Roll Forming Handbook / Edited by George T. Halmos

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5.10 Number of Passes .............................................................. 5 -64

Art or Science

†

Factors Inﬂuencing the

Number of Passes

†

Calculating the Number of Passes

5.11 Flower Diagram ................................................................. 5 -73

Cross-Section at Each Pass

†

Forming in the

First Pass Rolls

†

Forming in the Last

Pass

†

Adjusting Angles by the Operator

5.12 Roll Design ......................................................................... 5 -78

Strip Width Applied to Roll Design

†

Traps

†

Lead-In

Flanges

†

Finalizing Roll Edge Radii at

Bend Lines

†

Applying Additional Radii

to the Rolls

†

Relief

†

Interlocking

†

End

Gap

†

Roll Length

†

Scoring (Grooving)

Rolls

†

Cutouts for Embossments and

Other Protrusions

†

Overbending

5.13 Calculating Roll Dimensions Manually ........................... 5 -94

5.14 Computer-Aided Roll Design ........................................... 5 -95

5.15 Examples ............................................................................. 5 -100

5.16 Roll Marking System ......................................................... 5 -103

5.17 Roll Orientation ................................................................. 5 -107

5.18 Setup Charts ....................................................................... 5 -107

Rolls and Spacers

†

Shims

†

Bending

Charts

†

Good Setup Chart Set

†

Updating the Setup Chart

References ....................................................................................... 5 -111

5.1 Roll Design Process

During roll forming,aﬂat strip is gradually formed to the ﬁnished shape (Figure 5.1). The forming is

almost exclusively accomplished by rotating contoured rollswhile the material “passes” through the mill.

The aim is to form the required shape repeatedly within the speciﬁed tolerances, with the least amount

of “incremental steps,”or, in other words, with the least amount of passes. “Too quick”forming (too few

FIGURE 5.1 Gradual forming of astrip into ﬁnished section. (Courtesy of data MSoftware GmbH.)

Roll Forming Handbook5 -2

passes) will distortthe product because of the unacceptable level of stresses generated in the metal. To o

manypasses will makethe tooling and the process uneconomical (see Figure 5.2).

After choosing orientation of the part(the position as it exits from the last pass), the designer

establishes the number of passes required to form each bend. Forexample, ashortlip maybeformed in

one, two,three, or morepasses, depending on numerousfactors. The bending mayormay not be

completed in consecutive passes. For example, a90 8 lip may be formed in the ﬁrst three consecutive

passes at 30, 60, and 908 ,orintwo passes up to 608 and the last 308 bend (to makeit90 8 )will be formed in

the last pass.

Then the designer establishes the ﬂow of material. The forming may commence at the center with a

bead (see Figure 5.3a) or at the edges with the shortlip (Figure5.3b) with the bead formed at later passes.

Rolls are expensive. If the designer suggests too manypasses to “play it safe,”then the tooling will be

noncompetitive. On the other hand, having less than the optimum number of passes may result in costly

roll rework, additional or roll replacement, highscrap,highsetup time, or having to discard the complete

set of rolls. The ﬁrst and still commonly used “scientiﬁc” method for establishing the number of passes

was described by Fred Gradous [55] “…The experiencedroll designer doodles abit, gazes at the ceiling

and quite positivelysays,‘Ican do it in ten’.”This approach works remarkably well and most designers,

based on their experience, will opt for moreorless the same number of passes. Sincethe 1980s and 1990s,

some computer-aided roll design systems havebeen able to recommend the number of passes or calculate

the stresses created by forming.However,todate, it is still the rolldesigner’srole to accept or modify the

computer calculations and to establish the correct number of passes.

Oncethe number of passes, sequence,and amount of forming is settled, the designer will prepare a

section drawing at each forming pass. Forexample, if the designer elected to form the section shown in

FIGURE 5.2 Toofew passes form inferior products, but too manypasses are prohibitively expensive.

FIGURE 5.3 Forming of this section can starteither with the center bead (a) or with the outside lip (b).

Roll Design 5 -3

Figure5.3a by starting with the center bead,

formed in twopasses, followed with the outside

lip (three passes) and ﬁnally the larger legs in ﬁve

passes, then the sequence of forming,split for

better clarity, will be as shown in Figure5.4. If

the cross-sections are superimposed onto each

other, then aso-called “ﬂower diagram” is

created (Figure 5.5). The top,side, and three-

dimensional views (Figure 5.115)can easily be

generated by the computer.When the designer is

satisﬁed withthe ﬂowofthe material, roll design

can commence.

