Unimat Modifications
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The majority of the machining for Conway has taken place on a Unimat 4. As supplied the lathe uses a pair of belts to change the speed, with optional intermediate pulley for the slower speeds. After a while a knock began to develop and on further inspection this was found to be in the bush for the intermediate pulley. Meanwhile Alistair had a variable speed DC motor and controller from a previous project that would be just perfect - can you guess where this is going? A quick outline sketch was made - actually it was on the back of the envelope!
So to start with here's a picture of the original unit. The pulley on the top left was the one with the knackered bush.
Taking the bits of old motor mounts that were with the variable speed motor I knocked up a revised arrangement that would fit nicely. I then used a bit of scrap aluminum sheet (actually a piece of an old sign) to make a guard around the belt. This is a toothed belt which ensures that the belt won't slip - this was one of the problems with the unimat. Here are a couple of pictures of the assembly. The unit can be removed by 2 bolts which allows the old motor unit to be replaced quickly if needed.
The pulley is simply screwed to the headstock spindle. So after boring this was threaded M14x1. A new longer belt was acquired and then the unit fitted. Here are a few pics of the fitted unit. Thee first is a general view of the setup. The second shows an end view of new unit complete with belt and pulleys.
The controller came from a kit that Alistair built. An important factor in the choice of controller was that it ensures that the torque remains constant even while running at low speeds. The controller is fitted into an MDF box and is shown in the pic below.
The white piece of paper attached to the controller is a speed dial. The controller was calibrated using a very simple yet ingenious method, which I found on the net, using a series of patterned discs. Have a look at http://www.jeffree.co.uk/Pages/speedmeasurement.html for more info.