Roll Forming Handbook / Edited by George T. Halmos

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Because of the method of testing,some tables show a“minimum bending diameter” to thickness ratio.

This has to be converted to bending radius. For example, if the table shows that the minimum bending

diameter is t ¼ d ; then it means that t ¼ 2 r ; or r ¼ t = 2 :

Folding the material back on itself by 1808 bending is called “hemming.” Some materials can be formed

with very small, even 0(zero) inside radius without cracking.However,itissuggested to avoid hemming

with 0inside radius even if the material does not crack. The operators will havedifﬁculties in setting the

mill roll gap to form exactly twotimes the material thickness (Figure 5.16a). If the gap is larger than

two thicknesses ð 2 t Þ ; then the hem remains open (Figure5.16b). If the strip thickness or the roll pressure

increases, then the edge can be elongated. This can create acamber in the product, makethe edge wavy or

“open up”the edge of the strip (Figure5.16c). If the edge opens up,the operator may apply even more

pressure, compounding the problems. Therefore, for hems, asmall “tear drop”shape is recommended

(Figure5.16d–f).

To achieveavery sharp,orevena0inside radius, grooving (scoring) can be applied beforeforming.

The grooving rolls are reducing the material thickness by about 30% (Figure5.17a). Grooving rolls wear

faster than the rest of the rolls, therefore to facilitate replacement, it is astute to make them as separate

rolls, or makethem from morewear-resistant material.

If this type of thickness reduction/grooving is used by the rolldesigner,then the customer should be

advised that after forming,aportion of the corner will be partially “folded over.” This fold, which looks

like asmall crack, will reducethe fatigue strength of the product. The application of scoring should also

be avoided when prepainted material is formed.

Another wayofachieving sharp inside bend is to form agroove into the strip as shown in Figure5.17b.

Because the material thickness is not substantially reduced at the bend line (it is not scored), less fatigue

strength reduction would be expected.

Forcoatedmaterial, attention should also be paid to the behavior of the coating. Tables and standards

provide good guidelines for the safely attainable minimum bend radii.

If the recommended minimum bending radius:thickness ratio is followed and the zinc or other coating

still cracks or peels off, then the problemmost probably lies in the coating process. In this case, roll

modiﬁcation cannot eliminate the cracking or peeling of the coating.

When the zinc coatingcracks, the life of the

ﬁnished product willbeshortened despite the

sacriﬁcial properties of the zinc. The change in

product life can be occasionally noticed on the

formed edges of farm rooﬁng ribs and other

outdoor products.

Most paints and laminates do not restrict the

bending radius. However,some types of less pliable

paints (e.g., acrylic) and some laminates will crack

or haze at the bend lines. Thus, very small cracks,

similar to the above-mentioned metallic coating

cracks, can lead to premature corrosion.

FIGURE 5.16 Hem(1808 fold) designs.

FIGURE 5.17 Twomethods to form sharp inside radii.

Roll Forming Handbook5 -12

Cracking of paints and laminates can frequently be eliminated by preheating the strip to 125 to 1708 F

(50 to 758 C), depending on the type of paint. Heating makes the paint or laminate pliable but not too soft

for forming. It is advisable to interfacethe preheating process with the mill drive. When the mill slows

down or stops, then the electrical or gas preheating should be adjusted or turned off. Restarting of the

mill should be delayed until the material reaches the required temperature. This type of interfacing is not

required if preheating is accomplished by pulling the coil throughahot water tank.

Low temperaturemakes some paints less ﬂexible. Therefore,coilstransported or stored at low

temperatures, particularly below freezingpoint, may have to be brought up to room temperatureto

avoid cracking during roll forming.

5.2.10.2 Large Radius

Averylarge bending radius creates other kinds of

problems. The stresses at the outside ﬁbers are in

proportion to the r : t ratio.Ifthe “r”islarge, then

the stress can be so low that the elastic deformation

(and springback) will be more signiﬁcant than the

permanent one.

Therefore, it is very difﬁcult to roll form metals

with reasonable tolerances if the bending radius is

10 to 100 times (or more) the material thickness

(Figure5.18a,b). The change in thickness and

mechanical properties of the metal, even within a

coil, will signiﬁcantly inﬂuence the radius. Therefore, designing rolls to form aproduct with alarge

radius is much more difﬁcult than with asmall radius. It maythus be necessarytoresorttoan

approximation to the large radius using aseries of ﬂats and sharp corners (Figure5.18c). In most cases, a

better result can be achievedbyusing constant radius roll design and overbending the forming angles at

each forming pass (see Section 5.9.3 and Section 5.9.5)

5.3 Product Orientation and Other

Operations in the Line

5.3.1 Orientation

Beforeestablishing the number of passes and commencing roll design, the orientation (position) of the

ﬁnished section has to be established. As the section exits from the last pass, orientation is inﬂuenced by

manyfactors. Onceorientation is established, it will dictate the tool design and will inﬂuence other

operations such as cutoff, punching,embossing, curving,and packaging.Some of the factors inﬂuencing,

or inﬂuencedby, the orientation are listed in the following sections.

