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Drawing References:

   BAB [A] 161  General Plan and Detail of the Calculating Part (See Elevation "L")
   BAB [A] 163  Elevation of Difference Engine No. 2, 1/4 size
   BAB [A] 164  Plan of Difference Engine No. 2 (1/4 size)
   BAB [A] 176  Plan of Calculating Part of Difference Engine with the means of conveying
                    numbers to Stereotype sectors


'Framing ' refers to the fixed structure supporting the working parts of the engine. Framing includes the two base supports which run the full length of the engine and on which the engine rests, vertical framing supports, upper and lower horizontal frames, bearing plates supporting the calculating axes, printer shaft mountings which straddle the two base supports, rack mountings, and the cam stack pillars, fixed cam stack framing plates, cam shaft thrust bearing, and the drive shaft mounting.


The calculating axes are contained between bearing plates which span, front to rear, the upper and lower horizontal frames. The upper frame is shown resting on the uprights (163) and the lower frame (roughly midway up the engine) is shown with open comers let in to the uprights (161, 176). The upper frame is shown as a single tray (164), presumably a one piece casting. There is no indication on the elevation (163) of piece part assembly of the frame. Similarly, the lower tray is shown as a one-piece part with no indication of sub-assembly (161, 176). The bearing plates rest on machined surfaces running the full length of the frames (176 shows lower frame; also tracings B6. No fixing is shown for the upper ( 163) or lower frames ( 161, 176) to the uprights. (The fixing on the top left of 161 is for the rack support.) Positions for the fixing holes for the bearing plates are also not shown.

There is an inconsistency in the dimensions given for the overall length of the frame as annotated at the bottom of 161 and the dimension scaled from the arrangement as drawn and calculated from the geometry of the calculating mechanism. The distance between figure wheel axes derived from 171 is 6.98 inches [see section C[. Seven such pitches give the distance between the first and last figure wheel axes as 48.86 inches Adding to this the scaled dimensions to each end of the frame (5 inches to the right and 8.14 inches to the left) gives a figure of 62 inches for the overall length of the frame. The annotation at the bottom of 161 gives 62.25 inches for this same overall dimension a discrepancy of 0.25 inches. The scaled figure of 62 inches was used and the annotation ignored. (A similar inconsistency between annotation and scaled dimension occurs in the specification of the drive shaft bearing shown on 159 and 163. in this case the annotation was regarded as superseding the drafted version.).


Drawing Reference: 377F31
   Frame Ends (Left and Right):          311 A-D
   Framing Pieces (Front and Rear):      312 A-D

Because of the difficulty casting large one-piece framing pieces, and the difficulty machining the surfaces for the bearing plates, especially on the upper frame, the two framing trays were each made from four separate parts: a pair of end pieces (short dimension of the engine, left and right), and a pair of members (long dimension, front and rear).

The original drawings do not show an end elevation. Curves were added to the upper and lower end pieces modelled on those shown for the front lower member (163). Apart from appearances, the additional material increases the shortest diagonal dimension and strengthens the corner.

The four end pieces are made from one pattern. All four pieces are different. The three other frame ends are cast from the same pattern without loose pieces and differently machined. The upper and lower frame ends have different vertical depths: the lower right hand frame end (B) takes the main drive bearing and loose pieces are added to the pattern to provide for this: the top left frame end (D) has mounting holes for the printing mechanism, a central circular clearance cutout, and a rectangular cutout. The need for clearance cutouts for the printer (D only) became evident after the drawings had been sent out for manufacture and while the printer drawings were being progressed. The frame ends were sent for additional machining after they were first delivered.

No provision is made in the original design for securing the engine while being transported. Additional fixings were provided on the top frame ends to take transportation bars. Eight debars strap the framing to the steel base on which the engine rests. The transportation fixings are omitted on the lower frame ends by removing the loose pieces from the pattern when casting. The transportation bosses are not visible to normal viewing from ground level and can only be seen if the machine is viewed from above.

