Model Engineers' Workshop|May 2020
Users of this lathe will no doubt be aware that limited simple dividing by 2, 3, 4, 6 and 12 can be accomplished using the pre-drilled holes in the mandrel flange along with a simple accessory which bolts onto two pre-drilled and tapped in the headstock housing. If you have not got one, then drawings for this accessory were published in MEW 135, February 2008.
Divisions of 2, 4 and 6 are quite useful on their own as along with a suitable filing rest, one can produce spanner flats on a bar, a square end on a bar or a hexagon for use as a nut. But what if you need to divide by 8, say for a cover with 8 fixing holes? Well, you could rig up something to engage with the 32 teeth gear wheel semi-permanently mounted on the mandrel inside the gearbox. This same device would also give you 16 and 32 divisions if you need them.
Life gets somewhat more difficult, though, if you want something else. Suppose you want to mark graduations around a disk with a M6 tapped hole through the centre with a view to being able to adjust the device, whatever it may be, by 0.1mm. M6 has a pitch of 1mm, therefore, if you could divide the disk into 10, then you could adjust by 0.1mm and hence problem solved. But how do you do it? Or, as I did recently, I wanted 40 long graduations of 5mm length around the periphery of a ring, with a further 40 short graduations of 3mm length to subdivide the main 40 divisions. The answer is to use a dividing mechanism mounted on the rear of the mandrel and which uses the lathe changewheels as the master device. Even so, there is problem in that I required 80 graduations, and the largest changewheel available is 64 teeth. The answer is to use the 40 teeth wheel and a detent which either slots in between two teeth (thus giving 40 graduations) or sits astride a tooth (thus giving a further 40 graduations spaced between the original markings). Taken to it’s logical conclusion, the greatest number of graduations this method can give is 128 (2 x the largest changewheel of 64 teeth). Even so, there are a number of divisions which cannot be achieved, and for these it will be necessary to use some form of gearing using two or more changewheels. The design presented here is the simple version using a single changewheel.
The essential ingredients of such a device are firstly some means of rigidly attaching a changewheel to the rear of the lathe mandrel, and secondly, rigidly attaching a detent to the lathe itself such that the detent can lock securely into the changewheel thus preventing its rotation whilst suitable marking of the workpiece proceeds. It was this latter ingredient which caused me most concern as I ended up using the gearbox cover itself to locate the detent. This cover, being hinged could be considered to be somewhat flexible and hence could contribute to some inaccuracy, however, on my lathe, once the gearbox cover lock is engaged, it does seem reasonably rigid. The cover itself is reasonably thick at about 1.75mm; indeed, it is thick enough for there to be three 4mm tapped holes which are used to hold in place the protective shield for the end of the mandrel. Finally, the actual strain on it is, I think, quite small, and in the end it worked satisfactorily.
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