Roll Forming Handbook / Edited by George T. Halmos

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Nonwelded, closed sections can be also curved,

but the minimum curving radius can be inﬂuenced

by the deformation of the cross-section. Figure4.91

shows afew typical curved, closed sections with

various degrees of curving difﬁculties.

4.11.6 Curving Asymmetrical

Sections

Comparedwith symmetrical sections, it is more

difﬁcult to curve sections that are not symmetrical

to the plane of curving The stressed and com-

pressed elements of the cross-section moreorless

balanceeach other in asymmetrical section, but

they will deform asymmetrical section. The difﬁ-

culties to curve asy mmetrical, thicker walled, hot

rolled sections are well known. In both hot rolled

and roll formed cases, the simplest waytominimize

problems createdbythe unbalanced residual

stresses is to add additional rolls to the curving unit.

4.11.7 Curving by Controlled

Buckling

Buckling of curved products is unsightly and

unacceptable. Curving by preplanned corruga-

tions, however,isone of the ﬁrst and simplest

methods used by tinsmiths (Figure4.92).

Roofhip caps made from roll formed panels

havebeen curvedinasimilar fashion, using

multiple press hits. Building siding sheets are also

curvedtorelatively small radii by multiple press

hits (Figure4.93).

Continuous curving by cross-corrugation is used

to manufacture arched building components.

Sections up to 7to8in. (175 to 200 mm) in depth

havebeen curvedinthis waybyusing presses or

curving rolls(Figure4.94).

It is important that during curving,the original

bend lines should not be distorted [233]. Bend

lines distorted by the curving press die can reduce

the strength of the section by as much as 35 to

40%. The preservation of the longitudinal roll

formed bend lines is critical to all curving process.

4.11.8 Spiral Forming

Rings andmanyother circular productsare

frequently made to exit from the curving mill in

aslightly spiral waytosimplify cutting.Ifthe spiral

(helix) angle of these products is small, and the

contact lines

FIGURE 4.82 Curving rolls contacting the products

in aline only.

SPRINGBACK

FIGURE 4.83 In special cases, the product is wrapped

around the male roll.

radius

setting

entry roll

(b)

radius

stabilizing

roll

(a)

FIGURE 4.84 Radius stabilizing roll (b) can reduce

radius ﬂuctuation (a) when elastic roll is used for

curving.

Roll Forming Handbook4 -36

stresses in the material remainbelowthe elastic

limit, then the product springs back to the

circular shape after cutting to length.

The spiral forming of ﬂat and formed sections

for pipes has been well known for along time.

The great advantage of the helical pipe manu-

facturing method is that by changing the helix

angle, an inﬁnite number of diameters can be

produced fromthe same starting blank width

(Figure4.95 ). The smallest pipe diameter is

usually limited by the joint strength, while the

largest diameter is limited by the stiffness of the

pipe wall and the available space. Joints are

usually lock seamed or welded.

Smooth walledventilation pipes, ﬂexible

electrical conduits, and ﬂexible water hoses can

be curvedwith shoes.

The spiral curving of corrugated culvertpipes

is accomplished by threesets of curving rolls. The

angle of the rollsisadjustable to suit the helix

angle, while the corrugation pitch distance is

automatically kept the same (Figure4.96).

4.11.9 Curving with Shoes or Plugs

To curve rollformed sections with rollsissimple,

but buckling,crippling,ortwisting of the section

restricts the minim um curv ing diameter.To

curve products to smaller diameters, at room

temperature,curving shoes or plugs are required.

Curving shoes usually envelop the full cross-

section and forcethe parttocurve.One set

of shoes is required for each diameter.Shoes are

not adjustable or may havealimited adjustability

if made out of segments. The curving shoes can

be expensive, but in manycases,the shoes

provide the only route to curve the product to

the speciﬁed, small radius.

