Roll Forming Handbook / Edited by George T. Halmos

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The removal and reinstallation of such a

split spacers may take less than 1min after

the operator-side stand is pulled out by a

shortdistance.

Split spacers are moreexpensivethan

one-piecespacers, but the extra cost can be

quickly recovered by reducing the non-

productive time.

5.7.5 Shims

(a) One-Piece Shims .Shims are made out of

hard“shim”stock. The bores can be about

0.005 to 0.060 in. (0.13 to 1.5 mm) larger

than the shaft diameters. Keyways should

be cut into the shims. The outside diameter

should be aminimum 1in. (25 mm) larger

than the bore. If the shims or spacers are

placed between largediameterrolls

(Figure 5.83), then their O.D. should be

considerably larger to avoid excessive tilt-

ing or bending of the rolls.

Shimsshouldbeavoided wherever

possible. Shimscannotbeproperly

marked, and they can be easily mixed up

and lost. The edges of the thin shims can

easily fold over,creating aspace of double

shim thickness between rolls.

(b) Split Shims.Quick-change shims can be easily made by splitting the shims at one side as shown in

Figure5.84a. The use of horseshoe shims (Figure 5.84b) is deﬁnitely not recommended for the

reasons explained in Section 5.7.4b.

5.8 Calculating Strip Width

5.8.1 Straight and Curved Elements

To calculate the blank size, or in other words, strip or coil width, the ﬁnal cross-section is divided into

straight and curvedelements. Forblank size calculation purpose, it is assumed that the lengths of the

straight elements do not change during roll forming. Calculating the length of acurved element by using

the length of its center line (theoretical neutral axis) gives an incorrect, larger starting width than the

required width.

Foraccurate strip width calculation, it is assumed that the theoretical neutral axis of the bent element

moves from the half thickness location closer to the inside (Figure5.85a). The new location of the

“relocated” neutral axis is expressed by the bend allowance(“ k ”). Forexample, the “ k ”factor for alow

carbon, annealed steel having abend radius equal to the material thickness ð r : t ¼ 1 Þ is about 0.33. This

means that the new neutral axis, used to calculate the blank size, will be 0.33 times the metal thickness

distancefrom the inside surfaceofthe bent element instead of 0.5 times.

In reality,the material also gets thinner at the corners as shown in Figure 5.85b. The stresses created by

the bending are over the yield limit and they cannot change abruptly to zerostress at the end of the

curvedelement. The changeover is gradual, which means aslight thickness change in the straight element

too.However,the empirical “ k ”factor used for the curvedelement provides an accurate blank size (strip

width), with the assumption that the length and the thickness of the straight element are not changing.

FIGURE 5.83 Between large diameter rolls, large

diameter shims or spacers should be used to avoid tilting

of the rolls.

Roll Forming Handbook5 -52

The main inﬂuencing factors on the empirical

“ k ”factor are the inside radius, material thickness,

and mechanical properties of the metal.

Thelargerthe r : t ratio, thecloserthe “ k ”isto0.5.

The larger the yield and tensile strength and the

lowerthe elongation, the closer the “ k ”isto0.5.

The “ k ”factor has been used to calculate blank

size of press brake-formed parts for along time.

Therefore, alarge number of equations and tables

are available to calculate the length of the bend

sections. Almost all tables are different and give

slightly different results. Most calculated blank

sizes are very close to the actual ones, but the ﬁnal

blank size has to be proven by testing.

An equation to establish the “ k ”factor devel-

oped by the author and used in the RollFormer’s

Guide software is shown in Equation 5.6:

k ¼ 0 : 567

þ 0 : 25

1 : 2

þ 1

 1 þ

2 : 5

250U

1 : 41

!

ð 5 : 6imperialÞ

where

¼ inside bend radius (in.)

t ¼ metal thickness (in.)

Y ¼ yield strength (ksi)

U ¼ tensile strength (ksi)

5.8.2 Manual Calculation of Strip

Width

Each cross-sectionconsists of straight and curved

element. The manual calculation of the strip

width required for acurvedelement is shown

below (see Figure 5.86).

t ¼ thickness

k ¼ bend allowance

¼ inside bend radius

¼ neutral axis radius

¼ R

þ k

L ¼ length of the bent element

¼ angle of bending in degrees

L ¼ 2 R

360

L ¼ 2 ð R

þ ktÞ

360

FIGURE 5.84 Horseshoe shims are not recommended,

but split shims can be used.

