The internal surface, when its accompanying cylinder is of moderate depth, is turned with the flat tool and armrest. At increased depths, the tool is used in a long handle; it may be held upon the armrest in the usual manner, or its long handle may be held directly beneath the right fore arm, which then presses upon it. For still greater depths, a strong, long handled flat tool is used upon the tee rest alone, placed parallel with the work. The management of the tools is the same as that described for the external surface.

Fig. 425. Fig. 426. Fig. 427.

The Internal Surface 400301

The internal surface and cylinder are finished together, to produce their internal corner. This is readily formed, for the sides and cutting ends of the flat and right side tools, being less than right angles, their cutting ends when working upon the surface, may be carried to the extreme circumference or corner, without their sharpened sides touching the cylinder. While in like manner, their sides may advance cutting to the extreme end of the cylinder, without their cutting ends taking effect upon the surface. The tools may therefore he applied, each to its particular superficies exclusively, or either tool may travel along that into the corner, and then by alteration of the horizontal angle at which it is held, it may be withdrawn, with its side or end cutting along the other. The truth of the internal surface and the accuracy of the corner formed with the cylinder, are tested by a small steel square, fig. 425, or by the turning square with sliding blade, which also transfers the measure for the depth in fitting one piece to another. The stock of the turning square, is held in contact across the mouth of the aperture, and the steel blade at right angles to it, is pushed down until it touches the surface fig. 426; the greatest depth is then sought, by traversing the blade of the turning square from the edge towards the center, the blade is then fixed by the set screw, and the whole surface reduced to this level.