Hi Randy,

I knew there must be a trick to vacuum forming skins and it looks like you
have found it. I, however, had already ruined two skins so I was not eager
to experiment any further. The materials for the press brake cost me $40
while another sheet of 6061-T6 would have cost me $70+shipping. I am
forwarding your technique to the Sonex List in case someone wants to give it
a try.

What engine are you going to use in your Zenith? I just ordered a Jabiru
3300 and can't wait for it to arrive.

Thanks again for the info

David Koelzer


-----Original Message-----
From: Randy L. Thwing [mailto:n4546v@...]
Sent: Wednesday, October 25, 2000 11:02 AM
To: david.koelzer@...
Subject: vacuum forming skins


Dave:
I'm building a Zenith 701 and am on the Zenair list. Someone sent a
post with your site mentioned, I went there and read of your problems with
vacuum forming which were identical to mine. I wrote a short post to the
list on how I successfully accomplished vacuum forming with kitchen sink
technology and am attaching it for your information. It does work well with
proper technique. If you have any questions, I would do my best to answer
them. Nice site.
Regards,
Randy L. Thwing, Zenith 701, plans




Date: Jan 03, 2000



I formed the flaparon skins using the ZA method and measurements in the
701 construction manual, and they came out OK. I then used this method,
which is to position the trailing edges (top and bottom ) a certain
measurement
apart, set a piece of wood a certain height forward of the nose,
and lay a piece of plywood on the metal and press your weight on it until
contact with the wood prevents further downward movement. When I made my
elevator skin, it didn't work out with the nose radius and the trailing edge
measurements. That's when my friend and helper: John (doesn't cut and run
when it gets tough) Clements suggested Vacuum forming the skin. His
previous experience was one time, successfully forming a nose skin of 2024
about .025 thick for a Piper Comanche. He described the process used and we
started.

As the .016 of most Zenair skins is considerably thinner, we were
concerned as to how it would act. In a fit of uncharacteristic sanity, we
decided to do a test piece. Now usually when practice is suggested in our
circles, the response is "Practice! we don't need no stinking practice!" and
on to ruin! We cut a wide piece of previously ruined wing skin to the
elevator dimensions, the trailing edges were duct taped, then garbage bags
were taped to the periphery of each open end. Duct tape is used in every
reference to "tape". A slit was cut in one bag and the standard hose of a
small 6 gallon shopvac was inserted and taped in place. When all was ready,
we ignited the shopvac, and "varoom", watched in horror as from trailing
edge to nose in about 2 seconds, the two pieces vacuum formed into pancake
like pieces with a nose radius of about .010". Too much vacuum, to quickly.
The garbage bag excess had also sucked into the center of the assembly, no
good! Here is what finally worked:

First, measure the ribs you will use in the assembly and locate and mark
the apex of the curve of the nose, which will be the center of the skin
bend. From this point measure aft along the top curvature of the rib to
where the trailing edge will be, then measure the bottom of the rib aft to
the point where the trailing edge will be. Transfer these measurements to
the flat skin material. You will have three lines on your skin blank, one
near the center that is your rib nose apex or center, and a line out near
each edge which designate the trailing edge position. If you have the extra
material, I recommend cutting you sheet 10 or 20 MM beyond the trailing edge
lines so slight deviations to the theory can be corrected later, that is,
you can trim it so the trailing edges match up. On ZA airfoils, the bottom
trailing edge is always bent on the brake downward 8 or 10 degrees, do this
after forming the nose, and trailing edges are properly located and matched.
The distance of the upper skin from the nose center to the trailing edge
is shorter than the bottom distance on the 701 elevator. You tape the
tailing edges together, spanwise, with the tape sealing the joint, with the
longer lower length sticking beyond the shorter top length by what ever the
measured difference between the two is. Example if the bottom measurement
is 600 MM (from nose center to trailing edge) and the top measurement is 580
MM, the top skin is taped 20MM (the difference) short of the bottom skin
trailing edge line, and will locate the bend in the right place.

The trailing edges are now taped (and sealed) spanwise, forming a large
bow, straining at the tape joint. Take your garbage bags and tape them
around the open ends of the bow, message the tape against the thin plastic
to make sure you have a good seal all around. As garbage bags are not
shaped like your part, you will have much excess bulging here and there. If
you don't do something with this, it will be sucked inward when vacuum is
applied, no good! Gather up all loose excess bag and twist it like a rubber
band in a balsa flying model for a few turns, take this twisted excess, pull
it outward, and tape it to the bench which will hold it outward during
forming.

