The mark 1 assembly line. At the right-hand end is the advancer, ready to push a pair of wheel bricks along the track (its rack is the grey object sticking out from the model to the right); in the centre is the brick positioner, loaded with a plate; and at the left-hand end is the press.
This was the original assembly line I made. Like almost any first version, it was bulky and inefficient. I did at that stage possess the RCX, but did not use it in the control of this model - instead I used the Technic Control Centre, a 9V battery box with the capacity to control 3 independent motors in either direction and store a preprogrammed sequence of moves. The assembly line was designed to build small Lego cars, consisting of two 2x2 thin plates of the sort with attached small wheels held together by a 2x6 thin plate - see the photo below.
The product of the mark 1 & 2 assembly lines.
The assembly line consisted of three mechanisms: the advancer, the brick positioner and the press.
The advancer moved the partially-constructed car through the assembly line along a track of smooth (studless) thin plates 2 studs wide, on which the wheel plates stayed due to the wheels on either side of them. In front of and behind the wheels was a spacer consisting of a 1x4 smooth thin plate with a 1x1 thick brick at either end. It was simply a Lego saddle resting on the same track as the wheels driven by a motor through a gearbox and rack and pinion. The retracted rack of the advancer can be seen sticking out of the right-hand end of the assembly line in the overview photo.
The brick positioner laid the 2x6 thin plate on top of the wheel bricks, ready to be squeezed together by the press. It contained two motors; one, geared down a lot, lifted up a 2x4 section of the track on which the wheels would be resting at the time, while the other, not geared down very much and with an O-ring as the first step in the drive train (so that when it reached its limit it would stop without damaging any of the rest of the structure), moved a pair of forks supporting the 2x6 plate over the rising wheels. When the wheels reached the level of the plate they rose a little further, lifting the plate off the forks; both motors were then run in the opposite direction to before, which quickly retracted the forks before the wheels had got down to that level. Once the moving section of track had descended to the level of the rest of the assembly line, the process could continue. The brick positioner is the central unit in the overview photo.
The press performed the last act of the assembly process, that of squeezing the parts of the product together. I encountered great difficulties in trying to make a mechanism that would exert sufficient force on the bricks to force them together in a way comparable to that achieved by hand. I tried a simple rack and pinion mechanism and then a pneumatic one, alone and in combination with levers to increase the force, but nothing was strong enough. Then my father suggested a screw press using two sets of Lego worm gears and racks; this did exert enough force, but needed quite a large gearbox for a motor to drive it and a lot of bracing to prevent it from forcing itself apart. Unfortunately, despite my best efforts even in the end it almost constantly broke: in order to have teeth meshing on both sides of each screw I had to stagger one set of racks on each side by half a dot and reverse the direction of the dots; I used the half-dot properties of Technic girders and the small 1x1 thin plates with a double-sided dot sticking out from them, usually used for mounting headlights on cars, to reverse the dot direction, but the resulting structure was rather weak. This was one of many flaws I fixed in the mark 2 version. The press can be seen on the left hand side of the overview photo.
Once the operator had successfully made a car, the advancer would be retracted and a new set of components would be loaded.