The core-box plane, shown in Fig. 35, while not indispensable, will be found to be a very rapid working and useful tool for making semicircular core boxes up to 2½ inches in diameter. By using the extension sides, one of which is shown in the illustration, and two pairs of which are always furnished, this tool will work accurately a concave semicircle up to 10 inches in diameter.

The core-box plane is constructed upon the principle that if the sides of a right angle lie upon the extremities of a diameter of a circle, the vertex of the right angle will lie upon the circumference of the circle. This is illustrated in Fig. 36, from which it will be seen that if the block of wood has been worked to a perfect semicircle, and the edges of the blades of a try-square or right-angled triangle touch the semicircular curve at its extremities, the right angle or corner will touch the arc at some point, as b, ear A, the angles abc, def, and ghi all being right angles.

Fig. 35.

Fig. 36.

To this kind of plane the objection is often made that it abrades and wears off the corners of the semicircle as it is being worked out. This, however, can be practically avoided if the following instructions are followed:

Carefully lay out the block from which the core box is to be worked, from a center line on the face of the block, describing on each end of the block a semicircle of the required radius; connect the extremes of the two end area by straight linee on the face of the block, as shown in Pig. 37. Two very thin strips of hard wood are tacked along these lines, just outside of the wood to be cut away.asshownat a and at bin Fig.38. These strips form rests for the sides of the plane while the heavier part of the work is being done. After working out the semicircle as far as the strips will allow, as shown by the dotted arc acb, the strips are removed, when the work can be finished without materially affecting the corners at a and b.

When making the finishing cuts with this plane, care must be taken to adjust the cutter centrally, i. c, so that it will cut equally to both right and left; otherwise the work will not be correct. If, however, the work has been done with care, the finishing may be completed with coarse, and lastly with fine, sandpaper held on a cylindrical block of radius slightly less than that of the required core box.

Fig. 37.

Fig. 38.