Roll Forming Handbook / Edited by George T. Halmos
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In spite of all precautions, it will be adifficult
taskfor the operator to keep the“stretch”
uniform thus keepingthe hole centers to tight
tolerances. For example, excessiverollpressure
created by incorrect setup or significant increase
in strip thickness can increase a100-in. (2500-
mm) long section by 1to1.25 in. (25 to 32 mm),
thus increasing the distancebetween prepunched
holes.
9.3.2.3 Hole Shape
In addition to the deformation caused by the
“stretching-compressing” described above, the
elongation of finished productsmay change
the shape of the hole (see Figure9.18). Holes
close to the bending line can be deformed by
stresses across the direction of roll forming as
shown on Figure9.19.
It is difficult to form “lips” shorter than 4to
6times material thickness. Shortlips between
punched holes and bend lines maynot be bent
to the same degree as the longer segments
(Figure9.20).
9.3.2.4 Hole-to-Cut-Edge Distance
The minimum hole-to-cut-edge distance will
depend on the method of punching.
In the case of rotary piercing and cutting
product to length, the cut may be made at any
position, without consideringthe hole locations.
However,using the pick-up type of die posi-
tioning,the hole-to-cut-edge distancecan be
made uniform, but the product length tolerance
will be influencedbythe accuracy of the hole
locations.
If holes are prepunched and the location of
cutting is determined by apositive pick-up pin
entering into the hole, then the hole-to-cut-edge
TABLE 9.6 RotaryPrepunched
before after
roll forming
FIGURE 9.16 Holes close to the strip edges may deform.
after forming
pre punched
FIGURE 9.17 Hole center distances can increase in the
direction of roll forming.
after formingas punched
FIGURE 9.18 Hole shape can change duringroll
forming.
Roll Forming Handbook9 -14
distanceisestablished by the mechanical means.
If thereisonly one hole close to the cut edge,
then the size of the pick-up pin may influence the
distance. If the pick-up pin enters another hole
further away from the location of the cut, then
the minimum distancemay be influencedby
the stiffness of the product. Buckling between
the hole and the cut edge can result in loose
tolerances.
If the punch is incorporated into the cutoff
die, then the minimum distancewill be dictated
by the minimum die blade thickness, which is
required to prevent breakage and to provide
maintenancefreeoperation.
9.3.2.5 Postpunched Hole-to-Bend-Line
Distance
When holes are punched after forming,then
the distancebetween the hole and bend lines is
frequently dictated by the space required for
punch or die.
If the “legs” are facing up (see Figure 9.21),
then the hole-to-bend-line distanceisrestricted
by the safe clearance that the punch can enter
between the legs, and by the minimum flat area
required for the die to supportthe section. The
same figure shows the condition when the legs
are down. In this case, the minimum distanceis
restricted by the minimum wall thickness of
the die to fit between the downward legs of the
products. In special cases, the die can be made to
the same profileasthe roll formed section that
will be punched (as shown on Figure9.22).
9.3.2.6 Direction of Burr
The “slug”generated during punching must be
disposed. Therefore, the punch is usually placed
aboveand thedie underneath thesection.
Otherwise, slugs remaining on the surface of
the products can damage the rolls or other dies.
Forthe above reason, burr is usually directed
downward. If one surface of aproduct must be
“burr free,”then the rolldesigner usually forms
the legs of the products in the predetermined
direction. Forexample, to keep the surfaceof
shelvesfreeofburr,the punch must enter from
the top; therefore, the legs of the shelvesmust be
formed downward. An alternative method is to
squeeze the burr back to the flat surface during
roll forming.
before after
roll forming
FIGURE 9.19 Holes too close to the bending line may
deform.
t
too small
min 4-6t
t
FIGURE 9.20 Edge of holes should be at least 4to6t
distance from the bend line.
max dia
punch
FIGURE 9.21 Clearance between the punch and product
determines the maximum hole size when punched with
“legs up.”
Designing Products for Roll Forming 9 -15
9.3.3 RotaryPunching
The distances between the holes punched by a
properly designed rotary unit are uniform and
are within atight tolerance. However,changing
(adjusting) the distancebetween the holes is a
difficult and complex procedure.
