Figure 2-66.Lap seams
Figure 2-67.Layout of lap seams for riveting.
GROOVED SEAMS are useful in the fabrication
of cylindrical shapes. There are two types of grooved
seams-the outside grooved seam and the inside
grooved seam (fig. 2-68). The allowance for a grooved
seam is three times the width (W in fig. 2-68) of the
lock, one half of this amount being added to each edge.
For example, if you are to have a 1/4-inch grooved
seam, 3 x 1/4 = 3/4 inch, or the total allowance; 1/2 of
3/4 inch = 3/8 inch, or the allowance that you are to
add to each edge.
The PITTSBURGH LOCK SEAM (fig. 2-69) is a
comer lock seam. Figure 2-69 shows a cross section
of the two pieces of metal to be joined and a cross
section of the finished seam. This seam is used as a
lengthwise seam at comers of square and rectangular
pipes and elbows as well as fittings and ducts. This
seam can be made in a brake but it has proved to be so
universal in use that special forming machines have
been designed and are available. It appears to be quite
complicated, but like lap and grooved seams, it
Figure 2-68.Grooved seams
Figure 2-69.Pittsburgh lock seam.
consists of only two pieces. The two parts are the
flanged, or single, edge and the pocket that forms the
lock The pocket is formed when the flanged edge is
inserted into the pocket, and the extended edge is
turned over the inserted edge to complete the lock. The
method of assembling and locking a Pittsburgh seam
is shown in figures 2-70 and 2-71.
The allowance for the pocket is W + W + 3/16
inch. W is the width or depth of the pocket. The width
of the flanged edge must be less than W. For example,
if you are laying out a 1/4-inch Pittsburgh leek seam
(fig. 2-72), your total allowance should be 1/4 + 1/4 +
3/16 inch, or 11/16 inch for the edge on which you are
laying out the pocket and 3/16 inch on the flanged