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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.


The bobbin is about 8" long with 1/2" shoulders of 2.1/2" diameter, the body being 1.3/4" diameter. Four holes are drilled in each end. These need to be accurately drilled so that they are evenly distanced from each other and the centre, (draw a circle with a compass and divide it into 4). They do not have to go all the way through, only through the shoulder with enough clearance to drill holes in from the side to meet up with them. Failure to do this accurately on the Mk I bodger's muddle created a small amount of vibration in use. The diameter of the bobbin body can be a matter of experiment. If you use a smaller diameter you will get less stroke per twist thus raising the rate that the "point of twist" travels to the extremities of the bows. The same effect can be achieved by using a thicker cord on the bows. Using the present dimensions with 6mm sash-cord instead of 3mm, gave me a useable travel of about a foot! The current dimensions give me all the travel I can use on my longest treadle.


 

The Shuttle is basically a strong box section partly cut away around the bobbin. The bows are separated by about 12" and held in place by 18swg sheet steel brackets screwed to the shuttle. The internal clearance of the shuttle around the supporting cross-member is 2.1/4". I made mine from 3/4" pine floorboard. Anything that is strong and unlikely to split will do. Glue and screw them together. It will have to stand compression by the bows from each end and downward pressure from the treadle. I padded the underside of the shuttle where the bobbin might hit it, with leather strips glued to the bottom of the cutouts.

 

Stringing the bows takes quite a lot of cord, about 5M of 3mm plus several metres for the drive cord. Start at one end of one bow, down to the shuttle, in one end hole, out that side hole, back into the next side hole and out to the other bow at the same end. Then reverse this using the other pair of holes and end back at the point where you started. Repeat this for the other end of the bows. I drilled 2 holes in each end of each bow. Where the cord ends in a knot, I passed a nail through the knot so that it cannot slip through the hole.

Vital statistics;

Cross-member 2x2" by length of lathe bed.

Bow 1.1/4" x 1.1/4" x 4'6" 2 off. (Ash or Yew).

Bobbin 8"x 2.1/2" (body 1.3/4" diameter, see text).

Shuttle (top) 14" x 3.3/4" x 3/4" (sides) 14" x 4.1/2" x 3/4" 2 off. (bottom) 3.3/4" x 2.1/4" x 3/4" 2 off. Metal brackets to hold bows in place.

10M of 3mm cord.

Anyone is free to copy this design for his or her own personal use and to freely modify the dimensions and detail to suite his or her needs. I reserve the copyright on this article and on the "Bodger's Muddle". This is done not for selfish reasons but by publishing the design in the public domain I effectively deny anyone the ability to take out a patent on it. If anyone requires detailed plans or wishes to discuss any aspect of the "Bodger's Muddle" post a question on the bulletin board, linked from the top of the front page.