Fig. 1774.
Fig. 1775.
[Fig. 1774] represents an extension bit, being adjustable for diameter by reason of having its cutting edge upon a piece that can be moved endways in the holder or stem. This piece is ruled with lines on its face so that it may be set to the required size. Its upper edge is serrated with notches into which a dish screw or worm meshes, so that by revolving the worm the bit piece is moved farther out on the spur or wing side or end, it being obvious that the spur must meet the walls of the hole. A better form of extension bit for the end grain of wood is shown in [Fig. 1775], the cutting edge being a curve to adapt it to severing the fibre in end-grained wood, as was explained with reference to [Good’s auger bit].
Fig. 1776.
[Fig. 1776] represents a drill for stone work, whose edge is made curved to steady it. This tool is caused to cut by hammer blows, being slightly revolved upon its axis after each blow, hence the curved shape of its cutting edge causes it to sink a dish-shaped recess in the work which holds that end steady. The end of the tool is spread because the corners are subject to rapid wear, especially when used upon hard stone, and the sides of the drill would bend or jam in the walls of the hole in the absence of the clearance caused by the spread. To prevent undue abrasion water is used.