5.3.2 Forming

Unless other factors override it, the simplicity in forming will dictate the roll design: the simpler the

forming, the fewer the number of passes. Some of the typical examples shown in Figure 5.19 indicate how

the orientation inﬂuencesthe number of passes or the internal stresses.

5.3.3 Cutoff

The strokeofmost cutoffpresses employedinthe roll forming lines is in avertically downward direction.

The roll designer maychange the orientation of the section to avoid or minimize the chances of

distorting the product by the cutoffblade and minimize the product/blade contact during cutoff.

FIGURE 5.18 Roll forming products with large radius

is difﬁcult.

Roll Design 5 -13

Long contact with the product will wear the blade quickly and will create “galling.” Forexample,

galling will drastically reduce the blade life when a2-in. (50-mm) vertical leg is cut offwitha

vertically moving blade. The designer can trytoreduce the shear contact by changing the contact

angle. The blade/product contact angle can be changed by either “rotating”the product (different

orientation) or changing the traveldirection of the blade (Figure 5.20). A15to30 8 blade travel

direction (Figure5.21a,b) is quite common when cutting off“U” or “C”channels or other sections

with vertical legs.

The most frequently used methods to avoid or minimize section deformation and blade galling during

cutoffare to prenotch the strip or to change the blade travel from vertical (908 )toanangular (70 to 808 )

direction. Cutoffdie designers occasionally employcircular or avarietyofcam generated shear blade

motions [188–193].

Some presses can be rotated around the axis of formed product travel. This will simplify the cutoffdie

design and section orientation can be dictated by other,sometimes morecritical factors. If the cutoffdies

also haveother functions such as punching,embossing,swaging,and so on, then the direction of the

stroke and the design of the die may be dictated by these factors.

5.3.4 Other Operations in the Line

The technologyofother operations in the line may determine product orientation. However,slender and

relatively pliable products maystill be roll formed at the optimum orientation, and the product can then

be twisted over along distance to the position (orientation) best suited for the secondaryoperation.

(a) Punching and Notching.During punching,the downward movement of the punch will dispose of

the slug below the surface of the strip.Punching upwards can be dangerous and should be

avoided because anyslug accidentally remaining on the surfaceofthe strip can damage both the

product and the tooling.

FIGURE 5.19 Simplicityinforming frequently dictates partorientation.

FIGURE 5.20 The length of contact between the cutoffblade and the material changes with the angle.

Roll Forming Handbook5 -14

The burr is also in the direction of the

travel of the punch. Figure5.22 shows

the result of the punching operation. Figure

5.23 is aphotograph of ashelf. The burr

must be in the downward direction, and the

punches above the product. Therefore, the

forming of the longitudinal shelf legs is

dictatedbythe directioninpunching

downwards (Figure 5.24). In some cases, a

burr facing the wrongdirection can be

“squashed”backbypassingthe strip

through apair of rolls.

FIGURE 5.21 Cutoffblade travels at an angle to minimize galling.

FIGURE 5.22 Punching ahole creates burr.

Roll Design 5 -15

(b) RotaryPunching.Tofacilitate slug removal,

similar to the standard punching process,

the rotarypunches are usually installed in

the top roll and the dies in the bottom roll.

However,owing to the rotary nature of

punching, if required,the directionof

the burr can easily be changed upwards

(Figure 5.25).

Bending.These processes do not generate

slugs; therefore, they can protrude fromthe

strip up or down.These protrusions will

not inﬂuence the orientation of the pro-

duct. The roll designer must leaveagroove

in the rolls for the protrusions to pass

through.

Wing benders and end benders (wipers)

can operate either up or down, but because

of habit, easier producttransfer,and for the

sake of better visual observation, the

bendersusually operateupwards.This

means that ashelf with the ends bent up in the line has to be rollformed with legs up (Figure

5.26). In continuous shelf or similar product operation, the 908 single bends across the forming can

be formed within the cutoffdie (Figure 5.27). To removethe slug,the blade moves downwards and

the die forms the end bend upwards. This cutoffmethod stipulates forming the shelves with legs up

as shown in Figure5.26.

(d) Mechanical Joining of Different Materials.Itisnot unusual for two or three differentmetallic or

nonmetallic materials to be mechanically joined in the roll forming line. Given the continuous

nature of roll forming, some of these mechanical joints (Figure5.28a) are created by forming one

material abovethe other (“capping”). In this situation, usually the more complex partmade in a

rollforming mill is at the bottom, and the slightly formed cap,formed by asecond mill, is at the top.

