any pre-punching operations that a job might benefit from or require. Punching typically is not recommended on tube mills. This is because of inconsistencies with so-called round-to-shape material movement. It can be very difficult
to “lock” the hole location in a round shape and then reshape it into a square,
hoping the hole ends up in the correct position. Moreover, punched holes may
warp during the reshaping phase.
Pre-punching in welded roll forming is usually better because it allows you to
accurately locate hole features. The punched holes also tend to hold their shape
during forming, if those holes are put in the right location. If the hole is near a
corner, however, the hole shape still can change.
Finally, consider processing speed. This hinges mainly on pre-punching operations. Since pre-punching is not normally performed on tube mills, these mills
have been designed to run up to 750 feet per minute (FPM), depending on the
product OD and wall thickness. If a pre-punching job is running on a welded roll
forming mill, processing speeds typically are 100 FPM or less, though they can
be improved with special tooling like a rotary pierce tool, which looks like a roll
with punches sticking out of it. These punch or pierce the material as the roll rotates. (Rotary piercing does have limits and may not work in some applications.)
If pre-punching is not required, then a welded roll forming system can process at the same speeds as a tube mill. To achieve and maintain these speeds,
however, a welded roll form system needs some tube mill specialty equipment
added, such as an accumulator and high-performance cutoff.
Current and Future Production Needs
So again, which is best? If you have only simple shapes to form, a tube mill may
be the way to go; if you have complex shapes with multiple radii, holes, and
other features, a welded roll forming system is the way to go.
But what if you have a simple shape with tight tolerances? What if those radii
need to be consistent from corner to corner? What if you need sides to be precisely flat, with no chance of crowning? In these cases, a welded roll forming
system may be more suitable for the job, unless product specifications can be
As with anything else, choosing a roll forming system involves considering
product requirements and weighing those against your current and future production needs.
Brian Kopack is application engineering manager at Formtek Inc.,
Which Is Best?
For round shapes, a tube mill probably is the best choice. But what if you only
want to make squares or rectangles? What if you want to have prepunched
To determine the best machine, consider the strengths and weaknesses of
each. Tube mills specialize in simple shapes. Besides round shapes, they’re capable of producing squares, rectangles, and ellipses.
If you’re looking to form a few complicated shapes, the tube mill may not be
the best choice. Forming some shapes on the tube mill simply isn’t possible,
but a welded roll forming mill can form highly complicated shapes quite readily.
Also consider the outside corner radii of those shapes. In some cases, a tube
mill can form those radii down to 2 times the material thickness, but this isn’t
typical. Usually a tube mill can form radii down to between 2. 5 to 3 times the
Welded roll forming mills have a different tooling setup that allows for tight,
accurate radii. Specifically, it has to do with how the forming rolls engage the
material on the corner. On a welded roll forming mill, both the female and the
male roll engage the corner, making forming very exact. Typically, radii can be
formed down to less than 2 times the material thickness.
Also consider the consistency of the corner radius, as measured from
corner to corner on the
roll formed product.
Shapes formed on a tube
mill may have discrepan-cies between opposite
and/or adjacent corners.
This occurs because of
one-sided bending, with
just one roll (not both
male and female) fully
engaged with the material.
Male and female roll
engagement in welded
roll form systems has
another benefit: It helps
produce consistent flat
sides or features, with
no crowning or other reshaping. Because tooling on a tube mill gives
the workpiece no inside
support, flat sides or
features tend to reshape
during the process and
emerge from the tube
mill with a crown.
Also take into account
This step beam is formed to near net shape using a
welded roll form system.
A forming flower shows how a workpiece forms in a tube mill. To the right are typical
shapes that can be formed on a tube mill. (A tube mill also can form rectangles,
which aren’t shown.)
This typical sizing section of a tube mill has round roll
stands with specialty reshaping stands toward the exit.
Material flow is left to right.
In this welded roll form system, material flow is left to
right. (The welding area is out of the frame.)