The varietyofapproaches to form even a

simple section described in this chapter high-

lights the fact that rolldesign is not governed by

exact rules or equations. Given the same cross-

section to 10 designers, they will most probably

end up with 10 different sets of rolls. Some of

these roll sets will form good products, while

others will not.

Theprobability of having agoodset of

“operator-friendly” rolls is to followthe basic

rules: envision asmooth ﬂowofmaterial; do not

be skimpy with the number of forming passes;

avoid too shorthorizontal distances and too

smalllead-inﬂanges; and consider allthe

requirements. However,regardless of how well

the set of rolls are designed, it will only work

properly if the rolls are properly made, and the

mill does not havebent, loose shafts or misaligned shoulders. In addition, the material to be formed has

to be suitable for the process, the right type of lubricant must be used, and awell-trained operator must

be assigned to set up the rollsand operates the line. If conditions are suitable, then agood set of rollsmay

produce millions of feet of good-qualityproducts.

The details of this basic roll design procedureare described in the subsequent sections of this chapter.

5.2 Cross-Section

5.2.1 Complexity

The cross-section of the rollformed product is the most signiﬁcant factor in roll design.

The varietyofshapes is unlimited. The shapes can be arbitrarily classiﬁed as simple (open), closed,

medium complex, very complex,and panel (Figure5.6).

5.2.2 Section Depth

The depth of the section (Figure5.7) is the maximum vertical dimension of the proﬁle as it exits from

the last pass. The depth (sometimes called “leg length”or“leg height” or “depth of corrugation”) has

great inﬂuence on the number of passes. In certain cases such as “folding over,” the maximum vertical

dimension can be at another pass ahead of the last pass (Figure5.58). The deeper the section, the more

passes are required for forming.

FIGURE 5.5 The usual (superimposed) ﬂower diagram.

FIGURE 5.4 Split ﬂower diagram.

Roll Forming Handbook5 -4

Figure5.8 shows the theoretical ﬂowofthe strip

formed into a“U” channel. The length of the bend

line, traveling in astraight line from point A

to point Bis“‘ .” The edge of the strip travels in

ahelical pattern for the same length as well as

upward by the height of the leg ð h Þ : The length of

this travel ð s Þ is:

s ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃ

‘

þ c

where c is the length of arc from point FtoD,

therefore:

s ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

‘

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

‘

þ 2 : 4674h

so the elongation (difference in length between “ ‘ ”and “ s ”) expressed as apercentage, in theorywillbe

e ¼

s 2 ‘

‘

100% ð 5 : 1 Þ

FIGURE 5.6 Cross-sections with different complexity.

FIGURE 5.7 Shallow and deep sections.

FIGURE 5.8 Schematic representation of the theoretical edge travel of astrip edge in helix pattern.

Roll Design 5 -5

Examples

1. If a1-in. highsection is formed in four passes in amill having ahorizontal distance(between

passes) of m ¼ 14 in:; then the theoretical elongation ð e Þ can be calculated as follows:

h ¼ 1in : four passes m ¼ 14 in: therefore ‘ ¼ 4 £ 14 ¼ 56

s ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

þð2 : 4674 £ 1

¼ 56: 022 and e ¼

56: 022 2 56

100% ¼ 0 : 04%

2. If the height is increased to 2in. and is formed under the same condition, then

h ¼ 2in : four passes m ¼ 14 in:

s ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

þð2 : 4674 £ 2

¼ 56: 088 and e ¼

56: 088 2 56

100% ¼ 0 : 16%

3. If the section height is increased to 4in., then

h ¼ 4in : four passes m ¼ 14 in:

s ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

56 þð2 : 4674 £ 4

¼ 56: 351 and e ¼

56: 351 2 56

100% ¼ 0 : 63%

These examples show that increasing the leg height by 2in., the elongation theoretically will be

fourfold, and by increasing the leg height by 4in., the elongation will be 16 times higher than the

original value.

