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The framework is straightforward.  The sides are pairs of  24½" angle girders connected by five 5½"x2½" flanged plates.
Five identical horizontal trays are formed from pairs of 18½" angle girders joined by four 5½"x2½" flanged plates with three holes between the plates. Note that the bolts holding these are not in the outermost holes so as not to interfere with the rods passing behind. Use a drift to ensure the plates are accurately positioned so that gears meshing on shafts carried in adjacent plates will be precisely spaced and not bind.  Each of these plates has three pairs of 2½" strips, bolted over the front, middle and rear rows of holes to form reinforced bearings for the main vertical shafts. Four flat trunnions are used for bracing where each tray is bolted to the uprights. The trays are spaced apart 5" from each other vertically.

In Babbage's terminology, the space between a pair of these trays, where one digit's worth of mechanism fits, is referred to as a cage. The bottom cage contains the units digit on any given axis, with successively higher digits occupying cages 2, 3, and 4 above that.

Robinson's Difference Engine #1
Click for a larger version
One further pair of 18½" angle girders at the top provide for mounting of the large crank bearing.   Handles can be added as shown as the finished machine is very heavy, so these come in useful for transportation.   After taking the picture above I found that in actual operation, greater stability is required front to back, and so it is advisable to add some 12½" angle girders, suitably braced, at the sides on the bottom to prevent tilting. Rubber feet are also advisable to prevent scratching tabletops.

It's imperative to make sure the frame is absolutely square in all dimensions, absolutely rigid, and that all the bearings align perfectly for the long rods that form the axes. Time spent on this will be repaid later. Everything must run very freely for the machine as a whole to operate reliably. Any binding will result in something slipping and messing up the adjustment.

 > Framework
    Arrangement of axes and drive system
    Decimal digit storage

    Digit readout and addition
    Carry detection and propagation
    Setup and adjustment
    Parts list

Last modified:  30 July 2004

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© 2003, 2004 Tim Robinson