HAND WOOD-WORKING TOOLS 427 



(d) The screw-driver bit. This bit is like the blade end 

 of a screw-driver, and is used for driving large screws. 



The mechanical principle underlying the brace and bit 

 is that of the wheel and axle. The circle (called the sweep) 

 formed by a complete revolution of the brace corresponds to 

 the wheel, and the circle made by the revolution of the bit 

 corresponds to the axle. 



492. Description of Plane. Another important tool is 

 the plane, and since it "cuts" its mechanical principle is that 

 of the wedge. The first planes used were comparatively 

 crude and were little more than heavy, thick wedge-shaped 

 cutters. The plane has been improved, however, until today 

 it is a very delicate tool consisting of a body in the bottom 

 of which there is a slit, called the throat, through which the 

 cutting piece, called the plane iron or blade, projects. The 

 end of the plane iron is sharpened on a bevel to a cutting 

 edge. A flat, curved piece of steel, called a cap iron, is 

 fastened against the plane iron about iV in. from the cutting 

 edge by a short, heavy screw, called the plane iron screw. 

 The cap iron serves to stiffen the plane iron, and also bends 

 and breaks the shaving, thereby preventing a splitting action 

 in front of the cutting edge. Just back of the throat of the 

 plane is the frog, fastened to the bottom by screws. The 

 object of the frog is to hold the plane and cap iron in place, 

 and to carry the thumb-screws by which the plane iron is 

 adjusted. The plane iron and cap iron are held firmly in 

 place against the front by means of a clamp worked by a 

 cam. The vertical adjustment of the blade regulates the 

 thickness of the shaving and is made by means of a thumb- 

 screw on the underside of the frog; the horizontal adjustment 

 of the plane iron is made by a lever just under the plane iron. 



