By Brian Kopack
The welding of streel strip into a product, such as round tube or pipe, box frames, and structural members, is a multibillion-dollar
industry that had its beginnings more than a century ago. Many items that we use today, like furniture tubing, oil pipe, and fuel lines, are manufactured on welded tube mills.
In the last 20 years OEMs have been asked to apply the principles learned from tube mills to the
much larger roll forming processing market. This
has created another technology area today known
as welded roll forming.
Companies may move from nonwelded open
shapes to welded roll formed shapes for many reasons. Welded roll formed shapes have structural
strength and integrity, help eliminate secondary
operations in downstream manufacturing, and can
offer savings in steel and construction labor. While
there are benefits, there are also costs, which typically involve an investment in capital equipment and
skilled people who know how to use the technology.
What are the differences between a tube mill and
a welded roll form system? Which one should you
consider? Can the same parts be run on both systems? Will the part quality be affected?
These questions, asked often within the industry,
are plagued with many conditional aspects that
cloud the final answer. Intricate details often are required to help machine builders focus on the best
In the classic sense, a tube mill is a kind of welded
roll form system; however, not every welded roll
form system is a tube mill. Tube mills are welded
roll form systems that have been fine-tuned to run a
specific diameter range, typically at fast speeds (see
Diameter range is mentioned because a tube mill
welds a round product. Progressive male and female roller dies (or rolls) shape the incoming strip
for welding a specific diameter.
On all welded roll form systems, including tube
mills, welding can occur with processes like high-frequency (HF) induction, HF contact, electric resistance, and laser beam welding. For the sake of this
discussion, it is assumed that the correct welding
process has been determined based on the specific
Figure 2 shows the typical forming flower of a
tube mill forming the strip into a round product just
before welding. From there the round product can
stay round or can be sized or reshaped into a square
In a tube mill, the forming machine forms the strip
into a round, weldable product. The forming machine has two main parts: breakdown and fin-pass
sections. Once welded, the tube can be left round,
though it undergoes further forming to size it to a
more accurate outside diameter. The sizing section
shown in Figure 3 has round roll stands with specialty reshaping stands toward the exit and a double
turks-head to finish. Used for straightening, a
turks-head has two pairs of rolls, one arranged vertically
and the other horizontally.
Defining a welded roll forming system is a little
more difficult. Again, in the classical sense, a tube
mill is one kind of a welded roll forming system. But
if someone refers to a “welded roll forming system,”
that person is probably not talking about a tube
mill, but other roll forming machines able to form
various, often highly complex shapes to within tight
Much like forming on the tube mill, with breakdown and fin-pass sections, welded roll forming
systems have a similar forming setup, with the fin-pass section occurring in the last few stands before
welding (see Figure 4).
When working with nonround shapes, such as a
step beam (see Figure 5), the roll forming system
tends to form the shape as is before welding. Some
in the industry call this near-net-shape forming. Others call this the form square-weld square process.
Once the shape is formed and welded, most recommend at least two additional passes to work the
welded shape once more to finalize the dimensions.
Tube mills tend to have alternating driven forming
pass and idle side pass patterns. These patterns
help alleviate springback from the previous forming
This specialty pass progression has led tube mill
builders to design systems with a given diameter
range to have between five and nine driven forming
passes, with their sister idle side passes. The actual pass count often is a function of the ratio of the
workpiece diameter to the material thickness, material yield strength, as well as the minimum to maximum range for both material OD and thickness.
Welded roll forming mills tend to have mostly
driven passes, though using idle side passes is becoming more common. Many factors come into
play when determining the most robust pass count
for stable welding. Pass counts depend not only on
material wall thickness and yield strength, but also
on the overall complexity of the shape to be formed.
The pass count also depends on material movement during the process, as well as the experience
of the roll tooling designer.
Welded roll forming systems versus tube mills:
What’s the difference?
Every tube mill is a welded roll form system, but not vice versa
A tube mill specializes in the forming of round shapes,
but it also can form simple shapes like squares and