Having come to the conclusion (See my pages on boring small holes) that the only
practical way of boring small holes is to use one piece cutters and the only economical
way of doing this is to produce them from round high speed steel in the workshop
itself, let us consider the requirements. I should though add that at the very smallest
sizes grinding by hand will be difficult, but not impossible, and some assisted
method is to be preferred. I will therefore illustrate the requirements using one
of my controlled feed grinding rests.
The first task it though to produce a holder for each diameter of high speed steel
being used, Sk. 1 showing a drawing for these, However, the viewer may need to change
the dimensions to suit the lathe on which they are to be used. The holder is bored
in one corner to take the cutter so that it can be mounted in anyone of the four
possible positions as the situation demands. Typically, when used with a very small
parting tool for parting off diameters of say 2mm or less, the action will need to
take place very close to the chucks jaws. Having mentioned a very small parting tool,
this highlights that the system can be used not only for boring tools but also for
others. These commented on at the end.
Whilst I realise that most workshops will only have an off hand grinder for sharpening
tools, and doing it largely free hand, the method will be essentially the same as
that that I illustrate. If being done free hand I would strongly advise the viewer
to place the cutter in its holder and hold this whilst grinding, providing you with
something larger to hold and making it easier to control. This also keeping the hands
further from the grinding wheel.
The boring tools will produce a minimum hole sizes a little above the diameter of
the high speed steel blank on which it is being formed.
When forming a boring tool the working end is similar to a knife tool used on external
diameters, Sk. 2A, , there are though two exceptions.
1. Due to the fact that when turning external diameters the curvature of the work
being produced adds to the clearance provided by the tool but in the case of internal
diameters the curvature reduces the clearance, the front clearance (4) has to be
increased, at least in the case of those for smaller holes. At much larger diameters
(say 25mm plus) this is less of a problem and a clearance angle of 5° will suit but
at smaller diameters I would suggest 10°. Secondary clearance (Z) will also be required,
even when used for larger holes, to cope with the internal curvature of the hole
being bored, see Sk. 2B.