Well, I’ve completed a number of “required” projects and now I can get back to the “fun stuff”. The Sidewinder lathe is taking on “a life of its own”. I’ve done some re-examining of my goals for this project. Here’s the current list of things that I’d like to accomplish by building this lathe:
- design and construct a lathe that is human powered, but still capable of a range of speeds that will allow for high quality execution of the work on a reasonbly broad range of diameters, up to eighteen inches;
- be able to turn pieces up to sixty inches in length on a fixed bed (perhaps longer with a bed extension);
- minimize the footprint, by positioning the driving wheel parallel to the lathe bed;
- utilize contemporary work holding devices
In short, the goal is to build a big lathe that will be capable of turning large parts using modern chucking while occupying a small amount of floor space and – DO IT WITHOUT A MOTOR!
Okay, so I’ve got to start somewhere. And, it seems to me that the logical place to begin is with the drive mechanism. This, it would seem, would consist of the power supply (Me!), the power transmission apparatus, and the speed control system. Alright, you’re right to ask the question – if I’m using a treadle, which is driven by human power, why do I need a speed control system? Can’t I just increase or decrease the treadle speed as required? Well yeah, I could. But if anyone has every ridden an old single speed bike up a hill, you’d immediately know why some method of speed control (or more appropriately power control) would be so desirable.
I plan to use a large drive wheel, forty-two inches in diameter. From working on my springpole lathe, I know that about one hundred treadle strokes per minute is a pretty comfortable pace. The math is realatively straightforward, I’ll be able to generate a pretty good amount of speed. Ahh, here’s the problem. The springpole lathe, you see, is all about torque. It is a “torque monster” (as Underhill says, great for cutting multiple threads). The “big wheel, high gear, high speed” lathe won’t be very torquey. It’ll be kind like when Honda brought out their 250 cc four cylinder motorcycles to the race track. Everyone thought their little rubber bands would break (but they beat everything in sight). However, when I’m roughing in a large piece of stock, I’m going want to run at a slower speed with adequate power (torque). Therefore, the need for some type of gear change mechanism.
So for the past few days, I’ve been thinking about methods of gear changing that I might use. The whole issue is pretty well complicated by the fact that I have a right angle change of direction in the “power supply” line. I thought about a step pulley arrangement but that would require the use of some kind of tensioning device (idler, etc.). That device would not only take up valuable space, but it would also alter the direction of the power transmission (spliced rope or round leather belting). Then there’s the problem of the type of belting I would use between the two step pulleys, as I invision them as being supported at both ends, not cantilevered. Then, EUREKA! – a flash of insight. Why not use two cone pulleys with a transfer idler? Truly variably speed. After a little simple mathematics; Speed of drive pulley X Diameter of drive pulley / Diameter of driven pulley = Output speed, I figure that if I pump the treadle 100 times per minute, I can realize spindle speeds of between 175 rpm and 2950 rpm by simply moving the transfer idler. Here’s a diagram of the “theory”,
So…If there’s anyone out there in “Lalaland” who’s as goofy as I am, let me know if you think that my theory holds up. I’d be most appreciative of any input.
Obviously, this little project isn’t going to be completed within the next couple of days, so I’ll keep you posted.