Different bore depths for the same diameter would at first sight seem unnecessary
but those for a shallower depth would of course be more robust and could be pushed
harder when used in industry.
High speed steel boring tools are available commercially but only in a very limited
range as far as I know. On that basis they will have to be ground in the workshop,
what then are the options? Ground from square toolbits is one but frequently requires
a lot of metal to be removed making it very laborious and time consuming, I know,
as it was a method I used in my early workshop days. However, ground from round tool
bits and held in a simple holder will considerably reduce the grinding to be done
and there will be a saving in cost also as it uses much less tool steel, Photograph
6. These are for use at a minimum diameter just larger than the diameter of the tool
bit being used.. See “Finally” next page.
Using the boring tool
Largely, the process, follows that for turning an outside diameter but with a number
of minor (?) differences.
1. The removal of metal being made within a hole the process is difficult to see,
almost impossible in the case of smaller holes. Unfortunately, there is little that
can be done to minimize this problem, it is a case of listen to the sound being produced
and taking note of the feel of the leadscrew handwheel. If they are not as expected
then remove the tool and investigate.
2. With the boring tool being much more fragile than a conventional knife tool it
is bound to deflect under load unless the depth of cut and feed rate are kept well
down making the process a very time consuming one. Therefore, some deflection must
be accepted as normal and suitable measures taken.
As an example let us assume a depth of cut of 0.25mm has been set but the tool deflects
0.05mm so that only 0.20mm is removed. If now the tool is set a further 0.25mm deep
it will in actual fact be being called to remove 0.30mm in which case the deflection
will be even more resulting in the depth of cut and amount of deflection increasing
with each pass.
To limit this, one of two things, or both, should be done. This is, after the tool
has entered the bore is should be remove slowly allowing the tool to cut both on
entering and leaving the bore. This second cut, known as a spring cut, will largely
eliminate the effect of the tool deflecting so the next cut can be set deeper once