Like starting to drive for the first time I dreaded the hill start, but lived on a hill with a railway line crossing it half way down, with level crossing gates, and according to Murphy's law, nine times out of ten the gate would be closed when I started up the hill. So with car modelling and the metal body, I knew how the originals were made, and my first example for what was then the Montague Motor Museum, called for polished aluminium as the final finish. This was OK with minimal bashing of the metal around a hard wood pattern to stretch the aluminium to make simple one piece shapes for the 1920s Alvis and Vauxhall models in the relatively small one twentieth scale. However, when I moved to the larger one fifteenth scale I turned every which way to come up with a more comprehensive solution. Aluminium was OK but has always been a problem to solder or otherwise fix parts together. I tried aluminium filled resin/fibre glass, hot metal spray into rubber moulds, and electro forming on resin moulds among other methods. All produced reasonable results, but not the all-encompassing answer I was looking for to the problem.
Then, after reading a book on silver smithing, I came across the technique of crimping metal, making a small ‘U’ shape on the edge of the metal sheet with round nosed pliers, and knocking this flat from the out side edge, then to flattening the mound that remained, which would reduce the length of the out side edge by thickening the sheet metal This on annealed (softened) copper, brass and/or aluminium. Up until this time I always thought that if you hit a piece of sheet metal with a hammer, you would make it thinner, so this was a complete revelation, and as it were, the answer to the maiden’s prayer. With the ability to soft or hard solder brass and copper sheet with ease, and to be able to shape it by making it thicker or thinner at will, all things (shapes) now seemed possible. The technique is so simple it seems ridiculous not to have thought of it before.
After you anneal the metal to soften it, almost anything else you do to it will harden it again, in fact if you do nothing to it, it will age harden on it’s own account in time. When you crimp the edge to form a mound, say half an inch into the sheet, then flatten the out side edge with a hammer, the out side edge will form into its self, as the sides of the mound are ‘almost’ vertical (‘almost’ is important, as to make them vertical, is to fold the metal when it is hit which is not wanted) because it is soft and has nowhere else to go. Thus the distance between two points each side of the mound is now shorter by the height of the mound just flattened, or nearly so. If one now hits the mound, it cannot flatten out to it’s original flat form, because the out side edge is now work hardened, so the metal has to work back into it’s self, thus making it thicker.
And so for 30 years I produced more than 200 models of cars of the period up to the mid 1930s using this simple technique, to form bodies and fenders, all of which, had a break at the fire wall. This meant that after the chassis was built, which included the fire wall, the separate components, of body, hood and fenders could be made and fitted to the chassis one at a time. Then came the pontoon fenders of the late 1930s such as the 812 Cord and Bugatti Type 57 SC Atlantic of 1937 and 1938 respectively. The swept fender in general has a cross section form of a gentle angle on the inside with a short rollover from the top to the out side, which can be accommodated, with the aid of much crimping on a single sheet of copper. An example of this are the Duesenberg fenders.
Where the overhang of the out side is more pronounced, as with the fenders on the Bugatti Type 41 Weinberger Royale, they can be made in two sections, by the same method, of crimping the out side edge and silver soldering the two halves together down the relatively unworked centre line. These are long narrow strips of metal with most of the metal working on one end, namely the very front or at the back the rear of the fender.
Pontoon fenders call for a working all round a comparatively short length of metal, and two parts to match exactly for soldering together. Furthermore, if taken as two parts, the forming is not excessive, being only half of the top radius of the fender at most. Stretching metal, rather than compressing it, as in crimping has two advantages, one it takes less time, and two there are no irregularities, dents, to hammer out. Thus formed in my mind the use of matched dies to produce the fender parts.
Having made use of polyester resin, fibre glass, glass cloth and aluminium filler, in my early experiments in building car bodies, and knowing of the strength of these materials, I investigated further, and discovered the wax sheet used in the pattern making industry. This is available in adhesive sheet form, in a number of thicknesses, down to a few thousandths of an inch.
As all body and fender work in sheet metal first needs a hard wood pattern to be made, over which the annealed sheet copper or brass, is then worked with a hammer, I decided to try making female moulds of the hard wood pontoon fenders patterns instead.
