Chapter 8 - Roll Forming
Roll forming is a continuous bending operation in which sheet or strip metal is plastically
deformed along a linear axis. Tandem sets of rolls (known as roll stations) shape the
metal stock in a series of progressive stages until the desired cross-sectional
configuration is obtained. See Figure 1. Because of the progressive manner in which
bending takes place, there is little or no change in, cross-sectional area of the
workpiece.
Roll forming is ideal for producing parts with long lengths
or in large quantities. It can also produce multiple length parts from the same set of
tooling. Virtually any material that can be formed by sheet forming techniques can be roll
formed. The process typically runs at speeds from 6 to 600 ft./min., depending upon the
desired configuration, tolerances required, additional ancillary or in-line operations,
and material being formed. For example, a soft aluminum can be roll formed much more
rapidly than the same shape in titanium.
Figure 2. Roll forming's versatility.
Figure 2. Roll forming's versatility.
Equipment Considerations
Roll forming mills generally fall into two categories,
outboard and inboard mills. Outboard mills have housings that support both ends of the
roll tooling shafts. If the shafts are supported at one end only (in cantilever fashion),
the mill is said to be of the inboard variety.
Inboard mills typically are used for thinner materials and
for strip edge forming. Sometimes, both inboard and outboard features are incorporated
into the same roll forming mill.
Figure 3. A roll forming machine
A roll forming machine (see Figure 3) includes a drive
system to power the roll stations and drive the material through the mill: a brake that
prevents "coasting" after shutdown: a coolant/lubricant system to reduce roll
wear and scuffing (roll marks) and to keep the rolls cool; and a straightener to remove
bow, sweep, or twist.
Turret-type, rafted, double-high, and side-by-side forming
mills provide roll formers opportunities for quick changeover.
Operation
In addition to the roll forming mill itself, a roll forming
line typically can be divided into the following major parts: material entry section,
cutoff press, and exit section. See Figure 4.
At the entry section of a roll forming line, material is
usually fed from a continuous coil, although it may be fed in sheet form, transferred
directly from another operation (such as piercing). Many roll forming systems incorporate
material handling devices for in-line storage of three to five coils. These systems
increase production efficiency by minimizing downtime required for coil changeover. Some
of the most efficient systems for long runs include in-line coil accumulators and strip
end joiners to eliminate coil changeover time.
Prenotch or prepunch presses can be used to punch hole
patterns into the material prior to forming, making it possible to locate holes that would
be difficult or impossible to punch after forming due to the final part configuration.
Another major element of a roll forming line is the cutoff
press where the continuous section is sheared to length. Due to the continuous nature of
the roll forming process, flying die cutoffs are frequently used. For some shapes, such as
heavy parts formed from thick gage material, the line actually may be stopped while the
roll formed section is saw cut to length.
Cutoff may be performed prior to the forming operation
(precut) or after the material is formed (postcut). Precut systems tend to produce more
end flare than post cut systems. End flare is inherent in the process and typically causes
one end to spring open and one end to spring closed as residual forming stresses are
released.
The exit section of the roll forming line is the fourth
stage. Normally the roll formed shape exits the roll forming line onto a table or roller
conveyor where it is removed manually. Almost any secondary processing needed, such as
bending, cutting, parts cleaning, welding and joining, finishing, assembly and
fabrication, can be done after the shape is formed.
Lubricants are typically used to lubricate the work
material during forming. If residual lubricant left on the part is of concern, check with
your roll former.
For some aircraft, aerospace and other applications,
aluminum alloys and some steels, it is necessary to heat treat the material prior to
forming.
Figure 5. This brass part, used as a shower bar, is
supplied prenotched, roll formed and cut to length. Note how the lock seam mechanically
fastens the material edges to form the hollow shape.
Figure 5. This brass part, used as a shower bar, is
supplied prenotched, roll formed and cut to length. Note how the lock seam mechanically
fastens the material edges to form the hollow shape.
Roll formed shapes can be open or hollow. If a hollow shape
is to be produced, a variety of methods can be used to join the free edges of the strip.
One of the most common techniques is known as a lock seam, which uses the roll form
tooling to mechanically fasten the two edges. See Figure 5.
Resistance, electric induction and high-powered CO2
laser welders are common welding techniques used for joining the free edges of the strip.
See Figure 6. For further specifics on welding, refer to Chapter 14.