FIGURE 5.85 The “ k ”factor used in calculations of

blank sizes compensate for the actual change of length at

the bend lines.

Roll Design 5 -53

L ¼

180

ð R

þ ktÞ

The equation to calculate the strip width “ L ”

required for abent element is:

L ¼ 0 : 0174533ð R

þ ktÞ

ð 5 : 7 Þ

If H and

are given (Figure5.87a–c)

tan

þ t

¼ R

tan

¼ðR

þ t Þ tan

¼ a

þ b

¼ tan

ð R

þ R

þ t Þ

if R

¼ R

; then a

þ b

¼ð2 R þ t Þ tan

During strip width calculation, it is assumed that

thelengths of thestraight elementsare not

changing during forming. However,ifthe lengths

of the straight elements are not shown on the

drawings, they havetobecalculated. Forthe

lengthymanual calculations, see atypical cross-

section shown in Figure5.88 and the equations

below.

If R

¼ R

¼ R ; then a

¼ b

and a

¼ b

V ¼ L cos

þð2 R þ t Þ sin

ð 5 : 8 Þ

If R

– R

L ¼

sin

2 R

tan

2 ð R

þ t Þ tan

ð 5 : 9 Þ

If R

¼ R

L ¼

sin

2 ð 2 R þ t Þ tan

ð 5 : 10Þ

If H ; A

; and

are given, and A ¼ A

2 a

A ¼ A

2 ð R þ t Þ tan

ð 5 : 11Þ

If H ; B

; and

are given

B ¼ B

2 ð R

þ t Þ tan

ð 5 : 12Þ

If H ; A

; and

are given, and A ¼ A

2 a

A ¼ A

2 R

tan

ð 5 : 13Þ

and

B ¼ B

2 R

tan

ð 5 : 14Þ

FIGURE 5.86 Calculating the length of curved element

(“L ”) for computing strip width.

FIGURE 5.87 Calculating blank size.

Roll Forming Handbook5 -54

5.8.3 Manual Calculation of Blank Size: Example

Figure5.89 shows atypical cross-section prepared for manual blank size calculation. The corresponding

calculations are shown in Table 5.4.

With the widespread application of computers and CAD software, these time-consuming manual

calculations are less and less frequently required.

5.9 Bend Lines

5.9.1 Bent Elements

The cross-section of rollformed parts can be divided into straight and curvedelements (Figure

5.90). The curved elements from the original ﬂat position can be bent in different ways. They can

be formed with constant arc length or with constant radius or with amixture of constant arc length

and constant radius.

FIGURE 5.88 Schematics illustrating blank size calculations.

Roll Design 5 -55

5.9.2 Constant Arc Length

Forming

Thefulllengthofthe bent element ð L Þ

designated for the bending is formed at each

pass to aprogressively smaller and smaller radius

(Figure5.91).

Oncethe “ L ”iscalculated (Equation 5.7), the

inside radius ( R

)ateach pass can be calculated

from the total angle of the bend (

)atthat pass.

(For details, see Figure 5.86 where“k ”isthe bend

allowanceand “ t ”isthe thickness of the material).

¼ 57: 2958

2 kt ð 5 : 15Þ

5.9.3 Constant Radius Forming

One or moresegments of the element to be bent are formed to the ﬁnal radius at each pass (Figure5.92).

The roll designer selects the segment of the arc element, which is bent in aspeciﬁc pass. The segment to be

FIGURE 5.89 Manual blank size calculation of across-section.

TABLE 5.4 Manual Blank Size Calculation

#Calculation Length

10.450-R 0.300

20.017453*(R þ k*T)*90 0.314 (5.7)

31.05-2*R 0.750

40.017453*(R þ k*T)*90 0.314

51-R 0.850

60.017453*(R þ k*T)*60 0.209

71.2/sin60-(2*R þ T)/tg30 1.126 (5.10)

80.017453*(R þ k*T)*60 0.209

92-P-R-T (P ¼ 0.953) 0.747 (for “P,” see Figure5.91)

10 0.017453*(R þ k*T)*270 0.940

Total blank (strip) width 5.759

FIGURE 5.90 All cross-sections are divided into straight

and curved elements.

