BBBBflt] «BlJIUrIrlr - Clpdigital.org

162 Fbrging-Stamping - Heat Treating
Impulse Blades.
Fig. 7-c shows two typical examples of impulse
blades. These are fabricated by either of two methods
(a) milling from bar stock, whereby the port is
milled out on revolving drum fixtures and (b) drop
forging and hot swaging with suitable dies in a forging
press concluding with milling operations for base
and edges. Close limits of precision are required in
either case and each blade is subjected to a V guage
inspection to verify the pitch thickness. The finish is
such that the curved surfaces nest together to proper
pitch without milling or other machine finish. Individual
blades of a set may depart from standard in
thickness of pitch not to exceed .005-in., but in all
cases blades guaging plus, must be compensated for
pelative Displacement setweci
JACENT SECTIONS MijsT NOT £,CEEE.
the DiTEEPENCE BETWEEN THE ABOVE
TOLERANCE'
FIG. 8—Sketch of multiple exhaust blade, giving an idea of
the precision required.
by corresponding blades guaging minus, so that the
algebraic sum of all departures from standard for a
full circle or row shall be zero.
Low Pressure Blades of Parallel Section.
Where the necessities of design do not require extreme
length for a low pressure blade out are beyond
the range of physical qualities of manganese copper,
standard practice employs a drop forged blade of
straight parallel section, for moving rows usually of 5
per cent nickel steel, of which Fig. 7-D shows a typical
example. For stationary rows a parallel section in
manganese copper is used. Although the latter blade
is essentially of parallel section, its base is reinforced
May, 1925
by upset thickening to afford additional strength at
and near the root.
Forged Low Pressure Reaction Blades of
Special Types.
In recent designs of Westinghouse turbines of
large capacities, low pressure blades of 20 to 24 inch
length and even longer have been used. Owing to
the stresses due to high peripheral speeds, also owing:
to radial angularity between the long blades, departure
in certain instances from a straight parallel blade
section has been effected in two particulars: A—the
blade has been tapered to keep down the weight and
stresses due to centrifugal force, and B—the blade has
been twisted or warped for two reasons in order that:
First—the inlet angle may permit the steam to be received
tangentially at all points throughout the length
of the blade, and, second—the passage between the
blades may be of such shape throughout the length of
the blade as to give the requisite nozzle effect in guiding
the steam through the blades for best efficiency.
Various stages in the course of manufacture of a
multiple exhaust blade are shown in Fig. 7-E. On
the bottom is shown the blank sheared to length from
round bar stock. Next is shown the blank after being
broken down under a Bradley hammer in preparation
for the forging die. Next is the forging untrimmed
after the first drop. Next is the finished forging with
the flash trimmed off, and at the top is the blade fully
machined. Not only is the forging of these blades an
undertaking of unusual refinement, but the machining
requires a high degree of precision in workmanship,
as well as specially developed devices and fixtures.
The sketch of a typical multiple exhaust blade. Fig.
8, shows certain dimensions and characteristic tolerances
from which the required standard of precision
may be judged.
Preparing Models of Dies.
The Keller Mechanical Engineering Corporation
of Brooklyn, New York, was instrumental in working
out methods of preparing models and cutting drop
forge dies for blades for the multiple exhaust design,
an undertaking of no small difficulty or magnitude.
Each cross section of blade to be modeled has its
appropriate contour gauge made of '^ in. sheet steel
and fitted with extreme precision to lapped and
ground steel templates or master pieces. Bottom
halves of these contour guages are shown mounted in
a plate. Fig. 9-A, so that each section bears its proper
relation to its neighbors, vertically, horizontally and
laterally, and these guages form the directing curves
from which by the aid of straight edges, the warped
surfaces are generated in modeling clay, Fig. 9-B.
From this clay model a plaster cast is taken, Fig. 9-C.
and around this plaster cast a cement model Fig. 9-D,
was poured, which when thoroughly hard becomes
available as the model for cutting the bottom die in
an automatic engraving machine. A cast iron frame
encloses the cement model to protect it from breakage
or injury in use. Before being set to work in the
engraving machine, the cement model bottom die is
used in shaping a model for the upper die. Fig. 9-E
shows the model partly prepared with modeling clay,
accurately shaped templates of sheet zinc resting on
the cement form and bedded in the modeling clay
provide the directing curves for generating the sur-