sides, but the back is more important as brakes bend from the back side.
Here’s the same drawing, executed on a larger sheet of aluminum. We made the measurements and used our straightedge to draw the lines. We’ll cut from this side, but we’ll also make the same marks on the white backside for the bending process.
4: Cut away the parts you don’t need. We used the big freestanding shear to cut the basic rectangle and then our corner notcher to get rid of the edge pieces. We like the nice sharp cuts the shear and the notcher make, but we used aircraft shears to take out the triangles from the ends of the box and make the end cuts for the riveting tabs. You can do the whole project with shears if that’s what you’ve got.
Most of our cuts are right angles from the edge of the metal, so we were able to use the notching shear. This gave us sharp and accurate cuts.
When the cutting is done, you can see that the aluminum matches the pattern we made. We cut out the triangles with ordinary aircraft shears, and made snips at either end.
5: When you’re done cutting, it’s time to bend. The order in which you make your bends is critical. Start from the outside and work your way in. First, bend the bottom lip on either side.
Start bending with the long sides at the edge, then bend the end lips up.
6: Next, bend the bottom lip on either end of the box. This is where you made those two cuts on either end. Those cuts have to be able to fold into each other, so press the center one in a little before the next bends.
This is where you need to use the moveable fingers on the box and pan brake you couldn’t make these bends without allowing some space for the previous bends to travel through the brake.
7: Now make the two central bends along the long axis of the box. This is where you have to adjust the dies (the teeth) in the brake to allow for the end lip bends you made before. Most adjustable brakes allow you to make space for prior bends. If your brake does not have that capability, you might have to skip step 6 and hand-bend the end lips later with pliers.
8: This step is tricky. You need to re-adjust the brake dies to fit the shorter central end bends, but even with the brake adjusted for the bends, you won’t be able to get a full 90 degrees into the sheetmetal. You’ll get 45 degrees before you run into your prior bends. Bend both ends as far as possible. Finish up the bending by pressing the ends in, holding the bend location firmly against your workbench to make a nice crease. You might even be able to get a dolly in there to bend against. You might have to bend open the lips to get everything to fit, but they’ll bend back again.
This picture shows the fingers of the brake adjusted for 3-D bending. Even with this adjustment, we’ll only be able to make this bend about 45 degrees in the brake.
9: Finish up the bending by pressing the ends in, holding the bend location firmly against your workbench to make a nice crease. You might even be able to get a dolly in there to bend against. You might have to bend open the lips to get everything to fit, but they’ll bend back again.
To hold all the bends in place, put a hole in each corner through all the layers stacked up there, and then pop-rivet the joint.
10: Your box is now almost done. Take it over to your drill press and clamp the corners so they’re nice and tight and square. Then drill a hole in each corner. It should pass through the three layers of aluminum stacked up there. Put a poprivet in each hole and that completes your structure. Make sure your poprivet size matches your drill bit!
When you’re done, you’ve got a neat box suitable for a switchplate in a race car or to cover something like a fuel pump in a hot rod or custom. The lines drawn on the surface come off when you peel the protective plastic sheet.
Your box has an open bottom, so it’s good for use as a switch box in a hot rod or race car, or to cover something less pretty in the trunk or engine bay. Truthfully, you could make a serviceable box without the diagonal cuts and with one less set of lips just cut a square out of each corner of the original rectangle. It’ll bend up easier. But look at the amount of metal in the corners you can now weld (carefully!) or braze up those edges, put a lid on the open side, and have a fluid-tight box for an overflow tank.
Optional: If you have a bead roller, why not put some ribs or a raised or recessed step in your box? While it’s still a flat piece of aluminum, but after you’ve got the pattern marked for cutting and bending, go ahead and use your roller to put some shape in the metal. It’ll make this strong design even stronger, and it looks good, too!
Making a Bulkhead
Cutting straight lines in sheetmetal can be a challenge without a big shear, but cutting long curved lines has to be done by hand, and you want it to fit well. Bulkheads tend to be large flat or curved sheets with intricate work at the edges to fit whatever installation you’re covering.
Most bulkheads that are put into hot rods or race cars are made from aluminum it’s easier to work with, it’s lighter, and it shines up nicely. If you’re planning to make a firewall, use sheet steel. You want that extra protection.