Changing the number of passes or the horizontal distancehas similar effects.

h ¼ 2in : two passes m ¼ 14 in:

s ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

þð2 : 4674 £ 2

¼ 28: 176 and e ¼

28: 176 2 28

100% ¼ 0 : 63%

hencehalf as manypasses quadruples the elongation

h ¼ 2in : four passes m ¼ 7in :

s ¼

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

þð2 : 4674 £ 2

¼ 28: 176 and e ¼

28: 176 2 28

100% ¼ 0 : 63%

If the horizontal distanceisreduced to half, then the elongation (strain) will increase four times.

The above mathematical approach, based on the edge traveling on asmooth helix demonstrates

the inﬂuence of the leg height, number of passes, and the horizontal distance on the strain

developed during forming.

In reality, the edge travel (ﬂow) will not be smooth, as illustrated in Figure5.9a,b.Therefore, the

actual elongation in almost all cases will be higher than the one calculated by Equation 5.1. If the

strain (elongation) exceeds the elastic limit, then the permanently strained edges of the ﬁnished

product will be wavy or the product will haveabow, camber or twist. Because this limit moreor

less coincides with Y (yield stress), one can conclude that under the same condition, materials

with higher Y (yield stress) and lower E (elastic modulus) will less likely be wavy.

5.2.3 Width of Flat Elements

The width of the ﬂat, nonformed section usually does not inﬂuence the number of passes (Figure5.10).

However,the wider the ﬂat (not formed) section and the thinner the material, the higher the chance that

Roll Forming Handbook5 -6

waviness will occur. Regardless of howwell

designed therollsare, they cannot eradicate

waviness. In “Designing Products for Roll Form-

ing”(Chapter 9), several methods are suggested to

reduce or eliminate edge or center waviness.

Wide, ﬂat, nonformed elements can create other

difﬁculties during roll forming. If only one side of

the product is formed and the rest of the strip is a

wide, ﬂat element, then asmall pressure on that

ﬂatelement canmakeitlonger. Thelonger

elementwillcreate acamber, twist, or edge

waviness.

During roll forming of wide, corrugated panels,

the forming usually starts at the center rib.Inthese

cases, in addition to the progressivechange of the

bend angles from pass to pass, arelatively wide (up

to 25-in. or 635-mm) ﬂat sections at each side has

to be moved in the same plane sideways closer and

closer to the center (Figure5.11a,b). Shifting athin

ﬂat sheet sideways without creating large wavesis

not an easy task.

5.2.4 Formed or Stretched Grooves

Frequently,relatively shallow grooves(ﬂutes) are

incorporated in the section design to improve

appearance, minimize center waviness or add

strength to the product. The groovescan be either

formed or stretched into the ﬂat area (Figure5.12).

If the groovesare formed, then they require

additionalblank material (wider strip). The

stretched grooves take material from the adjacent

areas and no additional material is required. The

formed or stretched groovescan be formed in the

ﬁrst or in the last passes or in between. The pass in

which the grooves are formed usually depends on

the type of product, the anticipated shaft deﬂec-

tion, and the experience of the designer.

When the grooves are made in the ﬁrst passes,

they are usually “formed” because the material

“slips” in from the sides. If the ﬂutes are stretched

in the last passes, then the section is “grabbed” by the rollsatthe larger cells and the material will not slip

sidewaysinto the grooves.

Another method is to stretch all groovesinone special pass. Because stretching requires considerably

higher forces than roll forming,the stretching of groovesusually requires larger shaft diameters. When

the material is stretched across the section, it will “shrink”inthe longitudinal direction. The shortened

length will help to minimize or eliminate waviness.

Stretched grooves help moretoeliminate waviness, but their depth is limited by the elongation of the

material. If the elongation of the material restricts the depth of the grooves, then acombinationofboth

methods can be used. The groovecan be formed to acertain depth, for example, twothirds of the ﬁnal

FIGURE 5.9 Actual ﬂow is different from the “theo-

retical” ﬂow.

FIGURE 5.10 Width of the ﬂat, nonformed sections

has no inﬂuence on the number of passes.

Roll Design 5 -7

depth and the last one third of the depth is stretched. This combination willprovide the right depth

without cracking the material.

The stretched grooves frequently create residual stresses, which result in across-bowofthe section

(Figure5.13a). Often, compensation is needed andeitherthe sectionwillbeformedinthe opposite direction

during grooving(Figure 5.13b) or,morefrequently, asubsequentbending in theoppositedirection is

FIGURE 5.11 Wide, ﬂat sections of apanel (a) or asection (b) movesideways to the center during forming.

FIGURE 5.12 Formed grooves (a) and “stretched-in”grooves (b).

