However, when attempting a very accurate final diameter, it may be necessary to
infeed the tool a second, and maybe third, time before setting on more depth. The
reason for this caution can best be explained by an example.
Going back to our setting of depths of 0.25mm resulting in a deflection of 0.05mm.
If at the final stage a further 0.08mm depth is set on, as measurement of the diameter
indicates, the tool will actually be requesting 0.13mm to be removed (0.08mm plus
the deflection of 0.05mm). As this is only about half of the depth of previous cuts
the deflection will be less than 0.05mm resulting in more than 0.08mm being removed
and the hole therefore oversize. When nearing the required diameter do take account
of tool deflection by feeding the tool in a second , maybe a third and fourth time
before measuring and setting on an increased depth using the cross slide.
3. A rather more obscure reason for tool deflection occurs when a bore is being made
that is near the cutters minimum. In this case with some cutter shapes, such as those
shown on the previous page, there is little room for the swarf being produced and
it is carried over the tools shank with the rotation of the workpiece. This can then
be forced between the cutter shank and the bore being made resulting in deflection
of the cutter, if you have not firmly secured the cutter I have known the tool to
be physically moved.
This problem is most likely with blind holes in which case the swarf should be removed
as far as is possible after each cut, perhaps even removing the tool a number of
times during each pass.
4. Occasionally, you may find that the tool is refusing to cut cleanly even though
it would appear that it should do. This occurs when a bore close to the cutters
minimum is being attempted and often appears to start satisfactorily but soon ceases
to cut properly when perhaps no more than 1/2mm deep. This is because the secondary
clearance of the cutter is making contact with the bore, which may be difficult to
Rather than looking for a smaller cutter, or grinding more from the secondary clearance
of the cutter being used, raising the tool within the bore will frequently overcome
the problem as it increases the clearance between tool and bore. In the same way
that a knife tool will work well below centre height a boring tool will work when
Finally, for guidance on producing the small boring tools seen on the previous page,
see my pages on forming small boring tools. Other small tools are also illustrated.