6. SPECIAL PURPOSE ULTRA-LIGHTWEIGHT LADDER

(Editor's note: This section had not been written by Coase, and Petty does not know the method of construction.  Very clear diagrams had been drawn, however, and these are reproduced as figures 12 - 14. As the reproduced diagrams are not as clear as the originals the following interpretation of them is given.)

The ladder was constructed using 10cwt, 7/14 construction cable and had rungs of 6" overall length with a rung pitch of 11", the distance between the wire centres being 5 1/4".  The rungs were oval in cross-section, 3/8" x approximately 11/16" (the latter figure is not given on the drawing and has been obtained by measurement of the rung as drawn) but whether this cross-section was purchased or formed from circular tubing is not known.  Similarly the wall thickness of the tubing is not shown but appears to be approximately 14 gauge.  (Don Coase was a draughtsman and it is assumed that his original diagram was drawn at least approximately to scale.)

Each rung was prepared by drilling, through one side only, two holes at 5 1/4" centres with a No. 32 drill.  On the opposite side of the rung was cut an 1/8" wide slot, 7/32" deep and at an angle of 45° to the length of the rung.  This is shown in figure 12.

The method of rung fixing appears to have been as follows.  Rungs and 1/4" diameter copper ferrules were threaded alternately on to the wire rope, then at 11" centres the ferrules were squashed flat and soldered.  A rung was centred over each ferrule by threading the flattened ferrule through the slot in the bottom of the rung and finally the rung was clamped by flattening the end tightly over the ferrule.  This method of fixing would necessitate the ladder being hung

Figure 12

from the correct end so that the rung was forcing the ferrule against the drilled hole and not against the slot.  The fixing of the rungs is shown in figure 13.

Figure 13

The third diagram, figure 14, shows the jig used for slotting and drilling the rungs.  It was manufactured from 3/4" x 1/8" angle iron and made to fix on a bench vice in place of the hardened jaws.  The inside measurements of the angle iron were cut to 13/16" x 11/32" from the 3/4" x 3/4" and they are so arranged in the vice that they form, in cross-section, a hollow rectangle into which the rung fits.  Appropriate guide slots and drilling hole were than made.

Figure 14

Note:  A ladder of very similar dimensions is used by the Shepton Mallet Caving Club for normal use but these use the 'taper pin' method of rung fixing. 'The method of manufacture has been fully described in Caving Report No. 3A.

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