Roll Forming Handbook / Edited by George T. Halmos

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The shape of the side-rolls is just as important as the shape of the main rolls. To provide reasonably

“smooth”forming,the diameters should also be adequately large. Side-rolls are usually designed the

same wayasthe main rolls. However,some of the side-rolls used for overbending can create aproblem

when they are only partially engaged. Figure5.102ashows atypical example wherethe side-rolls designed

for the maximum 108 overbend are set to form only arequired 5 8 overbend. Special Delta overbend rolls

havebeen developed to avoid this problem. The rolls, hinged around the bend line, are “tilted” at the

required angle (Figure 5.103).

When aside-roll is applied to one element to the section, the created bending moment may curve the

adjacent element too (Figure5.104a). To avoid this problem, the other elements of the section should also

FIGURE 5.100 Continued

Roll Forming Handbook5 -62

be supported (Figure 5.104b,c). Special side-roll

stands supporting the bottom of the section and

rotatinginorout will eliminate allthese

problems.

5.9.8 Cluster-Rolls

To better control the forming process, the side-

roll stands can be located at, and working in

conjunction withthe main rolls. These types of

side-rolls are called “cluster” rolls. The cluster-

rollsare efﬁcient. They requireaspecial “bridge”stand to supportthe rolls between the main roll shafts.

Atypical cluster-roll arrangement is shown in Figure 5.105.Cluster-rolls are also shown in Figure 5.176.

FIGURE 5.101 Side-roll pass squeezed between main rolls maycreate too highinternal stresses.

FIGURE 5.102 Conical side-rolls maycurve straight legs

while bending at the corner.

FIGURE 5.103 Side rolls “tilting” around bend lines eliminate edge curving problem shown in Figure 5.102.

Roll Design 5 -63

5.9.9 Slides and Other Forming

Devices

During the roll setup and testing,the setup

person occasionally ﬁnds away to eliminate

problems by pushing or squeezing the section

with a2£ 4pieceofwood or by other means.

This temporarysolution is usually replaced with

side-rolls or occasionally with slides made from

bronze,plastic or othermaterial.Bronze,

hardened steel or other guides are also employed

to guide the strip from pass to pass, especially in

the case of precut or deeply notched sections.

Similar guides may be used to help he strip or

sectiontoenter into othertools such as

straighteners, cutoffdies.

These slides (guides) are not considered as

forming passes and are not partofthe ﬂower

diagram.

5.10 Number of Passes

5.10.1 ArtorScience

One of the most frequently askedquestion in the rollforming industryisthe number of passes required

to form asection. To answer is not easy.

The required number of passes is inﬂuenced by manyfactors including competitivepricing.

Astandardmethod of calculating the optimum number of passes required does not exist. Most

designers’results are based on experience.The most commonly used process is described by

F. Graduous [55]: “The experiencedroll designer doodles abit, gazes at the ceiling and quite

positivelysays: ‘I can do it in ten’. ”

5.10.2 Factors Inﬂuencing the Number of Passes

Most of the factors inﬂuencing the number of passes havebeen discussed in Section 5.2 to Section 5.5.

Additional factors include the following:

(a) Material Thickness.The thickness of the product has an inﬂuence, but the effect is controversial.

Many designers believe that thicker products require morepasses. This is valid only if the mill is

FIGURE 5.104 Supporting the horizontal surface in (b) and (c) eliminates buckling caused by the bending

moments (a).

FIGURE 5.105 Cluster-rolls.

Roll Forming Handbook5 -64

not strong enoughtoaccomplish the required forming at each pass. If the mill is strong enough,

which it should be in the majorities of cases, then the thicker the material, the less passes are

required. This effect can be explained by the fact that the thickness does not limit elongation

(strain), but thicker material will less likely be wavy when compressed in the longitudinal

direction (Figure5.106).

This effect is conﬁrmed by manycases where narrow sections or wide panels can be

satisfactorily formed from different thickness of materials down to alimit. Material thinner than

this limit requires more passes for good forming,otherwise the product will be wavy or will have

other problems.

