Bodger's Muddle

Written by Hugh Spencer on .

Originally published in the Bodger's Gazette.

For many of us, the use of a pole is impractical in the modern postage stamp gardens and mini garages beloved of today's developers. While I agree that a 20ft pole is nice, transporting it certainly is not. Mike Abbot's comments about shorter poles are quite correct, you just don't get the long stroke. Some of the less well endowed lathes in action looked like riding a bike that is too small and running in bottom gear. It involves a lot of frantic peddling and very little progress. The use of a bungy-cord is a simple but limited substitute, as is a bicycle inner tube. If you have a short bed to the lathe you end up with a short bungy and the same problems as a short pole. Some have tried a single bow and bobbin as demonstrated at the last two AGMs. This gives a long stroke but suffers from a "soft action", "snatchy return" and lack of lateral adjustment.

All this set me thinking and after nearly two years of cogitating I finally committed my ideas to wood. Firstly I would mount the bow so that it would traverse on a sliding shuttle so that I can have the cord descend directly above where I wanted it to turn on the work. Secondly, it must exploit the flex of the bow to a far greater extent than the examples demonstrated at the AGM so as to give a much firmer action. Finally the return stroke must not suffer from "snatching".

So why does the simple bow and twisted cord not suffice?

The simple bow relies entirely on the twisting of the cord to shorten its length. This requires an awful lot of twists to cause even a minor deflection of the bow. Pre-tensioning the cord merely causes the treadle to slam into the lathe bed and makes mounting work on the lathe a bit of a wild operation.

A bow tends to be lashed to a cross-member above the bed of the lathe. Allowing it to traverse along this cross-member would mean having enough room at either end of the lathe for it to overhang. My intention was that it would not increase the "footprint" of the lathe beyond that of the bungy powered lathe as I still want it to fit in my garage.

The "snatch" is due to the fact that, with a single bow, the bobbin actually gets pulled downwards several inches and it is this rather than the untwisting of the cord that gives the primary impetus to the recoil on the return stroke.

To this end I arrived at the Bodger's Muddle. This consists of a double-opposed bow, transversely mounted on a shuttle, running on a cross-member above the lathe. By mounting it so that the bows are orientated at right angles to the bed, the footprint of the lathe remains the same as before. The use of two bows gives twice the length of "pole" to flex. This is further amplified by the use of two fulcrums, as opposed to the one of a traditional pole, so for a 4'6" pair of bows you should get an effective length of 18' with respect to the deflection. The bows are horizontally opposed with the bobbin strung between them. As the bobbin turns it twists the cords of each bow around the other. The 'point of twist' travels away from the bobbin towards the extremities of the bows. The action of the twist is now not the same. Instead of just shortening the cord by twisting, the point of twist is drawing the bows towards each other just like an archer drawing a bow. The only way to eliminate the snatch entirely would be to have the bobbin running in a bearing so that it is spatially static. However, in practice the horizontal forces exerted on the bobbin mean that it is almost static with the downward deflection being a much smaller component of the recoil. Most of the recoil is transmitted via the twisting action because the two bows only have one way of releasing the tension and that is to untwist the cords.

I used Yew for the bows an old pine floorboard for the shuttle, Hornbeam for the bobbin and an old computer shelf bracket to fashion the bow holders. The uprights and cross-member were cleaved from B&Qs 2"x2" sawn timber section but I have already done my penance for that. I would recommend that however you attach the bows to the shuttle, you do not drill holes in them at centre or compromise their integrity in any other way as the forces involved at the fulcrums are phenomenal.