Front and Rear Framing Pieces

The bearing plates span the front and rear framing members and are fixed to a raised machined surface running along the top of the inside bottom flange of the angle (tracings B6). The height of this machined surface was taken as l/8th inch. This dimension is taken from the depth of the internal pad on the upright intended to receive the lower frame (176). Pads for pulley brackets are required on right hand side only, front and rear. Only these two pads were machined. Redundant pads on left front and rear are unsighted and were left intact after casting.

The four long framing pieces were similarly cast from one pattern and differently machined. Differences between upper and lower members include: vertical depth; the omission of curves on the underside of the upper framing pieces (A and C, 163); and the location of bearing plate holes.

The depth of the upper frame is shown as less than that of the lower frame and it was thought at one stage that the upper frame would need to be strengthened. The idea of strengthening the upper frame arose from attempts to separate and apportion the load of the calculating mechanism between the upper and lower frames. For example, assigning to the upper frame, via the counter balancing springs, the weight of the warning shaft assemblies, sector and figure wheel shafts, figure wheel shafts and zeroing arms; assigning to the lower frame, via the figure wheel supports, the weight of the figure wheels. Separating the loads in this way suggests that the upper frame is more heavily loaded than the lower frame and that the aesthetic of a more slender upper frame conflicts with the load distribution. However, these considerations ignore the structural role of the figure wheel supports which couple to two frame assemblies and stiffen the whole structure. The twenty four figure wheel supports (three per axis), fixed between the upper and lower bearing plates, act as pillars or struts giving rigidity to the structure. The overall length of the figure wheel supports was tied to close tolerances to aid uniform distribution of load between the two frames. Measures to strengthen the upper frame were subsequently abandoned and the original dimensions were adhered to.


The uprights were extended to the full height of the engine to allow the upper tray to be let in and still retain the overall height for the calculating axes. The frame ends have three fixings to the verticals at each comer. The two long frame members screw to the frame ends and are lapped. Rxing is by tapped recessed cheese head screws.

Rack Mounting

Drawing References:
   BAB [A] 160   [Untitled]
   BAB [A] 161   General Plan and Detail of the Calculating Part (See Elevation "L")
   BAB [A] 163   Elevation of Difference Engine No. 2 (1/4 size)
   BAB [A] 171   Difference Engine No.2 Addition Carriage and mode of Driving the Axes

Sets of toothed racks meshing with toothed quadrants (171) provide the reciprocating rotary motion for the odd and even figure wheel axes, the odd and even sector axes and the odd and even warning resets. The racks are mounted on sliders which run in channels machined in the rack mountings. The sliders are driven by links to the pivoted cam followers. The linear reciprocating motion of the racks is produced from the rotating cams by the arrangement of cam followers, links, and sliders.

Details of the rack mountings are well defined though details are not immediately obvious from the two dense drawings 160 and 161. The racks for the odd and even figure wheel axes are mounted in a single machined casting (160). The rack mounting for the odd and even warning reset axes is similarly one machined casting. However, the odd and even sector axes have separate rack mountings. There are therefore four separate cast rack mountings in all.

The figure wheel axes and the warning axes are in line so the first odd warning axis (seventh difference, extreme right) is masked in 163. However, 163 shows the bevel drive for the first odd carry shaft indicating the inclusion of a cany mechanism here. The intention to include a full carry mechanism for the seventh difference is confirmed by 161 and 164.

Nonetheless, the carry mechanism for this axis was considered to be redundant. The warning and carry axes were therefore omitted. The half-boss encircling the redundant warning axis was also omitted from the figure wheel rack mounting (F531). The profile of the curve for the right hand end of the figure wheel rack mounting was taken from elevation 163.

The figure wheel rack mounting is shown fixed at the base only. Three additional mounting ears were added to the top of the casting to provide additional back-to-front support These secure the figure wheel rack mounting to the odd/even rack mounting (F331). The additional support was provided as a precaution against strain from side thrust in the event of jams or even resistance during normal use.

Bearing Plates (F371.2.3)

The bearing plates straddle the front and rear members of the horizontal frames and support the figure wheel axes, sector wheel axes, carry and warning axes, locks, and zero stops.

