Al Tubing Bender

[DAN FURGASON]

Last spring Dan Elsberg of Cornell made mention of a tubing bender he had
designed and fabricated. He e-mailed me a discription and asked me to forward
it to the list. 

Dan Elsberg wrote: 

Sorry it took me several months to get back to you about the tube bender.
I was way busy during the school year, but have now graduated and can put
down some basic directions.  (I am no longer on the DIY-EFI list, but if
you could forward this to the list, I would appreciate it.)

The tube bender is basically the exact same thing as one of the small hand
brake line benders.  There needs to be a round mandril which fits half of
the tube in it.  The diameter of this mandril will define the diameter of
the 'u's that you bend.  We made this by rough cutting a piece of mild
steel on the lathe.  (Take a piece of round stock, chuck it up, cut in a
semicircle gouge that is "one radius of the tubing that you want to bend"
deep.)  This should be close to the final round shape, but not any deeper.
Then take a ball end mill of the same diameter as your tubing and put the
mandril piece on a dividing head and finish cutting the semi-circle groove
to the perfect shape.  Make sure you run the mill at the slowest speed to
begin with and go slowly.  Once the tip starts cutting, you are asking
everything to do some hard work-- things will wobble and make a lot of
noise and smoke.  Next we cut the mandril off from the excess length of
stock.  Then we drilled four holes in the mandril through the flat sides to
fit the lever arm into and reamed one hole in the middle for the
pivot/axle.

The outside or smaller mandril was made from a half piece of tubing that
was bent out a bit to fit around the aluminum to be bent.  It was welded to
a few pieces of plate to make a box shaped assembly that had room for a
hole in order to attach it to the base plate of the bender.

The next piece was a base plate (we used 1/2" thick steel).  We drilled and
reamed two holes in it for the pivots of the two mandril pieces.  The
spacing should be relatively accurate so that when the two mandrils fit
together, there is a perfect circle (otherwise the aluminum will squish out
the gap.)  The outside mandril should actually meet the inside mandril at a
point.  One corner will be right against the inside circle and the part
will trail off on a tangent.
                          __
                         /  \
                      _ ( () ) <----inside mandril (a circle)

  outside mandril--> | | \__/
                     | |
                     |_|

                    Top View (correct)


                      _   __
                     | | /  \
                     | |( () ) <----inside mandril (a circle)

  outside mandril--> |_| \__/


                    Top View (incorrect)

These drawings are not to scale-- the inside mandril is much larger that
the outside mandril.

Finally you need to make something that will pull the tubing around the
inside mandril as it is bent.  On the hand benders this is made of several
pieces of plate held together and pivots in and out of the plane of the
u-bends.  This will take a lot of force so make it beefy.  This piece bolts
to the inside mandril.

The aluminum will have to be normalized to take it down to T0.  I think
that dillsburg or other folks sell tubing that is already soft, but we had
to put our 6061-T6 pieces in an oven and treat them.  (I don't remember the
recipe, but you can look it up in a metals or heat treating handbook.)  It
is all just easier if you buy the right stuff to begin with.  I have been
told that the aluminum will actually time harden back up to somewhere
between T0 and T6, but I don't know how long this takes or any details.

Now assemble the parts on the base plate using round stock as the
pivots/axles (we used 1" mild steel).  Fashion yourself a lever arm that
can drop into the holes drilled in the inside mandril (goes into two holes
at a time).  The lever should be about 6 feet long.

Lube everything up with WD40 and place the aluminum tube in so that the
part that pulls the tube around is lined flat against the outside mandril.
The tube should stick about an inch past the end of the pulling part.  Turn
the inside mandril with the lever arm and it will pull the aluminum around
and bend it as it goes.  You will have to experiment with the exact
placements of all the parts, because the aluminum is prone to locally
buckling and flatening.

Sorry that this may be a little hard to follow, but you really should buy
one of the brake line benders to get an idea of how this really works.  The
whole thing took us about 2 weeks to manufacture and tune.  There is also
about 60 to 80 pounds worth of steel between the mandril, the base plate,
and the scrap left on the shop floor.

I mentioned this several months ago, but if anyone wants to purchase some
U's we would be willing to figure out a reasonable price-- the shops we
found who were willing to do it wanted some outrageous price for it.

(Just found the email I sent out months ago:
We got tired of the high prices and low availability of Aluminum u-bends so
we made a mandril bender.  It was a _lot_ of work.  (about a week of
machining and then another couple of days playing around with the
technique.)  Anyway if anyone needs 1-3/8"x0.058 we may be able to work
something out that would be a lot cheaper than the estimates we got from
various other sources (one wanted $275 to bend 6 semi-circles (180 degrees)
including the labor and material).  E-mail me directly if you want to talk
about it.
)

Good luck and happy machining,
--dan

--
Dan Elsberg                                                dhe1 at cornell.edu
203 Williams St. Apt. 1                                            277-9503
Ithaca, NY  14850