Description of Layout Work
Laying out is the planning of the work on the surface of the material
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
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
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 layout a workpiece. A
poor job of lay out will result in a scrap part. You will need to know the
sequence on the test.
Other tools used to do layout work include precision steel rules,
layout rules, squares, bevel protractors, surface gages, height gages,
scribers, hermaphrodite calipers, dividers, prick punches, center punches,
hammers, V-blocks, parallels, angle plates, and a variety of clamps. These
layout tools are described in your text. Study them carefully before
attempting the review questions.
Figure 1. Part to be laid out.
Sequence for laying out
|1. Locate and scribe base lines.
|2. Locate all circle and arc centerlines.
|3. Scribe in all circles and arcs.
|4. Locate and scribe in angular lines.
|5. Complete all other object lines.
*Note: Steps 4 and 5 may be done in