The relative positions of the addition and carry mechanism components are shown on 164 and 171. The detail on 171 shows the carry axis handed to the right Plan 164 shows all carry axes i.e odd and even axes handed to the right. The mirroring correction requires the odd numbered axes to be opposite handed i.e with the carry axes handed to the left. Plans 171 and 176 show the positions of figure wheel supports, locks and zero stops for a consistently right-handed arrangement. The mounting holes for the locks, figure wheel zero stops, figure wheel supports, carry axis and sector wheel zero stops were drawn to the modified scheme required by the mirroring correction. Plan 176 shows odd and even bearing plates identical. The shape of the odd axes bearing plates have been altered to accommodate the back-to-back arrangement of the zero stops. The position of the locks is unchanged. The upshot is that the odd and even bearing plates are handed differently.

The upper and lower bearing plates have small but significant differences. The counter-bores are handed in the case of the fixing holes for the figure wheel supports and the sector zero stops. The counter-bores for the bearing plate fixing holes are not handed. Similarly, the shaft bearing holes for the cany axes, warning axes, sector wheel axes, and figure wheel axes are counter-bored but not handed. The pair of bearing plates for the constant difference (first odd axis) has the warning and carry axes shaft bearing holes omitted as the cany mechanism for this axis was considered to be redundant.

Base Supports

Drawing References:
   BAB [A] 163, BAB [A] 165, BAB [A] 167
   F381 A and B

The base supports consist of two long cast members with a basic right angle cross section running the length of the machine front and rear. Elevation 163 shows the right hand vertical framing piece resting on a boxed platform cast into the base support. The left vertical rests on a cantilevered platfonm. The cam stack pillars are also shown resting on a cantilevered platform (163 and 167). The right cam stack pillars rest on cylindrical bosses on the end mounts (167, F383).

The three [A] drawings also specify three shaft mountings which support the printer drive shaft along its length. The shaft mountings straddle the two base supports. One shaft mounting is provided at each end of the base supports with the third midway along.

A fourth shaft mounting was provided just inside left of the printer drive gear to give additional support to the printer drive shaft. The three shaft mountings to the left are identical (F382). The right hand end mounting (F383) has two bosses for the cam stack pillars and two half-bossed holes for the sliding bars support (167 left).

Each of the base supports was thickened in four places by the addition of pads on the lower surface of the base supports. These are positioned at each far end and under the verticals.

The purpose of the added thickness is to cover the protrusion of the cam stack pillar bolt (167) and to provide a machined surface for levelling jacks.

Cam Stack Support

Elevation 163 shows the cam stack pillars secured below to the base supports. No upper fixing is shown and the drive shaft assembly provides the only other securing for the cam stack assembly. Upper framing ties were added keep the drive shaft aligned and to reduce the risk of the machine breaking its back when moved. 164 and 159 show plan views of the cam stack framing plates. The upper framing plate (F424) was extended by the addition of two ears similar to the those shown for the lower framing plate (163, 159). The upper framing ties fix the ears to the verticals to secure the upper portion of the cam stack.

Drive Shaft Bearing (F421)

There is an inconsistency in the dimensions given for the drive shaft bearing as shown in 159 and 163. The drive shaft bearing is drawn full size on 159 (faint, lower left) with the dimension to the shaft centre as 5.25 inches. 163 shows same dimension at 4.5 inches. The discrepancy is accounted for by reference to the manuscript annotations on 159 for the thicknesses of the framing plates, and comparing these to the thicknesses as drawn on 160. For example, the bottom cam stack framing plate is drawn 0.5 inches thick in 160; manuscript annotation on 159 indicates that the plate should be increased to I inch. Similarly, the upper framing plate is drawn 0.5 inches thick on 160 but the annotation on 159 calls for 0.75 inches. The middle framing plate is drawn and specified at 0.5 inches. The discrepancy between the plate thicknesses as annotated and the thicknesses as drawn is 0.75 inches which is the difference between the distances to the shaft centre as shown variously on 159 and 163. The annotated thicknesses were used in preference to the drawn thicknesses and the distance to the shaft centre on the drive shaft bearing was specified at 4.5 inches (F421). The plate thicknesses appear to have been amended by annotation on 159 without amending the shaft bearing (159) or the cam stack layout (160) which had already been drawn. 163 is drawn to the annotated dimensions and was used as the basis of the layout for the machine.

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