I don't know about then but now, not having a CNC setup, I would couple the mill table to a universal dividing head using a spiral pulley and a wire instead of gears. Od course that means a spiral has to be made thus creating a new problem.

Is it specifically a logarithmic relationship of just not linear?

Yup US National Bureau of Standards developed an attachment for 9" SouthBend lathes under contract to the Navy Aeronautics Bureau. Drawings can be found at **LINK** along with a lot of other SouthBend related information. Presumably to expand production of such components to meet wartime needs by exploiting the large number of SouthBend and similar machines in schools and training establishments.

Specialist machines do it differently. One way is by chasing using a non linear master. Another is to fit a matching feed screw to the slide of a standard precision bench lathe driving in the usual manner through a shaft from a gear train. Far as I know the mould makers lathes with sliding lead screws can only do linear pitch changes .

Clive

Edited By Wolfie on 17/02/2013 13:35:21

There are a lot of ways of achieving the non linear motion needed to generate the screw thread. In the Workshop Technology Series No 440 – Mechanical Movements – there are numerous examples of non linear gear systems using eccentric gears, elliptical gears, "snail" gears etc. Also a simple crank mechanism will give nonlinear motion, as will a whole range of cams. It would be fairly straightforward to disconnect the lead screw driving the table and then to substitute a mechanism based on one of the above, geared to the spindle that is rotating the screw. By changing cam profiles or gear shapes you could generate just about any non linearity that you want, and if the machine becomes a special, just doing this – so what , you're going to be making hundreds or thousands of them.

Keith

Edited By Keith Long on 16/02/2013 18:54:52

Edited By Wolfie on 17/02/2013 13:36:16

Screw shown in pictures was just thread milled in a very simple jig on a milling machine . Common shaft , blank on one end and master on the other end . Shaft arranged to slide and rotate in a support block . Fixed pin or roller engages in master such that rotating the master by hand or under power causes blank to follow required profile under milling cutter .

Master would generally be made larger diameter than finished workpiece . It could be precision made but more often than not masters were quite crude – even as crude as a strip of steel wound round a bar .

Advantage of doing it this way , apart from simplicity , is that set up can be used many times for either multiple cuts on one blank or the making of production series of parts – in all cases without any risk of loss of register between master , blank and cutter .

The making of all sorts of funny screws , cams and gears was common in the days of mechanical analogue computers .

Some of theses screws , cams and gears could have generated shapes but more often they were made as one off's using hand made masters .

Sometimes masters were made very big and also sometimes geared or worked via lever arms to get better workpiece control and to reduce errors .

Regards ,

Michael Williams .

Well done Wolfie; have a gold star

Neil

Brian

The Workshop Technology Series is produced by Tee Publishing. Just looked on their on-line list and coundn't see it. My copy ,I think, was a download as a pdf, but I can't remember from where. It might even have been picked up at a model engineering exhibition. It doesn't have an ISBN number on it anywhere so no help there either I'm afraid. Another source that might be worth googling for is "Ingenious Mechanisms for Engineers and Designers". I believe it very much on the same lines, wierd gear and cam devices and multi-element linkages.

Keith