(b) Continuity of the Strip .Continuous forming provides asmoother ﬂowofthe material from pass

to pass. The ﬂowisdifferent when the partisprecut. The leading edge of the strip will slightly

spring back after leaving the forming pass, and will travelpractically in astraight line until the

front end hits the lead-in ﬂange of the next pass of rolls. Abruptforming thereforetakesplace

on entrytothe next pass. The rest of the precut strip is formed as acontinuous section, but a

similar “jump”happens everytime when the front end of the strip contacts the next forming

pass. Similarly,there will be difference in the ﬂowwhen the tail end of the strip exits from the

passes.

To minimize abrupt forming, precut strips are usually roll formed with two or three more

passes than the same section formed from acontinuous strip.The front and tail ends of precut

sections frequently havemoreﬂarethan the same section formed from continuous strip cut to

length after forming.

Applying astraightener or curving unit after forming precut strips is either not practical or

quite complicated.

The deeper the notch, the more similar the effect is to precutting (Figure5.107 and Table 5.5).

Holes punched in the web havelittle inﬂuence on the number of passes. However,holes

punched in the elements which are under tension during forming can greatly inﬂuencethe

required number of passes. In extreme cases, products with large cutouts close to the elements

exposed to highest strain require30, 50, or 80% more passes to be formed than the same section

without holes. Insufﬁcient number of passes will distortthe shape of the holes.

Very large cutouts, like complex miter prenotches will makeroll design moredifﬁcult. At the

point of alarge cutout, the remaining strip is veryweak, but it should neither be permanently

stretched nor compressed during forming. This cross-sectional weakness will also create problems

when the strip passes through aloop after punching,when the material is forced through the

straightener,and when the ﬁnished product is handled after forming.

The number of passes is established to keep the stresses at the edges of the strip at around or just

abovethe yield point. When wire webs or sections slit to the edges (Figure5.108)are rollformed,

FIGURE 5.106 The buckling strength of thickerand higher strength material is higher;therefore, it is less likely to

buckle under compression than the thin or low strength material.

Roll Design 5 -65

the edges are not constrained and highstresses are not developed. Therefore, roll forming these

types of starting materials requireless passes than the same section made from solid, continuous

strip.The lead-in ﬂanges of the rolls should be sufﬁciently large to ensureasmooth entryofthe

edges into the roll gap.However,the variation in the circumferential speed can change, say, a90 8

angle of the “ﬁngers” by 3to68 .Careshould be taken to avoid or minimize this problem.

(d) Straight or “Free-Flow” Pass Line.Almost all rolldesigners’experience,aswell as rolldesign

software,are based on forming sections in aroll forming mill which has all the bottom shafts in a

straight (usually horizontal) line. Astraight pass line is not necessaryideal for rollforming, as

explained in Chapter 15. If rolls are designed to suit the optimum material ﬂow (“free-ﬂow” or

“down-hill forming”), wherethe pass line is not astraight line, then the number of passes can be

reduced compared with the passes used with the present methods (Chapter 15).

FIGURE 5.107 Inﬂuence of holes and notches on the number of roll forming passes.

FIGURE 5.108 Noncontinuous edge will “open up”instead of “stretching,” thus often requires less passes to

form it.

Roll Forming Handbook5 -66

(e) Tabulated Form of Inﬂuencing Factors.The number of passes required to form aproduct within the

speciﬁed tolerances are inﬂuenced by numerousfactors as described in the previous sections.

Table 5.5 summarizes the overall effects of these factors and Table 5.6 to Table 5.8 provide the

numerical values for Equations 5.19.

5.10.3 Calculating the Number of Passes

Numerous attempts havebeen made for along time to bring “science”into rollforming or at least to

provide some numerical guidelines to calculate the number of passes. One of the earliest approacheswas

to calculate the distance from the edge of the strip to the edge of the ﬁnished product in asimple, straight

layout (Figure 5.109). Dividing this distancewith tangent 1 2 tangent 1.5 in. (0.0175 2 0.0262) gives a

guideline for the total mill length required for forming.Dividing the total lengths with the horizontal

spacing between passes will givethe required number of forming stations (see equation below).

Example

p ¼

ﬃﬃﬃﬃﬃ

2 a

¼ a

ﬃﬃ

¼ 1 : 414a ð 5 : 17Þ

if a ¼ 1 : 5 ; then p ¼ 1 : 414 £ 1 : 5 ¼ 2 : 12

tan

c ¼

tan

ð 5 : 18Þ

assuming

¼ 1 : 5 8 ; tan

¼ 0.0262, then c ¼ 2.12 4 0.0262 ¼ 80.96 ¼ distance from the ﬁrst pass to

the last pass.

