Lathe Setup, turning parallel, etc. plus a Between Centres Test Bar
This item is dual purpose, predominantly covering an essential part of the procedure
for setting up a lathe, and in so doing producing a between centres test bar.
It is understandable that a beginner may consider that a well made lathe will automatically
produce a parallel result. This is not so, and if the error is sufficient, satisfactory
work will be difficult to achieve. He or she may also think that, if such an error
exists, the lathe is at fault and there is little that can be done to rectify it,
for most lathe's this is not so.
First, it must be realised that no piece of metal is totally rigid and a typical
cast lathe bed is no exception, as a result, a major consideration is the effect
of mounting the lathe onto an uneven surface, distorting the lathe's bed in so doing.
However, this cause of a problem can also be used to advantage, with packing placed
under the lathe's feet, then any inherent error can be minimised. The approach provided
by Myford, and probably others, of a jacking screw under each mounting (Photograph.
1) rather than packing is to be preferred. By this method very slight bending of
the bed can easily be induced to overcome the errors present.
There is though an essential requirement for any method of adjustment that may be
adopted, and that is, the mounting must be sufficiently strong to create the necessary
forces to flex the lathe's bed. Of course, the force required will both bend the
lathe's bed and the mounting surface itself, and for it to work adequately the mounting
must have at least a comparable strength with that of the lathe. It must therefore
be mounted on a very rigid frame, ideally of welded construction, even so the frame
can be used with a wooden bench top, positioned above or below the frame.
Setting to turn Parallel
If you have read other explanations regarding setting a lathe to turn parallel you
may have come to the conclusion that the lathe must be mounted with its bed level
to a very high degree of accuracy, as frequently a precision level is deemed to be
required. This is not so and some articles do qualify this point. What is in fact
necessary is that there is no twist along the length of the bed, and the only easy
way to check this is by using a precision level so that if both ends are level then
the lathe's bed is not twisted. It would also be free of any twist if the bed sloped
by 1.000 degree at each end, but that would be difficult to measure.
Unfortunately, a precision level will be very expensive, especially when taking into
consideration that it will be used very infrequently. Fortunately, all is not lost
if a precision level is not to hand, as has been said, it is not necessary that the
bed is exactly level.
All pictures can be clicked on to provide a larger view