Possessing this feature in common are three distinct types of machines. The Brandt drill is a very satisfactory one, used, however, more often in Europe than in this country. The cutting is done by chilled steel teeth revolving very slowly (5 to 8 r.p.m.) and forced against the rock under very heavy pressure.
In a diamond drill carbons or black diamonds are set in a soft steel bit which is screwed to the end of the revolving cylinder or core barrel. In a bit for a common size (1 3/4 in. diam.) core eight diamonds are usually set, four projecting slightly outside and four inside. They must be set with just enough projection to give a thickness of cut such that the tool will not bind; and the process of setting them should be undertaken only by an experienced man.
In a shot machine the cutting is done by chilled steel shot poured in as required and ground against the rock by a soft steel cutting bit. Sometimes instead of round shot granular fragments are used. In a diamond or shot machine the pressure on the bit is comparatively small, usually not more than the weight of the revolving barrel; the speed is much higher than with the Brandt drill, being 200 r.p.m. to 400 r.p.m.
In any type of machine a stream of water is supplied through the revolving core barrel to the cutting edge to carry the d6bris away between the core barrel and the wall of the hole. When the rock is overlaid by earth or loose material it is necessary to sink a casing through such material after the manner described under Wash Borings, clean it out, and introduce the boring tools through it.
It is not the purpose here to discuss all of the advantages and disadvantages of the different methods, nor the various means of overcoming all the troubles encountered in difficult rock.
The most exhaustive treatise imaginable would not render it less necessary to employ a skilled runner to do the work. However, a few of the broader comparisons between diamond and shot machines may be made. A diamond drill will drill holes at any angle of inclination, while with a shot machine the holes must be very nearly vertical in order that the shot may stay at the cutting edge. In ordinary borings for exploring the foundation of a dam this limitation on the shot machine would count for very little, as it is rarely necessary to drill other than vertical holes. When an open seam is encountered the shot or granular fragments are often lost in the seam, in which case the drill runner may then cut teeth on the end of the bit and proceed until again in rock that will retain the shot.
With a diamond drill the maximum diameter of hole is about 15 in., and with a shot machine 30 in. or more. It is obvious that the large sizes are for the purpose of wells, elevator plungers or column foundations; strictly exploration purposes would be as well served by a 2-in. or 3-in. hole as they would by anything larger.
It should be remembered that in a very deep hole it may be necessary to step down in diameter, as with a common percussion drill on shorter holes, and allowance should be made for such a possibility. Both types of drill have very light rigs for use in mountainous country or in places where transportation is a serious question compared with the amount of boring required. However, for a number of holes at a site not too difficult of access a heavier machine run by a steam or gas engine is certainly advisable. There seems to be no marked difference between the two types as regards cost per foot, rate of progress or ability to recover satisfactory cores.