formed tubes are used increasingly in industries as diverse as automotive,
furniture, appliance, sporting goods, construction equipment, and
temporary buildings. With recent developments in new forming techniques
to produce accurate, finished parts at high volumes without scrap,
end formed parts are replacing some conventionally machined components.
applications practically are limitless. Ranging from the simple
to the complex, they include reduction of the outside diameter,
expansion of inside diameter, flaring, beading, and completely altering
the original shape.
Square and rectangular
tubing present unique challenges for end forming. Inline forming
is a traditional forming method for square and rectangular shapes;
however, segmented tooling has been growing in acceptance for forming
is a process in which a workpiece is held in a clamping device and
a tool is pushed over the workpiece to generate the desired profile.
While suitable for many applications, this method has several drawbacks.
cannot be adjusted for various sizes of tubing -- it is dedicated
to one size only.
The length of
the end form is determined by the travel of the tool and length
of the tube that is not clamped by the clamping dies. This distance
can cause the end form to be off-center from the original part.
Off-center end forms can render the finished product unusable --for
instance, a garage door track assembled from track sections with
off-center ends has transitions between each section. If the transitions,
or steps, between each section are too extreme, the garage door
mechanism cannot travel the length of the track.
Also, tool life
is a concern because of the friction forces involved and possible
presence of dirt particles or metal burrs.
for forming square or rectangular tube is to use segmented tooling.
The most common application is a slip fit in which the end of one
tube slips into the end of another tube.
is based on four dies closing simultaneously. This action applies
the work force all at once. In addition to completely altering the
original shape, segmented tooling can offer a better-defined transition
area with increased strength. An expanding mandrel can be used for
applications with complex profiles; a simpler fixed mandrel can
help direct the material flow in the desired areas.
dies, which are informally called expanding fingers, can be moved
into positions that define the exact internal profile of the tube.
After forming they can collapse a small amount, reducing the amount
of force required to remove the finished workpiece from the tooling.
In cases of
a less accurate internal profile but a complex outside shape, a
fixed plug can redirect the material flow to ensure correct coverage.
This type of mandrel can be made from a tool steel with a highly
polished surface or carbide coating to reduce wear caused by workpiece
removal. The contact area is smaller than that of an expanding mandrel,
and only a small surface will remain in contact at the end of cycle.
Material springback is also a factor in decreasing friction.
In both cases
-- whether using a fixed plug or an expanding mandrel -- the outside
dimensions that define the type of fit in the tube can be changed
easily because segmented tooling is suitable for a small range of
sizes around the nominal value that it was designed for.
generally is designed for universal drop-in, inside-outside (I/O)
tooling in both standard and large-barrel configurations. It can
accommodate tube sizes ranging from 1 by 1/4 in. to 4 by 4 in. The
standard angle is 14 degrees on the cone and outside of the dies.
Designing tooling around industry standards allows the use of standard
tool components for new applications.
When more reduction
is necessary, a larger angle can be used. This allows for a larger
opening to fit the new part into the forming dies. However, dies
with a custom angle require a custom cone with the same angle.
Based on the
close tolerances of profiled dimensions, stringent requirements
-- part straightness, equal step-down from the original tube to
profile on all sides (common centerlines), minimal taper condition
on the formed are -- create new challenges in the forming process.
does present some drawbacks, however, such as tool marks made on
the wall of the tube at the edges of the forming dies. Also, the
amount of reduction that can be achieved is limited.
Design for Welded Tube.
is in the use of welded tube. One side of the rectangle contains
the welded seam. Compared to the rest of the tube circumference,
the weld seam has significant differences in strength, hardness,
flow characteristics, and springback. The welded side and the opposite
side resist the forming forces with different values, resulting
in slightly different dimensions.
A die design
that allows the dies some degree of axial rotation is beneficial
for self-alignment for forming round parts. However, this is a drawback
with nonround parts. The small gap between the dies, and their ability
to rotate axially, allows the dies to close more on the side opposite
the weld seam. Square and rectangular parts formed with this type
of die are not symmetrical.
design -- one that is specifically intended for rectangular tube
-- has a guiding system that restricts axial rotation, helping to
ensure a proper approach of all four dies at 45 degrees. The guiding
system prevents axial pivoting of the dies, resulting in symmetrically
As parts are
fed manually into the machine, the dies contact the forming length
while the rest of the tube is free. This condition leads to parts
with deviation of the formed end toward the weld, as stress in the
tube is concentrated in a pattern dependent on the weld. Assembling
two or three of these sections together accentuates this deficiency.
A set of dies that contact both the end formed area and some length
of the unformed area help to decrease or eliminate these deviations.
Forming or Segmented Tooling?
forming continues to be a viable end-forming method, use of segmented
tooling is growing. In applications that have more stringent forming
requirements, segmented tooling can be a suitable alternative.
Joe Dean is
vice president of engineering with Aristo Machines Inc., 2400 Southeastern
Ave., Indianapolis, IN 46201-4161, phone 877-227-4786, fax 317-635-1336,
e-mail firstname.lastname@example.org, Web site www.aristomachines.com.
Aristo designs and manufactures end forming machines and inside-outside
tooling for end forming applications.