2O.
OPERATION OF CUTTING EQUIPMENT.
a. Setting up and shutting down a cutting outfit. The rules for setting up
and shutting down a cutting outfit are the same as for a welding outfit, in
practically every detail, and should be carefully adhered to. Particular care
should be taken to avoid oil, grease, or other easily oxidizable lubricant on
fittings.
b. Selection of cutting tip. The correct style and size ,of cutting tip to
use is determined, primarily, by the kind of material to cut, its thickness, and
its surface condition. Some materials are harder to cut than others and require
different cutting tips. The main factor that controls the size of the cutting
orifice to be used, however, is the thickness to be cut. Very rusty, scaly, or
painted surfaces offer greater resistance to the preheating flames and require a
higher intensity of heat. Refer to figure 16 as a guide in selecting the proper
tip.
c. Cutting oxygen pressure, cutting speed. The use of oversize cutting tips
and excessive oxygen and acetylene pressures is uneconomical. A cutting tip with a
large cutting oxygen orifice necessarily cuts a wider kerf and consumes more oxygen
than a tip with a smaller orifice, even if lower oxygen pressure is employed.
Cutting oxygen pressure, of itself, avails little beyond forcing sufficient oxygen
into the cut to oxidize the metal in its path and blow away the resulting oxide and
unconsumed molten iron. On the other hand, excessive pressure does cause the
cutting oxygen jet to swirl and spread out after leaving the tip, thus producing a
wider kerf with uneven sides and increasing oxygen consumption. For maximum
efficiency, cutting tips should be operated at, or near to, normal capacity but not
beyond. This includes normal gas pressures and cutting speeds that utilize the
cutting oxygen jet to best advantage for producing the quality of cut desired.
d. Starting the cut. In most cases, cuts are started at the edge of the
piece with the cutting torch held lightly, but steadily, so that the ends of the
preheating flame cones are about 1/16 inch above the surface of the material. When
a spot of metal at the top of the edge has been heated to a cherry red, the trigger
or lever controlling the cutting oxygen jet is pressed and cutting begins.
e. Piercing holes. When starting a cut in the metal away from the edge, or
in piercing a hole, more time is usually consumed in bringing the spot of metal to
the kindling temperature than with the edge starting method. After the spot is
sufficiently heated, the torch is raised about 1/2 inch above the normal position
for cutting and the cutting oxygen is turned on slowly. As soon as the plate is
perforated, the torch is again lowered to the normal height above the work and the
cut is completed. In piercing holes, care must be taken to avoid slag plugging the
cutting orifice. Drilled holes are used in certain cases, particularly, to form a
straight, smooth, starting edge for a heavy internal cut.
f. Hand cutting. In hand cutting, the torch is held steadily, but not
tightly, with the left hand supporting it a few inches back of the head and the
right hand on the handle in position to operate the cutting oxygen trigger or lever
with the forefinger. As in all manual arts, the results obtained depend largely
upon the skill of the operator. Fortunately, the operation of hand cutting is
relatively simple. The average conscientious operator soon acquires proficiency.
He must learn to hold the torch at the proper height above the work without
wavering and to advance it uniformly at correct speed along the line of cut. If
the torch head wavers from side to side a wider kerf will be made, the speed will
be reduced, and the oxygen consumption will be increased. If the torch is moved
along the line of cut at an irregular speed, the cutting will be slowed down and
the gas consumption will be increased. Poor quality cuts result in either case.
For best workmanship, the. operator should look down into the cut, as it
progresses, and watch the drag or amount which the cut curves backward in a
direction opposite to the travel. He should
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