Workshop Hints
Universal Mainspring Letting Down Tool.
From Mike McCreery
The idea of the letting down toot is to utilise the 4-jaws of a chuck- type tap wrench as the adjustable 'key' for the square arbor. The outer diameter of the chuck does not foul anything, assuming the dial has been removed.
The size of the tap wrench is chosen to cover the range of arbor sizes required. (eg Eclipse make 2 wrenches 3-5mm jaw and 4-8mm jaw.)
- Remove the tommy bar, which is usually retained by a grub screw.
- Drill an axial hole into the end of the handle through to the tommy bar hole.The diameter of the hole is chosen to be between the across flats and across peaks of the hex bar. For a 6mm AF bar a 6.5mm drill works well.
- Prepare a short stub (about 20mm) of hardened hex bar with a small tapered lead in one end. This may be easily done on a grinder.The size of the hex bar can be either 6mm or 0.25". 0.25" seems to be used in screwdrivers with interchangeable bits, but, in practice 6mm will also be satisfactory. Alternatively handles are available with a 0.25" square that can be fitted with a 6mm or 0.25" socket. The choice will depend on what is most readily available in the toolbox.
- Grip the wrench in a vice and, with a swift blow from a heavy hammer, insert the hex bar into the wrench.
In use, the chuck is adjusted to fit the square arbour, the spring tensioned with the handle to allow the click to be lifted, and the spring let down by allowing the handle to slip through the hand in a controlled manner.