Curvingshoes must envelopthe section

without restrictingmaterial ﬂow; they must

apply the pressure to the curve and prevent

waviness (buckling) in the compressed areaat

the same time. The latter action is similar to the

function of the hold-down ring used in deep

drawing; therefore, the application of spring or

hydraulicpressure wouldbelogical,but is

practically neverapplied. To reduce wear,the

curving shoes are occasionally used in combi-

nation withcurving rolls.

To prevent buckling of tubes during bending,

ﬂexible plugs are used. Similar plugs are used

incorrect

correct

rubber

FIGURE 4.85 Thelocat ionofthe el astic ro ll is

important.

improved designs

original

shape

FIGURE 4.86 Stiffeners can reducethe buckling of

inward forcing legs.

FIGURE 4.87 Containing the legs between the rolls

(ironing) will reduce buckling.

FIGURE 4.88 Legs facing out are under tension and

can bend further in or out after exiting from the cur ving

rolls.

Secondary Operations in the Roll Forming Line 4 -37

during curving roll formed products, but the

product slides between the inside plugs and

outside shoes or rolls. Because the plugs are

ﬂexible, they do not curve the product, but in

manycases, they are essential for preventing

buckling.The plugs can be anchoredatafair

distancefrom the curving equipment because the

anchor must reach into the inside of the product

whereitistill open.

4.11.10 The Effect of Roll Forming

on Curving

Roll formingmillinduces stresses into the

product. These residual stresses can distort the

straight product an dare compounded with

the stresses created during curving.For example,

in the case of asimple Uchannel, bend lines

travel straight in the roll former,whereas the edges

of the strip are traveling in alonger route.

Therefore, the edges are stretched during roll

(a)"HAT" section (b)"C" section

A-A

FIGURE 4.89 Product can be deformed during curving if curving tool cannot reach the bend lines.

FIGURE 4.90 Cl osed,weldedsection maydeform

during curving.

E1 D1 E2

RRRRR

D2 E3 D3

FIGURE 4.91 Difﬁculties varyfromeasy to difﬁcult (E1, E2, E3, D1, D2, and D3) when curving nonwelded closed

sections.

FIGURE 4.92 Regular prebuckling is more acceptable

and helps curving.

Roll Forming Handbook4 -38

forming, even if they arecompressedbackatthe

last pass. The channel will havethe natural

tendency of bendingdownwards as shownon

Figure 4.97a. If theUchannel is curved in the

directionofthe legs (legsin),thenthe already

compressed legs willbeexposedtoadditional

compression(Figure 4.97b).

To avoid or at least minimize the above-

mentioned effect, it is advantageous to pass the

material through the roll former with rolls

stretching the web and compressing the legs.

Roll forming in this manner requires aspecial

FIGURE 4.93 Building panels curved in press brake. (Courtesy of Metform International Ltd.)

FIGURE 4.94 Arched building components are cur ved

by forming indentations in the panels.

FIGURE 4.95 Differentdiameterspiralpipes canbemadefromthe same coil width by changing thehelix entryangle.

Secondary Operations in the Roll Forming Line 4 -39

mill that can inducestresses to help to curve

the Uchannel in the required direction (see

Chapter 15). This method can be moreuseful

when curving deep,light gauge channels, made

out of highstrength, low elongation materials,

product typical of HSLA or UHSS automotive

bumpers.

Asimilar approach can be followed with other

products, inducing the correct stresses to curve

or twist the product in the direction of the ﬁnal

curving or twisting .Unfortunately,thereare not

enoughdata or equations to accurately calculate

theantic ipated stresses.Therefore, designers

usually shyawayfromthese logical approaches.

4.11.11 Effects of Cutouts and Dimples

Rollforming is the most productive metal fabricating method; therefore, the trendistoincorporate

as manyother operations in the roll forming line as is economically feasible. The aim is to produce

ﬁnished products at the end of the roll forming line. To achieve this goal, products with prepierced

holes, notches, lances, dimples, louvers, or embossed holes are frequently required to roll form and

curve.Most of these ﬂat surface discontinuities willadversely effect curving.Stresses created by curving

will not be uniform, affecting and sometimes varying the radius of the curvedproduct.

