Fig. 1767. Fig. 1768. Fig. 1769.
In the smaller sizes of auger bits the twist of the spiral is made coarser, as in [Fig. 1767], which is necessary to provide sufficient strength to the tool. For the larger sizes the width of the top of the flute (t, [Fig. 1765]), or the land, as it is termed, is made narrow, as in [Fig. 1768], for holes not requiring to be very exact in their straightness, while for holes requiring to be straight and smooth they are made wider, as at d, in [Fig. 1769], and the wings a, b in the figure extend farther up the flutes so as to steady the tool in the walls of the hole and make them smoother. It is obvious that the conical screw requires to force or wedge itself into the wood, which in thin work is apt to split the wood, especially when it is provided with a double thread as it usually is (the top of one thread meeting the cutting edge a in [Fig. 1765], while the top of the other thread meets cutting edge b).
Fig. 1770.
In boring end-grain wood, or in other words lengthways of the grain of the wood, the thread is very apt to strip or pull out of the wood and clog the screw of the auger; especially is this the case in hard woods. This may be to a great extent avoided by cutting a spiral flute or groove along the thread, as in [Fig. 1770], which enables the screw to cut its way into the wood on first starting, acts to obviate the stripping and affords an easy means of cleaning. The groove also enables the screw to cut its way through knots and enables the auger to bore straight.
In boring holes that are parallel with the grain or fibre of the wood, much more pressure is required to keep the auger up to its cut and to prevent the thread cut by the auger point from pulling or stripping out of the wood, in which case it clogs the thread of the auger point and is very difficult to clean it out, especially in the case of hard woods.
