Laying out is the planning of the work on the surface of the material that is to be made into the finished part. It is the scribing of lines which indicates cuts to be made, the centerlines of holes to be drilled, and other details that guide the worker in completing the job.

The precision of the finished part depends largely on the care the worker takes in making the layout. The type of part determines the tools to be used. Castings, forgings, weldments, and rolled steels are laid out  to establish reference lines or planes which are used to set up the part on the machine, to outline the surfaces that are to be machined, and to show the amount of metal to be removed from each surface.

Castings and forgings usually have extra material on all surfaces which are to be machined, including cored holes. The first purpose of the layout is to see if enough material exists on all of these surfaces to "clean up" or "machine out."

Sometimes, a core will shift in the casting process and not enough material will be left to "clean up" to the finished surface requirement. When the core shift is minimal, the layout can be shifted in order to save the part. If the casting cannot be saved, at least the only additional cost that was put into the casting was the layout and not the machining time.

Another reason for laying out a workpiece to be machined is to locate and outline the position of holes and surfaces to be cut. A layout line will either confirm to the machine operator that he is correct in the cutter location, or alert the machine operator as to a possible error in locating the cutter to the part.

Although castings, forgings, and weldments are usually laid out before machining any surface, additional layout work is often done after some machining operations have been performed. In some cases, usually involving cold rolled material, the layout operation is performed after the part has been squared up.

Not all workpieces are laid out. Parts made from rolled bar stock and those parts having a relatively simple shape are typically machined directly without making a layout.

In layout work, a base or reference surface is selected from which to begin making the measurements. On a flat layout, this is commonly the smoothest, straightest edge on the piece. This surface must be kept clean and free from scratches, nicks, and burrs that would affect the accuracy of the reference surface and the layout work being done on it.

On some layouts, base lines instead of surfaces are used as reference points for measurements. Before the layout is begun, the workpiece is coated with a "bluing" or purple layout dye. This makes the scribed lines highly visible, thus contributing to the accuracy of the work. After the part has been machined, the layout dye can be washed off with lacquer thinner or polished off with abrasive cloth.

In order to ensure the accuracy of the work and to reduce the chance of errors, the various types of lines making up a layout must be laid out in a definite sequence. This sequence is outlined in the illustration at the end of this lesson. Make sure you fully understand the correct technique and sequence of operations to correctly lay out a workpiece. A poor job of layout will result in a scrap part.