Body: Applying the Skin
From The Scratch-Built Hot Rod
Chapter 9: Body - Applying the Skin
Now comes the fun part. After countless hours getting the chassis built and the body skeleton fabricated, it's time to start seeing what the car is going to really look like as we begin hanging the sheet metal skin.
|The skinning process takes place in two stages. In the first stage, all of the large flat body areas are covered with flat panels. In the second stage, all of the curved areas, connecting one flat panel to another, are fabricated. 18-gauge sheet metal is used throughout the fabrication, and we begin with two panels which will cover the front 3/4 of the roof. These panels are crowned at the center, where they join over a raised center rib member of the roof skeleton (Photo 9-1). Each panel is bent to form a curve along the front edge, to create a smooth transition from the roof to the front of the windshield frame. This bend is made by clamping the sheet metal to the work table under a length of 1" black pipe (Photo 9-2). A heavy metal hammer, also shown in Photo 9-2, is used to tap the metal around the pipe.|
|Photo 9-3 shows the front bend in the sheet metal and the two panels clamped to the skeleton as they are tack welded around the outer edge and down the center seam. With the major portion of the roof skin in place, the contour of the "crown" is established, and we can now duck to the underside of the roof and install additional 1x1 roof supports (Photo 9-4). If these supports are installed earlier, before the skin is in place, there is a good chance they might get positioned at the wrong height...either too high or too low. With the sheet metal panels in place, it is a simple matter to follow the contour of the crown as it tapers left-to-right and front-to-back, and position the roof supports correctly.|
|At the rear of the roof is a major curve, which transitions from the roof to the back skeleton. The sheet metal is cut to the correct length and width so that it will butt up to the two existing roof panels, and span the main corner supports of the skeleton. It is butted to the existing roof panels and held in place using a length of square tubing and clamps (Arrow "A" in Photo 9-5). A second length of square tubing (arrow "B" in Photo 9-5) is laid on the sheet metal 8"-12" from the front edge of the panel. Clamps are attached to this tubing and the skeleton sections below, and the tightening/bending process is begun as the sheet metal is drawn down to the curved skeleton sections. The tubing allows us to bend the sheet metal uniformly across the entire width of the panel. Without the tubing, the sheet metal can bow or become deformed if we try to bend just one small section at a time. By moving back and forth between the two end clamps, tightening each as we go, the metal can be curved in a uniform manner to minimize kinks and potential oil canning.|
|Photo 9-6 shows the next step in the bend's progress. As Tube "B" from our prior photo is drawn firmly against the skeleton supports, a third tube, "C", is added and clamps are positioned so that the balance of the sheet metal can be drawn toward the skeleton supports. Also note that clamps "D" are utilized to keep the sheet metal firmly against the skeleton supports once it has been drawn tight around the curve. Photo 9-7 shows the sheet metal fully formed around the skeleton supports and being held in place for tack welding. Note that wherever possible, the sheet metal is tack welded to the skeleton from the inside (Photo 9-8). If the panels are tack welded on the outside along the edge of the panel, it would make it very difficult to fit the next panel flush up against the first. So, try to keep abutting panel edges free of welds.|
|The next panel to go on is the lower section at the rear of the car. The panel is cut roughly to shape and then clamped firmly to the car (Photo 9-9). From the inside of the car, it is marked where it will fit around the driveshaft tube and the panel is cut to final shape (Photo 9-10).|
|The panel is once again clamped firmly to the skeleton (Photo 9-11) and welded in place (Photo 9-12). Note that the area between the lower panel and the upper curve has purposely been left open. This is the area where the hatch door will be located.|
|The door skins are made by clamping a blank of sheet metal over the door skeleton and tracing around the perimeter and window opening. The metal is then cut as shown in Photo 9-13. The panel is firmly clamped to the door skeleton (Photo 9-14), and tack welded around the edges (Photo 9-15).|
|With the clamps removed, the doors are welded up around the outside and around the window openings and the welds are ground smooth (Photo 9-16).|
|Next, the large side panels of the car are cut and installed. To create a side panel, the builder can either create a cardboard pattern or, as was done here, cut sheet metal roughly to shape, and then clamp it to the skeleton to draw the cut lines. Photo 9-17 shows the side panel cut to shape.