FIGURE 5.23 Shelf with holes.

FIGURE 5.24 Shelving is formed with “legs down”to

avoid burr on the top surface.

FIGURE 5.25 Direction of burr created by rotary

piercing can face down or up.

Roll Forming Handbook5 -16

The twoparts are tangentially brought

together and ﬁnally formed at the later part

of the mill. Different materials can also be

stitched (Figure 5.28b), clinched (Figure

5.28c),lock-seamed(Figure5.28d)or

joined together by other methods (Figure

5.28e–h). These operations will inﬂuence

the orientation of the part.

(e) Marking and Coining.Product identiﬁ-

cation (marking) and continuous or inter-

mittent coining (local thickness reduction)

of the material can easily be accomplished

on both topand bottom surfaces.If

marking is made by embossing or stamping identiﬁcations into the material, then the roll

designer must ensure that these markings are not obliterated at subsequent passes. Forbetter visual

observation, most markings, including attaching of adhesive labels, are applied to the top of the

product.

FIGURE 5.26 Shelves are formed with leg up if required for end bending.

FIGURE 5.27 Bending ends of products across the

direction of rolling.

FIGURE 5.28 Examples of joining different material mechanically without fasteners.

Roll Design 5 -17

(f) Curving and Sweeping.The distancefrom the roll forming mill bed (or from the ﬂoor to the pass

line height) does not usually allowfor the product to be curved downwards. Therefore, in most

cases, the product is curvedupwards or sideways. The direction of curving (sweeping) will dictate

the orientation of the product in the roll forming mill.

(g) Welding.When highfrequency arc or laser welding is used in the line, the weld is usually made at

the top of the product (Figure5.29a,b). Resistancewelding (spot or continuous) can be made at

anyposition (Figure 5.29c–e). Both the technologyand the visual checking of the welding process

often dictate the partorientation.

(h) Caulking and Applying Adhesives.Tominimize spillage, caulking of grooves or cavities is usually

made from the top (Figure5.30).

To facilitate visual observation, sprayed, roller coated or otherwise applied adhesive are usually,

but not exclusively,applied to the top surface.

(i) Foaming.When foaming is continuously applied to products such as insulated panels and doors,

the twoouter skins havetoberoll formed with their inside, usually horizontal surfaces, facing each

other.Orientation is less critical for parts cut beforebatch foaming using automated or manual

handling.

(j) Extruding.Ifrubber,plastic, or similar material is extruded onto the roll formed product, then the

simplicity in rollforming usually dictates the product orientation. If the product has to be cleaned

beforeentering the extrusion head, then the discharge of the cleansing liquid maybea

consideration (legs facing downwards).

(k) Other Processes.Other processes such as interleaving, packaging,automated material handling,

automated placement of the roll formed parts into the next operation, and others may havean

importantinﬂuenceonthe orientation of the product.

Products cut to length beforethese operations may be “ﬂipped over” after rollforming.

Occasionally,for better “nesting,” everysecond product is ﬂipped (turned) over during packaging.

(l) Visual Observation.Itisfrequently speciﬁed that critical painted or highgloss surfaces should be on

the top side to help visual observation.

If forming, leg lengths, hole locations,

or other features are moreimportant at

one edge than at the other,then the

critical edge should be at the operator side

of the mill.

FIGURE 5.29 Welded roll-formed products.

FIGURE 5.30 Caulking building panel in the line.

Roll Forming Handbook5 -18

5.4 Materials

5.4.1 Effect of Material on Roll Design

Materials are selected to suit the speciﬁed product requirements at the possible lowest price. However,the

mechanical properties, surface, deviation from thickness, width, straightness, and ﬂatness of the material

will inﬂuence rolldesign.

Adetailed description on the most frequently roll formed materials is in Chapter 6.

5.4.2 Mechanical Properties

During roll forming, the strain (elongation) of the outside ﬁber of the strip must remainbelowthe

uniform engineering strain. Therefore, the rolldesigner must knowthe yield strength, maximum tensile

strength, and elongation of the material. These data are usually available from the supplier.

In low carbon steel, the correlation between hardness and tensile strength is reasonably good.

Therefore, hardness is sometimes used to “check”the incoming material and “screen out”the too “hard”

or the too “soft” steel. However,“good hardness” does not guarantygood formability.

Cold work (plastic deformation under the annealing temperature) such as bending, tensioning,

compressing,and other operations will increase the yield and tensile strengths and reduce elongation.

Cold working of some materials, for example, certain stainless steels, zirconium, and other special alloys

signiﬁcantly increases the yield and tensile strength (Figure6.11).

