Introduction Edit

This page shows one way to make the little arch of the cheiroballistra or similar torsion engines. The little arch is the upper support strut that holds the field-frames rigid and at the exactly correct distance from each other.

Some improvements/options for the future:

  • The fork could also be made rectangular, which would make adjusting the distance of the field-frames easier
  • The long bar could be split in half and the fork formed from these
  • One half of the fork could be formed from the (unsplit) long bar, and the other half from a separate piece welded to the long bar

The process step-by-step Edit

Take a length of round steel bar around 12mm thick. Bend the bar around a round wooden log to form the crescent-shaped bend in the middle:

Making the little arch - 01

If you want to create a sharp bend, heat the bar first in a forge and use a smaller log.

Next attach the little arch to a vise and adjust the bend as necessary:

Making the little arch - 02

If the furnace in your forge is below the forge's bottom plate (as in mine), place a pierced steel plate on top of the furnace. This moves the fire to same level as the bottom plate. This allows heating the bend in the little arch easily:

Making the little arch - 03

Once fire is ready, place the bent portion of the little arch into the forge and heat it:

Making the little arch - 04

Use the round hole in the anvil to bend the corners of the crecent-shape part to the correct angle:

Making the little arch - 05

Making the little arch - 06

The basic form of the little arch is now ready, except for the forked ends. Heat the entire bar one section at a time, flattening it as you go:

Making the little arch - 07

Making the little arch - 08

Once all of the bar has been flattened, it's time to make the forked ends. Here we've bent a round bar (~10mm) so that it can fit inside the field-frame's pi-brackets, and flattened it:

Making the little arch - 09

We can now cut the long bar with the crescent-shaped bend into roughly correct length by comparing it and the forks to the little ladder:

Making the little arch - 10

Next cut a slot for the fork into the long bar:

Making the little arch - 11

Making the little arch - 12

Check the proper position for the fork using the little ladder as guidance and squeeze the "jaws" of the slot shut. This will hold the fork in place nicely:

Making the little arch - 13

Making the little arch - 14

Finally, attach the fork and the long bar to a vise, weld them together and remove what remains of the "jaws" of the slot:

Making the little arch - 15

Making the little arch - 16

At this point, the whole package should look like this:

Field-frames, little ladder and little arch - 01

Field-frames, little ladder and little arch - 02

Once the pi-brackets have been attached to the field-frame bars, slide the fork of the little arch into them. If necessary, file the forked ends so that they fit snugly against the field-frame bars:

Fitting little arch to field-frame pi-brackets - 01

Fitting little arch to field-frame pi-brackets - 02

When the fork fits nicely inside the field-frame, drill a pair of holes to each end:

Attaching pins to little arch ends - 01

The distance between the holes in each pair should match the width of the matching pi-brackets, so that the little arch is prevented from sliding in them. The holes should be placed at the same distance from the center of the little arch as the edges of the notches in the little ladder are. This ensures that the field-frames are exactly vertical, not leaning sideways, when assembled

Next take a nail and hammer it through the one of the holes so that the edge points downwards. The nail should just barely fit into the hole, so if necessary, file the nail down a bit at the sharp end and force it through. When the nail projects a few centimeters from the bottom, cut the head of the nail near the surface of the little arch, then beat down the remaining part and lightly file both ends as necessary. This process ensures a really tight fit without then need to sink the nail's head below the surface of the little arch. Finally cut down the nail to maximum length that still allows the fork to slip through the pi-brackets. Repeat the process for each end. The final result should look similar to this:

Attaching pins to little arch ends - 02

Attaching pins to little arch ends - 03

When the field-frames are assembled, the connection between little arch, pi-bracket and field-frame bar should look like this:

Attaching pins to little arch ends - 04

Attaching pins to little arch ends - 05