Model Engineers' Workshop|December 2019
A roller box is usually used on a capstan or turret lathe. Capstan and turret lathes serve the same purpose but differ slightly, both can carry a selection of tools on a turret which is a substitute for the tailstock of the centre lathe.
In both cases, the turret indexes automatically, to bring the next tool into operation, as the turret is racked to and fro.
The capstan lathe is usually smaller and lighter, and better suited to lighter repetitive work, and carries the turret on a saddle which can be fixed at a point along the lathe bed. In addition, the four way toolpost on the cross slide, a rear toolpost is often carried there. (this is often used for a combined chamfer and part off tool).
The turret lathe is a larger machine, intended for heavier work. Here, the turret moves along the lathe bed, under the action of the capstan handle, again indexing a fresh tool into position with each movement.
As an instance of this, a Herbert no 7 Preoptive turret lathe is about twice as large and heavy as a Ward 2D capstan lathe.
Both are intended for semi-automated, high volume, production, with unskilled or semi-skilled operators. The machine will have been prepared by a skilled setter.
They were the forerunners of automatic lathes and cnc machines.
Roller boxes (and the model engineer’s running down tool) are intended to overcome the problem of producing a taper when reducing the diameter of a long workpiece. Here, the outer end tends to deflect under the cutting pressure, leaving the outer end oversize, so that only the inner end, close to the chuck or collet will be at the required diameter.
The model engineer’s running down tool is a light duty version, using a vee shaped steady to prevent the material deflecting under the cutting load. They are all, in effect, a travelling steady with a cutting tool attached.
Roller boxes normally come in two versions, arranged for the rollers to lead, or to trail, the cutting tool. The roller leading type is suitable for reducing the diameter of round bar. The roller trailing type is used to change the size and section of non round material, such as hexagon bar.
The usual way of setting up a roller box is to turn, near the chuck, a short section of material to the required size, and then to set the rollers and cutting tool, to that size. This should be followed by another short trial cut to allow final adjustment, before setting the length stops for volume manufacture.
Making a reversible roller box had exercised my mind for some time, but the final inspiration of a means of doing it came from an illustration of a 3D printed steady on a lathe.
The plan was to have the rollers off set from the centreline of the roller carrier, so that depending which way it is fitted to the frame; the rollers would either lead or trail the cutting tool.
What follows is how it was made, using material that just happened to be waiting “to come in handy”. Because of this, in typical fashion, the design was more or less made up as it went along. The work was done from sketches and calculations on various scraps of paper, instead of drawings. Consequently, dimensions are contained within the text.
In any case, those reading this will be likely to have materials, facilities and machines which differ from mine, and so materials, sizes and methods may well have to be amended to produce a “bespoke” device for a particular workshop.
Some of the operations were not perfectly done, and possibly not even carried out in the best order!
Method and materials
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