Grooveshavetobeprovided in the curving tool to avoid ﬂattening of dimples, lances, or embossed

holes. These protrusions should be located far enoughfromthe bend corners to allow as wide as possible

contact surface between curving tool and material [232].

Hole-to-hole distances in the outside ﬁbers (away from the center of the curving radius) will be increased

and the holes will be elongated during curving.The opposite effect will occur in the holes at the inner ﬁber.

Holes that are too close to the edge can create additional problems. The small material left between the

holes and the strip edge will buckle when the material is compressed during curving and the hole will be

deformed.

Cutouts have one additional bad effect on curving. The portion of the product weakened by notches

or holes can easily buckle during curving (Figure4.98), especially when curving is made by rolls. If the

cutout is on one side only,then the cutout side will buckle morethan the other one, adding atwist to

the product. The problems with cutouts are usually less pronounced and it is easier to correct when

curving is accomplished with shoes. Burrs left on cutouts, however,may create excessivewear on

curving shoes.

FIGURE 4.96 Spiral curving of culvert pipes can be made with the same tooling.

(a)

(b)

FIGURE 4.97 The legs stretched during roll forming

contribute to bowing down of the ﬁnished product (a) that

makes cur ving of the channel in the opposite direction

(legs-in) moredifﬁcult.

Roll Forming Handbook4 -40

4.11.12 Alignment and Adjustments of Curving Tools

The roll forming mill and tooling must be well aligned to produce good-qualityproducts with tight

tolerances. Shaft shoulders, against which the rolls are positioned, should be aligned within 0.001 to

0.003 in. (0.025 to 0.075 mm) in the vertical direction and within 0.002 to 0.005 in. (0.05 to 0.125 mm)

in the horizontal direction. The shoulder-alignment tolerancerequirement depends on the product

tolerance, surface qualityrequirements and material thickness. To check such tight tolerances, a

straight edge and feeler gauges, or dial indicator,orlaser (or optical) alignment methods are required.

Similar accuracy is needed in everydirection in the curving equipment. Unfortunately,atmanyplants,

the additional units such as entryguides, side-rollstands, and straightening or curving units are not

accurately keyed to the mill base, but aligned solely by eyesight. Misaligned curving units create

additional stresses and problems, causing twist or achange in the radius. Therefore, it is important that

the operator is able to ﬁnely adjust the curving dies in anydirection. To ease setting and control, good,

easily readable scales should be installed in the direction of each movement.

If the curving unit is used for morethan one product formed at different pass heights, then in addition

to the ﬁne curvatureadjustment, aquick height adjustment is required to suit the position of the

products. In manycases, the best solution is to haveinterchangeable, separate curving units with dies for

each curedsection.

4.11.13 Controlling Radius and Curving to Variable Radii

The curving radius is affected by so manyfactors (some of them varyevenwithin one coil) that additional

tooling,equipment, and control devices are required to keep the radius within speciﬁed tolerances.

Additional tooling such as supporting,pushing, twisting rolls, or shoes may be required to avoid

postcurving deformations of sections being asymmetrical to the plane of curving.

Some asymmetrical shapes may require ﬁve to six additional rolls to the original threecurving rolls.

This multiple set of rollscan also assist to reduceﬂat (noncurved) lengths at notches and to keep bends at

the correct ang le.

straight

theoretical actual

FIGURE 4.98 Change in product cross-section, such as notches and holes, usually causes buckle.

Secondary Operations in the Roll Forming Line 4 -41

Forlong,heavy products with large radii,

asupporting roll or rolls may be required.

Without support, the weight of the product will

straighten out the tail end of the curved part

(Figure4.99).

