Following an off forum communication I promised to try and give a better explanation of how the tap and die holder for the Britan repetition lathe, and its internal dogs, work. First, here are all the parts:

The three parts to the right are the base unit. The adaptor top middle screws into the front of the unit and will then take one of the three split die holders. Alternatively the screwed bush bottom middle can be used, with sleeves, to hold a variety of taps.

The front holder and dog has a spindle that sits inside the fixed part, and is held axially by a spring loaded screw. The holder can rotate and move axially a few millimetres. It is not constrained in rotation, except for the action of the dogs.

To set the scene for an explanation of how the dogs work, assume that we are looking at the tool from the lathe spindle end and that the spindle is rotating clockwise. As the holder engages with the work it also begins to spin clockwise. This leads the dogs to engage, stopping rotation of the holder, like this:

Since the holder has stopped but the lathe spindle is still rotating the die/tap cuts a thread. The tailstock lever moves forwards keeping pace with the thread being cut. At some point a stop is reached and the body of the tool stops. The holder keeps cutting pulling it forward until the dogs just disengage:

At this point the holder starts spinning with the lathe spindle. Due to the matching tapers on the dogs the drive does not engage at each revolution but simply push the moveable dog out of the way, against the spring:

Now if the lathe spindle is reversed the holder will spin the other way. This means the dogs will engage on the opposite faces. Due to the tapers on the dogs they now engage rather than slip past each other:

This stops the holder rotating, with the consequence that it starts to wind off the thread just cut. The tailstock lever needs to be moved back in sympathy with the rate at which the thread is unscrewing, to stop the holder jamming.

I hope that makes more sense than my last explanation.

Andrew

Edited By Andrew Johnston on 18/08/2017 23:18:15