Roll Forming Handbook5 -8

required.Stretchingthe groove deeper than

required,thenpressingitbacktothe ﬁnal size may

reduce thecross-bow.

In some cases, such as farm (barn) rooﬁng

(Figure5.14), the cross-bowhas no detrimental

effect because the panel will lie ﬂat under its own

weight. Therefore, in these cases, it is not necessary

to take correctiveactions.

5.2.5 Cross-Sectional Tolerances

The customer,the product designer,and the roll

designer must agree on theinterpretation of

dimensions and tolerances speciﬁed in the draw-

ings before roll design commences.

Product tolerances will affect the roll design, the

number of passes, and thus the tooling costs. The tighter the tolerances, the morepasses (and better

equipment and tooling) are required.

As ageneral rule, the tighter the product tolerances, the greater the number of passes. The inﬁnite

varietyofcross-sections and tolerancecombinations make it impossible to put an exact numerical value

on the increased number of passes required, but experienced roll designers havea“feel” of howmany

extra passes will be required to achievethe required tolerances. These additional passes frequently include

the duplication of the last pass, incorrectly called the “ironing” pass.

5.2.6 Deviation from Straightness and Flatness Tolerances

Camber,bow,twist, cross-bow, herringbone effects, edge, or center waviness (Figure10.38 -c, -d, -e, -f, -g,

-h, -i, -j, -k, and -l) can be caused by the nature of roll forming (straining some parts of the section

beyond the yield limit), as well as imperfections in material, equipment and setup.Tooling has great

inﬂuence on residual stresses, which create deviation from straightness and ﬂatness. Good roll design

eliminates or minimizes them; bad design creates or exaggerates these problems.

5.2.7 Tolerance on Hole and Notch Locations

The rolldesigner must dictate the sequence of operations if holes, notches, embossments, or other

changes to the strip surface are speciﬁed. During forming,certain parts of the product are under

tension, while others are compressed. The difference between the overall length of the partbeforeand

after roll forming is inﬂuencedbythe roll design. In some cases, the designer can proceed with

prepunching the product, using the ﬁnal or estimated dimensions. However,inmost cases, especially

with multipunch dies, it is recommended that the rolls should be tested ﬁrst with“experimental”

FIGURE 5.13 Stretched groove creates cross-bow(a); compensation for the bow may be required (b).

FIGURE 5.14 Cross-bow is not critical whenthe

productlies ﬂat under its weight.

Roll Design 5 -9

prepunching.Test holes can be punched one-by-one, cut by laser beam, EDM, or by other methods.

Based on the hole location measurements before and after rollforming,adjustment can be made in the

prepunching pattern. Sometimes, these steps havetoberepeated until the ﬁnal prepunching

dimensions are established. However,itisimportant that the test should be carried out with the same

roll pressure.Undue large pressure can thin the material and elongate the product, thus giving false test

results on the hole locations.

Increased section depth, less passes, larger bend radii:thickness ratio,and larger increase in the pass-

to-pass roll diameter will extend the length of product compared with the length of strip before

forming.Ifthe ratio of the total area of the bent elements to the total areaofstraight elements is high,

and the section is formed with an adequate number of passes, then the length of the ﬁnished product

will be equal to the starting strip length, and in afew rarecases, length reduction have been reported.

The length reduction in these cross-sections can possibly be explained with ahighPoisson’s ratio

(or plastic:strain ratio) of the relatively thick material. If the Poisson’s ratio is high, then the ratio

of length reduction (in the direction of rolling) is relatively hightothe thickness reduction at the

bend lines.

5.2.8 Surface Appearance Tolerances

Judging the appearanceand establishing the level of “acceptable” surfacedeﬁciencies is very subjective.

Owing to the lack of practical standards and measuring methods, the extent of acceptable scratches,

marks, and other surface imperfections are usually established by “in-house”companystandards.

Therefore, agood understanding of the qualityrequirements between the customer and the supplier is

essential. Several “acceptable” and “nonacceptable” samples will help to evaluate the acceptabilityofthe

product.

Hot- or cold-rolledsteel is the least sensitivetosurface scratches. Steels withmetallic coating (zinc,

aluminum, tin, etc.) are relatively good for roll forming,but precautions should be takentoeliminate the

pick-up of the coating by the rolls. The best waytoeliminate pick-up is to use agood lubricant and to

minimize the surfacespeed differential between the strip and the rolls.