TABLE 5.5 Factors Inﬂuencing the Number of Passes

Factors Number of Pass Requirement

Decrease Increase

Depth of the section ShallowDeep

Bend lines Open Hidden

Section toleranceLoose Tight

Material Thickness

If mill is strong enough

If mill is not strong

Thicker Thinner

Thinner and thicker

Mechanical properties Low yield strength High yield strength

High elongation Low elongation

Surface Hot rolledsteel Prepainted or luster surface

Uniformity (twist, waviness,

camber,etc.)

Tighttolerancematerial Loose tolerancematerial

Continuity Continuous strip Precut

Other operations in

the line

Holes at the center No inﬂuence

Holes close to edges More passes

Notches Shallow(1to2extra)

Deeper (more passes)

Welding —2to 4ormore

Curving Same or less —

Mill characteristics Larger shaft diameter Maybeless passes —

For deep sections and panels

Horizontal distanceLess passes —

Large ——

Small More passes

Roll Design 5 -67

TABLE 5.6 Effect of Holes and Notches ( Z )onNumber of Passes (Equation 5.19)

For letter “Z” in the equation

pick one number only,

the highest value of all

applicable conditions

y=height

t=thickness

v=distance from edge

a0 0

b0 0

c0 0.07

d0.03 0.05

e0.050.05

f0.07 0.12

g0.07 0.12

h0.1 0.1

i0.12 0.15

j0.12 0.15 (if s ≥ 5t)

k0.15 0.2 –0.4 (*)

l0.25 0.3 –0.5 (*)

m0.5 0.6 –0.7

0.05

0.30

0.25

0.18 0.18 (pre-cut)

no hole

(*) The longer the slot and the smaller the dimension “v”, the larger the “Z” factor is.

Small perforation

edge continuous

Slotted limited edge continuity

edge not continuous

Wire

Pre-cut

TABLE 5.7 Effect of Shape ( S )onNumber of Passes (Equation 5.19)

Shape

Factor

111.05 1.05 1–1.05 1.1 2 1.2 1.4 1.5

Roll Forming Handbook5 -68

If the horizontal distancebetween passes is

HD ¼ 10 in., then according to this equation,

(80 4 10) þ 1 ¼ 11 passes will be required to

form the 1.5-in. deep “U” channel.

The calculation based on Equation 5.18 gives

some guidanceto“beginner”designersfor

simplesections butitis“crude,” unreliable

approach, which does not take other important

factors into consideration.

Other author [38] recommends to calculate

the number of passes by limiting the “resulting

edge tension”asshown in Figure5.110.

With the advanceofcomputer-aided rolldesign, additional approaches havebeen tried (see Section

5.14). One method has been based on calculating the stresses in the formed section to arrive at the

number of passes required. Another method is to establish the number of passes and prepare aﬂower

diagram ﬁrst, and then to calculate the developed stresses. If the stress between passes is abovethe

permitted level, then the forming can be modiﬁed. Another method of calculating the number of passes

with computer-aided rolldesign is based on empirical equations.

An empirical equation preparedbythe author,which provides guideline for the number of passes, is

incorporated in the Roll Former’sGuide

software.This, so far not fully tested equation is shown below:

n ¼ 3 : 16h

0 : 8

0 : 05

0 : 87



2 : 1

40U

0 : 15

s ð 1 þ 0 : 5 z Þþe þ f þ 5 zs ð 5 : 19 imperialÞ

n ¼ 0 : 237h

0 : 8

0 : 834

0 : 87



2 : 1

0 : 003U

0 : 15

s ð 1 þ 0 : 5 z Þþe þ f þ 5 zs ð 5 : 19 metricÞ

FIGURE 5.109 An early rough“straight line” approach

to calculate the number of passes.

TABLE 5.8 Effect of To lerance Factor ( F )onNumber of Passes (Equation 5.19)

Tolerance Factor

Loose (construction) 0

Medium 0.5–1.0

Tight (automotive) 1.1–1.7

Extremely tight 1.9–(2)

FIGURE 5.110 Calculating the number of passes from limiting edge stress.