The side panel will abut the door, so the crowns in their sheet metal must match. If the contour of the crown at the door edge does not match the contour of the crown along the door jamb, it will be very obvious to the naked eye when the door is in the closed position.To crown the side panel, a 3/8" spacer (see arrow) is tack welded to the door jamb at the same height as the 3/8" spacer located on the rear vertical of the door (Photo 9-18).
|The side panel is then clamped firmly on all sides EXCEPT the door jamb side (Photo 9-19), and welded in place (Photo 9-20).|
To make sure the contour of the side panel crown will match the contour of the door crown, the door is installed, and a length of 5" wide, 1/8" flat stock is clamped to the door (Photo 9-21). Then, from the inside of the car, the side panel sheet metal is pushed firmly against the 1/8" flat stock, and small tabs are tack welded from the skeleton to the sheet metal, to hold the sheet metal in position.
When the 1/8" flat stock is removed, the crown contour of the side panel should match perfectly with the crown contour along the edge of the door (Photo 9-22). The void between the sheet metal and the jamb is then filled with a combination of sheet metal strips and welding (Photo 9-23).
|As noted earlier, the operating rear hatch will be replaced later in the build process. But for those who might be interested, Photo 9-24 shows the hatch after being skinned with sheet metal, and Photo 9-25 shows the hatch hung in place.|
The firewall sheet metal is marked and cut to shape using a cardboard pattern. (Photo 9-26) The panel is then firmly clamped in place (Photo 9-27) and welded (Photo 9-28).
|The cowl gets a bit trickier than the prior panels, particularly because of the cowl corners, which require some mathematics. Most cowls from the 20's and 30's have a corner curve, which is uneven from the front to the back. That is, the radius of the corner curve is larger on the firewall end than the radius on the windshield (cockpit) end.
To build the cowl, one must first decide the radius of these two curves. For this car, the radius of the curve at the firewall end is 4" and the radius on the windshield end is 3". These curve radii are a matter of personal taste, and they can even be identical, front-to-back, if the builder wishes.To begin building the cowl, we need a short length of 1/8" thick x 1/2" wide flat stock steel to be used as a corner-curve-support at the windshield end of the curve (Photo 9-29). The length of this support is determined by the radius of your curve. The support is going to look like exactly one quarter of a full circle. Thus, the length of the support will be one quarter of the circumference of a circle having the radius you have chosen.
Here is a quickie formula to calculate the length of the support, where L=length and R=the radius of the curve. 3.14 is the value used for pi.
L= (3.14 (2 x R))/4
Or in longer hand: Length equals pi times twice the radius divided by 4.
For this particular car, having a curve radius of 3", the calculation looks like this:
(Sorry if the math may seem a little drawn out, I just want to make sure folks can follow it and use it.)
The curve support piece is cut to length and bent around 6" PVC pipe to form one quarter of a circle with a 3" radius. The support is then positioned and welded to the windshield post at the windshield end of the cowl (Photo 9-29).
Next, a pattern for cutting the sheet metal is made using posterboard or heavy paper stock. Cut the pattern paper 12" wide and draw a line down the center of the paper on both the top and bottom of the paper. Also, mark the center of the firewall curve, and the center of the curve support just welded to the windshield post. (Note that the accompanying pictures of the pattern process, 9-30 through 9-34, were taken during a prior cowl fabrication. The body shown in the pictures will appear slightly different than this body, but the process is exactly the same.)The pattern paper centerline is matched to the centers marked on the two curves, and the pattern is cut and trimmed to fit tightly up against the windshield post. Once it has been trimmed, it is taped in place (Photo 9-30). Note also in this picture the center mark "A" for the curve support and marks "B" noted on the pattern, representing the two ends of the curve support. The firewall end of the pattern paper is allowed to extend out over the firewall hoop (Photo 9-31).
|Holding the paper so that it can't move, a line is drawn on the underside of the paper following the outer edge of the firewall hoop. When you pull the pattern paper off, the underside should look like Photo 9-32. The pattern can then be trimmed (Photo 9-33), transferred to sheet metal, and the metal cut out and marked with a centerline (Photo 9-34).|
|The panel is then formed into the rough shape of the curve by bending it over 5" PVC pipe using a series of clamps. Once it is roughly shaped, it is positioned on the skeleton with the centerline on the panel lined up with center points of the two end curves (arrows), which were previously marked. The panel is bent to its final form around the two curves using clamps, and, if necessary, a bit of coaxing with a teardrop mallet (Photo 9-35).|
|The panel is then tack welded in place (Photo 9-36).
