the turbine bearing housing (or head cover if the turbine bearing housing is
removed) and the trammel points in the draft tube. The installation procedure will
vary with boring bar design, but usually the boring bar thrust bearing and a guide
bearing are mounted on the turbine bearing housing or directly to the headcover.
Another guide bearing is usually mounted in a spider type frame welded to the draft
tube liner. The guide bearings usually have some adjustment to allow moving the
bar to center. When the boring bar is centered and its guide bearings locked in
place, the boring arm and tool slide can be installed. The motion of the tool slide
should be perfectly parallel to the bar for vertical cuts and perpendicular to the bar
for horizontal cuts. If necessary, the tool slide should be shimmed where it mounts
to the boring arm to make it perfectly parallel or perpendicular.
The boring bar drive speed should be adjustable, using either an electric motor with
solid state frequency controls or a hydraulic system with some means of varying
flow. The cutting speed for boxing will depend on the type of material being
machined, the diameter of the ring, the depth of cut, and the feed rate of the tool
slide. The Machinery Handbook and other machining reference books list a range
of recommended cutting speeds based on these factors. The optimum speed, cut
depth, and feed rate will have to be determined onsite through experimentation.
As discussed in the previous section, it is sometimes necessary to machine the facing
plates in order to make them true and parallel to each other. A boring bar with a
tool slide with a horizontal feed can be used to machine the facing plates' surfaces.
Because of the interrupted cut caused by the holes in the facing plates for the wicket
gate bushings, the cutting tool will see a great deal of shock. The cutting speed and
feed in most cases must be reduced to prevent breaking the tool bit.
Wicket gate bushing line boring
When it is necessary to line bore the wicket gate bushings, the installation of the
boring bar is critical. The bar must be made plumb, positioned at the correct radial
distance from the unit center, and in the correct angular position with reference to
the other gates. Whatever procedure is used to accomplish this, it must be repeated
16 to 24 times depending on the number of wicket gates.
In most cases, centering the boring bar in the intermediate bushing will be within the
tolerance for the radial distance from the unit center and the distance between gates.
The radial distance should be verified by measuring the distance from a single plumb
wire hung through the center of the unit to the boring bar. The distance between
gates should be measured between the boring bar and a wire hung through the last
hole bored. The first hole bored will be the reference.