This was undertaken by making the patterns in two parts split at the centre line. Each of these was then covered with sheet wax of the same thickness as the sheet metal to be used for the fenders, in this case .020in. (.5mm) thickness. Each of the patterns was then attached with double sided tape to a base plate of melamine chipboard with clearance around the pattern of about a half of an inch. Chipboard walls were then made and attached with masking tape to a height of about half an inch above the top of the pattern, and release agent applied to the inside of the box so formed. Into this was applied a gel coat mix of polyester resin, thexotropic paste and aluminium powder, of a consistency (thickness) not to run down the vertical surfaces of the mould box or sides of the pattern. When this was almost dry (hard but still tacky), glass cloth and chopped strand mat were worked over the surface with more polyester resin, in several coats, to a thickness of about a quarter of an inch. When each of these had almost set, more resin was mixed, together with saw dust and wood chips to fill the box to the top, so making a solid brick. This formed the female die.
When fully set the box was opened and the pattern removed, which left the wax sheet still attached to the inside of the die. A wall of wax sheet was now made around the negative form left by the pattern, to a height of about half an inch. Into this was now put a similar combination of resin gel coat and glass cloth to fill the form, also including a block of hard wood to stand about two inches above the top, so making the male die. The wood block is used to hammer on and for holding in the male die in the vice for working the metal over. When set, the male and female dies are separated, the wax removed, and the two halves cleaned up by removing any surplus resin and glass cloth from around the edges.
In use, a sheet of paper is placed between the dies to give a size and shape for the sheet copper required to form the fender. With the copper sheet cut to this size and then annealed, heated to red heat then quenched in cold water, is placed between the two dies and the male die given a very hard hit with a large hammer, to force the copper into the female form. If it is shallow, one can form the complete shape by a single or several annealings and application of the hammer. If more complex, then the sheet copper will reveal where it is necessary to apply the crimping technique to reduce the waste portion until it will fully form between the dies.
When the form is correct and a perfect fit to the original hard wood pattern, it is put to the pattern and trimmed and filed flat to the centre line. With both halves so made they will be a perfect match and can be soft or hard soldered together to form a complete fender. Which soldering method will depend on what comes next. If the fender is made away from the body to be fitted later, it can be hard soldered. However, if it is to be fitted directly to the body, then it is better to soft solder it, as the flame heat necessary for hard soldering will anneal and distort the sheet bodywork. In this case a thin sheet copper plate can be soldered inside covering the join to give it added strength.
With the 1938 2.9 Alfa Romeo, because it has no visible evidence on the outside of joins in the bodywork, such as at the fire wall, I was able to take this process one stage further, in that the complete body and fender parts were produced by this method and assembled on the master patterns to form a single body unit. This was them fitted out and bolted to the fully detailed chassis. Such are the body shapes of this family of cars, that when the individual parts of the bodies and fenders, are cut and opened up, for doors, wheels, hood and trunk etc., that they would have no way of holding their shape, with out much fitting and making of the frame work. By first making all of the panels and fitting them together on the master pattern, the complex shape is such that each panel gives strength to the next. When the openings are made in the fitting out stage, the shape is intact and the frame work can be added bit by bit.
This method of forming sheet metal between matched dies, is suitable for almost any shape, whether for car bodies, boats, or aircraft parts. Whatever shape that can be carved in wood, can then be reproduced in sheet metal, all with the aid of a spoon, to mix the resin and a hammer to compress the dies.
The more perfect the finish on the master patterns the less work is needed to finish the sheet metal work. My own preference for a finish is to turn to the original used on the cars of this period, i.e. cellulose. This has the advantage that each coat melts into the previous one, as against some of the modern finishes, where each coat is added on top of the last, in which case rubbing down and polishing can show up in rings, should one cut through one coat into the next.
The technique is first an etch primer, essential on copper and brass, followed with a grey primer, then several colour coats, rubbing down between each. This is then left for several days to fully harden and settle down, and finally cut back with a fine car polish to remove the glass gloss of the natural paint finish to a fine polished in depth sheen.
http://web.mac.com/gawingrove/iWeb/Home%20Page/Home%20Page%20.html
