The topic of tube bending
always seems to draw the most attention when it comes to chopper frame
fabrication. Other than some relatively expensive bending equipment it is not
really all that hard to do but that geometry and trig class you slept through has
come back to haunt you.
Keep a few things in mind:
Tube is not the same as pipe. Different dies are required to properly bend tube
or pipe. Pipe is not a suitable choice of material for a vehicle frame. A good low
cost bender for tubing will cost in the area of $300 to $700 (+) with the
additional cost of $150 to $275 (+) for die sets. With accessories, it is
possible to have $600 to $1200 tied up in a bender to do just one size of
tubing. Avoid the cheap pipe “bow” type benders where the material is held
between two rollers and a die presses in the middle forming the bend. These
type benders do not compress the tube to hold the sidewall shape of the tubing.
Let’s bend up a piece of
tube and see what is involved.
First step is to mark the
beginning of your bend. With most bender dies, the bend starts somewhere inside
the die. In the picture is a silver line showing where the actual bend will
start and the red line that is lined up with the outside edge of the bending
die. The little chunk of pipe around the tube has been squared up and makes a
handy tool for drawing a complete line all the way around the tube that is
being bent. Next step is to load the tube in the bending die. The second
picture shows the tube in the bender. This particular bender is a follow bar
type. The 2-1/2” square aluminum block below the tube is the follow bar. The
tube is inside the half round groove in the follow bar. The red line drawn in
the first picture is visible just in front of the aluminum follow bar. The
radius die is just above the tube. The silver line drawn in the first picture
is inside the die and follow bar (remember the bend will start 1/8” inside the
radius die. The next picture will make
a little more sense with what is going on.
There are a lot of different
bender styles that are suitable for the small shop. Some are hydraulic and some
are mechanical. Most benders bend tube in a horizontal plane. The picture shows
a hydraulic bender that bends the tube end upwards. A magnetic base dial angle
finder shows the angle of the tube as the radius rotates and bends the
tube. The silver line is now visible
just below the red line, which is still lined up the leading edge of the die.
This tube was being bent to 90 degrees. Since tube has a bit of memory or “spring
back”, the tube actually needs to be bent further than 90 degrees. The amount
of over-bend required varies for each tube type, diameter, wall thickness and
die radius. In this case, the tube required almost 4 degrees more to spring
back to 90 degrees.
Below is a picture of the
complete 90-degree bend. You may be able to tell where the bend actually
started visually but it is usually easier to feel. In the case of this 1-1/4” x
.125 wall DOM tube, the inner surface of the tube was compressed .050”. Unless some kind of internal die is used to
hold the inside diameter, the typical low cost bender will cause this tube wall
distortion. An expensive mandrel bender has the internal die (or mandrel) to
support the tube during the bending process).
This 90-degree tube now
becomes an important tool for future bends. There are a number of ways to put
it to use. One was is to determine tube length. The top rectangle (green…but hard
to tell in the picture) on the paper is the start of a full scale bend. The red
lower lines are a planned 15-degree bend. The “tool” is laid on the drawing and
the bend line is lined up with the bend line on the drawing. A line can now be
drawn following the actual bend of the tube as in the second picture.
With the tube moved, the
radius lines are now visible. Next, the
tube is lined up with the straight part of the lower angle and the radius lines
drawn in the last step. Now the bend line can be marked showing the end of the
bend.
In this last picture, the
beginning and end of the bend has been found. There are a lot of calculating
that can be done and software that figures tube length. However, a short piece
of flexible tubing (stripped off wire insulation in this case) makes a good
tool to lay on the center line of the bend area, the beginning and end of the
bend are marked and the flexible tube straightened out and measured. This is
the length of material used by the bend. Add on the length of tube before the
bend and after the bend and you have the full length needed to make the part.
Not super accurate but is really close if you only have a few bends to do. It
is always a good idea to leave a little excess tube to trim off after the bend
and be aware that the tube length will increase slightly by the compression of
the tube during bending. Some type benders can cause considerable increase and
a few test bends should be made to learn what to expect.
If the tube is going to have
more than one bend, it is often a good idea to tack weld a chunk of flat bar to
the end of the tube in line with the first bend (just lay the tube flat and
tack the bar flat with the same surface). Now, using a dial angle finder or
protractor with a bubble level, you have a reference for rotating the tube for
subsequent bends. Clamps are available
for purchase from fabricating suppliers (MechWerks has these) that attach to
tubing and provide a flat for attaching a dial angle finder. Once the clamp is
attached, the tube can be rotated a needed amount and the next bend will be in
the correct radial relationship to the first bend. Note that tubing has a lot
of built in stress from the forming process of making the tube. Bending causes
some of this stress to be released and may result in the radial degrees to be
off slightly. It is difficult to compensate for this twist but is something the
fabricator needs to be aware of. Sometimes, application of a heating torch is
the only way to pull tube back into place after it twists. Sometimes if the
tube can be fastened down securely, simple application of force can be used to
fix twist and take out small amounts of overbend.
