The impetus for producing item this was provided by a contact suggesting that an
article on boring small holes (10mm and smaller) would be a worthwhile subject but
whilst the main purpose of these pages will be boring small holes, larger holes will
also be included.
Boring small holes,why?
Why then bore small holes when the range of metric drills will provide hole sizes
in increments of 0.1mm (nominally 4 thou) and many of these gaps will include imperial/number/letter
size drills making the increments even smaller? The most obvious reason is that the
task in hand demands a hole size other than those provided by the drills available.
Even so, if the hole size required matches the size of an available drill there are
still reasons for boring rather than drilling. Most likely, the hole needs to be
very accurate in diameter, something that even a new drill cannot reliably guarantee.
In many cases where the hole size is that important the part fitting into it will
either be rotating or reciprocating and the much superior finish possible with the
process will reduce the eventual wear that results. Concentricity is another factor
as even if a drilled hole is started with a centre drill there is no guarantee that
it will remain on line, in this situation boring the hole is essential.
Subsequent manufacturing stages may also demand a bored hole, for example, if the
task that follows requires that the whole outer surface is available for machining
then the part may have to be held on a mandrel, in which case, the hole size can
be very important. This especially if more than one part is being made and they are
to be held on a Taper Stub Mandrel. The viewer should see my pages regarding mandrels
should he or she need additional information regarding their use, see also Sk. 1.
Drilled holes will invariably be over size though only very slightly with a good
quality drill, as purchased. A feature of this however is that the hole size will
reduce to the drill size just as it breaks through making the hole smaller at one
end than the other, if nothing else this can result in confusion. Because of this,
the use of drilled holes when using mandrels, especially the taper variety, can be