Follow these steps to make an aluminum bulkhead from 18 or 20 gauge aluminum:
1: Get a supply of thin cardboard from an art supply store. This material generally comes in 24 x 36- inch sheets and costs less than a dollar per sheet. It’s perfect for templates because it cuts easily, you can mark on it, and it holds up well enough to use over and over again. Also, get a good heavy-duty pair of paper shears to cut it.
2: Using a felt-tip marker and the cardboard, make a careful template of your bulkhead. Pay special attention to roll bar tubes or other obstructions you’ll have to work around. Cut the cardboard to fit perfectly up to all edges. Also, pay attention to the size of each piece of the bulkhead. This is primarily limited by the size of your brake, if you are planning any bends in the sheet. Also consider your ability to get the shape you want into the sheet with your bead roller.
Fit your cardboard template snugly up to all mating surfaces, especially under windshields.
Don’t be afraid to bend the template to work around corners and around obstructions such as roll cage components.
3: If you plan to have a removable access panel, now is the time to plan for it. Leave about 3/8 inch of overlap for all adjoining pieces. This is enough space to rivet, screw, or even weld the pieces together. If your template requires several pieces of cardboard, mark them carefully in relation to each other.
4: Transfer your template to the aluminum with a felt-tip marker. Mark the side of the line where you plan to cut. Check your marks and remeasure the car for perfect fit.
Leave about 3/8 to 1/2 inch for a recessed lip. This allows you to attach two pieces and still have a smooth surface. That’s yet another reason to have a bead roller.
When you’ve got your template just right, transfer your design to the sheetmetal.
5: Cut out your bulkhead pieces and mark your overlaps. Be sure to hit every edge with your deburring tool! Also, mark any shape you plan to put in the metal with the bead roller. Then take each piece to the roller and work the shape into the metal. This adds a professional look to your project and also helps make the bulkhead solid. A perfectly flat sheet wobbles back and forth, sometimes making noise—and it often looks like an amateur job. If you don’t have a roller, consider taking your bulkhead pieces to a metalshop and hiring them to do your bead work.
With your design on the sheetmetal you can cut with confidence. If it has a protected side, make sure that you’re laying the template for that side to be facing outward.
6: A special case of bead rolling happens on your overlap edges. You want to put a step down in the back side of each overlap. Make the step the same depth as the thickness of your sheet material. This way, your panels lay flat on each other and fit perfectly. This really enhances the professional look of your project.
Using a step die with your bead roller allows you to fit bulkhead panels together and still have a flush surface. The lip also gives you a place for rivets, brazing, Dzus fasteners, or screws.
7: When the pieces are cut and you’ve got your shape in the metal, you can make any necessary bends. Put each piece into your brake and bend it as closely as possible to the desired angle. Be sure that your bend lines line up if you’re working with multiple pieces that must fit together!
8: Finally, go ahead and install your bulkhead. If you’ve got nothing to affix the panels to, you might have to design a lip into your panels and attach them to the sides of the space. Alternately, you can cut some strips and bend them in the brake to make your own mounting lips. Rivet or screw the mounting lip to the perimeter of the bulkhead area, then attach the bulkhead to the lip. This is handy if you want to use Dzus fasteners or other removable fasteners to make your panels removable just remember to overlap about an inch for Dzus fasteners.
This shot shows an installed bulkhead panel held in place with pop rivets. You can see the panels make room for a roll bar brace at the top and share a bead-rolled riser. The panel on the left has a recessed edge to lay flush with the panel on the right, and everything lines up nicely.
Repairing a Rusty Floor
In this project, you’ll cut out a rusty patch of steel and cover it and the surrounding area with new steel. This is not a restoration-quality repair, but it will suffice to keep a car on the road. For the specific work, we found an old GMC pickup truck with a rust-perforated area beneath the driver’s right heel. We removed just the rustiest part of the steel and then covered the entire area with a new piece of sheetmetal. The material we used was .095-inch mild steel sheet. We corrugated the new metal on the bead roller to give it strength and stitched it into place.
1: Look at the rusty area from both sides to ensure that nothing critical is attached to the area and that no fuel or brake lines run nearby. Cooking your fuel lines while welding is a bad idea.