Prepainted metal, stainless steel, aluminum, or other metals with highluster havetobecarefully

formed to avoid scratches. Adifferentrolldesign, morepasses, different tool material or rollsurface ﬁnish

may be required to avoid damage to the surface.Different roll designs could include freerunning rolls,

wherethe surface speed of the roll is governed by the speed of the strip.Insome cases, such as the one

shown on Figure 5.45,anorientation change is suggested. In this case, the highluster surface would be

scratched by the rolls having substantial surface speed difference if it is rolled in the conventional way.

Rotatingthe product 908 into anew orientation will requiremore passes, but at the critical surface,the

strip speed and the roll speed will be almost identical thus minimizing the chances of surface scratching.

5.2.9 Length Tolerances

The length measuring method, the die accelerating system and the press havethe greatest inﬂuence on

length tolerance(see Chapter 3). However,other factors such as roll design can also inﬂuence length

tolerance. The surface speed differential at the driving surfaces can cause unevenrolling speed. Alarge

difference between the driven top and driven bottom rollsurface speeds can increase the length tolerance

by 8to10times.

Examples

Aroll forming mill, with one set of tooling has been used to form the shallow product(Figure5.15a), and

another set of tooling to form the deeper product (Figure5.15b). The measured length toleranceofthe

postcut shallowproduct was within ^ 0.125 in. (3 mm), acceptable for the roofs and walls of agricultural

buildings. Using the same coil and making no change to the equipment, but changing the tooling to the

deeper proﬁleincreased the length tolerance to ^ 1.5 to 2in. (40 to 50 mm).

Roll Forming Handbook5 -10

In this example, both the top and bottom rolls of the shallowproﬁle were5in. in diameter and were

running at the same surfacespeed. The length variation resulted from the small ﬂuctuation in length

sensing,die acceleration and cutoff-press cycle of the mechanical press.

Forthe 1.5-in. deep sections, the rolldesigner selected a5-in. diameter and an 8-in. diameter roll

combination. The 3-in. differenceindiameter is the result of the 1.5-in. section depth. The

circumferenceofthe 5-in. diameter rolls is 15.7 in. (399 mm), and the circumferenceofthe 8-in.

diameter roll is 25.13 in. (638.4 mm). The circumferenceofthe larger roll is 9.4 in. (239.4 mm) longer

than that of the small rolls. The shafts are rotating at the same rpm (1:1); thereforethere is an

approximately 9.4-in. (240-mm) surface length difference(slip) between the larger and the smaller rolls

at each revolution. Depending on the change in friction, pinch, and out-of-straightness of the shafts,

the speed of the panel changes continuously.When the cutting signal is given to the press and the

panel suddenly accelerates, the partbecomes too long.Ifthe panel suddenly decelerates, then the part

becomes too short.

The problemwas resolved by using similar diameter (same surface speed) urethane rollsinthe last two

passes. Without making anychange in the equipment or material, the length tolerancewas reduced from

1.5 to 2in. (38 to 55 mm) to ^ 0.25 in. (6.4 mm).

5.2.10 Bending Radius

It is the roll designer’sresponsibilitytoensurethat the product can be formed with the bending radii

shown on the product drawing.Ifaspeciﬁed bending radius is not attainable, then it must be brought

to the customer’s(or product designer’s) attention before roll design and blank size calculation

commences.

During roll forming, the outside of the bend line is under tension and the inside is compressed in a

direction transverse to the roll forming.The magnitude of the tension and compression is afunction of

the thickness/bending radius ratio and the mechanical properties of the material.

5.2.10.1 Small Radius

Bending most of the frequently used metals to asmall inside radius (inside radius ¼ 1or2times the

material thickness) results in permanent deformation with little springback. However,atoo-small radius

may crack the outside ﬁber of material having highyield and low elongation.

The minimum attainable inside bending radii are provided by the metal suppliers or are speciﬁed in

standards. Althoughthe minimum bending radii are usually established with apress-type of bending test,

it is safe to apply them to roll forming. Actually,inmost cases with rollforming,asmaller radius to

thickness ð r : t Þ ratio can be achieved.

FIGURE 5.15 Same top and bottom roll diameters (a) contribute to better length tolerance, large surface speed

difference between the top and bottom rolls (b) increases length tolerance.

Roll Design 5 -11