The basis of the design was originally published in an article of mine in a very
early copy (is 10) of the Model Engineers' Workshop magazine,and whilst it worked
well it had one weakness. This was, that the stop tends to be drawn back into the
lathe's bore as it is being secured. This making it difficult to set an accurate
position, though for most tasks this will be of little consequence but will comment
later regarding the problem. Even so, it seemed worthwhile to attempt to overcome
the problem, the result being the subject of this project.
In basic terms the purpose of a back stop is to provide a repeatable position for
the workpiece, or workpieces. For a single component this will permit it to be removed
for some purpose, measuring length for example, and then returned in exactly the
same axial position. With that achieved the top and leadscrew dials will still be
relevant enabling machining to progress with certainty.
For me, its most frequent use is where more than one component has to be accurately
made to the same length. In this case, they are all first made a little on the long
side and the first then machined to length as suggested above. With that done the
top slide and saddle are locked and the remaining parts machined to length using
the cross slide to face the end of each one in turn.
In the above the precise position of the back stop is relatively unimportant and
the tendency for it to move when being secured is of no importance, where then does
it become a problem. Another use for the back stop is to support the back of very
thin parts whilst held in the three or four jaw chuck, taking a typical example to
make this clear. Say we have to produce some 3mm thick washers. Initially, they will
have been turned from bar, drilled and parted off a little on the long side. They
then need returning to the lathe for the reverse side to be faced and the thickness
reduced to 3mm, this is where the problem arises.
I am sure that all workshop owners will realise that it is difficult to accurately
hold such a thin part so that when faced the thickness will be consistent. This is
where supporting the rear of the workpiece using a backstop fitted with a supporting
face becomes one way to proceed. I say one way as using soft jaws suitably bored
is another, in some cases preferable, especially if precision is a factor. However,
if a back stop is to be used then it will be necessary to position the support face
quite accurately, say within 2.5mm +0 – 0.5mm from the front face of the chuck's
jaws. This accuracy being difficult to set with the original design.