Perhaps a more obscure reason is where the hole is stepped and the step needs to
be flat rather than angled as would be the case with a drilled hole. A blind hole
with a flat bottom would also need to be bored, at least to finish its base, but
this is a rare requirement.
When boring holes with a top slide mounted tool there are clear limitations to the
sizes, diameter and depth, that can be achieved. Whilst it is difficult to place
precise values on these, especially depth, obviously the smaller the diameter the
shallower the depth that can be worked due to the fragile nature of the tool being
used. This is more evident with small holes and is likely to be one reason for the
initial request. As most types of tool are available in a range of sizes this article
will concentrate largely on the minimum diameter that they can work at.
There is though another major factor when boring very small holes that is not entirely
tool related, that is the available speeds on the lathe being used. Very small holes
will need very high speeds else it will be difficult to keep the feed per rev rate
small enough to avoid overloading the fragile cutter.
The Boring Tool Types
Brazed Tip Tooling.
This type of tool, seen in Photograph 1, is made in a range of sizes with 8mm square
shanks normally being the smallest and having a minimum bore diameter of nominally
12mm. Depth is more difficult to comment on as it will depend on the depth of cut
and the rate of feed but would suggest 50mm would be possible with an average depth
of cut and feed rates. Whilst they are essential for opening up pre cast holes in
cast iron they can be used with other materials.
Indexable Tipped Tooling.
There is a vast range in this category mostly outside the price range of the home
workshop owner. Fortunately though, more economic boring bars are becoming available,
typically that in Photograph 2. Minimum diameter for this is about 9mm with a nominal
depth of say 40mm. Like all the cutters I am suggesting, larger sizes are available.
Even smaller indexable boring bars are available, machining diameters as small as
4.6mm, these from suppliers to industry, they are of course much more expensive,
around £50 to £70 at the time I write this (2011), and even considerably more in