9.3.4 Notching
Discontinued edges haveatendency to flare.
To minimize the flare, more passes are usually
required. Among other factors, the flarewill
depend on how deep the notch is (Figure 5.31).
Notching also requires aprepiercing press and,
in most end-notch cases, adouble shear cutoff
die. Notching of the edges makes the welding or
lock seaming of strip edges of closed sections
difficult or often impossible. However,when the
end of the product does not haveasimple,
straight cut, then it is usually easier to prenotch
the parts before rollforming.Insome cases,
prenotching is the only practical waytoproduce
the part(Figure4.34 and Figure 9.23).
9.3.5 Lancing, Partial Punching,
Louvering, and Embossing
In these four processes, aformed section of the
product willprotrude out from the flat strip.
Lancing usually refers to aproduct feature that
is cut around asubstantial partofthe protrusion
and it is bent out of the original plane in a
straight line (Figure 4.44 and Figure4.45).
Partial punching refers to aprocedurethat is
similar to normal punching,except that the
“slug” remains attached to the parent strip and
it can be removed later on with alittle force. It is
similar to the “knock-out” holes punched into
electrical boxes.
Louvering is similar to lancing,but the bend line wherethe “louver” joins the strip is usually longer
and often has bent elements (Figure 4.46).
Embossing usually refers to aprocedurewhere the material is drawn out from the original plane
without discontinuing the strip (Figure 4.49).
If lancing,partial punching,louvering, or embossing is completed before or during roll forming,then
the roll designer must provide openings in the rolls for the protrudingparts to avoid crashing them back
to flat. Similarly,the protrusions must pass through the cutoffdie, which mayresult in alonger cutoffdie
stroke or asmall distortion at the cut end.
Providing spacefor the protrudingparts in the nonhorizontal partofthe roll formed section
may require awide roll gap.This gap can cause ausually insignificant change in the angle of the finished
partorinthe accuracyofthe bending radius (Figure 5.151).
max dia
punch
FIGURE 9.22 Die size restricts hole diameter if “legs”
are facing down.
slug
is
cut
out
in
the cut off
die
next
part
FIGURE 9.23 In some cases, the ends of the formed
partsmust be prenotched to facilitate cutting to length.
Roll Forming Handbook9 -16
9.3.6 Mitering
Mitering is frequently done on parts bent at angles such as picture frames, door frames (Figure 4.36), and
windowframes. Applying prepunching/prenotching to obtain the proper miter is sometimes complex, but
it has great advantages in the subsequent bending/assembly operations.
9.3.7 Slitting
Slitting the edges of the strip during or at the end of the rollforming operation is rarely performed
process, but extremelyseldom it can be useful if veryaccurate edge tolerances are required. Edge slitting
has been used to align edges beforehighfrequency welding of previously corrugated sections (e.g., helical
culvertpipes).
Center slitting is used when two products are made side-by-side from adouble width strip.Inthis
latter case, the product is slit in the middle or at the end of the forming process (Figure9.24). Forming
twopieces side-by-side increases productivityand can be useful when the same number of left- and right-
hand parts are required.
9.3.8 Cut-to-Length
The cut-to-length process is described in Chapter 4. The designer musttakethe capability of the existing
roll forminglineintoconsideration unless the companypurchasesnew and better lengthsensor, die
accelerator,new press, or acomplete new linetosuit the tighterlengthtolerance.Whilesectiondimensions
and tolerances can be influenced by the roll design,length tolerance is mainlythe functionofthe equipment.
9.3.9 Joining Different Materials
Different types of materials can be joined in the rollforming process. The difference can be in thickness,
mechanical properties, and type of material. Different types of steel, stainless steel, aluminum, plastic,
rubber,and other materials can be joined in the process.
