Harold Hall

Workshop Processes


Boring Small Holes on the Lathe. Harold Hall

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 difficult.