Radius control arms, photocells, or proximity

switches, which control curving rolls through

servo mechanisms, can keep the radius within

tight tolerances (see Figure 4.100).

By applying length sensors or measuring units and controlling the curving roll position through servo

mechanisms, or by using aprogrammable controller or computer,itispossible to produce partially

curvedparts. Components curvedtotwo or moreradii, or curvedtovariable radius (Figure 4.101a–c)

can also be manufactured.

4.11.14 Cutting to Length after Curving

The cutting of continuously curvedproducts can be classiﬁed in threegroups, namely:

(1) Cutting asegment of acircle

(2) Cutting afull circle

(3) Cutting helically wounded tubes to length

Similar to straight rollformed sections, curved products can be cut by shearing throughthe full cross-

sections or shearing apartial cross-sectionatprepierced or prenotched locations. The cutoffcan be single

deflection

control roll

self weight can

straighten out

curved panel end

FIGURE 4.99 Long,heavy products requiresupport

during curving.

radius control

arm

servo

motor

photocell

FIGURE 4.100 Curvature can be controlled by mechanical or electronic means.

(a) (b) (c)

FIGURE 4.101 Mechanically or PLCorcomputer-controlled units can curve one end only (a), or curve to two radii

(b) or to multiple radii (c) of asingle product.

Roll Forming Handbook4 -42

or double shearing,and it maybecombined with

additionaloperations such as piercing holes,

embossing,swaging,orbending.

The cutoffmethod can be shearing,saw cutting,

or cutting with ﬂame, laser,plasma jet, or other

means. Product length can be sensed or measured

with ﬂagswitch, photocell, proximityswitch,

positive stop,rotaryencoder,orlocating prepierced

holes,notches,orother methods appliedin

conventional roll forming lines.

When theproduct is roll formed andcurvedata

relatively lowspeed,afast-actingcutoffdie travels

tangentially to theproduct on straight railsonlyfor

ashort distance;therefore it maynot distortthe

section(Figure 4.102and Figure 4.104).

Small curving radii, highproduction speeds,

or slowerpresses frequently demand ﬂying cutoff

die traveling in acircular motion (Figure4.103). In

these cases, it is better to use asimple rotary ﬂying

press and die to cut products curvedtodifferent

radii. In these rotary(swinging) cutoffpresses,

both the position of the center pivot pin and the

length of arm are adjustable. In the case of lower

production speeds, ﬂying cutoffdies can be used,

traveling on curvedrails. However,separate curved

rails are required for each radius.

4.11.14.1 Cutting Segment of aCircle

Curved products manufactured in large quantities can usually be cut by positioning aconventional cutoff

press in the proper position. When the product is curved in the vertical plane, the conventional press is

tilted. Alternatively,unusual arrangements can be made such as tilting the entryend of the mill upwards.

When the curving is in the horizontal plane, the conventional press can be kept in the usual, vertical

position, but moved and rotated sideways in the path of the curvedproduct(Figure4.104). To suit

curvature in anyplane, special adjustable press bases can be built.

4.11.14.2 Cutting Products Curved in about 3608

Special pneumatically or hydraulically operated rotating presses, which swing around apivot point, are

used to cut circular products (Figure3.10). To minimize the danger of reentering the cut end into the

curving die after afull circle, the product is usually deﬂected sidewaysbyabuffer plate to create aspiral

shape. The cutoffdie is often positioned close to the curving tool. The product is guided (ﬂexed) sideways

in aspiral form. After one full circle, it enters the cut offdie instead of entering again into the curving die.

This deﬂection usually does not create permanent deformation and the ring springs back into the same

plane.

Automotive brake dust rings and manyother products are made in this way. After cutting to length, cut

ends of bicycle rims are be joined by welding or by other methods. The full circle product then can be slightly

stretched to the speciﬁed diameter and shaped to tighter tolerances.

