The cheiroballistra uses the little ladder as the lower support structure holding the field-frames at correct distance from each other, and for attaching the field-frames to the case. This article shows one easy way to make the steel parts of the little ladder. Making of the (probably wooden) cross-pieces and rungs is described in the assembling the little ladder article.
The process step-by-step Edit
Take two long pieces of relatively thin (~4mm) steel and mark the center, the ends of the little ladder beams and the width taper. Also leave some extra steel at the ends for the tenons:
Cut the width taper into the beams and mark the cross-piece and rung holes:
Heat the tenon ends in a forge and fold them lengthwise:
Reheat and forge the tenons until they're straight and rectangular:
Next file fairly deep and tight notches to the tenons for the field-frame bars:
The notches are needed for two reasons:
- Prevent the tenons from sliding in the pi-brackets during pullback (for full discussion see my first and second blog posting about the subject)
- Fix the field-frames to a correct distance from each other
While wedges alone can hold the field-frames in position for a while, the will eventually loosen without the notches.
Finally bend the beams slightly outward near the rungs and back inwards near the tenons:
The idea is to (have to) bend the beams slightly outwards when the little ladder is inserted into the pi-brackets: this ensures that the tenons lock tightly into place, even before any wedges are inserted.
Finally, make the outer holes for the rungs, and the hole for the cross-piece in the middle. The rungs holes are round, so they can be either drilled or punched. The cross-piece hole is rectangular, so punching with a rectangular punch is easiest. An alternative is to drill the hole and file it rectangular: