Roll Forming Handbook / Edited by George T. Halmos

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TABLE 5.9

Continued

FurnitrTop

FurnitrBack FurnitrStiffnr

Sill

Sill right side

Height

1.44

0.45

4.6 13.1

4.6

8.5

2.5

0.5

2.25

Thickness

0.026

0.018 0.375

0.35

0.24

0.24 0.06

UTS

55 55

Total angle

360

270

140

540 60

180

270 450

Shape

1–1.7

1.07

1.05

.45

Extra pass

Tol. 0–

0.5

0.75

1.7

1.6

Hole fact

0–20

.18

0.18

.18

passes

13.0

10.3

6.0

6.4

20.2 30.5

14.9

21.3

9.8

6.9 20.7

Actual

Swept 14

Garage Door Garage Door Garage Door

Guard Rail Guard Rail Guar

ail Rack Beam Rack Beam Rack Beam

Rack Beam Rack Leg

Auto

Height

1.2

1.71

1.82

3.187 3.187 3.187

Thickness

0.018

0.1

0.064 0.064

0.078

0.078 0.078 0.078

0.074 0.018

UTS

75 80

Total angle 750

450

730

190

360

360 180 450

Shape 1–1.7 1.22

1.2

1.1

1.05 1.2

Extra pass

Tol.

0–31

0.5

0.25

0.3

0.8 1.6

Hole fact

0–20

.18 0.18 0.12 0.18

No passes

24.8

15.7

17.3

12.1 12.3 12.0

23.9

20.8 26.9 23.6 15.7

17.2

Actual

Edges only

18 insufﬁcient

big ﬂar

Roll Forming Handbook5 -72

where

n ¼ number of passes (guideline only)

h ¼ maximum height of the section

t ¼ material thickness

¼ total degreeofformed angles at one side of the guide plane (whicheverside is larger)

Y ¼ yield strength in ksi (MPa)

U ¼ ultimate tensile strength in ksi (MPa)

z ¼ prepunched hole/notch and strip continuityfactor (0 to 2; see Table 5.6)

s ¼ shape factor (1–1.6) —(see Table 5.7)

e ¼ number of extra passes (0, 1, 2, etc.). For example, for scoring, grooving,duplicate “last pass,”and

so on

f ¼ tolerancefactor (0 to 2; see Table 5.8)

The accuracy of the guide number for passes depends on the correct input of numbers. Selecting the

correct input number requires some experience.

Table 5.9 provides several examples of the application on this empirical equation.

5.11 Flower Diagram

5.11.1 Cross-Section at Each Pass

After the section orientation has been selected and the number of passes established, the ﬁrst step is to

locate avertical guide plane (often called guideline). Aselected element of the cross-section will travel

through the roll forming mill in that vertical

plane. Typicalexamplesofthe effect of the

location of the vertical guide plane on aﬂower

diagram and the stresses developed in the section

are shown in Figure5.111.For manysections, the

selected element will travel in astraight horizontal

line in that plane, while others with aslight

increase in the pass line height if the roll diameters

are increased from pass to pass (Figure5.112a). In

other sections, the element traveling in the vertical

plane may movesigniﬁcantlyinthe vertical

direction if it is required for optimum roll design

(Figure5.112b).

In afew,exceptional occasions, it is advisable

to havemore than one guide plane. The ﬁrst

guide plane is used up to agiven pass and, from

the next pass onwards adifferent guide plane is

used. Examples for this unusual technique are

shown in Figure5.113.

After the guide plane is located, the next step is

to decide on the sequence and magnitude of

bending of each curvedelement at each pass.

Bending maystartatthe edges (as shown in

Figure5.3b), or at the center (Figure5.3a and

Figure5.5), or all bends can be formed at the

same time (Figure5.114 and Figure 5.120a).

After the bending angles have been established

FIGURE 5.111 The edge stress is related to the edge

travel shown in the ﬂower diagram. The position of the

vertical plane ( V

)inﬂuences the edge movement (“a”

worse, “d”best).

Roll Design 5 -73

and the cross-section at each pass is drawn,the designer reviews the access to the bend lines (open

or hidden), the overbend requirements to counteract springback and how the forming angles will be

adjusted by the operators by moving the top rollsdownorup.

The cross-sections at each pass (Figure5.1), superimposed from the ﬂat strip to the ﬁnished shape, is

called the standard“ﬂower diagram.”Typical ﬂower diagrams are shown on (Figure5.5, Figure5.44,

Figure5.111,Figure5.162,Figure5.173,Figure5.174 and Figure5.178). Occasionally,the large number

of cross-sectional lines obscures the sequence of forming. In this case, the “split ﬂower diagram”can be

helpful (Figure5.4, Figure5.44,Figure5.113,Figure 5.175,and Figure 5.177).

FIGURE 5.112 One point of the cross-section travels within astraight vertical guide plane almost horizontally (a)

or may travel up (b) or down.

FIGURE 5.113 In veryspecial cases, one vertical guide plane is used for the ﬁrst partofthe passes and another one

for the rest of the forming.

Roll Forming Handbook5 -74

With theadvancement of computers, it

becomes easier to generate top view,side view,

or three-dimensional view of the ﬂower (Figure

5.115). For an experienced roll designer,the

standard andoccasionally thesplitﬂower

diagrams are sufﬁcient.

Theﬂower diagramisone of themost

important aspects of roll design. It reveals the

anticipated ﬂowofthe material and the magni-

tude of the stresses generated by the forming

process. The designer tries to avoidlarge “jumps”

(markedas“b” in Figure 5.116a,b), resulting in

higher stresses at the edges or bend lines. The

moreeven the increments, the less chance there

is to run into problems. If necessary, additional

pass(es) can be added or some passes can be

eliminated. Shorter edge and bend line travel

distancewill generate less stress. The original

ﬂower diagram (Figure 5.117a) maybemodiﬁed

to reach into thecorners wherever possible

(Figure5.117b).

To ﬁnd the best, smooth, uniform ﬂow,the rolldesigners usually modify their ﬂower diagrams many

times beforethey are satisﬁed that undue stresses will not be created between anyofthe passes. Different

approaches to the ﬂower design are described in Section 5.14.

Manually changing aﬂower diagram of asimple cross-section is easy.However,the morecomplex the

section is, the moretime consuming the change becomes. The roll designers are always under pressure

and frequently behind schedule. Therefore, often the last ﬂower changes are not completed, only noted as

to what changes havetobemade during rolldesign.

One of the big advantages of the computer-aided roll design is the speed of changing the ﬂower

diagrams. The rolldesigners can review manyalternatives in ashorttime and select the one which

appears to be the best under the given circumstances. Upon completion of the ﬂower diagram, the roll

design can commencefrom pass to pass.

5.11.2 Forming in the First Pass Rolls

The ﬁrst pass rolls havemultiple functions. They “pull” the leading edge of the strip into the mill and

complete the ﬁrst forming. In some cases, they also guide the strip and can scorethe material for sharp

bending.

To pull the material into the mill is an important function. The maximum allowable forming angles at

all passes, including the ﬁrst one, depend on the thickness, mechanical properties of the material, roll

diameter,length of the bent elements, lubrication, and the available top shaft drive. However,inthe ﬁrst

pass, the maximum attainable angle often has to be reduced to suit the other functions, especially when

feeding the strip into the rolls. If the forming angle is too large, then the operator may havetocut tapers

on to twocorners of the leading edge of the coils. Considering the thousands of coils processed through

the mill, it is much less expensivetohavethe correct ﬁrst pass rolls than have the mill standing idle while

the corners of the coil ends are cut.

Some products, such as widepanels formed in the ﬁrst pass at the center are usually guided into

the mill by entryguides only.Ahead of the ﬁrst pass of the center formed panel, a6-to12-in. (150- to

300-mm) long “hump”can be observedwhere the coil enters into the ﬁrst pass center rolls. The length of

this free ﬂoating “hump”ﬂuctuates during forming. This length change is frequently associated with

buckling in and out of the material which may show up as wavesinthe product. The forming can be

FIGURE 5.114 All bends formed at each pass (only 4of

the actual 12 passes are shown).

Roll Design 5 -75

FIGURE 5.115

End, top

,side, and three-dimensional views of the ﬂower diagram. (Court

esy of Metform International Ltd.)

Roll Forming Handbook5 -76

improved by placing asolid “hump guide”underneath the hump,which will keep the hump uniform. It

is importantthat the twoparallel entryrolls, frequently applied ahead of the entryguide, as well as the

entryguide itself should be of sufﬁcient distancefrom the ﬁrst pass.

Forming of panels can often be improved or waviness avoided if the coil is fed in the ﬁrst pass at an

angle, up or down,tosuit the natural formation of the “hump.” Forexample, if the ﬁrst corrugation is

formed upwards, then it is frequently advantageous to lead the material from alower or upper levelup

into the ﬁrst pass at ashallowangle (5 to 15 in.) as shown in Figure5.118.Ifthe forming of the ﬁrst rib is

“up,”then the lead from alower level up is recommended and vice versa. Further help in forming can be

achievedbyusing smooth “sine wave”ﬁrst pass rolls (Figure5.119b,c). The section can obtain its angular

FIGURE 5.116 Big movements from pass to pass (b) creating highstresses should be avoided.

FIGURE 5.117 Flower diagrams slightly changed allowing the top roll to reach the bend lines at Pass #7.

FIGURE 5.118 Modiﬁed entryangle can reduce buckle and waves at the ﬁrst “rib”forming at the center of wide

panel (elements 1, 5, and 9are false bends).

FIGURE 5.119 Sinus wave-shaped false bending at the ﬁrst pass reduces internal stresses at the ﬁrst forming pass.

Roll Design 5 -77

shape in the next passes. Occasionally,the curvedapproach is used through several passes. Care should be

taken to avoid permanent bending of the straight elements by selecting asuitable large r : t ratio.

Narrowstrips are usually guided at their edges by the ﬁrst few pairs of rolls (see Figure5.121). In this

case, the entryguide is needed only to lead the strip in into the ﬁrst twopasses. Proper guiding of the strip

by the rolls is also required when its surface is scored for sharp bending.

5.11.3 Forming in the Last Pass

To reduce the cost of tooling,inmost cases, the ﬁnal overbend for springback is accomplished in the last

pass. The designer assumes that the product will springback and obtain its ﬁnal shape after exiting this

last pass. Better tolerances can be achieved if the last pass is aduplicate of the previous “ﬁnal shape”pass.

This last “duplicate”pass is frequently but incorrectly called the “ironing pass.”

5.11.4 Adjusting Angles by the Operator

Regardless of how manybends are made in each pass, the designer should giveachancetothe mill

operator to easily adjust the critical angles. Figure5.120 shows asplit ﬂower diagram. Bend lines marked

in black indicate achange in the bend angle at that pass.

Flower diagram (Figure5.120a) depicts adesign where everybend line is formed at everypass. In this

case, the operator adjusting the ﬁnal angle of the lips by moving the top rolls up or down will inadvertently

adjust the legs and vice versa. Flower diagram (Figure 5.120b) shows the correct approach. Moving the last

pass rollsupand down will change the angle of the lips only.Adjusting the pass beforethe last one will

modify the leg angles only.

5.12 Roll Design

5.12.1 Strip Width Applied to Roll Design

The strip width used for rolldesign always represents the calculated strip width plus the maximum

“plus” slitting tolerance. For example, if the calculated strip width is 2.000 in. and the strip width

toleranceis ^ 0.005 in. (50.800 ^ 0.127 mm), then the strip width used for roll design should be

2.005 in. (50.927 mm). Forwide panels with large “runouts,”the coil width used for roll design includes

the added 0.5 to 1in. (12 to 25 mm) runout at both sides or at one side only.

5.12.2 Traps

To avoid the strip “wandering” sideways between the top and bottom rollsand to obtain better tolerances,

the strip is frequently trapped in the rolls. Because the strip width includes the “plus” slitting tolerance, the

maximum movement of the strip to the left and right direction (wandering) between the traps will not

exceedthe total of the plus and minus slitting tolerances. Forexample, if the strip width is 3.000 ^ 0.005 in.

FIGURE 5.120 The roll designer should give opportunities to the operator to adjust critical angles one at atime.

Roll Forming Handbook5 -78

(76.2 ^ 0.127 mm), then the maximum width

between the traps will be 3.005 in. (76.327 mm).

If thecoilwidth happenstobe2.995 in.

(76.037 mm), then the coil can be tight against

one trap,leaving a0.010-in. (0.25-mm) gap at the

other side. This means that because of the strip

width tolerancealone, in this case, one leg can be

0.010 in. (0.25 mm) shorter than the other.

Different kinds of traps are used to retain the

strip edges in the rolls.

(a) Vertical Trap.Averticaltrap (Figure

5.121a,b) is used when the strip ends are

less than 15 to 208 to the horizontal. The

vertical trap surfaceofthe rollcan be two

to three times the material thickness,

continued in an 82 to 858 lead angle and

ending in aradius of aminimum two to

four times the material thickness. The

horizontal gapbetween thetop and

bottom rolls at the vertical trap is usually

0.002 to 0.003 in. (0.05 to 0.075 mm) or,

for thicker materials, 5to10% of the

thickness.

(b) AngularTrap .Anangular trap is

occasionally used for legs which are at

approximately 30 to 608 angle to the

horizontal. The application of the angular

traps is limited because the sharp small

trap is prone to damage (Figure5.122).

applied to the strip edges over 40 to 608

to the horizontal. Normally,the steep legs

of the section and the bend lines will

provide sufﬁcient guideline for the sec-

tion, but atrap can still be used if the

leg length tolerances are critical (Figure

5.123).