2: Carefully cut out the weak area. Use some good judgment here—you don’t want to cut out too much. You might want to scrub the affected area with naval jelly to remove surface rust or cover it with a product such as POR-15 to seal the rust. But surface rust is not a reason to cut out original structure. Unless you’re building a show car or concours restoration, your repair will hold up better with as much of the previous material in place as you can keep.
This floor is in dire need of a repair. The right thing to do would be to cut out the entire floor and make a new one, but the owner just wanted to get back on the road as soon as possible.
We cut the worst of the rust out of the floor, and we’ll cover the rest with new steel. We’ll have to work far enough out where there’s still good metal to weld up to our new piece.
3: Measure your replacement plates. If you’re replacing outer bodywork, cut the new plates exactly to shape. If you’re replacing a floor, make the replacement panel big enough to tie in to good sheetmetal away from the rusted area.
4: Because this patch covers parts of two panels that come together at an angle, you can either make one panel and bend it in a sheetmetal brake, or make two panels and weld them together at the joint. We elected to use two pieces for this job because the vertical panel covered a series of complex curves in the original stamping. If your project requires two panels as ours did in this exercise, cut each panel to shape and give it ridges in the bead roller. Then bend the panel to match the surrounding metal.
Russ made the new part in two pieces and used the bead roller to give the panels some shape. Then he tacked them into place and started welding.
If you elect to make a single panel, measure the total dimension required and cut the panel to size. Then mark and bend the panel in your sheetmetal brake before you use your bead roller to add structural ridges to the part. Place ridges in both planes of the panel before you weld it in its final location. You may need to work the panel a bit with your rubber mallet or body hammers to get a snug fit around existing curves.
5: Position the panel (or panels) in place and tack-weld them at their corners and in several locations along each edge to hold them in place and prevent warping. If you are using two panels, tack them to each other in several locations at this time.
6: Using the stitch-welding technique, run weld beads around the perimeter of the new metal, joining it to the surrounding structure. Be sure to skip around the entire perimeter and weld in stitches of about 2 inches. If the piece becomes too hot, take a break and let things cool down before continuing.
Russ uses a stitch welding technique to avoid warping the panels. He welds short stretches and moves around on the project. Eventually, all the seams will be welded.
7: If you used two pieces for your patch, wait until the perimeter is welded into place before you join the two new pieces together. Use the stitch welding technique for this joint, too.
8: In this project, we had to remove the area where the accelerator pedal is mounted onto the floor. When the new panel was in place, we drilled and re-mounted the pedal.
The final result will keep the driver’s feet off the pavement, but doesn’t do much to help the rest of that floor.
You can modify this project to fill gaps in bulkheads, replace truck beds, and create new floors and bulkheads for racing and specialpurpose cars.
Stretching a Fender
In this project, we’ll extend a wheel arch to accommodate larger tires. This is a popular drag racing modification for rear wheels. The subject car is a 1970 Chevrolet Camaro, but the principles apply to any car. This project involves cutting and welding a car’s bodywork, so plan on repainting if you undertake a similar project.
1: Measure how much additional space is present behind and in front of the fender lip inside the wheel well. You don’t want to exceed that space, but there’s usually quite a bit to work with. Plan how much extension you would like. In the case of this project, we extended the wheel arch by 4 inches.
You can extend a wheel arch as far as the inner structure of the wheel well will allow. This arch was stretched to within an inch of the inner wheel well.
Cutting a square makes it easy to remove strips from the front and back, and then welding the pieces back together again.
2: Measure and cut the outer fender skin in two squares around the wheel well. Leave a tab hanging down in the middle of the wheel arch so you have the original lines to match as you work. Leave plenty of material at the front and rear of the wheel well to get a good weld and smooth over when you’re done. Then cut an additional strip in front of and behind the square you just cut out. Each strip should be half the width of the extension you want.
3: Weld the rear half of the wheel arch to the new rear cut in the fender, then weld the front half into the new front cut. You’ll have your extension as a gap at the top of the wheel well, with the tab hanging down as your template.
The central tab hanging down as a template. You will weld in new steel here, and it’s handy to have a piece of the original as a guide.
4: Make patch panels to fit the gap at the center of the wheel arch. Make sure you account for the wheel arch lip. Weld the patch into the gap in your fender and grind all your welds smooth.
