These planes are illustrated in Figs. 32 and 33. They are made of different curvatures, and a set of assorted sizes, especially the rounds, are almost indispensable to the pattern maker for finishing semi-circular core boxes, for making fillets, and for working out curves of every description, both concave and convex. Core-Box Plane. The core-box plane, shown in Fig. 34, while not indispensable, will be found to be a very rapid working and useful tool for making semicircular core boxes up to 2 1/2 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.
Fig. 32. Round Plane.
Fig. 33. Hollow Plane.
The core-box plane is constructed upon the principle that if the sides of a right angle lie upon the extremities of the diameter of a circle, the vertex of the right angle will lie upon the circumference of the circle. This is illustrated in Fig. 35, 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, e, or h, and the angles abc, def, and g hi will all be right angles.
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 put into effect:
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 arcs by straight lines on the face of the block, as shown in Fig. 36. Two very thin strips of hard wood are tacked along these lines, just outside of the wood to be cut away, as shown at a and at b in Fig. 37. 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 affect-ing the corners at a and b.
Fig. 34. Core-Box Plane.
When making the finishing cuts with this plane, care must be taken to adjust the cutter centrally, i.e., 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.
This tool, illustrated in Fig. 38, will be found very convenient for smoothing out sunken panels, for letting in rapping and lifting plates, and for all depressions below the general surface of the pattern. It will plane the bottoms of recesses to a uniform depth from the surface of the work, and will work into angles and corners that otherwise could be reached only by the use of the paring chisel.
Fig. 38. Router Plane.