Sheet metal (1/8 inch and less in thickness) has
EDGE PREPARATION
such a large surface area per unit of weight that
Joint edge preparation and proper spacing
heat stresses tend to produce warping or buckling of
between the edges of the parts are important in any
t h e sheet. This and the contraction effect
welding operation. The thickness of the plates and
encountered on long seams are the main points to
the joint design determine the amount of edge
be considered in sheet metal welding.
preparation required. Sheet metal is easily melted
and does not require special edge preparation. The
The effect of welding a long seam (over 10 or 12
faces of the square edges can be butted together
inches) is to draw the seam together as the weld
and welded. This type of joint can be used on
progresses. If the edges of the seam are placed in
sheets up to 1/16 inch thick. For metal thicknesses
contact with each other throughout their length
from 1/16 to 1/4 inch thick, a slight root opening
before welding starts, the far ends of the seam will
between the parts is necessary to obtain complete
actually overlap before the weld is completed.
penetration if welded from both sides. Plate over
3/16 inch thick and welded from one side requires
One way of overcoming this effect is illustrated
beveled edges and a root opening as required by
in figure 7-22. The two pieces to be welded are
MIL-STD-22. For oxyacetylene welding on plate
placed with an increased allowance at the far end,
over 1/4 inch thick, the edges are beveled at an
and as the welding progresses, the two pieces are
angle of 35 to 45, making the groove angle from
drawn together. This allowance is generally one
70 to 90. These edges can be prepared by flame
metal thickness per foot of seam.
cutting, shearing, flame grooving, machining,
chipping, or grinding. In any case, the edge surfaces
Another method of controlling expansion and
should be free of oxides, scale, dirt, grease, or other
contraction is by the use of chill bars. Heavy pieces
foreign matter.
of metal are placed on either side of the weld. They
absorb the heat and keep it from spreading across
Plate from 3/8 to 1/2 inch can be satisfactorily
the whole surface area. Copper is commonly used
welded from one side only, but heavier sections
for chill bars because of its ability to absorb heat
should be welded by preparing the edges from both
rapidly. Welding jigs sometimes use this same
sides. Generally, butt joints prepared from both
principle to remove heat from the base metal. (See
sides permit easier welding, produce less distortion,
fig. 7-23.)
and ensure better qualities in the weld metal in
heavy sections than do joints made from one side
only.
TEMPERATURE CONTROL
The control of temperature before, during, and
EXPANSION AND CONTRACTION
after welding is often a matter of vital importance.
Preheating and postheating are specified for many
Heat causes metals to expand, and cooling
welds on many types of metals. Control of interpass
causes them to contract. Uneven heating will,
temperature is important in all multipass welds.
therefore, cause uneven expansion, or uneven
cooling will cause uneven contraction. Under such
conditions, stresses are set up within the metal.
T h e s e forces must be relieved, and unless
precautions are taken, warping or buckling of the
metal takes place. When cooling, if nothing is done
to take up the stress set up by the contraction
forces, further warping may result.
I f the
surrounding cool sections of the metal are too heavy
to permit this change in shape, the stresses remain
within the metal itself. Such stresses may cause
cracking while cooling or may remain within the
metal until further force is applied, as when the
piece is put into use.
Figure 7-22.--Allowance for a straight butt weld.
7-17