FIGURE 5.121 Ve rtical trap.

FIGURE 5.122 Angular trap.

FIGURE 5.123 Horizontal trap.

Roll Design 5 -79

(d) No Traps

As mentioned above,for bends over 45

to 608 ,traps are usually not required

because the bend line will keep the

section in track.

Traps cannot be used if variable size

products are to be formed without tool

change (Figure 5.124). In these cases, it

is advisable to guide the strip by the

entryguide and by afew guides between

the passes. The in-between pass guides

should be adjustable in widthand

height.

Wide building products with þ 3/16 in.

(5 mm) coil width tolerance and with

possible runout provisions are guided

only by the entryguide.

5.12.3 Lead-In Flanges

“Rawrolls” designed by simply enveloping the cross-section would form the section but in most cases, it

will be difﬁcult, if not impossible, to enter the strip into the gap between the rolls. To help to lead the strip

from one pass into the next pass, it is advisable to change the “raw”rolls by adding “lead-in ﬂange”

sections to them (Figure 5.125 and Figure 5.167).

Lead-in ﬂanges, which often include traps, are morecritical when forming sections with critical surface

ﬁnish, highstrength/highspringback material, very thin material or forming noncontinuous strip precut

blanks or strips withholes or notches. Leads are not required when section bending commences at the

center partofthe strip,and when the edges are not formed up or down as in the forming of wide panels

and similar products.

The lead-in ﬂange design is based on the geometryofthe strip edge, its direction (up or down), and the

degreeofbending at that pass. It is usually designed to have the outside edge of the strip (section) contact

the lead surface as it exits from the previous pass. The left side of Figure5.125 shows the “raw”roll design

(without lead-in ﬂange). The strip coming from the previous pass would hit the edge of the roll,creating

surface marks and extensive roll wear.The right-hand side of the same pass shows the bottom rollwith a

“lead” surface.The strip edge will contact the lead surface and will travelmoregently to its new position,

creating less internal stress. Longer lead in will reducethe stress and willless likely create waviness, ﬂare,

or other deviations form straightness and ﬂatness (Figure 5.126).

FIGURE 5.124 Angles with different leg lengths formed

by the same rolls cannot be trapped (a). Guides adjustable

in width and height are recommended between several

passes.

FIGURE 5.125 Unedited (raw) rolls at the left side and edited (with lead-in ﬂange) at the right side of rolls. The

thinner line represents the cross-section formed at the previous pass (a); (b) shows both sides lead-down and in (c),

the left side is lead-up while the right side lead-down.

Roll Forming Handbook5 -80

Depending on the direction of the strip edge

movement (up or down), the lead is placed either

on the bottom or on the top roll. The roll on

which the lead is placed by its rotating movement

will, in almost all cases, movethe strip edge away

from the shaft as shown in Figure 5.127.The

direction of the roll rotation (friction on the roll

surface)helps to move the strip into the roll gap.

(a) Lead-InFlange Design for Straight Edge.

The approximate edge travel(Figure5.128)in

the horizontal direction is between passes:

ET 1to2¼ L (cos

2 cos

)(edge travel

between Pass 1and Pass 2)

ET 6to7¼ L (cos

2 cos

)(edge travel

between Pass 6and Pass 7)

The increase in roll diameter to accommodate

the lead-in ﬂange depends on the horizontal

travelofthe edge from thepreviouspass,

materialthickness, roll diameter, springback,

and other factors. Figure5.129a,b shows that as

leg entryangle increases, for the same angle

increment between passes the guide roll diameter

has to be increased.

Occasionally,the lead angle is an extension of

the forming angle, as shown in Figure5.130.

Apartofthe radius at the rolledge of the

lead and horizontal roll surface intersection can

often be used to starttolead the edge of the

strip.Alarge radius tangent to the lead surface

only (Figure5.131a) is morepractical than a

smallerradiustangentialtobothsurfaces

(Figure5.131b). Afrequently used lead surface

has about two-thirdsstraight and one-third large

radius surface. Occasionally,itcan be necessary

to havealead radius that intrudes into the

straight portion of the section particularly when

forming to the vertical.

FIGURE 5.126 Long lead creates lower stresses in the

product.

FIGURE 5.127 Friction at the lead-in ﬂange surface

helps to movethe legs in the right direction.

FIGURE 5.128 Edge travel in the horizontal direction

between passes.

FIGURE 5.129 Formed angles (

and

)inﬂuencethe roll diameter increase required for the lead-in ﬂange.

Roll Design 5 -81