Roll Forming Handbook / Edited by George T. Halmos

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these wireswithacross-section of “segments of acircle”were cold reduced to ﬂat strips, slit

longitudinally into twohalves, corrugated and cut to length, all in one line. The forming speed was

700 ft/min. The length of the ﬁnished pieces was 1in. To meet the aboverequirements, 140 cuts were

made per second on both parallel exiting corrugated strips. The line produced about 16,800 pieces per

minute or about one million pieces per hour.

15.6.2 Roll Forming Noncontinuous

Bend Lines

Accordingtothe deﬁnition of roll forming, the

ﬂat metal strip is bent in continuous straight

lines. Continuityisassumedbecause of the

nature of roll forming. However, in some

applications, it would be better to discontinue

theribs to leave spaceoraﬂatareafor

punching,embossing or other operations. Some

manufacturer ﬂattened back the embossed ribs at

the end of transformer oil heat exchanger plates

or similar products (Figure15.10). However,a

process to discontinue the stiffener ribs at certain

point is better.Inthe April 1970 issue of Sheet

Metal Industries,G.H. Pugh described aprocess

developed by Joshua Bigwood where the shaft of

the stiffener rib embossing rollswere lifted up by

hydraulic cylinders to leave aspeciﬁc partofthe

transformer plate ﬂat. The shafts were pressed

back again when embossing was continued.

Asimilar discontinuous rib forming process

may be used when either the end of the products

requires special forming or when only acertain

partofthe product, such as the bed of the pick-up

truck (Figure 15.11)isroll formed.

15.6.3 Roll Forming Nonstraight

Bend Lines

In almost all rollformed products, the bend lines

areparallel. However,itispossibletoform

nonparallel lines by applying aspecial technique.

Atypical exampleofthistechniqueisthe

roll formed, pie-shaped grain bin roof sheets

(Figure15.12).

With special equipment developed for another

product, even the edges of curveddome-shaped

silo roofs can be rollformed (Figure15.13).

In the 1980s, B&K Machinery built arotary

embossing unit to form asnake-likeembossment

at the center of the standing seam panels shown in

Figure15.14 [315].

FIGURE 15.10 Roll formed ribs are “ﬂattened” at the

end of the transformer heat exchange panel.

FIGURE 15.11 “Interrupted rib forming” can be used

in pick-up truck beds.

FIGURE 15.12 Nonparallel roll formed ribs.

Roll Forming Handbook15-8

Apatent application has been ﬁled to roll form

wavy wall panels to produce panels with appearance

substantially different from the straight roll formed

wallpanels. Twoapproaches areshown on

Figure15.15.These technologies haveneverbeen

tried, and it is questionable whether they can be

used when deeper sections are roll formed utilizing

multiple passes. However,afew manufacturers

recently disclosed that they occasionally produce

productswithwavy bend lines, although not

intentionally.

15.7 Roll Forming Tools

One of the cited deﬁnitions of roll forming is to

form with “multiple pairs of rotating,contoured

rolls.”However,aswithall the rules, there are

exceptions:

Some roll forming “mills” haveonly asingle pair

of rolls.

Bronze shoes, plastic supports, ﬂat cam rolls and

other “tools” are frequently applied by the roll

forming industry. The most frequently used other

tools found around most rollforming mills are the

2 £ 4s.

FIGURE 15.14 Rotaryembossed sinus shaped bead.

FIGURE 15.15 Roll forming wavy rib wall panel.

FIGURE 15.13 Roll formed dome section.

Unusual, New,and Future Roll Forming Technologies 15-9

Urethane and other plastic rolls are occasionally incorporated in the forming process.

One manufacturer,byusing a“homemade”roll former,managed to roll form deep corrugated

sections in 6passes instead of the traditional 22 to 24 passes. They “preformed” the cross-section to its

full depth using astand of nondriven car tires (Figure15.16), allowing averylong entrysection between

the uncoiler and these car tire“rolls.”This application shows the ingenuityofbreakingwith traditions,

using roll material other than the tool steel and to some degree, the application of partial pull-through

roll forming.

15.8 Reducing the Thickness of the Starting Material

Another deﬁnition of roll forming is to form

without changing the thickness of the starting

strip.However,the thickness is already reduced at

the bend lines and occasionally some parts of the

strip are thinned or as sometimes called “coined.”

In other cases, the edges of thicker sections are

beveledbycold reduction to eliminate machining

(Figure15.17).

Whynot reducethickness if it has advantages?

Cold reduction passes are already installed in front

of some mills to provide the starting strips with

tight thickness tolerances across its full width. Cold reduction passes can also be installed at anypass,

which must “coin” (cold roll) apartofthe section to tight tolerance. Atypical application is to keep tight

toleranceonthe “double thickness”sections of the drawer slides (Figure 15.18). Asimilar idea was

described by Jacmorr Manufacturing Ltd. in the April 1990 issue of ModernMetals.