Roll Design 5 -69

TABLE 5.9

Number of

Passes Calculated with Equation 5.19

Bumper

WBumper

Windo

wspc Wi

ndowspc Screen Door Screen Door Drawer

sld Door

Frame

Furniture Stud

Auto

T-hanger

Height

.35 0.375 0.312 0.35

0.75

2.25 0.7 2.5 2.36

1.6

Thickness 0.075 0.06

0.047 0.017 0.017 0.024

0.024 0.096 0.048 0.074

0.036 0.146 0.011

95 160

65 55 40

UTS

205 180

80 65 50

tal angle 360 360 195

535

436

450

630

250

360

405 180 184 720

Shape

1–1.6 1.25 1.08 1.2

1.3

1.35

1.35 1.3

1.05 1.15 1.05 1.05

1.3

Extra pass

Tol. 0–

.8 1.8

.65

1.65

1.65 1.6

Hole fact

0–10

0.0

0.5

0.5 0.18 1.18

No passes

22.4 17.1

5.4

12.8

11.6

13.5

16.7

9.4 17.8 16.1 13.0

14.5 25.1

Actual

14.0 16 26

Barn Ve

nt Drip Starter Canopy

Guide Fan

1 : 5 £ 845

8 1

: 5 £ 836 8

1 : 5 £

554 8

£ 55

4 8 3

£ 3ribs 628

1 : 5 £ 480 8

Height

.05 5.4 0.4 1.3 1.6

3.25 1.5

1.5

1.26

1.5

Thickness 0.015 0.018 0.03

0.006 0.015 0.071 0.048 0.013

0.031

0.013

0.035

0.036

530165

UTS

55 55 55 40 18

60 60

110

Total angle 640 480 250

240 270 510 280

750

918

490

372

640

Shape

1–1.6 1.3 1.3 1.1 1.07

1.05 1.25

.05

1.05

.05

Extra pass

Tol. 0–20

.3 0.4

.5 0.5 1.15 0.75

0.2

Hole fact

0–10

.07

.18 0.18 0.18 0.1 0.18

No passes

32.1 15.4 21.1 12.0 10.5

17.9 15.5 16.1

16.0

19.3

21.4

22.1

16.8

Actual

14 passes

did not

work

Roll Forming Handbook5 -70

HatS

tep Beam

Auto

to UB

ookshelf

Tubulrar auto SS “U” chnl Furniture Side

Furniture Door

Height

0.456

.93 1.1

1.5 0.85 1.2 0.78 0.625

.36

1.44

0.94

Thickness 0.093 0.068 0.093

0.024 0.06 0.024 0.024 0.082

0.06

0.04 0.12

0.026

UTS

65 60 50

50 50 50 50

Total angel 180 270 180

270 270 360 450

880 90

300

360

270

Shape

1–1.6

.25 1.05 1.2

1.15 1.2 1.3 1.06

.07

1.07

Extra pass

Tol. 0–21

.7 0.6 1.7 1.2

1.2 1.2 1.2 1.6

1.2

1.7

0.5

Hole fact

0–10

0.15

.18

0.18

passes

9.9 23.4 12.9 #REF!

11.5 11.1 15.4 18.4

5.1

26.1

9.6

13.0

10.3

Actual

Recommended 10 14 20

1 : 68 £

4768

1 : 5 £ 5deck

0 : 75

£ 575 8

Farm

Roof Farm

Roof

Hidden

Fasten

4 £ 38

5 8

2 £ 3858

Farm

Roof

Garage Door Garage Doo

Height

1.68

1.50

.75

0.354

0.5

1.5

1.44 0.8

0.6 0.71 1.71

Thickness

0.024 0.026 0.015

0.032

0.015 0.03 0.026

0.026 0.018 0.032 0.02

0.018 0.018

UTS

55 55

Total angel 608

960

900

594

850 360 680

340

500 562 1200

1570 1570

Shape

1–1.6 1.05

1.05

1.25

1.2

1.22 1.22

Extra pass

Tol. 0–

0.20

0.7

0.2

0.2 0.2

0.2

0.75 0.7

Hole fact

0–10

0.0

.00

No passes

17.3

17.5

16.1

10.1

15.0 14.5 21.3

21.4

21.2 11.0

31.1 30.0 33.2

Actual

14 22

20 þ 5SR

bit wavy

Roll Design 5 -71