As cuts of 0.001mm, or even smaller, may be attempted, it is essential that the cutting
edge is not above centre, it will only rub. This is especially so at smaller diameters,
typically less than 5mm. As measuring this is all but impossible, the only way of
being sure the tool is not above centre is to deliberately place it below centre.
This can be easily done by facing a piece of metal ensuring that the very smallest
projection remains in the centre. Even so, too low, and turning very small diameters,
say 1mm, will become a problem
The tool infeed
This is made up of a number of factors. First, the feedscrew dial will have divisions
of 0.01mm or even larger, typically 0.025mm in some cases, making it difficult to
set on a depth of cut of 0.001mm with a degree of certainty. However, this is far
from the whole problem. Even if the dial had micro divisions that could be read ,
there is little chance that the cross slide would reliably traverse by this small
amount. There are a number of reasons why this is and we are into complex territory,
but will attempt to give the basic reasons.
First, sufficient force will need to be generated to overcome the friction in the
cross slide so as to cause it to move. If then the lead screw and nut are only just
touching this degree of force will only be developed when the leadscrew has been
rotated, and the materials compressed enough to create a force sufficient to overcome
the friction. The result of this is, that whilst the handle has been turned no movement
of the cross slide will result. In addition, there will also be the oil between the
screw and nut surfaces that will have a marginal effect.
Another, and complementary problem, is that if the cross slide movement is set so
that movement is quite easily made, then there will be little compression between
screw and nut. In this case, whilst the previous cut is being taken, vibration can
cause the handle to rotate leaving a gap between screw and nut. In this situation,
the handle will then need rotating to close the gap and, at the very small movements
being considered, it will be very difficult to determine when movement of the cross
The above can be partially overcome by making the cross slide stiffer, such that
having made an adjustment there is still some compression between screw and nut holding
it in place during the following machining operation. If the user prefers that the
cross slide movement should not be that stiff for every day use, it can be stiffened
temporarily by slightly applying the slide lock screw.
Such an approach can also be worthwhile when lesser degrees of accuracy are being
worked to, say within 0.025mm.