Control Line Racing - 2004/02

February 2004 155
FROM TIME TO time I am asked the same
question: How do I make a shutoff? I make
and use a rotary shutoff, which I first
observed Stoo Willoughby using. It may look
complicated, but the concept is fairly easy
once you understand the design.
It is made from aluminum and steel, and a
Delrin button activates the shutoff. You need
access to a lathe and a mill to make this
design, and some basic knowledge of how to
use them. If you have that, the design will
build quickly.
Place a .750-inch-diameter piece of
aluminum in the lathe. Drill the hole the
length of the shutoff, turn down the stem, and
turn the piece around and counterbore the
front per the drawing.
While at the lathe, make the plunger from
a piece of steel. This operation is probably the
hardest since the material is small and you
could encounter problems keeping the size of
the area that goes through the aluminum
shutoff body consistent.
Turn the plunger around, and drill and tap
the hole for the stud to hold the Delrin button
in place. After you have done this, the only
lathe operation left is to make the Delrin
button. Set these parts aside, and you are
ready to mill the shutoff body.
At the mill you will need to make a fixture
to hold the shutoff body by the stem. All you
need to do is drill a piece of aluminum and
then cut a slot through one side of the drilled
hole. Place the shutoff body in the fixture and
hold it using a mill vise. Mill the sides of the
body to the width indicated on the drawing.
After this is finished, you will need to travel
across the body to get the opening to the
desired width.
While the body is in the mill I use a .070-
inch end mill to cut the slots for the pinch
wire and for the steps that keep the wire in the
open position. This is a simple operation
because the .070 end mill is the size that will
fit into the center hole of the shutoff body.
This provides you with a center locator. The
center slot must be located off the hole;
otherwise, the pinch wire will not work
correctly.
Make passes from top to bottom with the
.070 end mill, cutting the slot for the pinch
wire. I suggest a good rpm and small cuts for
this size of end mill. I usually take no more
than .015 inch at a time as a cut. These end
mills are delicate and will break easily.
After you have cut the center slot, lower
the table and move over the required distance
to cut the first step that holds the pinch wire in
the open position. After you have done that,
back the table off to center again. Remember
CONTROL LINE RACING
Dave McDonald, Box 384, Daleville IN 47334; E-mail: [email protected]
Rotary Shutoff drawing by Dave McDonald
All of the parts used to make the rotary
shutoff are ready for assembly.
A rough-turned aluminum body is on the
left; a finished body is on the right.
02sig5.QXD 11/25/03 9:57 am Page 155
where the settings were when you do this, and
make the bottom step the same width as the
top. These should be mirror images. Cut the
bottom step just as you did the top one.
You are ready to locate the mounting holes
and the hole to secure the spring in place. You
can do this in the mill or by making a drill
fixture that goes into the counterbore area of
the shutoff body that has predrilled holes. If
you use this method, a drill press is all that is
required to drill the holes.
The mounting holes must have appropriate
clearance for the screws that are going to be
used to secure the shutoff. I recommend 2-56
flat-head machine screws. You will need to
relieve the holes for the screw heads. Drill the
hole in the shutoff for the spring with a
number-63 drill bit. When you have
completed these operations, deburr the body
and set it to the side.
Put the steel plunger in the mill to drill the
four spring-adjustment holes. These are also
made with a number-63 drill bit and are
drilled .031 inch from the edge of the plunger.
These holes will allow you to adjust the
amount of preload on the shutoff.
Since I have made more than 150 of these
shutoffs, to put the hole in the plunger to
accept the pinch wire I have made a steel drill
fixture that has a hole to accept the smallest
diameter of the plunger. I also make a cross
hole the appropriate distance from the end that
allows me to rapidly drill several of these
plungers without making setup changes. The
cross hole should be .055 inch in size—the
size of pinch wire that I use and recommend.
Now that you have machined the basic parts,
it is time to assemble the shutoff. Bend the
ends of the spring so that you have two tabs
that will go into the steel plunger and the
aluminum shutoff body. I use springs from
MSC Industrial Supply Co.; they are part
number 03307956. Make sure that you have
adequate preload on the spring; this ensures
that the shutoff will lock into place
automatically when it is open.
Insert the plunger into the shutoff body,
and push the pinch wire through the cross
hole. After you are satisfied with the amount
of preload on the spring, which you can adjust
by switching holes on the plunger and
adjusting the tension by pulling the spring
open as needed, put some cyanoacrylate glue
on the pinch wire and plunger. Be careful
when you do this because if the glue runs
down the plunger into the shutoff body, your
shutoff will not work.
Screw a 4-40 stud into the back of the
plunger, screw your Delrin button in place,
and you are almost finished with the shutoff.
Make the button slightly long until you see
exactly how big of a cheek cowl you are
going to have. The ideal length would be that
at which the button just clears the cowl when
the shutoff is in the open position. You should
also round the end of the button; it will work
better in your finger. Now you are ready to
install your shutoff.
Remember that I suggested using 2-56
machine screws for mounting. The way in
which I mount my shutoffs may be overkill,
but they stay in place and work. Where the
shutoff is mounted should be maple; don’t
rely on plywood or balsa to hold the shutoff
for the long haul because it will not.
Drill the main hole through which the
shutoff plunger will slide. When this is done,
you can locate your two mounting holes.
Obtain a small piece of brass, tap the inside
for 2-56, and turn the outside diameter to a
standard drill size. Epoxy these into the hard
maple; this will provide a good mounting
point for the machine screws.
The shutoff is a mechanical piece of
equipment, and you must check it periodically
to ensure its integrity. I don’t think wood
screws will work during a racing airplane’s
long service life—at least I have never had
any luck with them.
I like this shutoff design because it is
compact, it provides an aerodynamically
clean area, and it is a one-step open
operation. Simply push in the button, and
the spring does all the work. There’s no
pulling a wire off of a step and pushing it
forward. Although those types of designs are
good, this design is faster in the pits, and
any time saved there is basically free time.
Most races are won and lost in the pits,
so why not take every advantage by making
something that works faster?
I suggest that you videotape your pit
stops to see how slow you are and in which
areas you can improve. Pit crews should try
catching right-handed and filling lefthanded.
That will save time since they won’t
have to transfer the filling bulb to the other
hand and then put it down to hit the
propeller.
If you have any questions or need assistance
in making one of these shutoffs, drop me a
line. I will be happy to help in any way.
As always, your comments are solicited
and appreciated. MA