made from PVC components
Joe F. B9-0316
A very creditable, rugged
and lightweight set of wrists for your B9 may be fabricated from
off-the shelf PVC plumbing components. The cost is less than $100.
Per set. If alternate materials are used for the outer rings, or
if you share a sheet of PVC with your fellow builders this cost
may be cut more than half.
The dimensions are shown on the club standard blueprints. Thanks
again to Dave Painter for making that information available. Some
dimensional compromises, of course, have to be made to accommodate
the materials used, but they are relatively minor.
Fig. 1 The prototype
Fig 2 The PVC Plumbing Parts
Bill of Materials
& part # Lowes
6” PVC Sewer Cap Mueller Industries Inc
(B&K) 93330 122863
PVC Pipe Cap Charlotte Pipe &
Foundry CPF 116 1400 219
DWV Coupling Charlotte Pipe &
Foundry CPF 100 1400 23284 $1.93
PVC (Type I) Sheet 1/4" Thick, 36" X 36" McMaster-Carr
# 8747K183 $ 50.53
Fig 2 shows the plumbing parts used; the 6” pipe cap provides
the body of the wrists. A 6” sewer cap provides the “cuffs”
for attachment of the arms as well as the disks that shroud the
claws. The 4” coupling is cut two provide 2 inner trim rings
for the hole in the center of the wrists. Only one coupling is required
to make a set of wrists.
A lathe is by far the
easiest method to cut and trim the parts and insures that the hole
is truly centered and the parts fit together smoothly. Sawing and
sanding would work if you go slow and measure carefully.
The trim rings are cut
as flat strips from ¼” gray PVC sheet. A sheet metal
shear works very well for the type 1 PVC sheet from McMaster-Carr.
The rings are bent by
heating them in a oven to about 225 degrees F for 20 min.
When removed they will be limp as a noodle. Wrap them quickly about
the wrist and smooth them with your (gloved) hands until they set.
A word about PVC: Most
materials grow when heated. PVC is no exception, expect the strip
to grow about ¼” when hot. Over-lap the edges when
wrapping them. As they cool they will fall into place and leave
only a small gap to fill.
If you mess up, i.e.
you cut them too long, fear not! Return them to the oven and they
will magically flatten back out. Did I mention PVC has a memory
Trim the strip and try again.
Before you cement the
rings to the wrists drill and tap 4 holes 90 deg apart through the
sides of the wrists to secure the cuffs inside the wrist. These
holes are located so as to place them under the trim ring so they
are not visible on the finished product. Use the disk left over
from the sewer cap as the wrist centers. I spaced the mounted cuff
to allow the disk to turn freely within the wrist but not rattle
(about 1/8” slop)
rings can be a bit tricky, I used a simple homemade fixture to block
the ring at the correct height and insure it stayed straight.
Cement the ring using ordinary PVC plumber’s cement. Make
sure the surfaces are clean and apply PVC primer first. Don’t
fiddle with the cemented part too long. Cement it, locate it and
tie it tight around the circumference with wire and walk away for
at least an hour.
Figures 4 to 6 show
the wrist assembled prior to sanding the radius on the trim ring
and cementing the insert (that’s why it sticks out so far)
If you want the “classic” sharp-angled face on the wrists
they can be turned to shape, I personally liked the slightly rounded
face of the pipe cap better. The picture quality is poor due to
the limitations of my camera.
Fig 4 Edge view
Fig 5 Oblique view
Fig 6 Edge view
Fig 7 The completed (albeit blurry) wrist