The clearing handle is mounted near the centre of the right-hand frame support plate. The handle rotates about 30° between stops on the parallel bars of the plate. A non-return pawl ensures that the handle completes its full stroke.

The clearing handle drives a series of links and levers on the inside of the support plate. The clearing levers are not attached directly to the mechanism, but operate through a series of cams in such a way that the entire support plate can be easily removed.

A rotates forward through 90°, operating the rock frame release mechanism and allowing the numerals to return to zero.
B operates a "release plate" to clear any incomplete operations of the trigger and accumulator interlocks.
C raises the front column lock retainer shaft, releasing the segment lever locking hooks and allowing the touch-off bar to reset for a new operation.

A coil spring at the rear of lever C pulls the cams and levers back to their starting positions after clearing.

The rock frame is a major sub-assembly which houses the carry gears and the escapement mechanisms. The frame hangs from the intermediate gear shaft at the front of the machine, and rocks forward to disengage the gears and allow the numerals to return to zero.

The positioning mechanism must clamp the rock frame firmly into place during normal operation, so as to resist the outward pressures on the carry gears and the escapements, but must allow the whole assembly to swing forward easily when required for clearing. An "over-centre" or "toggle" linkage provides the necessary action.

The toggle mechanism is located under rock frame at the front of the machine. This view shows the essentials of the toggle linkage, with all of the addition mechanism removed.

Small "L"-shaped toggle links are rivetted to the front of the frame plates. The links are connected via a pivot rod to the main toggle levers, which are supported near their centres on a shaft attached to the rock frame. When the pivot rod drops "over centre" to the position shown, it locks the rock frame securely in its rearward position. (The lower rock frame spacer rod (not shown) provides a stop to prevent the toggle lever rotating further anti-clockwise).

The toggle linkage is repeated (with minor variations) in every column to ensure that the rock frame is clamped firmly across its full width.

To release the toggle, the rock frame actuating shaft (bottom right) is rotated 90° anti-clockwise by cam A in the clearing handle linkage.

The shaft carries a series of slotted actuating links in alternate columns. The links are supported on a rod attached to the frame. A pin on the front of the actuating link engages with a yoke on the rear of the toggle lever.

As the actuating shaft rotates, the links press forwards and downwards on the rear of the toggle levers, raising the pivot shaft "above centre" and swinging the rock frame forward. The forward travel is normally limited by the clearing mechanism, but a "safety" stop is provided if the offset arms on the small toggle links strike the underside of the toggle levers. The forward movement is only about 1/8" at the toggle links, and 1/16" at the carry gear shaft.

As the count advances, the increased tension in the carry spring is resisted at one end by the long arm of the escapement wheel pushing against the detent, and at the other end by the carry gear pushing against the accumulator gear. The accumulator gear is held fast by the pinion ratchet and the segment lever (or by the backstop during a downstroke).

Swinging the rock frame forward releases the carry gear from the accumulator, allowing the spring tension to drive the gear train and numerals back towards zero.

The driving force ceases when the stop pin reaches the escapement arm, but the gear train will tend to over-run due to inertia. The escapement wheel may be pushed "below zero", as there is no longer any spring tension holding the arm against the detent.

A more positive zero stop has to be provided to ensure that the carry gear and the numerals come to rest in a definite zero position. This in turn will ensure that the carry gear will re-engage cleanly with the accumulator when the rock frame is swung back after clearing.

The zero stop is a short vertical arm attached to the rock frame toggle lever via a spring-and-stop arrangement in its hub. As soon as the toggle starts to rise for clearing, the zero stop swings rearward into the path of the small flattened stop pins on the right-hand face of the carry gear.

When the carry gear is released, the zero stop catches one or other of the pins just before the main carry gear stop pin on the front face reaches the escapement arm. This ensures that there will always be a residual spring tension (from the pre-load) holding the gears and the escapement in definite known positions.

When the rock frame toggle drops back after clearing, the zero stop swings forward and clear of the stop pins so that the carry gear can rotate continuously.

A short keystroke will trip the "Controlled Key" mechanism and lock the accumulator and segment lever in mid-stroke. The operator may repeat the keystroke and correct the error, but may also simply abandon the calculation. In this situation, the clearing mechanism must be able to release the interlocks and allow the segment lever to rise.