Roll Forming Handbook5 -56

bent can be at one end of the curvedelement,

formingthe adjacent straight piecetothe

ﬁnished radius in the next pass and so on (Figure

5.92a). The bending maystartatboth ends of the

element to be bent, then forming the adjacent

ones and ﬁnally ﬁnishing up at the center (Figure

5.92b). Another possibilityistoform the center

parttothe ﬁnished radius, then forming the

adjacent segments on the next pass and so on

until the bending of the element to the speciﬁed

angle is completed (Figure5.92c), or the bending

sequence of the elements can be mixed (Figure

5.92d).

The length of segment to be formed at each

pass can be calculated in the following way.

Total length of the element to be bent can be

calculated by using Equation 5.7

L ¼

180

ð R

þ ktÞ

if the total angle to the bent is

(Figure5.93)and

the bending angle increment from one pass to the

next pass is

; then the segment length ( L

)tobe

bent to the ﬁnal R

is:

L ð 5 : 16Þ

Example

If the ﬁnal angle is 908 and the bend angle increase at agiven pass is 22.58 ,then the length of element to be

bent at that pass is (22.5 4 90)L or 0.25 times the total length of the curvedelement (Figure 5.93b). If it is

decided to bend the element at twoplaces in this pass, then each element will be bent by

22.5 4 2 ¼ 11.258 .Therefore, the length of each half element will be 0.125L (Figure 5.93c).

FIGURE 5.91 Constant arc bending in three passes (dark

sections are formed).

FIGURE 5.92 Constant radius bending in three passes.

Roll Design 5 -57

5.9.4 Mixture of Constant Arc Length and Constant Radius Forming

Occasionally,althoughrarely, the designer uses constant arc length up to the point wherearollcan fully

reachthe inside of the bend line, then switches to the constant radius for the next sequence of the bend

line (Figure5.94).

5.9.5 Which Method to Use

No straightforward rule exists to dictate which approach to take except in afew cases. Forexample, the

constant arc method is recommended when blind corners are formed. In ablind corner,the male die

cannot reachinto the inside of the bend line. Therefore, it is not possible to specify that only one segment

of the bend line should be formed to the speciﬁed radius.

Constant arc method is also favoredwhen several bend lines are formed at the same time, such as the case

with siding,rooﬁng, and manyother sections. The larger radii used in the constant arc method allows a

“slip in”or“slip-out” of the material in the ﬁrst few passes of forming, instead of trapping it with the

smaller,ﬁnal radius.

In principle, constant radius bending can be used only when both the male and female rolls are in

contact with the bend line. Therefore, constant arc bending is recommended in all “free”bending cases.

The following two examples show twoproducts whereforming with constant radius has deﬁnite

advantages. The ﬁrst one, Figure5.95ashows aproduct with very large radius (large r : t ). Using constant

arc method requires so much overbending at the last pass (Figure5.95b) that the section would not ﬁt

into the mill. Using constant arc, each segment of the arc can be overbent apercentage of the total

FIGURE 5.93 Calculating the length of bent elements formed with constant arc or constant radius.

FIGURE 5.94 Forming with combination of constant arc and constant radius.

Roll Forming Handbook5 -58

required overbending. Therefore,the last pass

overbend will only be the fraction of the total

overbend (Figure 5.95c).

The otherexampleshows ahighstrength

material exhibiting 258 springback formed into a

“U” channel. Using constant arc length forming,

after reaching the 908 ,a25 8 overbend would be

required. 258 overbend cannot be made in narrow

sections because the “legs” will touch each other.

If the forming is made in, say, 10 passes and

constant radius forming is used, then each arc

segment at each pass should be overbent by

25 4 10 ¼ 2.58 .This means that at the last pass

the overbend will need to be only 2.58 and not

258 (Figure5.96).

Choosing between constant arc and constant

radius, or using amixture of the two is up to the

designer and frequently is dictated by his/her experience. Therefore, recreating rolldrawings by

measuring the used, worn rolls, and “double guessing” the method used by the designer of the original set

of rollsismuch more complex, difﬁcult and time consuming than to design acomplete set of new rolls.

