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Tube Bender Plans
Tube Notcher Plans
Why use round tube?
What's the difference
between tube and pipe?
So how do you choose which
type of tube to
use?
How is the tube bent?
How do I work out where to bend the
tube?
Bend Allowance Calculator
What about those tricky looking tube joints?
Useful tube information
Sand
bending thin wall exhaust tube
Instructional videos and DVDs
Tube Size vs. Strength
"Why would you choose to work with round
tube?"
Well, for a start, round tube is stronger
both in torsion and deflection than square tube of the same weight. Admittedly, butt-welding square tube is simplicity itself, but mitring round
tube, given a little thought and patience, is also very simple....once you
know how! And once it's done, how good does it look!
"What's the difference between tube and
pipe?"
The purpose of pipe
(CHS - Circular Hollow Section), is to
carry either gas or fluids. Its size refers to its Nominal Bore, as it is
this dimension that is used to calculate the flow.
CHS is rolled and then welded from thick,
mild (low-carbon) steel and withstands being pressurised
OK,
but
CHS is not intended for structural use. It
is identifiable by a visible internal weld ridge. It
is not dimensionally
accurate material either and is often visibly
out-of-round with a scaly black appearance.
Structural or mechanical tube on the other hand, is (at least,
externally) dimensionally accurate and has a bright finish. It is also
made from mild steel, and sometimes rolled and
welded too. The welded variety of tube (ERW - Electric Resistance
Welded) begins as a bright strip of steel which is shaped between rollers and has a
visible blue weld seam along its length. Some ERW tube ends up being Drawn Over a Mandrel (DOM, or sometimes, Cold Drawn
Welded), which removes the internal weld ridge, and mechanically strengthens
and perfects the whole tube to its finished dimension.
Note: In the U.S. ERW can also be called CREW (Cold Rolled Electric Welded).
Another type of mild steel tube is Cold Drawn Seamless, which
begins as a solid bar of carbon steel,
and, as the name implies, is
drawn over a mandrel to form the tube section.
This is a very strong form of structural tube and can be strengthened by
further cold working.
There are other families of tube, but only one is of any interest to us for
buggy building: 4130 chrome moly tube.
4130 is
carbon steel alloyed with chromium and
molybdenum, and has a justified reputation as a
tough material
for highly stressed structures such as aircraft frames, dragster
chassis etc.
Its main advantage is its
superior resistance to bending, therefore,
it can be used in thinner wall
sections, thus producing lighter structures than those made from similar
sized mild steel tube. It is not
(as many
people believe)
any lighter than other types of
steel tube. Unless you have a set of laboratory
scales, all steel tube weighs the same (size for size).
4130 does have its drawbacks though. Due to its complex chemistry, it is
recommended that it be either
brazed, or
TIG
welded, and may require pre
and post heat treatment to
help relieve the stresses built up
during the welding process. Failure to do this can cause
catastrophic HAZ (heat affected zone) failure.
The difference in strength between CDS and 4130
tube, is not as great as some think. CDS is typically 470mpa, and
4130 is typically 650mpa.
CHS
is around 250-350mpa. Section for section, CroMo is no
lighter than any other steel, and
all steel has
the same modulus of elasticity anyway (Young’s
Modulus), and are therefore equally stiff.
It is only its ability to resist permanent set that
sets CroMo apart.
"So how do you choose which
type of tube to
use?"
Ordinary ERW tube is ideal for
use in areas of tension or compression, but if it has to
be bent, make sure the
tube's weld seam is situated in
a neutral axis. Any tubes subjected to beam
(side) loads, would be better in either CDW, DOM or
CDS. All of these mild tubes are both easy to bend and weld, requiring
no specialist tools or techniques.
Most production motorcycle frames are just made out of
1.2mm to 1.6mm [.047" to .065"] wall ERW (cut one up and see for
yourself).