The "release plate" is a round bar with a series of rearward-facing fingers. The release plate pivots between the frame support plates, and is operated by cam B of the clearing mechanism. As the clearing handle is pulled forward, the fingers press downwards on the yokes connecting the accumulator locking hooks to the trigger. The trigger is disarmed as it would be at the bottom of a normal keystroke, the locking hook is raised clear of the accumulator, and the segment lever rises and completes the cycle.

The Model H release plate has a second set of forward-facing fingers which lift the pinion ratchet reverse locks during clearing. These fingers are not present in the Model J.

The accumulator latch is an additional detent lever (the closest black lever in the illustration) that was first introduced with the Model J. Its purpose is to assist the re-engagement of the carry gear by preventing the accumulator from moving out of position during clearing.

The latch lever is mounted on the same shaft as the subtraction cutoff lever, and is spring-loaded clockwise. A tooth at the forward end of the latch is shaped to fit between the teeth of the accumulator gear.

During normal operation, a pin on the top rear of the rock frame side plate pushes rearward on a small "latch lifter" mounted on the backstop shaft. The latch lifter raises the latch and holds it clear of the accumulator while ever the rock frame is engaged.

As soon as the rock frame starts to move forward for clearing, the latch drops onto the accumulator gear and stops it being "bounced" out of position by the reaction forces when the carry gear is released.

While this cures one problem it creates another, in that the latch will prevent the accumulator from advancing if it needs to clear an incomplete keystroke. To solve this problem, the latch is raised momentarily by the release plate at full stroke (via the rearward vertical extension at the top left) to allow the accumulator to advance. (The numerals will not advance, since they are already disconnected from the accumulator).

This view shows the rock frame toggle lever and zero stop assembled between two rock frame side plates. The zero stop is visible through the opening in closer plate. With the rock frame toggle engaged, the zero stop is held forward and clear of the carry gear stop pins.

When the rock frame toggle is raised for clearing, the toggle lever rotates the zero stop rearward. The pin on the top right-hand side of the zero stop will strike the lower arm of the carry lever locking dog, rotating it anti-clockwise and pushing the upper arm out from under the carry lever.

While there are more than a few complications involved in clearing incomplete keystrokes and possible delayed carries, it is not at all obvious why this particular interlock should be necessary. Suggestions from readers would be welcome.

The rock frame actuating shaft has a second "detaining toggle" mechanism in the leftmost column. (The left or "overflow" column has an accumulator and a modified carry mechanism, but no keys or segment lever).

The illustration shows the rock frame engaged and the detaining toggle linkage raised. A coil spring (top left) holds the actuating shaft rearwards.

When the actuating shaft rotates forward, the detaining toggle drops over-centre (assisted by the lower rear spring). The downward arm near the centre pivot provides a stop against the lower frame spacer shaft. The clearing lever and cam return to their rest positions, leaving the actuating shaft and the rock frame held forward by the detaining toggle.

The rock frame is pivoted on the same shaft as the intermediate gear. As the frame rotates forward, the intermediate gear rotates a corresponding distance in the same direction (as the carry gear is held fast by the zero stop). The numeral wheels rotate the same distance backwards, leaving the zero numerals slightly (but definitely) offset in the viewing windows. This provides a visual signal that the machine is clear and ready for use.

The detaining toggle is released by pulling rearwards on the release link (centre left), which raises the toggle links above centre via the bell crank at the rear pivot. The actuating shaft spring then pulls the mechanism back to the position in the previous view.

The detaining toggle release bar is another 1/8" x 1/4" bar pivoted between the frame support plates near the bottom rear of the machine. A round shaft of 0.088" diameter is supported above the bar on a series of fixed arms, and is connected to the rear end of the detaining toggle release link.

A spring near the detaining toggle hold the release link forward, which holds the round shaft against the tails of the brake levers in every column (visible through the frame cutout).

When any key is pressed, the segment lever roller pushes the brake lever rearwards. On the first key press after clearing, the brake will push the release bar rearward, pulling on the release link and raising the detaining toggle. A firmer key pressure is required to break the toggle (against the tension of the actuating shaft spring), providing a tactile signal that the machine has started from clear.

As the detaining toggle rises, it pushes the release link rearward, throwing the spring-loaded hammer momentarily against the bell. The bell is attached to the left-hand support plate, adjacent to the large round cutout, and provides the third (audible) zero signal.