5.9.6 False Bend

Occasionally,itisnecessarytobend and then restraighten elements which will eventually be straight after

roll forming are completed. This method is applied during false bending.

When aradius is “ﬂattened” at alater stage of forming (Figure5.97a,b)

To reachotherwise hidden bend lines (as shown in Figure5.98a)

When the leg angles are adjusted by this type of bending (Figure5.98b–d)

When it is anticipated that the forming of “legs” would also bend the adjacent shortstraight

element especially in the case of thick material (Figure5.99)

False bends require unusual blank size calculation. During the bend, the material stretches. The increase

of the element length can be calculated by using the “ k ”factor.However,when the same element is

straightened, it does not “contract.”Inthe opposite, it stretches again. Therefore, aﬁnished width of a

straight element containing afalse bend made with asharp radius will be slightly longer than before

bending and ﬂattening.

To compensate for this width increase caused by the false bend, aslightly smaller than the normally

applied “ k ”factor should be tried for the ﬁrst test. If the false bend is made with large radius and the “ k ”

factor is 0.5 or close to it, adjustment in the blank width calculation is not required. The properly calculated

blank for asection withsharp false bend will be slightly less than the same section without false bend.

FIGURE 5.95 Forming large radius with constant arc and overbending at each pass requireless overbending at the

last pass then forming with constant arc.

FIGURE 5.96 Overbending UHSS “U” channel at each

pass using constant radius method.

Roll Design 5 -59

5.9.7 Side-Rolls

In addition to the conventional forming rolls mounted on parallel, horizontal shafts, side-rolls are

frequently used for manyforming operation. Side-rolls havesimilar functions as the main rollsand are

equally important. Side-rolls havehad abad reputation in the industryfor along time. The reason is that

the usually conservativeroll designers have aimed to complete the forming with the main rolls. If tool

testing revealed that additional forming or guiding was required, then often the setup man tried to cure

the problem in the least expensiveway.Flimsy side-rollstands withcam followers frequently made the

product acceptable. However,itiscumbersome to properly setup and adjust these side-roll stands.

Side-rolls do the same work as the main rolls. Side-rolls are extensively and successfully used in tube

forming and should be morewidely used in roll forming.Side-rolls should be incorporated in the original

roll design. The forming withside or cluster-rolls should be aseparate “pass” in the ﬂower diagram.

FIGURE 5.97 “False bend” is ﬂattened at alater pass.

FIGURE 5.98 Angle / can be adjusted by the degree of “ﬂattening.”

FIGURE 5.99 Eliminating cross-bow with false bend in narrow,closed sections formed from thick material.

Roll Forming Handbook5 -60

The side-roll stands should be of good quality, easily,and accurately adjustable. Side-rolls are repeatedly

used to control variable springback, and often they are adjusted more frequently then the main rolls.

There is no rule that all shafts must be in horizontal position. Shaft positions should be selected for

optimum forming, reaching inside and enveloping the outside of the bends and to minimize speed

differential between the rolls and the formed sections. “Free-running”rolls will adopt the speed of the

strip,will reduce the friction, and hence will less likely to scratch the surface of the product.

Side-rolls, with afew exceptions, are not driven. Therefore, they must be used in conjunction with

driven main rolls. However,ifthe “free-running” nature represents arestriction,then it is recommended

to drivethe side-rolls.

Typical side-rolls are shown in Figure 5.100a–fand Figure5.26.Side-rolls are usually placed between

main passes. To be effective and to avoid undue residual stresses, the horizontal distances between the

main passes and the side-rolls should be sufﬁciently long.Itisacommon mistaketo“squeeze” side-rolls

between the main passes already having ashorthorizontal distance(Figure 5.101). This arrangement can

increase ﬂare, create waviness, and other problems. Ideally,side-rolls should be positioned at the same

horizontal distanceasthe main stands.

FIGURE 5.100 Typical side-roll stand forming or overbending.(a, b, and c) “U” channels. (d) Hat section.

(Courtesy of Delta Engineering Inc.) (e) Lips. (Courtesy of Delta Engineering Inc.) (f)Building panel. (Courtesy of

Metform International Ltd.)

Roll Design 5 -61