Panelling a TC ash frame – Part 2

2 May

In Part 1 of his article (see Issue 35) Bob Lyell started on the front quarter panels to (as he put it) “gain experience and confidence”. Part 2 sees him moving to the rear quarter panels. Over to Bob……………

With the front quarter panels successfully completed, it was time to tackle the rear ones and whilst the same metal working principles applied, achieving the more pronounced curves would prove significantly more challenging, but still possible.

However before getting started I took the opportunity to trial fit each rear wing to check for gaps between it and the wheel arch cut out in the ash frame, particularly in the rear corner where the body curves over the crown of the wing to follow its inner face. So visible and attractive on the finished car the best possible fit must be achieved now either by sanding to remove high spots or gap filling by adding small strips of wood, wing piping will only disguise so much.

Checking for rear wing to tub fit.

Actually I went so far as to fully pre-fit and drill the holes for the wing securing screws because it was so much easier to clamp, adjust and align the wing without the inner wheel arch or outer skin in place. I figured I would be able to find the holes again.

Continuing without an inner wheel arch, for ease of clamping, I lightly block sanded the outer faces to check for lumps or hollows (there were none) and to get a feel for the shape. Then I made a paper pattern about 1 inch oversize which for my body measured exactly 1 metre from front to back, allowing the blank to be cut from the width of an Aluminium sheet whilst leaving the wheel arch cut out to provide material for a door skin.

Next I jig sawed another clamping strip from 9mm plywood, shaped to exactly follow the top edge of the flat vertical face from the top hinge to the side screen slot. Once satisfied that the Aluminium was correctly positioned I clamped it in place and added 2 location screws in excess material. One just below the bottom door hinge, the other, to locate the top of the panel, into a wooden block screwed into an existing hole, drilled for a hood tacking fixing screw. These screws would allow the panel to be located now whilst flat and exactly repositioned throughout most of the panel forming process. Being in excess material, the holes would disappear when the panel edges were trimmed back to their finished size.

Panel location screws; additional block for upper/rear one and lower/front location screw in place.

The final preparatory job was to make and fit a tubular steel cross brace between the tip of the bull horn where it could share the 2 mounting screws and the opposite body iron corner below the door hinge, secured with a G clamp. Without it the ash frame flexed when I hammered against it.

I decided to start with the complex shaped top edge between the door aperture and the side screen slot and to anneal the Aluminium before striking the first blow. At least by attempting the most difficult part first, if I failed I would not be throwing away any other hard work. I roughly cut out the side screen slot to split the gentle curve into 2 more manageable lengths and then worked forwards each time along the curved top which wants to roll straight, through the external corner which required a lot of shrinking, then down the easy straight slope to finish through the tight internal corner with much stretching.

I found it very important (and difficult) to identify exactly where the flat face ended and the curved edge started so I could clamp the plywood with its edge no more than a couple of millimetres back. This prevented the Aluminium from bulging back out behind the hammer blows whist trying to work the metal forwards. As the shape was progressively formed the Aluminium had to be worked to fit tight against the Ash, so the first run of blows with a plastic headed mallet had to be as close as possible to edge of the plywood, not easy. I found that it helped to also pull the panel edge over with a mole grip whilst striking and to listen for the reassuringly hard sound of striking against wood as opposed to a hollow sound when it is not touching.

From the side screen cut out to the external corner it was easy to roll the metal over, however like a rolled up paper it wanted to form a straight line and lift away from the gently curved top edge of the wood at each end. I resorted to clamping both ends down hard with soft faced G clamps in order to achieve a tight fit which forced the centre portion to ripple. After annealing, these were removed by shrinking with a slapping tool before working the metal over the sharp edge and down the inner face, with more shrinking. This finished profile was then sufficiently rigid to hold its shape without springing back.

Forcing the panel to follow the curve of the Ash.

