Panelling a TC ash frame – Part 1

Purchased as much for the challenge and hopefully the satisfaction of its restoration, I always intended to panel the body tub and doors of TC 2628 myself.

My starting point was a new Ash frame, carefully aligned and bolted to a, by now, accurate chassis sitting in a reasonably well equipped workshop, including a 3 foot hand operated folder. I had read Michael Sherrell’s book, watched various helpful videos on YouTube and had some experience, having previously panelled a Caterham Seven, but that was 20 years ago.

Believing that sheet metal can only be cut, bent, stretched or shrunk, I decided to start with the front quarter panel to provide a gentle introduction to all four disciplines whilst I gained experience and confidence. Of course, being a relatively small panel, if it went wrong I could always scrap it and start again; fortunately I wouldn’t need to. My chosen material was Aluminium half hard grade 1.2mm thick. I purchased 2 sheets of 1m x 2m, sufficient for the whole job.

First I inspected the Ash frame, running a straight edge along the outer face of each timber, looking for any bumps or hollows; if there had been any bumps I would have removed them by hand block sanding. One small hollow was identified but not filled, instead it would be dealt with in the metal. I lightly sand anyway because it helps me to understand the shape and curves that I will need to persuade the aluminium to follow. I also filled the 2 rebates for the rubber door buffers with hard wood blocks to provide a continuous clean sharp edge.

The doors were hung and the gap measured, I had a consistent 6mm all round, which sounds generous but 2 layers of tight fitting Aluminium and a finisher would reduce it to a snug 2.6mm.

A paper pattern about 1 inch oversize, a little more where it goes under the scuttle and above the lower door hinge was used to cut out an Aluminium blank, using a nibbler to prevent any distortion.

Commencing with the vertical bulkhead fold, I decided to form this off the car as I considered the relatively thin and poorly supported Ash rail insufficiently rigid for use as a former. As the bulkhead front face is flat and the outer face close to a constant radius the resulting fold when curved should match the corner of the Ash.

I measured the angle with an adjustable square (much less than 90 degrees) placed the panel in the folder 1 inch from the edge and pulled it to the same angle, having now introduced stiffness to the panel, I marked a line a generous 11/16 inch from the outer edge of the fold and trimmed off the excess.

My technique is to use an old pair of Gilbow shears; aviation snips are fine for roughing out but I do not like the marks which the blade serrations leave. 11/16 was to ensure that the heads of the securing pins would be hidden by the second Ash frame when rebated and screwed to the front face.

Photo 01 – showing the rippling of the folded front face, later to be removed by annealing and slapping, as described in later text and shown in photo 03.

With the folded edge clamped to the front face in the middle I gradually pulled the panel with G clamps against a strip of 50mm x 9mm plywood on the outside until it touched the curved outer face from top to bottom. The Ash, checked by measurement, did not move; the inner corner of the Aluminium touched the Ash from top to bottom and the folded front face gently rippled.

My decision not to buy a shrinker based on high cost for little use was justified.

Before removing the clamps I also secured the panel to the Ash frame with 2 small wood screws so that upon each refit it would be in exactly the same place and during work it could not move; both are vital. One was through the excess material along the top edge under the scuttle screwing into the screen mount; the second in the excess material above the bottom door hinge screwing into the outer face of the vertical hinge rail.

Photo 02 – showing one of the small wood screws used to secure the panel to the ash frame.

The ripples were removed by annealing and slapping. Folding and rippling the edge has work hardened it, annealing by warming and allowing it to cool naturally will soften it back to its original condition; this can be repeated many times. Of course, insufficient temperature will have no effect and sadly too much will melt your hard work.

Removed from the Ash frame for fear of damaging the wood I apply a thin wipe of liquid soap and heat with the very tip of a welding torch flame so the (number 2) nozzle is about 4 inches from the metal. Heat until the soap turns brown, but only the area to be worked; leaving the folded edge cooler and therefore harder will help prevent it moving.

Once cool, the brown soap residue should be removed with thinners and a lot of rubbing.

