I was playing in my shed today*....
I built this contraption on my milling machine, an assortment of gears mounted on the end of the mill table to couple the movement of the mill table to a rotary table which is mounted with the axis horizontal. The mill feed screw has a 5mm pitch and for example with 1:1 coupling I am able to mill a helix with a 5mm pitch, right now I have set it up ratio of 20:65 for a 16.25mm pitch. I have allowed for compound gearing to get widely different ratios for particular jobs. The gears are 'borrowed' from my Drummond lathe and there is quite a selection of them to choose from. There are a lot of gears in use but three of them are idlers where a single idler would have been enough but I do not have any near the required size, there must be an odd number of idlers for a right hand thread. Working out the design to mount so many idlers was the major challenge of this project.
In action...
... the mill power feed can drive the action or I can use the handle which will be necessary for any larger diameter work as there is a minimum speed that the power feed has sufficient torque.
The end result...
As you can see this is in wood but it should work equally well in plastic or metals.
There are a number of things that could be better, I need to make spacers to have the gears align properly and I really should have machined that raw painted end off the vertical bracket!
*It took much more than a day to make all the bits!!!
