Of Slots and Shims

Of Slots and Shims (The History of the Humble Hinge) by Professor Gerry Freed

The evolution, this century, of the automobile as a means of transport is an exciting visual and practical experience for all car buffs. Less obvious, perhaps has been the changes in manufacturing technique which in their way have had a much more profound impact. This has not only been in the transformation from horseless carriage to MPV but more on the social outcomes of capital, labour and skill deployment in their production, servicing and repair Its effect on collectors is also apparent as the industry globally is building fixed life, throw away products which are ill suited to become 'classics'.
Citroen products have been a good platform on which to observe those changes. The models were replaced so rarely and when they were, they were leaders in their technology. The distance between each one was so long in time and technology that the changes stand out dramatically. An exciting story is revealed by zooming in close and focusing on the evolution of the humble door hinge.
Looking at the period of the Model A, the early post W.W.I era, the door and the frame in which it fitted were made of wood, not a material of great precision. It was at a time when repetitive moulding and fitting of timber was practiced but not considered appropriate to auto construction. It was accepted that assembly of a mechanical device was more a role for individual craftsmen with the skill to fashion and fit components together.
The door was fitted to the frame by hand planing and sanding. The hinge, a repetitively produced metal component of relatively stable tolerances came with holes pre drilled. The door was aligned in the frame and marked and drilled to take tapered screws. Those tapers averaged out any drilling misalignment. The frame was similarly drilled and the door screwed in position. A few trial swings and if the hinges were not swinging freely or the door mis-aligned, shims were used or the holes stretched with a file. Eighty years later we can, at a high cost, repair and reproduce these assemblies. The capital investment was negligible and the cost now is in the value of craftsman skills, which were once cheap. Of course, no two doors or sets of hinge screw holes are interchangeable without refitting - a feature revisited much later with the fibreglass Bijou.
The reason for going to all this trouble, is simply that the door has an infinite number of positions it can occupy within the frame, only a small range of which offer the symmetry to the eye and the functionality of closing smoothly and snugly. The task of the artisan, completed by defining the 12 hinge screw hole locations, is to constrain the door to fit within the acceptable range. The measurements, the execution and the judgment of success were all done, door by door, by the craftsman as the work progressed. If you want to build a lot of cars that way, you need large numbers of skilled people. They need the training and the factory needs the metrology to calibrate the acceptance levels of each individual eye to a repeatable standard.
'Near enough is good enough' means different things to different people.
Production of a growing range of consumer products by these methods, exploded the numbers in the community that acquired these craft skills and Fordism ensured they were exploited to the full and beyond.

You see the next generation appear in the Traction and perfected in the DS series. By then the components were metal and pressed or prefabricated. Those component production processes were not accurate enough to allow interchangeability of body parts without some compromise. The eye can spot a misalignment of a front door, as seen by the shape and width of the rear door front seal, that is exposed as a tapered black stripe. A skew, top to bottom, of 2-3 mm's is discernible, as is a similar difference in the gaps at the front and rear of the door. The tolerances in pressing and in assembly of complex structures, was simply not good enough for any door to hang on any frame with that accuracy. The technical solution was simple, elegant and very expensive. The hinge fixing points were made floating by holding a pre threaded fixing block in a loose bracket, finally to be located and held by the friction of tightened bolts. Fixing holes in the hinge were stamped as slots. Overlapping parts had slots at right angles where possible. The parts were then held in position so that the overlapping slots left a roundish hole through which a bolt and washers could be fitted and clamped. Similarly the fixing blocks were moved to take up the tolerances in the pressed components and clamped tight. So the door hinges could then be mass produced, as were the door and its frame. To get them all to fit so as to secure and swing the door acceptably, was again a challenge to reduce the infinite ambiguity of the door - frame location by human intervention. Armed with spacers or shims to adjust height, slotted bolt holes and floating mounting blocks any operator could fix any door, on any frame. The judgment of the operator was minimised by the use of pre set jigs that located the door acceptably before the operator secured them (not available for us restorers).
The process was capital intensive to machine and press the parts and was also labour intensive to fit them all and to carry out the alignment. As the DS was complex, this labour usage was unaffordable because it appeared in 1955 just at the time when semi-skilled labour costs were soaring. All this was reflected in that model's fundamental inability to earn profits. Assembly remained a difficult process to carry out within a fixed time on a transfer line, again adding to operating costs. For restorers, the technique is less expensive to replicate then its predecessor. Spares exist or can be fabricated to broad tolerances. The jigs may not exist but the process can be carried out with patience and a good eye. No special craftsmanship is called for.

The present manufacturing generation began with the BX. Machines were introduced with built in continuous measurements of the part being worked, so that tool wear was compensated. The machines stopped only as parts moved towards the limits of permitted tolerance. Body and door shells were robot welded, with the components and the welds all computer located to a precision that allows complete interchangeability, without absorbing tolerances by manual assembly. This meant that the two components of the door hinge could be welded to the door and frames and assembled by the insertion of a pin. Once this level of repetitive accuracy was achieved, no labour was needed but a great deal of capital was required for the component manufacture to tight and repeatable tolerances, for the welding jigs and robots and the now much simplified and faster transfer lines. Tolerances on door alignment of 0.5mm are now industry standard, without human intervention or perhaps because there is none. During the 1970's as the Japanese applied these techniques, Honda could be seen to have overtaken Rolls Royce in build quality, as they still used the first generation methods. Each and every Accord could be assembled by machine to a closer door fit than a coach builder's craftsman could guarantee, producing cars one at a time, at much higher costs.

Now, as the GM Chief Executive said recently, "factories build cars, not people". PSA, while operating in France, have to meet social obligations and to provide employment in both quality and quantity. This leaves them in a catch-up mode on build accuracy and repeatability relative to manufacturers in countries like Korea, where new plants can be invested in for each model, using fewer and fewer staff.

The unfortunate by-product for us is that craftsmanship and patience are not enough to repair a vehicle built this way. A new door will only fit, if the frame meets the original tolerances. They can only be measured and re-established with manufacturer supplied data, jigs and tools. Gone also is the use of a generic mild steel. The panels used are of specialised heat treated and rolled steel alloys which create the pressing accuracy and panel strength, but which cannot be repaired by simple manual welding and heating without losing the intended performance. Inevitably, while the model is current, the manufacturer will have nominated centres will have the tools and investment to repair close to factory design data. Once the model's use by date has passed the capability is not retained and so the cost of repair becomes unacceptable relative to its replacement value - a damaged door will create a scrapped car. In small remote markets like Australia the investment in that degree of product support is hard to justify. Those that have repaired BX rust around the door hinges will appreciate the challenge - just try a Xantia!

Will enough of these contemporary cars survive long enough for some of them to acquire so high a collector's value, that long term repair and re-manufacture costs are acceptable?

©gerry freed 1996,1997,2004

Last updated Mercredi 18 Fevrier 2004

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