If using 4130 for items such as A-arms or trailing arms, they can be tack
welded together in
their jig, using a MIG welder, but it's highly recommended
to have them fully
TIG welded by a competent specialist welder.
"How is the tube bent?"
To be honest, if you only need
some tubes bent to construct the Rorty R6 or Rorty R82s (just
twelve
bends in the whole chassis) then the simplest thing to do, is take the
drawings to a professional bender, or fabrication shop
and have them make the bends for you.
On the other hand, if you
are going to have a go at them yourself, it's not that difficult,
especially with the
Rorty Tube Benders (BTW, the
No.1 Bender plans are free with the R6, R15 and R82s buggy manuals).
There are different types of benders, and some are totally unsuitable for
bending tube.
Pipe benders,
believe it or not, are only intended for bending CHS
pipe. Do not attempt to
bend structural tube with one of these. They are designed for thick wall sections,
and do not produce accurate, pretty, or strong bends. A
pipe bender will crease thin
wall structural tube because
the OD of pipe doesn't match the OD of any structural tube!
The OD of Ø25.4mm [1.0"] ERW tube measures, funnily enough, Ø25.4mm [1.0"],
but the OD of 25.4mm [1.0"] CHS or Schedule pipe actually measures Ø33.7mm
[1.34"].
A proper tube bender must be used when working with structural or mechanical tube, but
there are several different designs with varying costs and ease of use. All
but production plants use a simple form of hand operated tube bender, of
which there are two basic types. The
compression type
draws the tube around a fixed former, and the
rotary type
rotates the former
around the clamped tube. Both work well, and produce virtually perfect,
distortion free bends.
"How do I work out where to bend the
tube?"
All bends have a start and
finish point, and the measurements and radius are taken from the centre line
of the tube. For example the CLR (centre line radius) of the bend in a 38mm [1.5"] tube is
commonly 165mm [6.5"], but the radius of the inside of the
bent tube is actually 146mm
[5.75"].
"What about those tricky looking joints ?"
The nicely-fitting tube
joints are called
fishmouths,
and when done accurately, are extremely strong. I knew a motorcycle frame
maker in the UK, who would braze all his frame joints.
One day he showed me how
tight his fishmouth joints were before brazing them. He took a short length of
tube with a fishmouth in one end, and stood it vertically on a horizontal
piece of tube that was held in his vice. He then proceeded to pour some 5W
(very thin)
hydraulic
oil into the vertical tube. We must have stood chatting for about ten
minutes while he held the top of the tube steady with his finger tip. Not a single
drop of oil leaked out of the fishmouth!
Fishmouths can be cut with a
holesaw notcher and like everything else
in life, several
types exist. There are manual notchers that literally munch out part of the
tube when the notcher handle is pulled down. Then there are the ever more
popular variety that uses an electric drill and a series of holesaws to match the
diameters of tubes being cut.
You can make your own holesaw notcher from these notcher plans.
I often use templates either in conjunction with one of the holesaw type
notchers, or just with the bench grinder and die grinder. The templates are created using a
computer program, and then printed on paper. The
template is then carefully cut out
with scissors, and taped to the tube, ensuring the printed centre line
aligns with the centre line of the tube.
After that, it's a simple task of
drawing around the template and cutting or grinding the fishmouth to shape.
Alternatively, the template can be stuck onto the tube, and the fishmouth
ground to the printed line with a carborundum or diamond
coated fishmouth grinder.
You'll need a few other gadgets to help make life easier, like a good
quality angle finder with a magnetic base, and a tube
centreline marker, which you can make from off cuts.
Useful Information
CLR = Centre Line Radius.
DOB = Degree Of Bend.
Length of tube in a bend = CLR X
DOB X .01745
or, to put it another way:
L = (R X .01745) x Z,
L is the length,
R is radius of the pipe bend,
Z is the angle of the bend.
Setback =
radius X tangent
½ angle of bend.
Circumference = 3.1416 X
diameter.
Bend Allowance Calculator
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