Hitting metal against a hard face reduces its thickness causing it to expand, exactly the opposite of what we are trying to achieve. Slapping the peaks of the ripples with a wide flat face tool will compress them into the adjacent “valley” to increase the panel thickness and shrink the metal along its length, exactly what we need.

The Ash rail proved strong enough for this more gentle work, not difficult but there is a technique. The tool, often made from an old file which has been de-tempered, bent and ground smooth is also called a bumper or flipper. There is a certain satisfaction to making your own tools.

Carefully repositioned, the panel must be clamped as before, prior to slapping, to prevent any distortion of the outer face.

Photo 03 – ‘slapping’ to remove the ripples (look back at photo 01).

Next comes the bottom fold, with the panel repositioned using the 2 wood screws I clamped it tight against the substantial bottom rail and marked the corner onto the Aluminium. With the panel removed and laid flat it did not describe a straight line, indicating that it must be formed in situ. I cut the edge 11/16 inch beyond the line, repositioned and clamped the same plywood strip along its entire length as close as possible to the edge of the Ash. Then, avoiding the hollow, used a nylon headed mallet to move it 10/15 degrees just sufficient to reveal the fold edge.

My technique is to strike the Aluminium with the hammer head square to the panel in a glancing blow away from the fold to make the sharpest crease and almost pull the metal across. Simply hitting the edge will cause the outer face to buckle out away from the Ash, a difficult problem to correct.

I then removed the panel and continued the fold across the gap where the hollow occurred by positioning the panel between a flat work surface on its outside face and a 1 inch square steel bar on the inside so its sharp corner sat in the fold line at either end to bridge the gap. Tightly clamped the fold could be continued in the same way and to the same angle as on the car.

Photo 04 – showing panel removed and positioned between a flat work surface on its outside face and a 1 inch square steel bar on the inside.

As before, softened by annealing and replaced, the fold was increased to 45 degrees with the hollow treated in the same way off the car, annealed again and taken to 70 degrees; ripples started to form. A final anneal allowed the fold to be finished with a slapper until the ripples disappeared and the edge was tight against the Ash rail.

Being an early car, the door aperture treatment required only a simple fold, no welding. First step was to mark the edge of the wood onto the panel and cut 5/8 inch beyond it along the 2 straights and the gentle top curve but reduced to 7/16 inch around the 2 tight curves because the wider the fold the more difficult it is to stretch the metal without it splitting. Then I cut plywood templates for clamping the panel around each curve 1/8 inch outside of what would be the fold edge. Forming the fold was a repeat of the previous 4 stages but this time the technique is very different because the metal is being stretched.

Clamped hard and struck just inside the edge of each curved fold with the edge of a nylon hammer head will succeed in starting to shape the corner. The Aluminium will curve because it only stretches where it is hit. Do not be greedy, once the metal starts to move the panel must be removed and annealed, striking the second time a little further in and repeating until the striking point reaches the edge. The 2 straight folds can be treated in the same way as the lower edge fold described above.

For the short section of rear wheel arch I only folded the metal half way because in its finished form it will be folded over the additional thickness of the inner wheel arch.

The two lower external corners were simply finished by cutting, to form a neat edge to edge joint. Where the door aperture finishes at the top, I cut level with the bottom edge of the small additional wood block, with the panel side left sufficiently high to allow the scuttle top to overlap. At the lower door hinge I decided to manage the overlap with the rear panel by making a horizontal cut 1.2mm deep, level with the top of the hinge rebate and blending it out sufficient for the panel edge to sit into it.

Photo 05 – managing the overlap with the rear panel

Finally the door buffer notches were cut out with the panel in place using a junior hacksaw and square file, before marking out and drilling the holes for the panel securing pins.

For corrosion prevention in each area where the Aluminium panel touched the ash, I decided to key the surface with Scotch-Brite and paint with aerosol acid etch primer.

BOB LYELL

Ed’s Note: Thank you Bob for an interesting practical ‘how you did it’ article and I hope that it will be possible to have a follow up.

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