Based on past experience,the rolldesigner can usually judge the anticipated formabilityofthe material,

including the minimum radius and springback. However,the highly work-hardening materials may have

similar mechanical properties to mild steel before roll forming,but by the time bending is completed, the

properties are closer to highstrength steels. The result can be a15to25 8 springback at a90 8 bend instead of

1to28 normally associated with material having similar starting mechanical properties.

Materials having highyield strength, small difference between tensile and yield stresses, and small

elongation require moreforming passes.

5.4.3 Springback

In addition to the mechanical properties of the material, springback is inﬂuenced by the r : t ratio,the

method of forming, and by the elastic modulus ð E Þ of the material (Table 6.1; see also Section 5.12.12).

5.4.4 Flare

(a) General Observations

Flare is achange in the cross-section at the cut ends of arollformed product.

The angular change can be towards or away from the center line of the section. In the case of

lower strength materials, not too-deep sections and especially with precut blanks usually the

front end is “ﬂared in”and the tail end is “ﬂared out.”

Deep or highstrength materials usually ﬂareout at both ends (Figure10.38c).

Noticeable ﬂaremay startonly 0.5 to 1in. (12 to 25 mm) or,inextreme cases, 24 to 36 to 48 in.

(600 to 900 to 1200 mm) from the ends of the ﬁnished section.

The section width increase caused by the ﬂarecan be from afew thousands of an inch (0.025 to

0.1 mm) to 0.5 to 1in. (12 to 25 mm).

The worst ﬂareobservedbythe author was approximately 4in. (100 mm) in width.

The ﬂare is usually larger when precut blanks are roll formed.

Flare does not depend on the method of cutting.Continuously formed sections cut with ﬂame,

saw, or cutoffdie will show the same ﬂare.

The deeper the section, the larger the ﬂarewill be.

Roll Design 5 -19

The higher strength the material, the larger the ﬂarewill be.

The more abrupt the forming is (fewer passes, shorter horizontal distances), the larger the ﬂare

will be.

Deep notches at the edges will create ﬂare, similar to precutting (Figure5.31a,b)

(b) Problems Caused by the Flare.Itisobvious that achange in width and angle can create problems

in joining parts together (likeassembling components), butting them side-by-side (likebuilding

panels) can create product application and appearance problems.

continuous, but become unbalancedassoon as the section is cut. The author has neither

sufﬁcient data to specify these internal stresses nor information on how to eliminate them in

everyproﬁle. However,the factors inﬂuencing ﬂarecan frequently be identiﬁed.

(d) Increasing, Reducing, or Eliminating Flare.Flare in certain proﬁles will be increased if:

Notenoughforming passes are used, especially to form the edges.

Horizontal centers are too close, especially in deep sections.

FIGURE 5.31 Deep cutouts havethe same effect on ﬂareascut ends. (a) Schematic representation; (b) cutouts

between passes. (Courtesy of Delta Engineering Inc.)

Roll Forming Handbook5 -20

Side-roll passes, placed betweenthe

main passes, used for edge forming or

overbending are at atoo shorthorizon-

tal distance.

Inward travel of the edges is extensive

at the last passes (Figure5.32)and, after

the last pass, it changes suddenly in the

direction of the ﬂow.

Manyrollsuppliers claim that they havemethods

to eliminate ﬂare. Some of the recommended and

often proven techniques are:

Usemore passes, mainly to form the

edges.

Useamill with larger horizontal centers

especially when forming deeper sections.

Overbend slightly and then bend back

the legs as shown in Figure5.33.This

will giveanopposite “bending moment”

to the edges.

According to one report, asimilar

effect hasbeen achieved usingan

unusual“concave” side(dovetailed)

main top roll and adjustable side rolls

(Figure5.34).

Use“gentle” forming at the last few

passes.

Apply twoidentical “last passes.”

Usethree roll straighteners designed

for the edges only.

Apply forcetosqueeze (crash) the out-

side legs and the bend lines. In some

cases, varying the pressure will change

the ﬂarefrom outwardstozero,oreven

inwardsatone end and opposite at the

other end of the product.

Preform bendswith slightly larger

than the required ﬁnal radius, then

reducethe radius in the last forming

pass(es) [414].

5.4.5 Material Tolerances

5.4.5.1 Thickness

The material must be able to pass through the gap

between the rollswithout thickness reduction.

Therefore, rollgap is calculated as the sum of the

maximum speciﬁed thicknessplusmaximum

FIGURE 5.32 Toomuch forming at the last pass will

increase ﬂare.

FIGURE 5.33 Overbending before the last pass and

bending out at the last pass usually reduces ﬂare.

FIGURE 5.34 This unusual“overbending” creates

similar effects as the one shown in Figure 5.33.

Roll Design 5 -21