4.11.14.3 Cutting Helically Wound Pipes to Length

The spiral forming is apractical method of producing pipes to inﬁnite number of diameters using simple

tooling.Toavoid the necessityofmaking acutoffdie for each diameter and deforming the thin-wall tubes

or pipes, manypipes are cut to length by ﬂy ing saw, laser,orother similar cutting methods.

finish

start

FIGURE 4.103 Cutoffpress travels in an arc.

start

finish

Radius Deformed

by Straight Cutting

Original Radius

FIGURE 4.102 Short straightcutting-die travel may be

used for cutting products curved to large radius.

Secondary Operations in the Roll Forming Line 4 -43

The sawunit travels at the same speed as the pipes moving longitudinally,while the pipe also

rotates. The sawblade enters the pipe at one point, and after one full rotation the pipe is cut through.

Figure4.106 shows aﬂying sawcutoffarrangement for culvertpipes.

4.11.15 Tool Material and Lubrication

Good curving rollsare made of at least the same qualitytool steel as the forming rolls. They are to be

machined, heat-treated,andpolishedin thesamefashionasthe forming rolls.Occasionally,curvingrolls are

plated to match the coating offorming rolls. Bronze rolls are used (albeit infrequently) to minimize scufﬁng

and surface marks.

Curving shoes are occasionally made from hardened tool steel, manganese steel, or from tungsten

carbide inserts.The most frequently used abrasion-resistant curving shoe material is the AMPCO bronze.

FIGURE 4.105 When the product is curved into afull circle, deﬂector plates prevent the cut end from reentering

the curving tool.

CUT OFF

PRESS

ROLL FORMING MILL

CURVING

UNIT

FIGURE 4.104 Presses having straightorcurved die rails can be moved sideways to suit the path of the curved

products.

Roll Forming Handbook4 -44

Aluminum bronze or other material can be used to curve aluminum. Wear-resistant plastic or other

materials can also be used for curving shoes. However,burrs, sharp edges, notches, and holes can damage

plastic shoes. Fortesting, sometimes well-lubricated hardwood blocks are sufﬁcient.

Applying lubricant to the roll curving tool is as important as applying it to forming rolls. It is, however,

still moreimportant, even essential, to use lubricant when curving shoes are used.

Good lubricant, compatible with both the formed material and the curving rollsorshoes, willreduce

tool wear,eliminate or minimize surface marks, and contribute to even curvature.

4.11.16 Curving by Reducing the Thickness of Material

The increased material thickness at the inside of the curvedproduct and the decreased thickness of

the outside section during curving havebeen mentioned in the early par tofthis section. Utilizing this

thickness change, it is possible to curve asection only by reducing the thickness of the outer partof

the cross-section beforecurving .The thin, reduced cross-section will be longer than the thick one. The

ratio of the length change will be equal to the ratio of the thickness change. To equalize the stresses

created by the differencesinthe length, the product will curve itself.

This procedure is applied, for example, in the manufacturing of auger (Archimedes) screws, in the

transfer of granular material and liquid. This method is veryseldom utilized in the curving of roll formed

products, because roll forming mills are usually not designed to change material thickness. However,the

procedure can be used if the mill has adequate strength. In some instances, such as curving Uchannels

with leg in, mills havebeen designed with alast pass that has larger diameter shafts than the others. This

last pass is used to reduce the thickness (thus elongate) the web of the Uchannel, inducing acurvature.

Controlled thickness change (see Figure4.107)induces the curvatureautomatically [232] but sizing rolls

would still be required to assure constant radius.

The thickness reduction at the entryside of the rollformer is moreapplicable to symmetrical than to

asymmetrical sections, because only the waviness created by the length differential is to be considered

during roll forming. In the case of asymmetrical section, camber introduced by the difference in edge

length will create considerable rollforming problems.

FIGURE 4.106 Flying sawcuts offculvert pipes. (Courtesy of Paciﬁc Roller Dies. With permission.)

Secondary Operations in the Roll Forming Line 4 -45