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(The Twister... pg.2)
Clamps (Two):
3”x 4” x 1 ½” overall. Each has mating
halves. Note “A” and “B” halves (sides)
on the plan. Cut wood with grain horizontal for the “A”
sides if wood strength doubtful. (Clamps can be made from ¾”-
thick pieces joined together. A method of cutting and assembly
is illustrated on the plan.) The plan shows the clamps with
an arc radius of 3 ½ inches, and an arc length of 2 ½
inches. (These clamps will accommodate twisting sheets with
a larger radius. Large-radius sheets can be ground off to fit
the clamps.) Hold the clamp halves together in a vise and fasten
the hinge to one end with wood screws. Mount the hinge flush
with or set in from the edge of the clamp. Turn the clamp over
in the vice and fasten the draw hasp to the top of the clamp
with wood screws. Mount the hasp body on the “A”
side, and the hook on the “B” side of the clamp,
as shown on the plan. If using the National brand arm-type hasp,
the flat spring-steel “arm” of the hasp will straighten
under tension, so leave a gap between the hasp and the hook
when mounting. If using the loop-type draw clamp, watch that
the hook base, when attached to the top of the clamp, does not
overhang the edge of the clamp on the side of the clamp abutting
the back plate. If it does, it will bind with the backplate.
The hook base can be filed off in any case.
Fastening the Clamps to the Backplates:
Position the first clamp on the left backplate (the one
with the 9/64” hole near the top) with the “A”
side on the right and such that the carriage bolt head is centered
in the arcing slot of the pivoting clamp. The bottom edge of
the clamp should be flush with the bottom edge of the backplate
so that the hinge does not impinge on the backplate. Holding
it in this position, turn it over, and drill two pilot holes
with a #6 countersink wood bit through the back side of the
back plate and into the clamp “A” side, about ¾”
from the top and bottom of the clamp, and about ¾”
in from the outside edge, and fasten the “A” side
to the back plate with the 1 5/8” flathead wood screws.
Countersink the screw heads. For the right pivoting clamp, position
the second clamp on the remaining back plate with the “A”
side on the left, with the carriage bolt head centered in the
arcing slot. Fasten to the back plate as above.
Install the pivoting clamps on the frame uprights. Do not put
a washer between the backplate and the frame upright. Install
a washer and nut on each bolt, tighten moderately, and check
for binding or resistance. If binding occurs, it could mean
the bolt hole isn’t straight, or the clamp mounting screws
are not below flush with the back surface of the back plate,
or that the frame upright is cupped or warped.
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Pull-Down Strap
Anchor.
(4” x 5/8” mending plate) This will be used to anchor
the Velcro pull-down strap. Mount it flat, with three washers
between it and the edge of the base plate to create a gap for
the strap. Fasten to the center of the edge of the base plate
on the side adjacent to the pivoting clamp “A” sides
with pan-head wood screws.
Pull-Down Strap.
(Velcro 18” heavy-duty cinch strap) Pass the strap down
through the gap between the strap anchor and the base plate
so that the lead end points away from you with the “fuzzy”
side facing down. The buckle of the strap will be brought beneath
and up over the twisting sheets, so that the tag-end is pulled
up through the back of the buckle and down on the outside facing
the rear of the jig to cinch.
Drill the Left Pivoting
Clamp Stop-Pin hole.
Remove the right pivoting clamp. Put the left clamp in the vertical
position. Use a 9/64” bit to drill through the hole near
the top of the backplate into and through the frame upright.
This hole is for a 10 d nail stop pin.
Mark the Locations for and Drill the Right Pivoting Clamp Stop-Pin
Holes.
See first photo under “Fabrication and Assembly.”
Remove the left pivoting clamp. Reinstall the right pivoting
clamp, and rotate it away from you, counter-clockwise, so that
the hinge is facing you and the “B” side of the
clamp is facing up and is parallel to the base plate. Align
the centerline you drew on the bottom edge of the backplate
with the 90-degree line you drew across the face of the frame
upright. Use a pencil to draw a line on the frame upright along
the upward-facing edge of the clamp “B” side. The
line parallels the 90-degree line you drew across the face of
the upright, and intersects the vertical centerline of the frame
upright at 90 degrees. Next, rotate the pivoting clamp to align
the centerline on the bottom edge of the backplate with the
80-degree line on the lower right quadrant of the upright. Again
draw a line on the frame upright along the edge of the clamp
“B” side, facing up, as you did with the 90-degree
line. Repeat this for the 70-, 60-, 50- and 40-degree positions.