For the external corner, by progressively striking from the start of the curved edge I was able to force the panel to gradually curve hard against the wood without a push back bulge forming. Due to the amount of aluminium to be shrunk, the small ripples formed by striking out from the clamping strip needed to be encouraged to grow by simultaneously working them from the open edge into folds with round nosed pliers or a similar tool. It became a continuous process of forming ripples and then shrinking them back with frequent annealing because this amount of manipulation quickly work hardened the metal. Trimming the metal back as soon as I could determine where the finished edge would be, also helped by simply reducing the volume. On one side I greedily formed a ripple so large that I could neither shrink it nor re-form it into two smaller ones. The only way out was to lose it by cutting out a wedge of material and forming a butt joint for welding after the panel was finally secured in place, an annoying but fortunately not disastrous mistake.

Butt joint ready for welding.

Fortunately, the next part, the edge down to the internal corner is straight, making the job much easier. For the internal corner my technique was simply to keep the metal soft by frequent annealing, to not be greedy and to keep checking that the Aluminium really is hard against the Ash, as it is so easy to fool yourself and bridge the corner. Again, trimming back as early as possible helped. I didn’t worry about the rebate at this stage, I just left sufficient material.

To complete the sharp fold down the inner face I annealed the panel and clamped it down tight with G clamps and small strips of wood along the top edge and worked it over with a plastic mallet. The additional stretching next to the internal corner presents a high risk of splitting because the Aluminium is now getting very thin and any cut or nick in the edge can quickly propagate into a split.

For the rebate I decided to intentionally make a (junior hacksaw) cut in the Aluminium to leave a flap of metal, form the rebate and then fold the flap back over to make a butt joint around its edge ready for welding, I just couldn’t see any other way. To my surprise, persuading the Aluminium to follow the rebate proved easy after annealing by using a homemade hardwood (Lignum Vitae) tool shaped like a chisel but with a soft tip and by using slightly heavier hammer blows.

Working the Aluminium into the rebate before cutting and folding over a flap to finish.

To finish this area, I formed the vertical fold between the hinges and was pleased to find that the panel could still be removed.

Back to the top rail, from the side screen cut out rearwards to the sharp square step (although I have now seen bodies without this feature) was a combination of serious panel clamping against the side, soft faced G clamps on top and my new hardwood chisel tool to work the metal into the corner of the step. Afraid of wrecking the panel and all of my hard work and being committed to welding for other reasons I had considered making another saw cut and filling the gap by welding in a small triangle of metal, but I decided to be brave and go for it.

The rear corner’s simple curve could be pulled around by hand before clamping it tight against the flat rear plywood panel. By using the cut edge of the Aluminium to draw a pencil line on the plywood it was easy to measure back to the edge of the rebate and allowing 1/8 inch for a flange mark where to cut and fold. As I was still able to remove the panel I formed the corner in my hand operated folder.

Cut, folded and a snug fit in the rebate.

To form the curved top fold which goes under the hood tacking corner I first made a simple clamping strip from 25mm x 4mm mild steel. After annealing the Aluminium this strip was aligned with the top edge, the temporary panel locating block removed and the metal progressively worked over to form ripples which could be shrunk back to leave a smooth face for the tacking corner to sit on.

After the first run of hammer blows it looks scary, but it will shrink down flat.

I had now reached my last opportunity to remove the panel, so it was a case of marking the final cut edge around the wheel arch, down the back panel and across the bottom door hinge. Removed and cut to final size, the order of assembly was inner wheel arch, front quarter panel, rear quarter panel and finish its return edges. Then mark out and drill for nails and try to rediscover the wing securing screw holes.

With both rear quarter panels secured in place I was able to finish on an easy note by just measuring the panel edge to panel edge gap to determine where to guillotine and fold the flat rear panel (exactly as described in Sherrell’s book) to give a tight edge to edge fit with no gaps or visible fixings, but noting that the top fold is less than 90 degrees.

Edge to edge. Clean, tight, flush and ready for nailing.

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