Drilling and Boring
long hole boring
By far the most daunting task facing the instrument maker is that of producing a straight hole of small diameter through a long piece of wood. The majority of pipe making manuals show twist drills that have been lengthened by silver soldering commercial bits to silver steel rod which although they cut fast and efficiently will persist in following the grain and may even in bad cases break out through the side of the work in hand.
It is necessary to use what is classified as a "d" bit, a form of drill that will cut true irrespective of the grain to produce a bore with little or no run out. Two main types fulfill this criteria and are shown below and are a straight forward type whereby the cutting end filed at a half diameter is relieved in both planes (assuming anti-clockwise direction of the piece being bored) and the parrot nosed style where the end of the bit is domed before filing or milling to a half section and relieved as shown with a semicircular section.
If boring through a short section then it is enough to hold the work in the jaws of the chuck but once this starts to get beyond around 200mm in length then some form of support is necessary other wise there is a real danger of the work being thrown from the chuck causing physical injury. This support is achieved in one of two ways either by using a steady to support the free end in which case it will be of the three or four jaw kind or more rarely a circular plate with a series of circular tapered cutouts to allow access. A good and easily made form utilises scrap bearings set in wooden blocks and clamped to the lathe bed. The second method is to use a hollow center which is really the only method suitable for producing offset bores in stocks and multi-bored sections of the pipes.
fig i: "D" bit drill
fig ii: Parrot style drill
A variation when boring holes of some length that will be reamed to conical form is to step drill from the beginning an example of which is shown in figure iii. Here one end of a larger drill is reduced to provide a start for the next size down and so on. For pieces that are bored at constant diameter a parallel reamer is used to bring this accurately to it's final size which may take the form of a "d" style bit with guiding spigot or a commercial fluted reamer silver soldered / braized to a silver steel rod. This latter form can only take out the barest minimum of material but if used with plenty of oil (linseed / almond) produces a highly polished and perfect bore.
For creating a tapered or conical bore the piece is first bored with the normal "d" bits either at a constant diameter which is then step bored using increasing sizes of either flat bits or reamers or by initially step boring and then removing these with a tapered reamer.
fig iii: Step drill for starting next bore size down
fig iv: Parallel reamer