To support a piece of work so that its chosen plane of reference may be parallel or perpendicular to the top of the table, a number of chocks and bars are used. Fig. 154 shows specimens of chocks C and of rectangular bars B. These are used in pairs and each pair must be exact counterparts.

Among the tools much used on the marking-off table are the following:

(1) Steel rule (Fig. 155). Made in various lengths. Opposite faces and edges are ground parallel.

Fig. 155.

(2) Straight edge. A long steel rule without graduations. Used to test plane surfaces and to establish straight lines as references for measurements.

Fig. 156.

Fig. 157.

(3) Combination level and square (Fig. 156.) Used as a square and a level. The auxiliary center head is used to locate the centers of cylindrical work. The ordinary steel square is also much used.

(4) Dividers (Fig. 157).

Outside.

Inside.

Fig. 158.

(5) Firm-joint calipers (Fig. 158). For measuring diameters, thicknesses, etc.

Fig. 159.

Fig. 160.

(6) Center punch (Fig. 159). This marks holes to be drilled, and marks points for locating lines on work.

(7) Scribers (Fig. 160). These have hardened steel points for scratching lines on metal.

(8) Surface gage (Fig. 161). This is used in adjusting work on the marking-off table, and for locating points or lines on the work. It consists essentially of a base, a post and a scriber. The under side of the base is planed to rest firmly on the table, and is grooved to rest symmetrically on a cylindrical surface. The scriber and post are so mounted and controlled by clamp screws that the scriber points may be adjusted to any position desired within the reach of the instrument.

Fig. 161.