If the machine be fitted with metal strip brushes, frayed ends should be clipped square with a pair of shears, the ends thoroughly cleaned from any dirt or carbonized oil, and replaced in their holders. Gauze and wire brushes require a little more attention. When their position on the commutator has been well adjusted and looked after, so that little or no sparking has taken place, it is generally only necessary to wipe them, clean the brushes and clip off the fringed edges and corners with the shears, or a pair of strong scissors. If, however, the machine has been sparking, the faces will be worn or burnt away, and probably fused. If such be the case, they will need to be put in the filing clamp, and filed true. A convenient method of trimming carbon brushes, or of bedding a complete new set of metal brushes, is to bind a piece of sandpaper, face outwards, around the commutator after the current has been shut off, and then mount the carbon or metal brushes in the holders, adjusting the tension of the springs so that the brushes bear with a moderately strong pressure upon the sandpaper. Then let the machine run slowly until the ends of the brushes are ground to the proper form. Care should be taken, however, that the metal dust given off does not get into the commutator connections or armature windings, or short circuiting will result. If the contact faces of the brushes are very dirty and covered with a coating of carbonized oil, etc., it will be necessary to clean them with benzoline or soda solution before replacing. [Illustration: Fig. 710.--Bissell brush gear. The brushes are held in the brushholders radially and work equally well with armature running in either direction. Brushes can be renewed and adjustment made while machine is in operation.] Ques. Describe a filing clamp. Ans. As usually constructed, it consists of two pieces of metal, both shaped at one end to the correct angle, to which the brushes must be filed. One of the pieces of metal (the back part) has a groove sufficiently large to accommodate the brush, which is clamped in position by the other piece of metal and a pinching screw. If the clamp be not supplied with the machine a convenient substitute can be made out of two pieces of wood about the same width as the brush. One end of each piece is sawn to the correct angle, and the brush placed between the two. In filing, the brush is fixed in the clamp, with the toe or tip projecting slightly over the edge of the clamp, and the latter being fixed in a vise, the brush is filed by single strokes of a smooth file made outwards, the file being raised from contact with the brush when making the back stroke. [Illustration: Fig. 711.--Jig for filing brushes to the correct bevel; used with copper brushes to fit them to the commutator.]