Prepare the shopvac. The regular diameter hose provides too much
suction, too quickly. I wound up bushing down the hose by taping a 3/8"
I.D. rubber tubing into the standard hose, just used tape, worked OK. This
causes the Vac motor to labor a bit, as airflow has been restricted, but it
won't be on long. Now you'd think that once you launch the vacuum, form
your part, you just shut off the switch and you're finished. Wrong! You hit
the "off" switch, but the time it takes the vac to spool down, it continues
to create vacuum, which can ruin (overbend) your part. We tried clamping
the tube with needle-nosed vise grips for instant shutoff, but the better
method is to unplug the shopvac suction hose at the canister, start the vac,
slowly bring the hose connector up to it's canister plug in point, and let
it suck through the hose. When the appropriate shutoff point is reached,
your assistant jerks the hose from the canister port, thereby causing
instant loss of suction, the "off" switch is hit, and the motor spools down
at it's leisure, not affecting you. You slit one side garbage bag and
insert the small tube into your assembly and tape it in place to seal.
Locate it near the center of the nose, not near the trailing edge. When
vacuum is applied, the metal compresses from the trailing edge and races
forward. You don't want the hose in the rear area where the metal, top and
bottom, will be sucked together completely, it could cause a bump.
The real trick is how to tell when bend occurs, and how much more you
need to bend. You have this great taped up mess on your bench, but like all
bending of metal there is springback, and with this thin stuff, there is a
lot of spring! The last thing you want to do is overbend the nose radius.
If you overbend, it is almost impossible to redo, so we want to stop far
before overbending occurs. The last thing you make is a ruler, at least the
height of your assembly, with clear, big, marks on it with Inches
identified, and divided into 1/4" increments. Make this ruler on square
material, I used the white, blank, back side of a plastic "House for sale"
sign, with black magic marker lines on it. A regular ruler can be tipped,
this big square rule sits flat on the bench, held vertically by your
assistant, next to the end of your assembly opposite of where the van tube
enters.

Now it's time to bend metal. One person operates the shopvac, prepared
to provide suction and cancel suction on command, you or other assistant,
holds the rule next to the other end of the assembly. Note the height
measurement of the top of the bowed metal against the rule say 12". We are
going to apply vacuum, and suck this metal down to a height of say 8" right?
And STOP there. Fire off the shopvac, and slowly apply vacuum, the assembly
will start to collapse. As it passes the inch markings on the rule, watch
as it approaches the 8" mark. When it hits the 8" mark, yell "stop" for
vacuum to be removed. The assembly will begin to expand. If it expands
back to the start measurement (12" in this example) no bend has occurred.
Repeat this process by pulling the metal down to lower and lower heights.
The first time the metal fails to rise back to the 12" mark, you have begun
At the first sign of bending you need to check for amount of bend. You
hate to dismount this whole mess, so have a rib, or pattern or bending block
the correct rib size handy. Once bending has occurred, go to the end where
the rule resides, shove the nose of your rib into your bent skin, garbage
bags in place and all, kind of pull the skin around your rib with your hands
and you will get a real good idea if you've gone far enough. REMEMBER: you
don't need to bend your nose skin enough to exactly match the radius of the
nose rib! You want to stop far short of this. You just want enough
curvature "set" into the metal so you can form or pull it into place with
the usual straps etc. By checking the edge with your sample rib, you can
see easily if the amount of bend is adequate. If not enough, launch the
shopvac again and suck it down to the next quarter inch mark. Do a LITTLE
AT A TIME, work slowly. It's actually fun to watch it happen.
I don't know if this is the best way to do it, this is the way I did it
and it worked out fine. I'm sure others more experienced have a more
"polished" technique. Work slow, measure often. I have tried to describe
my experience without pictures, sorry it's so windy but I've tried to be
thorough. Remember, I offered no more than kitchen sink technology, with 3
Stooges execution! Good Luck
Regards,
Randy,701/plans/60Franklin/50.000135% complete
P.S. If this works out, I will privately send to anyone interested,
pictures of me using my gravity powered, lightening hole flanging press!