Larger wheels and tires usually run both to the inside and the outside. This project was undertaken as part of a comprehensive modification plan that included tubbing the rear end of this Camaro to fit huge wheel wells where the rear seat and trunk used to be. The entire rear framework of the car was removed and replaced with a fabricated chassis. However, stretching a wheel well doesn’t require all that work. This can be undertaken as a lone modification, working within the confines of the inner wheel well.
Weld all the parts back in and fill the gaps in the wheel arch with new steel. Bend in the lip, or make a hem if you’re planning to roll the lip anyway.
You can see how this stretch has brought the wheel’s arch almost to the front of the inner wheel well.
The net effect of this project is that these large drag racing tires fit and look good under this fender.
Shrinking and Stretching Curves in Sheetmetal
Stretching is just the opposite of shrinking, and it’s generally used for creating a curve in metal. There are lots of tools to help you do this, including English wheels, planishing tools, shot bags, and even your basic hammer and anvil. When you’re stretching sheetmetal, smoothness is key, and most of these tools are designed to make long, smooth stretches in sheetmetal.
When you stretch a given area of sheetmetal, you make it thinner and larger. You still have the same amount of metal it’s just spread over a greater area. The effect is that the metal wants to curve when you do this, because it’s not stretching evenly. You can use this attribute of metal to create just the shape you want.
This air-powered planishing setup will put a curve in sheet metal if you work long enough, but is mainly used to smooth light imperfections in sheetmetal. We used it to smooth our fender after hot-shrinking dents.
This is the stretcher side of the same tool. You can see how the second bend is starting to pull the metal away from the machine.
This is the shrinker side of the shrinker/stretcher tool. You can tell because the metal is curving in toward the machine.
It’s easy to rip aluminum with the stretcher. If you’ve got a choice, use the shrinker side of the tool on this material.
Because most people don’t own an English wheel, we’ll use a more conventional shrinker/stretcher tool. Any tool supply store has these for sale. This tool has small jaws that grab the lip of your piece of sheetmetal and press it together or pull it apart a little bit. By repeated application, the metal starts to curve as the length at the edge is stretched or shrunk.You can use this tool in combination with an English wheel or lead shot bag and the planishing tool for forming fenders, headlight buckets, or any kind of curved shape you need.
If you have a choice, the shrinking application is somewhat safer to use on sheet aluminum. Because sheet aluminum tears easily, it’s easy to rip it apart on the stretcher. Mild steel sheetmetal is more forgiving. The shrinker will leave the sheetmetal with a corrugated surface at the shrunk edge, but a quick trip through the planishing tool will smooth this out.
1: As described in Chapter 5, make your flat sheetmetal bulkhead and get it right up to the edge of the bodywork.
2: Make a cardboard template showing the curved edge of the bulkhead to be followed with the stretched and shrunk piece.
3: Get a strip of aluminum or steel and put a 90 degree lengthwise bend in it in your brake. Choose a side that will be mounted to the bodywork and the side that will be the mounting lip for the bulkhead. Assuming that the bulkhead panel is flat, or nearly so, you will stretch and shrink the lip to follow the curve of the template.
4: Use your shrinking die to work concave bends into the metal, and the stretching die to pull the metal for convex bends. Be careful if you’re working with steel, and be extra careful with aluminum aluminum feels nice and soft to work with, but you can easily rip it with the stretcher.
We’re stretching this panel to curve around, then we’ll fold that lip into a hem on the bead roller.
Here you can see the corrugation left by the shrinking tool. You can work that smooth again with the planishing tool.
5: Work patiently and evenly, stopping frequently to match your work against the template you created. When it gets close, spend some time with the planishing tool to smooth out the wrinkles made by the shrinking die. You’ll lose a little curve, but you can work that back in, and having a smooth surface makes your project look better.
6: When your piece matches the template, match it up to the installation site in the car. You can screw or rivet it into place. The piece will be somewhat damaged by work hardening, so welding it into place is not the best idea, but you can weld it into place if the materials are the same. Using screws or rivets also makes undoing (or redoing) your work easier down the road.
The finished curved panel with a folded hem. You can create a unique wrap-around dash or other interior curves using this technique.
Written by Russell Nyberg & Jeffery Zurschmeide and Posted with Permission of CarTechBooks
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