Aword of caution is necessaryhowever; the forces required to reduce thickness can be 10 to 100 times

higher than the forces required for forming. Therefore, cold reduction should be carried out only with

passes speciﬁcally designed for that purpose.

FIGURE 15.16 Preforming ﬂat starting material to full section depth reduces the number of passes required for

forming.

FIGURE 15.17 Cold reductionofthe stripedge

eliminates costly bevelmachining .

Roll Forming Handbook15-10

Aspecial roll former was built by Yo der to roll form astrip with variable thickness in late 1970s is now

described by D.E. Beecher at the 1st International Conference on RotaryMetal-Working Processes in

November 1979.

15.9 Forming at Elevated Temperature

The widely used expression “coldrollforming”isinaccurate. Roll forming can be implemented at a

temperature higher than room temperature. Forexample, to avoid cracking of paints or laminates at sharp

bend lines, the strip (or just the bend lines) can be preheated up to 130 to 2308 F(55 to1108 C) before

entering into the forming rolls. The coatingwill be soft enoughnot to crack, but hard enoughnot to be

stripped offthe metal.

Hot roll forming of titanium at Boeing is described in the June 1971 issue of the Metalworking

Production.The titanium strip was preheated to about 14708 F(8008 C) and was passed through aspecial

roll forming mill. In this case, titanium was roll formed at ahightemperature because it could not be

formed at room temperature. However,hot roll forming has lots of other advantages.

Theforcesrequiredfor cold reductionare veryhigh. Theyield pointofthe metalcan be reduced

drasticallybyincreasingthe temperature. At 15008 C, theyield strengthofsteel is about1/5 to 1/10 of the

yield strength at room temperature. This characteristic of metalwas used by thesmithsfor thousandsof

years, andthatisthe reason why“I” beams, “U”channels, andother shapes,thick plates,and coilsare still

hotrolledtoday.

So,why not hot roll form sections? Is it simply because the sheet metal industryhas ahistoryand habit

of working at room temperature?

The trend will be to install induction, highfrequency,gas or other preheating methods in front of the

roll forming mill to heat the full cross-section, or just the bend lines.

Roll forming metals at elevated temperature willrequire:

Less passes (most probably)

Less HP

Smaller shaft diameters

Smaller rolls (because of smaller shaft diameters)

Shaft/roll cooling system

However,hot roll forming can doubtlessly extend the range of roll forming to and beyond 1in. (25 mm).

There is also achance that tension and hot rollforming can be combined to produce moreeconomical

products.

15.10 Hot Roll Forming Variable Cross-Sections

The cold reduction technologyofstrips described in Section 15.8 has been used for along time but it

requires highforces. Reducing selected parts of astrip is morefeasible at hightemperature. Figure15.19

FIGURE 15.18 Cold reduction of drawer slide section provides tight tolerances at critical points.

Unusual, New,and Future Roll Forming Technologies 15-11

shows atypical example, whereby thinning the walls of asection, similarly to the webs of the “I” beams,

moreeconomical and lower priced products can be produced.

15.11 Hot Thickness Reduction along the Length

of the Strip

Products are designed to withstand the maximum

stresses or deﬂections. However,the stresses are

usually not uniformly distributed along the length

of the products. As aresult, usually the uniform

cross-sections are “overdesigned” all along the

product lengths except at the maximum stress

location.

Efforts havebeen made to optimize the weight of

the products by changing their cross-section along

their length to suit the varying stresses. Typical

examples are the tapered light poles and changed

columncross-sections from the bottom to the top

of the buildings. Airplane manufacturers reduce the

weight of the airplanes by etching out the unwanted

thicknesses of materials at low stress areas. The

automotive industryuses “custom tailored”blanks

by welding together thicker and thinner sheets that

will be formed into load bearing sections. In case of

the roll formed section, the common waytoreduce

the weight of the uniform cross-section parts is to

cut out material, for example, by punching holes.

Reducing the weight and cost of the product can

be achievedeasier by reducing the strip thickness

at low stress areas. It is possible to reduce the strip

thickness in aroll forming line built for this

purpose by changing the gaps between the rolls in

thickness reduction stands located ahead of the roll

former (Figure15.20). By varying the gaps with

proper control, strips can be rolled with thickness

varying in the longitudinal direction.

The variable thickness strip can then rollformed

in the traditional way.A“U” channel rollformed

with this technologyisshown in Figure15.21 but

the process can be used for other cross-sections and

having the thick–thin section variations at any

desiredlocations.