The cowl's lower side piece is fabricated in conjunction with a small strip of sheet metal, which transitions from the front of the door opening to the front of the windshield post. This transition piece will follow the shape of the door crown contour on the door side, and transition to the straight contour of the windshield post on the engine side. This transition piece also extends up to the roof to cover the side of the windshield post.With the door in place, a small 3/8" spacer is tack welded at the edge of the front door jamb to match the highest point in the crown of the door. The transition piece (arrow) is then clamped in place over the spacer as shown in Photo 9-37. Additional spacers are then added above and below the first, to ensure the contour of the transition piece matches the contour of the door crown at the front edge of the door. The transition piece is tack welded in place.
Next, the side panel is cut to shape and tack welded to the firewall hoop at the front, and to the transition piece at the rear (Photo 9-38). The voids along the jamb edge are filled and welded (Photo 9-39). The final piece of the cowl is the top section. This section is made by first creating a posterboard pattern. The pattern outline is transferred to sheet metal, cut and tack welded in place (Photo 9-40).
Curves and corners
Shaping with stumps
|As the sheet metal fabrication moves into the second phase where complex curves and corners need to be formed, there are two tools to consider adding to your shop. The first tool is not only very helpful, it is dirt cheap: the wooden stump.
As was noted in Chapter 6 (Introduction to Scratch Building) stumps have been a mainstay of sheet metal fabrication throughout much of the automobile's history. This was particularly true in European countries, and remains true today in some coachbuilding shops.Two different stumps were cut and used for this project (Photo 9-41). Both are approximately 36" tall, which is a comfortable workbench height. The stumps were cut from 18"-20" diameter oak tree trunks, and they are debarked.
After trimming both ends to get the stumps sitting flat and level, a chainsaw is used to carve various shapes into the top of the stumps. The shapes are then smoothed using a 3" portable plane and a 7" angle grinder with coarse sandpaper. The shaping of the stumps is not to create an exact "buck" to shape the metal over, but rather to create a few universal shapes which can then be used to form the metal in many different ways.
The stump shapes used for this project are shown in Photo 9-42 and 9-43. Photo 9-44 shows the author getting in a little practice with the stumps, and photo 9-45 shows some shaping being done with a stump and a teardrop mallet.
|An English wheel was purchased to try out with this project. This is a relatively inexpensive unit from Harbor Freight. As shown in the photo, there are a number of modifications that need to be made to an entry-level wheel such as this, including some significant reinforcement of the frame to eliminate flexing (Photo 9-46). The English wheel is handy, and does speed up the shaping work. However, it is not absolutely essential. All of the cars shown in the Chapter 7 galleries were fabricated without a single builder using an English wheel. Obviously, the more professional shops rely heavily on their English wheels to reduce labor time. But for the first-time scratch builder, there are many other tools that might be considered more essential for the task.|
Top corner curve
|The first curve to be tackled is the top corner curve, which transitions from the side of the body to the top of the roof. This is the area shown at the arrow in Photo 9-47. The length for the curve can be measured easily on the skeleton, but the width must be calculated using the formula shown above. The curve covers one quarter of a full circle. So, the width of the work piece is determined by calculating one quarter of the circumference of a circle having a radius of 3". In this case, 4 3/4" (rounded). Once cut, the work piece is clamped to 5" PVC pipe, by laying 1x1 square tubing down the centerline of the workpiece, as shown in Photo 9-48. The metal is then forced around the curve by laying an 18" block of 2x4 on the sheet metal, and hammering up and down the length of sheet metal until it wraps fairly closely around the PVC. Note that the 5" PVC pipe is a circle with a 2 1/2" radius, while the curve we want has a 3 inch radius. This works to our advantage, however, since the metal retains a good bit of its "memory", and even when hammered around this smaller cylinder will spring back to a slightly larger size.|
After the workpiece is roughly shaped around the PVC pipe, we remove it and make use of 6" diameter well casing to do our final forming. The well casing has exactly the radius we are creating (3") and the workpiece is hammered to conform with the inside surface of the well casing using a plastic teardrop mallet, as shown in Photo 9-49. The resulting curve is shown in Photo 9-50. The curve section is then clamped to the body and tack welded in place (Photo 9-51).
