John Wolff's Web Museum

Marchant Figurematic - Overview

The Marchant Figurematic uses a complex and unusual mechanism based on proportional and differential gearing. Some of its distinguishing features include:

• Every column of the machine incorporates a ten-speed gearbox with three drive shafts and five selectors, which drives the corresponding register dial at a speed proportional to the number selected.
• The tens-carry mechanism is contained entirely within the main register in the carriage, using an additive (ie, differential) gearing mechanism based on two planetary gearsets.
• Multiplier digits are entered on a separate keypad in left-to-right or as-written order (ie, starting with the most significant). The calculation is performed "on the fly" and is completed as soon as the last digit is entered.
• The division mechanism does not wait for overdraft, but uses a predictive trip mechanism based on an analog magnitude comparator to avoid unnecessary machine cycles.
• The drive train includes 28 gears on 15 shafts, six dog clutches, and three different reversing mechanisms. Overall, the machine contains about four thousand manufactured parts.
• The machine performs a complete addition cycle in less than one-third of a second, with a peak rate of over one thousand additions per minute.

This page gives an overview of the layout and construction, and the basic sequence of operation.

Removing the covers.

The Figurematic covers are held in place by an ingenious system of overlapping flanges with no visible screws.

To remove the covers, roll the machine onto its back and remove the four screws in the corners of the base plate. Catch the front panel as it flies off, and slide the base sideways to disengage it from the back cover. Loosen the screws in the three small brackets under the front of the keyboard escutcheon, lift the front of the escutcheon and pull on the rods at either side to release the rear. Lower the machine, lift off the back panel, and pull the sides straight off. Remove the carriage cover from the back with two screws at each end.

Although the space under the numeric keyboard is empty, the rest of the machine is very densely packed. A few things can be seen to move as the machine cycles, but on the whole the mechanism reveals very few clues as to its operation.

The frame.

With the mechanism removed, we see that the machine is built upon a substantial pressed-metal frame of 0.078" steel. The frame plates are held together by interlocking tabs and a generous quantity of 3/16" bolts. A rear base plate (not shown) supports the drive motor. There are four smaller sub-frame plates in the control unit area on the right of the machine.

The frame sits on a light pressed-metal base plate with an absorbent lining. The side covers are substantial aluminium die castings which are located by the rubber grommets at each corner of the frame.

The calculating mechanism is built up from a number of separate modules which are supported by the frame. Although the mechanism looks impenetrable on first aquaintance, there is actually very little holding it together. The major modules can be removed and replaced relatively easily, once you know how they are connected.

The main modules.

The mechanism consists of four main modules:

• the carriage (rear), which contains the counter register, the main accumulator register, and the carry mechanism.
• the control unit (right), which contains the drive train and the mechanical control logic.
• the selector unit (front left), which sets up the gear ratios in each column according to the input from the keyboard. (The selector unit also houses the division trip mechanism).
• the actuator unit (front centre), which contains the proportional gearing that drives the main register.

Basic sequence of operation.

At its most basic level, the mechanism operates on a four-stage cycle:

1. Manual setting. Pressing a numeral key positions a "selection cam" near the bottom of the selector unit, and a check dial (the "keyboard dial") near the top.
2. Powered setting. Pressing a function key releases the setting clutch and starts the motor. The setting clutch rotates the setting camshaft (the "setting line") exactly half a turn, and then disengages. The setting line operates the selector mechanism, which sets up and locks the gear ratio for each column according to the position of its selection cam, and "dips" the carriage to bring the main register into engagement with the actuator.
3. Addition. As the setting line comes to a stop, it engages the main clutch to drive the actuator unit, which in turn drives the register columns via the selected gear ratios. The drive passes through a reversing mechanism for subtraction or division. The main clutch rotates either:
   • half a turn for a single addition cycle
   • a known number of half-turns for multiplication
   • an (as-yet) unknown number of half-turns for division.
4. Restore. On completion of the main clutch cycles, the setting clutch re-engages and completes another half-turn, raising the carriage and restoring the machine to its original state in readiness for a new operation.

(Note: when the Marchant manual speaks of "opening" or "releasing" a clutch, it means releasing the dog which stops the clutch, thus engaging the drive).