An important preliminary in a machine shop is the lying off and marking of work. Forgings, castings and other work to be machined must be marked to indicate the location of holes to be drilled, the axis or axes of hollow parts to be bored out, and in general the limits which guide the machinst in cutting away superfluous metal to bring the work to the finish and dimensions required by the drawing. For example, Fig. 152 shows a casting of two small cylinders cast in a single piece for a steam-launch engine. This casting is in the rough just as it was received from the foundry. The two pieces of wood WW have been placed in the cylinder ends to aid in marking. The casting must be so marked that the machinist can bore the cylinders to bring the axes the required distance apart, face the cylinder ends to give the cylinders the required length, and in short machine off all surfaces which require machining, and drill all holes, as required by the drawing.

The marking must be done from some point, line or plane of reference which is usually chosen to agree with the axes of reference shown on the drawing. From this point, line, or plane, all holes and machined surfaces are located on the work.

In the casting here shown a plane of reference is chosen which includes the axes of the cylinders as determined by the outer surfaces of the cylinders. This plane is located on the casting by the line Klmk, which completely girdles the casting. The line is scratched by a scriber and further marked at intervals by the center punch to avoid its obliteration. The axes of the two cylinders are then located and marked by aid of the wood pieces W, and from these axes all other marks are located. All dimensions are supplied by the drawings.

Fig. 152. - Casting Marked for Machining.

Castings and forgings are often whitewashed or chalked where their reference planes are to be marked, as this helps the workmen to see the marks readily.

The machining on the casting here shown is begun by planing the face MG and then the face LK of the two steam chests. These finished surfaces give convenient parallel planes for the transfer of measurements. Also the face MG forms a base for securing the casting on the machine which bores the two cylinders.