Rear corner curve and cap
|This particular curve on a scratch-built body is probably the most daunting challenge to the amateur coachbuilder, and the one curve that convinces most novices they could never build their own steel body. It's the compound-complex curve running up the back corner of the body, and finishing off with a cap at the top. It transitions from the side panel to the rear panel, and also transitions to the roof of the car (Photo 9-52).
The easiest approach for the novice is to break down these difficult curves into many smaller curves, shaping and forming each section one at a time. It is far more time-consuming than how a professional shop would approach such a task, but it is a much easier technique for the amateur to master.This particular curve is started with a couple of the primary sections (Photo 9-53). The metal is shaped using a combination of PVC pipe, stumps, a beater bag, hammer and dolly, and on this project, an English wheel. (The English wheel speeds up the process but the task can be accomplished without it.)
The import thing is to do one piece at a time, tack weld it in place, and then move on to the next piece. This way, the shape at the edge of the second piece will match up with the contour along the edge of the first piece.
Also, as the pieces progress, it is sometimes helpful to first cut a pattern for the next piece using posterboard. This will give you the general size of the piece, and cut down on the amount of edge trimming that must be done to ensure the abutting pieces fit together without any large gaps. Don't make a great effort to cut the pieces to the correct shape right at the beginning; it's much more efficient to cut to a rough shape and then do a good deal of your bending and shaping.
The bending, hammering and shaping will compress and flatten the metal, altering the outside dimensions of the workpiece. If you have ever flattened pizza dough or pie crust with a rolling pin, you understand the effect. As you push down the center of the dough, the outside spreads out farther and farther in the pan. The same thing happens with metal, although to a much more limited degree; as the metal is worked, the outside dimensions will change. The pieces will need to be trimmed or ground along the edges to allow them to fit tightly against the abutting sections.
|Note also that the individual sections are only minimally tacked in place. Now and then, a piece may need to be removed and redone if the overall curve is not forming up correctly. You need enough tack welds to prevent any movement, but not so many that it makes removal difficult in the event of an error. As the sections progress upwards toward the cap, the pieces will need to become smaller and more pie-shaped. The pieces will also require more shaping work to ensure that they fit properly and maintain the correct curvature (Photo 9-54). Here, posterboard patterns become almost essential. The final cap pieces (Photo 9-55) will require a good deal of shaping work with the stumps and beater bag, and many trial mock-ups to check the fit and shape. These final pieces will also require a good deal of edge trimming as the metal is stretched and formed. Don't expect your first attempt to survive; it may take two or three cap pieces before you get it right.|
|With the difficult cap pieces in place, the lower segment of the rear curve is finished off by fabricating more individual sections (Photo 9-56). Then, it is on to completing the other rear corner of the body (Photo 9-57). Make no mistake, this is a slow and tedious process. Each of these rear corners took approximately 20-24 hours to complete, and they still need to be welded and ground smooth. Don't become discouraged if your progress is slow, and plan your project work accordingly.|
|There will be a visor over the windshield, but before that can be fabricated, the electric windshield wiper must be installed to ensure the visor will not create any clearance problems. A wiper is being installed only on the driver's side. This happens to be a Mr. Roadster single-speed wiper with the automatic park feature (Photo 9-58). After determining the best wiper mounting location, a hole is drilled through the top section of the windshield frame (Photo 9-59).|
|The wiper motor is bolted in place (Photo 9-60), and the wiper arm is attached (Photo 9-61).|
|The main panel of the visor is cut roughly to shape, and is mocked up using some temporary supports and clamps (Photo 9-62). The underside of the panel is checked to make sure there is clearance for the wiper arm to operate (Photo 9-63).|
|The most difficult part of the visor fabrication is designing a corner piece that will smoothly transition from the body to the visor (Photo 9-64). After trying out a number of paper patterns, the final piece is cut and shaped (Photo 9-65).|
|The piece is then tack welded in place with a similar piece fabricated and welded at the opposite end of the visor. The front panel is temporarily put in place, and the outline of the corner piece is marked on the back side of the panel (Photo 9-66). The front panel is then cut and temporarily installed on the car, so that a 1/8" x 1" piece of flat stock can be welded along the bottom inside edge of the visor to support it across the entire width of the windshield (Photo 9-67).|
Body skin completed
Photos 9-68 through 9-73 show the completed sheet metal work prior to final welding of all the seams and the beginning of the paint preparation work. Note that the grill shell shown in these shots is an aftermarket unit and was not fabricated by the builder.
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