Figure 6. Many roll formers use resistance and electric
induction welding techniques to produce shapes with a closed profile.
Advantages and Limitations of the Process
- Cold roll formed shapes can offer superior surface finish.
Sharp, clean contours can be maintained. The absence of die marks on the material often
eliminates the need for additional finishing.
- Almost unlimited part lengths are possible. The only
limitations on part length are dictated by material handling and shipping capabilities.
- Once tooling is made, almost any length and multiple lengths
can be produced from the same set of tooling.
- Hollow or semi-hollow shapes can be produced with relatively
thin walls. Although it is not usually feasible to roll form extremely large components
made from thin material (such as rectangular air ducts), roll forming can effectively be
utilized to form the edges of flat material which is later bent into large sheet metal
ducts.
- The high speed, continuous nature of roll forming lends
itself to economic production of large volumes of parts. It cannot, however, normally be
used to produce shapes of varying cross-section or parts which have different dimensions
on one end than on the other.
- Many additional operations, such as punching, notching,
welding, and bending, that otherwise would have to be performed as secondary operations
can be incorporated into the roll forming line, reducing handling and processing costs.
For example, labels can be applied to the shape as an in-line process.
- Prepunching in-line allows holes or slots to be included in
the shape that cannot be done as a secondary operation, because of their location or
features of the part.
- Parts can be swept into a continuous radius or rolled into a
circular ring such as a bicycle rim.
- Roll formed materials generally have a strength advantage
over competing processes in structural rigidity applications.
- The same tooling can be used to roll a shape out of
different materials.
- Almost any bendable material can be roll formed. Since roll
formed parts are made from sheet metal, the design of the product is limited to material
of constant thickness and does not provide the opportunity to strengthen bends with
fillets such as in hot rolled shapes or extrusions.
- Two different materials can be formed simultaneously to
produce a clad shape in one operation.
- Two distinct parts can be run together to form one assembly.
Tooling
Several computer aided tooling design systems are being
used to generate what are known as flower diagrams, depicting the anticipated flow of
material through the dies. Development and analysis of a flower diagram helps assure a
smooth flow of material from the first to the last pass and permits maximum control over
fixed dimensions while roll forming.
Figure 7. To assist the roll tooling designer, a
drawing known as a flower diagram is developed by superimposing drawings of part cross
sections at each roll station. A flower diagram, such as the one shown here, depicts the
anticipated flow of material through the die.
Figure 7. To assist the roll tooling designer, a
drawing known as a flower diagram is developed by superimposing drawings of part cross
sections at each roll station. A flower diagram, such as the one shown here, depicts the
anticipated flow of material through the die.
These CAD systems can augment the roll tooling designer's
productivity by allowing modification of roll profiles to see what effect a new tool
profile will have on the finished shape, without having to machine the tooling. The number
and configuration of the roll tooling stations are mathematically defined in the computer
with output to a numerically controlled lathe, which cuts the rolls.
CAD/CAM roll design systems in use today can scientifically
produce tooling designs for forming almost any profile. Some of the systems take into
account flare, twist, curvature, "oil-canning" and residual stresses that will
be present in the work material.
Tolerances
Dimensional variations in roll formed parts are based on
material, equipment and application. Dimensions vary due to material spring-back,
variations in material width and thickness, material properties, tooling quality and wear,
machine condition and setup, and operator skill.
Whenever possible, supply a sample assembly drawing to
illustrate the end use of the part and areas where tighter tolerances are required. The
drawing of the shape should become part of the purchase order once an agreement is reached
with the roll former. For more information see the chapter on drafting.
While roll formers report that tolerances tighter than
those cited below routinely are achieved, the greatest economies usually are realized when
specified tolerances are as generous as possible. Often, dimensional problems can be
avoided by ordering the raw material to be formed with somewhat tighter than commercial
quality tolerances.
The following are general guidelines only. Far tighter
tolerances are possible but may add to the cost. If more restrictive tolerances are
required, the designer should define them clearly and discuss them with the custom roll
formed shape producer.