The joining of different materials can be accomplished by mechanic means (such as stitching,
clinching,orcrimping), resistance, or anyother type of welding,adhesive bonding,foaming,extrusion
(such as extruding rubber or plastic on aroll formed section), brazing,soldering,and other ways
(Figure 5.28, Figure 4.132 to Figure4.137). Knowing these possibilities, the designer may come up witha
better product, but must be aware of the restrictions. Restrictions can be in the process speed, cutting to
length, or forming the parent material after joining with another material.
slit line
slit line
as roll
formed
part
FIGURE 9.24 Twoparts roll formedfromone common coil can be parted (slit) beforethe first forming pass,
in between, or after the last roll forming pass.
Designing Products for Roll Forming 9 -17
9.3.10 Bending across the Rolling Direction
Althoughrollforming is usually associated with bending the metal at bend lines parallel to the direction
of roll forming, it is possible to bend the finished section at 908 or at other angle to the direction of
rolling. Atypical example is the shorter ends of ametal shelf. The shorter ends can be bent in the line
at 908 to the direction of roll forming after the forming of the longitudinal stiffener elements at the two
edges. This bending can be accomplished in aseparate operation, after cutting to length, with an end
(wing) bender (Figure 4.47, Figure4.50 to Figure 4.54) or with another roll forming mill running at 908
(Figure 4.48 and Figure4.49) to the direction of the first mill. Alternatively,the continuously made parts
can be cut offand the ends bent 908 in one hit in the cutoffdie.
9.3.11 Roll Forming Nonparallel or
Nonstraight Bend Lines
With special equipment or arrangement, it is
possible to roll form sections with nonparallel
bend lines. Atypical example is the pie-shaped
roof sheets of grain storage bins (Figure9.25).
Special equipment can be used to rollform edge
ribs on acurved“orange peel” segment of the
silo roof.
9.3.12 Curving and Sweeping
This process is described in details in Chapter 4. The product designer must consider the minimum
curving (sweeping) radius for each cross section, material, and thickness. Minor design variations such
as bending radius, stiffeners and others can influence the minimum curving radius. Holelocations and
cut-outs can influencethe curving radius or may requirespecial tooling.
9.3.13 Coining
Coining,orreducing the thickness, is not atypical rollforming operation, but occasionally it is required.
It requires more force to makethe material thinner than to form it. Therefore, the right equipment
(proper shaft diameters, bearings, drives etc.) must be available for the process.
9.3.14 Marking
Products can be marked in various ways such as stamping,embossing,laser marking, inking or
painting,labeling and others. Mechanical marking, such as stamping and embossing can be included in
FIGURE9.25 Specialprocessisusedtoroll form
nonparallel ribs.
FIGURE 9.26 Wavy edges reduce the qualityofthe weld.
Roll Forming Handbook9 -18
the cutofforprepunching press or performed by
rotary markers. Marking withpresses places the
mark in aspecific location while rotary marking
usually places the marks in aline at uniform
distancefromeach other,but not at aspecified
distancefrom the cut ends.
9.3.15 Welding, Brazing, and
Soldering
These continuous processes can be incorporated
into the continuous rollforming process. Non-
continuous resistancewelding or arc welding at
discrete position canbeaccomplished after
cutting to length although“flying”guns may be
applicable in certaincases.Weldingrequires
properly matched surfaces or edges, waviness
in one or both surfaces will create problems
(Figure9.26). The deeper the section is, the more
passes and longer horizontal pass distances are
required; otherwise, significant edge waviness cab
be experienced (Figure9.27). If possible, for the
same cross section, it is preferable to weld on the
longer side (Figure 9.28).
Forwelds wherethe edge alignment is critical,
additional bend linesadjacenttothe weld
(Figure9.29a) will eliminate or minimize edge
waviness. In other cases, for example, for arc
welding,edge forming (shown in Figure9.29b)
is suggested. In both cases, the additional bend
lines will increase the strength of the section.
However,inmanycontinuous welding pro-
cesses, adiscontinuity, such as aprepunched
hole (Figure9.30)can create amajor problem.
FIGURE 9.28 It is better to roll form asection (like one
shown in Figure 9.27)with the weld line on the longer side.
FIGURE 9.27 The edges of deeper sections (b) will be wavier than shallow sections (a) unless morepasses and
longer center distances are used.