Remove the pivoting clamp. Next, drill a hole with a 9/64”
bit at each degree-line in the upper left quadrant. Make the
bottom of each hole right at each line, so that the stop pin
will stop the pivoting clamp at each line. The 90-degree line
hole will be toward the outer end of the line, near the edge
of the frame upright. Each succeeding hole should be moved in,
so that the pattern roughly forms an arc, as shown in the photo
and on the plan.
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Reinstall the pivoting clamps. Do not put washers between the
backplates and the frame uprights.
“Twisting
Sheets”: (Two).
“Lexan” polycarbonate
.093 gauge (3/32”). Commonly available in precut sheets
11” x 14”. Have your supplier cut it into strips
2 1/2”x 11”. It can also be cut with a radial-blade
(fine tooth) table saw or with a jig saw, or by scoring deeply
with a utility knife and breaking it over a table-edge. Remove
the protective plastic after cutting and remove any roughness
from the edges. Forming is done on a sheet metal vent pipe (”stove
pipe”) or a form in an oven at 325 degrees F., one sheet
at a time. Sheet metal vent pipes are commonly available in
diameters of five to ten inches and two- to three-foot lengths.
A seven-inch diameter pipe is a good size to start with, unless
you know that you want a larger-radius form. Cut the pipe to
14 inches long with tin snips. Place the pipe in the oven on
an open rack, front to back, chock to prevent movement if necessary,
and preheat the oven to 325 degrees. Place the polycarbonate
on the top centerline of the pipe. It should conform in a few
minutes. If it doesn’t conform in a few minutes, raise
the heat just a little. If it starts to bubble, open the oven
door to cool it. As soon as it conforms, using heat-resistant
mitts or leather gloves, remove the pipe from the heat, and
allow it to air cool. If the polycarbonate sheet begins to curl,
hold it down with your gloves. The sheet will slide off the
pipe after it cools. If overheated, polycarbonate will bubble,
curl, and shrink. The bubbles and pocks will make undesirable
indentations in balsa laminations. (As a precaution, open a
window and remove any pet birds from the oven area when heating
plastic.)
If a stove pipe of a desired diameter is not available, or,
if you want to make twisting sheets of various radii without
buying a lot of pipes, you can make a forming “buck”
of wood and sheet metal with replaceable ends for different
radii.
Making a Forming “Buck” for Forming
the “Twisting Sheets”.
The “buck” is a wood book-end frame with a rounded
sheet metal “roof.” The frame consists of two wood
end pieces with a round top of the desired radius mounted on
two wood rails. The ends can be cut from stock plywood “rounds”
or from stock 1” x 8” or 1” x 10” pine
or similar. The rails are made from 1” x 2” pine.
The ends measure seven inches wide at the base, and four inches
high at the sides to the beginning of the rounded top. The top
is cut to the desired radius. You can use a stove pipe for the
sheet metal, or flat sheet galvanized or stainless steel, 20-
to 24-gauge. The sheet metal is cut to 7” x 14,”
and is screwed to the frame ends in the middle and at the corners.
Holes are drilled in the sheet metal at the edge in the middle
of the frame span, and uncoated wire is inserted through the
holes and wrapped under the rails to pull down the sides of
the sheet metal. The wood frame can withstand the preheating
and heating time without igniting.
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Mark the “Twisting Sheet” Forming and Reference
Lines
After forming the twisting sheets, use a sharp-pointed black
permanent marker to draw lines on the underside surface of one
twisting sheet bisecting the sheet horizontally and vertically.
Next, using a protractor, draw a diagonal line bottom-left to
upper-right as viewed from the top at 6 and 12 degrees to the
horizontal centerline, using the intersection of the bisecting
lines as the centerpoint. These are forming reference lines.
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Install the twisting sheets in the pivoting clamps. (Grind off
any spots that bind with the clamps.) Close the draw hasps.
Using the marking pen, draw a line on the surface of the top
twisting sheet at each end along the face of the pivoting clamps.
(See picture above.) These are visual reference lines.
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