15.12 Welding Hot Roll Formed Sections

Sectionsthat havealready been preheated can be more readily ﬁnished as welded structures

(Figure15.22). Pressurewelding of the hot ﬂat parts of asection (Figure 15.23), or anyother welding

technologythat is presently used, requires considerably less energyinput than is needed for welding cold

sections. The tendency for residual stresses, cracks, and distortion can be signiﬁcantly reduced. Also,this

FIGURE 15.19 Reducing the strip thickness at speciﬁc

segments of the strip provides moreeconomical sections.

Roll Forming Handbook15-12

method with controlled rate of cooling can be used to weld steels with hig hcarbon content that prone to

cracking.

The same hot rolling technologycan be applied to makewelded tubes.

15.13 Other “Hot” Processes

15.13.1 Hot Curving (Sweeping)

During curving,the outsideﬁbers areunder

tension, the inside ones under compression.The

minimum curving radius (maximum sweep num-

ber) for aproduct is limited by ,among other

factors, the shape of the section and the metal

thickness. Products curvedbelowacritical radius

become distorted by the buckling of the com-

pressed elements. Curving at hightemperature

allows moredeformation at the compressed parts

and longer elongation at the tensioned elements; as

aresult, the product can be curvedtoasmaller

radius than those curvedatroom temperature.

Tubes are hot bent for along time to asmaller

radius than can be achieved at room temperature.

15.14 In-Line Soldering, Brazing, and Heat Treating

The industryisalready using some of the “in-line”heating processes. Thin wall lock-seamed tubes are

brazed at the exit end by passing the tubes through an induction coil.Soldering and elevated temperature

adhesive bonding can be achievedwith the same technology.

Some metals, such as brass, stainless steel, zirconium, and nickel alloys, work harden at averyhighrate.

The above-mentioned in-line heating technology could be used for “ﬂash”annealing of the partially or

fully formed sections. The similar equipment can be used for in-line heat treating.

Presently,aluminum alloys, which can be “precipitations hardened” at room temperature, are

processed in the following sequence; the coils are cross-corrugatedfor better air passage and are recoiled,

they are heated to the required temperature,quenched in water,stored in refrigerators (to avoid

hardening at room temperaturebefore roll forming), and then roll formed after ﬂattening out the cross-

corrugations.

FIGURE 15.20 Changing the thickness along the strip produces “custom-tailored” thicknesses for certain products.

FIGURE15.21 Custom-tailored roll formed “U”

channel from strip with variable thicknesses.

Unusual, New,and Future Roll Forming Technologies 15-13

This whole process can be simpliﬁed and be made less expensivebyinstalling,just ahead of the mill, a

short, hightemperature ﬂash heat-treating oven for the continuously running strips, followed by

quenching the hot strip with water and roll forms it in an “all-in-one line process,”producing the parts just

in time (JIT).

The trend will be to use more hot roll forming and “in-line” heating of the strips or products for heat-

treating,bonding,orother purposes.

15.15 Equipment and Tooling Requirements

for Hot Roll Forming

Forhot roll forming, additional equipment must be added to the line. Equipment for heating,possibly

for forced cooling and sometimes pickling,isrequired. The type and capacity of the equipment will be

inﬂuencedbythe type of metal formed, the required maximum and lowest practical temperature,the

heat capacity of the cross-section, and the forming speed.

15.15.1 Heating and Cooling Equipment

Depending on the requirements, heating can be completed before, during, or after rollforming.

Conventional heating technologycan be used. However,instead of soaking the coils for along time in a

furnace, the moving strip or formed section can be ﬂash heated.

In some instances, the full cross-section must be heated, in others, only the bend lines or edges are

preheated. High-speed edge heating,such as high-frequency or induction heating,can be used for forge

welding of edges of closed sections.

FIGURE 15.22 Roll formed, preheated, pressure welded cross-section.

Roll Forming Handbook15-14

Water cooling of the ﬁnished section may be

required to quench the section after heat treat-

ment,orwhenrequiredfor parthandling.

Additional equipment, such as awalking beam

runout table to allow spacefor air cooling of the

parts, may be needed at the exit end of the line.

15.15.2 Special Mill Design

Thin metals, about 0.050 in. (1.25 mm) or less,

may be hot roll formed in conventional roll

forming mills. However,thicker material may

requirespecial roll forming mills. These hot roll

forming mills must havewater-cooled shafts; heat-

resistant rollsthat are partially insulated from the

shafts, special bearings, different lubricants for

bearings, gears,and other components,water

cooling and re-circulating system, heat-insulating

cover, and often an exhaust system.

If metal thickness reduction is substantial, then the forming speed changes caused by the thickness

reduction from pass to pass havetobecontrolled differently from the conventional rollforming mills.

The speed control of these mills will be similar to the ones used for sheet rolling at the mills (see also

Section 15.15.4).