Cross-sectional Dimensions
+/- 0.031 in. (0.787 mm) for fractional dimensions
+/- 0.010 in. (0.254 mm) for decimal dimensions
+/- 1 degree for angular dimensions
Straightness (bow or camber)
0.015 in. (0.381 mm) maximum deviation per foot of length
Length
- part thickness 0.015 in. (0.381 mm) to 0.025 in. (0.635
mm)
+/- 0.020 in. (0.508 mm) for parts up to 36 in. (0.9 m) long
+/- 0.047 in. (1.19 mm) for parts from 36 in. (0.9 m) to 96 in. (2.4 m) long
+/- 0.093 in. (2.36 mm) for parts from 96 in. (2.4 m) to 144 in. (3.7 m) long
- part thickness 0.026 in. (0.660 mm) to 0.104 in. (2.64
mm)
+/- 0.015 in. (0.381 mm) for parts up to 36 in. (0.9 m) long
+/- 0.030 in. (0.762 mm) for parts from 36 in. (0.9 m) to 96 in. (2.4 m) long
+/- 0.060 in. (1.52 mm) for parts from 96 in. (2.4 m) to 144 in (3.7 m) long
- part thickness 0.105 in. (2.67 mm) and greater
+/- 0.125 in. (0.32 mm) for parts up to 96 in. (2.4 m) long
+/- 0.250 in. (6.35 mm) for parts from 96 in. (2.4 m) to 360 in. (9.1 m) long
+/- 0.500 in. (12.7 mm) for parts longer than 360 in. (9.1 m)
Design Considerations
See Figure 8 for roll forming design tips.
Roll formed shapes should not be too deep. Profiles that
are deep require larger machines and larger diameter forming rolls which are more
expensive. One rule-of-thumb for average size machines is that maximum form depth should
be four inches. Greater depths are possible but require the use of larger machines and
more expensive tooling.
Parts should have uniform thickness throughout since the
raw stock is sheet or strip. Thickness may be increase by folding the material back on
itself.
Fig. 8 Effective design considerations for roll formed parts.
If wide, flat areas are required at the edge of a part,
consider using small stiffening ribs. The part will stay flatter and be much stronger.
When planning a leg, as with an angle or channel, the
length of the leg should not be less than three times the material thickness (3T). Legs
shorter than 3T are not easily formed because it is difficult to get enough leverage to
bend the leg up. This also applies when hemming or bending the material back on itself.
To plan pre-piercing pattern is critical and not repetitive
within the part, try to design it to have the minimum number of hole or notch patterns
within the part.
If a pre-piercing pattern is used that requires holes in a
specific area relative to the end of the part, try to keep it more than 0.5 (12.7 mm) in.
but within 4 in. (101.6 mm) of the end of the part.
Use maximum bend radii permissible. An inside bend radius
less than the material thickness will lessen roll life and increase power requirements.
Design parts to be as symmetrical as possible to eliminate
twist in the finished shape.
Design parts so that holes, slots and notches are not
distorted due to placement too close to or directly on a bend line. It is desirable to
have the edge of a hole or slot at least three times the material thickness away from the
tangent point of the nearest bend.
Do not specify tolerances that are closer than necessary.
Doing so will greatly increase the cost of both the tooling and the finished part.
Because roll formed shapes have uniform cross sections,
they can be bent easily. When rings or segments of rings are required, shapes can be
curved to uniform radii at the rolling machines without wrinkles and without disturbing a
prefinished surface. Helices are also possible.
Consider material elongation in designing parts for rings
to help eliminate wrinkles and fractures. Usually, the more elongation a material has, the
easier it is to bend. Where curves are not a constant radius, the uniformity of roll
formed shapes makes them ideal for stretching or tangent bending.
If piercing, notching or tabbing is required at either or
both ends of the part, keep the pattern of holes and notches close to the end of the part,
so that these operations need not repeat throughout the full length of the part. Often
this is less expensive and more desirable than pre-piercing because better accuracy from
the end of the part can be obtained.
Welded dimples or projections, tabs, stops and raised areas
can also be formed.
Almost any material that can be obtained as coil or sheet
can be roll formed. The material should be as ductile as design strength will allow to
permit sharp corners and easy bending. When high-strength alloy steels, heat-resistant
steels, titanium and other alloys are used, bend radii specified by the mill should be
followed. In many circumstances, bend radii can be reduced.
Go to the Design Guidelines Overview
Go to the Glossary
Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
Purchase the new Third Edition of Design Guidelines for Metal Stampings and Fabrications copyright © 2004 Precision Metalforming Association at Marketplace today!
|