(b)
(a)
FIGURE 9.29 Additionalbendliens reduce edge
waviness.
Designing Products for Roll Forming 9 -19
In these cases, relocating the weld line away from
the prepunched holes can be the right solution
(Figure9.31).
9.3.16 Coating
Owing to the differenceinprocess speeds, coating
linesare veryseldomincorporatedintoroll
forming lines, with the exception of plastic/rubber
extrusion. However, painting individual roll
formed parts or assemblies are frequent. The
foresight of the designer can help the subsequent
coating processes.Holes for hooks to hang the
product can be incorporated in the design. Other
prepunched holes for drainage and other features
facilitate themetallicornonmetallic coating
process.
9.3.17 Adhesive Bonding,
Caulking, and Foaming
The method of adhesive application can influ-
ence the product design. In the production line
both adhesive and caulking material is usually
applied from the top.Toapply foam into the
individual pieces or assembled products, such as
insulated doors, an appropriate opening has to
be provided in the product. Forcontinuous
foam application, such as insulated wall panels,
the design of the join can be critical. During
foaming,considerable inside pressuredevelops.
The designer has to knowhow the large flat
panels will be supported against “bulging out”
during the foaming process.
9.4 Profiles Manufactured in Different Sizes
9.4.1 Spiral Pipes, Panels, and “U” Channels
To meet different load or other requirements economically,the designer frequently has the option to
select from similar shapes formed from different thickness of metal or made to avarietyofsizes. The
tooling cost to manufactureaprofile in more than one size varies from practically nothing to the cost of a
full set of tooling for each size.
Thedesignerofthe roll formed profilegreatlyinfluences thetooling andproduction costs
by specifying the correct, variable dimensions before tool design starts. For example, different diameters
of corrugated spiral pipes can be produced with the same tooling by simply changing the entryangle of the
strip (Figure 4.95).
A36-in. (914.4-mm) wide building profilecan be converted to 35.43 in. (900 mm) metric coverage
with minimum extra tooling cost if the rollsare split at the right placeand an insertprepared when the
roll set is made.
cut out
weld line
FIGURE 9.31 Designing productwith the weld seam
away from holes simplifies welding.
weld line
cut out
FIGURE 9.30 Prepunched holes, notches can create
welding problems.
Roll Forming Handbook9 -20
Using the conventionally designed rollsshown on Figure9.32,the length of the legs of the “U”
channels can be varied within acertain limit, just by varying the blank size. If for tight tolerances, the legs
are “trapped” (or registered) in the rolls, the length of the legs cannot be changed but leg length
tolerances will be much tighter (Figure 9.32b).
9.4.2 “C” Channels and “Z” Sections
In the case of “C”channels, which are basically “U” channels withlegs bent inwards, the above-
mentioned change cannot be achievedassimply when traditional roll forming line with shafts
supported at both ends is used. The inward facing legs of the “C”channels are usually formed in the
early passes, restricting size changes in the subsequent passes. However,both “U” and “C” channels can
be reasonably easily manufactured in multiple widths within specified limits by changing the position
of rolls(see dimension “H” in Figure 9.33). Frequently both types of “C”and “Z” sections are rolled
with one combination set of rolls. However,inconventional mills changing the “W” dimension often
requires acomplete set of tooling for each size when standard mill is used.
Special purpose, quick change, duplex roll forming mills havebeen developed specifically for this
purpose. With these mills, unlimited varieties of “C”channels can be manufactured between the specified
maximum and minimum web widths, leg heights, and lip lengths without tool change. The changeover
time from one dimension to another can be as shortas1to3minutes.
The forming of a“C” channel is asymmetrical process, bending both legs upwards. The rolling of a
“Z” section is an asymmetrical process because one edge of the strip is forced up and the other one down
during forming. The forcing of one leg up and the other down induces asymmetrical stresses into the
finished “Z” section. The result is atwist in the finished product. To eliminate or minimize this “twist”
and to keep the edge of the strip at about the same height, roll designers prefer to shape the “Z” section at
an angle as shown in Figure 5.45.