15.15.3 Other Equipment Requirements

If cutoffdies are used, then their cooling has to be considered. When heavy sections are roll formed, saw

cutting typically is the most economical method to cut products to length. Roller conveyors, coolant

recirculating pumps, and other components also are exposed to the heat.

15.15.4 Roll Design

The traditional roll design is based on the assumption that the thickness of the material does not change

during roll forming.

The existing manual or computer-aided roll design method can be applied to hot roll forming if

the thickness of the material is not changed, or if the thickness reduction is restrictedtoasmall

portion of the cross-section. However,because of the considerably lowerstrength and higher

elongation at hightemperatures, fewer passes will be required to obtain the ﬁnished shapes, especially

when thick material is formed.

During roll forming,ifasubstantial partofthe cross-section is reduced in thickness, then the ﬂowof

material will be different. In these cases, the rolldesign could be ahybrid between the traditional roll

forming and the hot section rolling roll design. The substantial thickness reduction also will change the

speed of the strip entering and exiting the thickness-reduction pass. The speed change necessitates

changing the mill drive system.

15.15.5 Hot Roll Forming Conclusion

Hot roll forming often requires considerable additional spaceand capital investment for preheating the

material and cooling the product, and different roll forming equipment maybeneeded. However,hot roll

forming can provide the most economical manufacturing method for manyproducts as well as extending

the maximum thickness limit of forming to 2in. (50 mm) or more.

FIGURE 15.23 Roll formed, edge-heated, pressure

welded tubular section.

Unusual, New,and Future Roll Forming Technologies 15-15

The biggest obstacle in introducing hot roll forming is not the cost of the equipment, but rather the

resistanceofmetal fabricators to change. Eventually,perhaps this resistance will be overcome as the

beneﬁts of the process are seen to outweighthe additional costs.

15.16 Press Tooling for Conventional Roll Forming Lines

Dramatic changes in prepunching or postpunching,notching,orcutoffdies are not expected. However,

the already available compact high-speed hydraulic presses will allow operation at 600 to 800 ft/min

(180 to 245 m/min) and faster.These small presses, along withtechnologyborrowed from the turret

punch system, will speed up tool change and the N/C or computer control will provide versatility.

15.17 Computer-Controlled Roll Forming Lines

The application of N/C, PLC, computers, and other devices will greatly increase in the near future.Some

lines will operate accordingtothe prepared programs, while others will be based on data received from

sensors, and will adjust the mill, tooling,and other componentstomaintain uniform quality.

Forexample, in the case of athree-roll curving

unit, asensor can measurethe curving radius

during the curving process (Figure 15.24). The

data feedback can be used to adjust the curving

rolls to maintain the speciﬁed radius.

Curving rolls, controlled by servo motor and

governed by preprogrammed data, are already

used to form parts with variable radii to tight

tolerances (Figure15.25). Some dimensions of

the products, for example, the width of a“U”

channel, can be held tightly by either measuring

or sensing the width. The pass inﬂuencing the

width can be adjusted either manually by push

buttons or automatically through servo motors

(see Figure15.26).

When aroll forming mill with good tooling is

properly set up,itcan produce parts to very tight

tolerances without anyadjustment from consist-

ent, uniform material. However,owing to ﬂuctu-

ation in strip dimensions,dev iationfrom

straightness andﬂatness, andvariations in

mechanical properties,the material is never

uniform. Therefore, operators regularly have to

makeadjustments to the mill. Onceenoughdata

is available to knowwhat mill adjustment has to

be made when the ﬁnished product deviates from

the speciﬁed shape, length, or straightness, a

properly programmed computer can adjust the

mill using servo motors. Servo motor adjusted

shafts or side rollers can keep formed angles

within tight tolerances.

Servomotor controlled straightenershave

already been developed to keep products straight

at the end of the line.

FIGURE15.24 Sensor- co ntrolled, servomotor

adjusted curving unit.

FIGURE 15.25 Computer-controlled curving unit can

curve sections with different radii.

Roll Forming Handbook15-16

Computers or similar devices are used by manycompanies to control productlengths and quantities,

ﬁrepresses, change punching pattern, control packaging,and to displaytroubles and maintenance

schedules. Increasingly,the function of the operators will be takenoverbycomputerized or automated

devices. It will also makeoperating several lines by one operator possible; apracticealready used by afew

advanced companies. Whether the line is simple or highly sophisticated, skillful, motivated, and highly

educated operators will always be needed.

References

[291] Halmos, G. Rollforming beyond year 2000, SME Conference,December 1993.

[315] Osbeck, G. 1981. The Embosser. Fabtech Int., Proceedings ,pp. 201–208.

[432] Newtechnologyinthe 90’sand its impact on your work. CSSBI Conference, Aylmer,Quebec,June

1993, unpublished.