The forming of the “Z” section at an angle does not lend itself to width (web length) adjustments and
each section size require aseparate set of rolls. Therefore, this method is usually used only when large run
quantities are required.
However,avarietyofspecial quick changeover “duplex” mills are available to rollform both the “C”
and “Z” sections with different web,leg,and lip dimensions.
9.4.3 Building Panels
In the case of ribbed (corrugated) building panels, adding (or removing) two ribs will keep the section
symmetrical; only the width of the starting material has to be changed. Adding (or removing) one rib to
the width may create minor asymmetryproblems and makes roll forming with the same set of tooling
somewhat moredifficult.
FIGURE 9.32 The tolerance on leg length is affected by the roll design.
Designing Products for Roll Forming 9 -21
When only the panel (product) edges are roll formed, quick changeover duplex mills can produce
panels withany width within the maximum-minimum limitations of the mill.
9.5 Design of Specific Products
Because of its highproductivityand versatility, roll forming is used to manufacturealmost every
type of product. Several articles about roll forming automotive,appliance, building,grain storage
bin, storage, and other products are published in the technical magazines and conference
proceedings.
9.6 Dimensioning and Tolerancing
9.6.1 Dimensioning
After aproduct is designed, the idea has to be communicated to the manufacturing plant. The
communication is usually in the form of drawings. Forroll formed sheet metal products, national or
international drafting standards are not available. As aresult, alarge percentage of the drawings are
inefficiently,inadequately or incorrectly dimensioned and toleranced. Sometimes, companies establish
(d)(c)
(b)
(a)
H
3
L
L
L
L
H
2
H
1
H
3
H
2
H
1
W
1
W
2
W
3
W
1
W
2
W
3
FIGURE 9.33 Changing the “H”,“L”,and “W” dimensions is aslowprocess in standard mills but it takes only
minutes in special mills.
Roll Forming Handbook9 -22
their own guidelines for dimensioning and tolerancing.However,dimensioning and tolerancing methods
are usually moreinfluenced by the design software and individual habits than by established standards.
Adimension is “a numerical value expressed in appropriate units of measureand indicated on a
drawing and in other documents along with lines, symbols, notes to define the size or geometric
characteristics, or both, of apartorapartfeature”[421].
Basic dimension can be defined as “a numerical value used to describe the theoreticallyexact size, profile,
orientation, or location of afeature or datum target. It is the basis from which permissible variations are
established by tolerances on other dimensions in
notes or in featurecontrol frames” [420].
Product designers and draft persons frequently
employ dimensions to suit theirown habit,
company’sstandard,oravailable space on the
drawing. Although it is important, product
dimensioningfrequentlydoesnot provide
information in aformat that is required for
manufacturing, strengthcalculationsortool
design.For these purposes, additional calcu-
lations are usually required; these can be time
consuming and the sourceoferrors.
To calculate thesectionalproperties
(strength), blank size (strip width) and to design
tools, the section usually is divided into straight
and curvedelements. For the straight elements,
the length, and for curvedelements, the bending
radius to the inside of the profile and the formed
angle are required.
These dimensioning methods, however,are
not suitable and are not used for dimensioning
roll formed products. Formanufacturing pur-
poses, especially to locate additional features
such as holes, notches and dimples, adifferent
method of dimensioningisrequired. It is
recommended that the ASME Y14.5M “Dimen-
sioning and To lerancing”standard[421] be used
as aguideline.
The application of the ASME standardfor roll
formed products should followthe guidelines
described below:
*
During roll forming, products are usually
“wrapped” around the male die. There-
fore, in most cases, it is preferable to
show insidedimensions as shownin
Figure9.34.
The final decision on applying dimen-
sions will be influenced by the method of
forming, accepted industrial standards, or
application. Forexample, tubular pro-
ducts (which are not completely formed
around amale die) are frequently defined
by theoutside dimension(OD).The
bad
bad
bad
good
good
good
FIGURE 9.34 Dimensioning of the inside surface is
preferred.
ref.
FIGURE 9.3 5 Tangent point dimensions have been
preferred by manual roll designers.
Designing Products for Roll Forming 9 -23