Roll Forming Handbook / Edited by George T. Halmos

Подождите немного. Документ загружается.

thickness toleranceplus maximum coating thickness plus the maximum depth of embossing (if any).

Roll gap ¼ max: thickness þ max: tolerance þ max: coating thickness þ max: embossing depth

(see Figure5.35).

Attempting to roll form thicker material than the speciﬁed maximum thickness could break the rolls,

damage the equipment, reduce the qualityofthe product, and create waviness, scratches, or other

problems.

The thickness also affects the blank size (strip width). The blank size and dimensions are calculated for

the mean thickness. If the thickness is at the maximum plus tolerance, then some dimensions will be

smaller.Ifthe thickness is at the minus tolerance, then some dimensions willbelarger.

Example

The proﬁle shown in Figure5.36 has eight 908 bends (giving atotal of 7208 in the bends). The material

thickness is 0.100 in. (2.54 mm), and r : t ¼ 1 :

The bending starts at the center rib.Atypical ^ 0.006-in. (0.15-mm) thickness toleranceallows the

thickness to ﬂuctuate between 0.106 in. (2.69 mm) and 0.094 in. (2.39 mm). Designing rolls for the

0.094-in. (2.39-mm) thick material and for the same product dimensions, the blank width would be

0.075 in. (1.91 mm) less.

The usual practiceistodesign the rollsfor one thickness and use the same blank size for both the

maximum and minimum thickness (in this example, nominal ^ 0.006 in. [0.15 mm]). As aresult, for a

proﬁle with one cell at each side of the center line, the twooutside lip dimension would ﬂuctuate by

^ 0.050 in. (1.27 mm). This is based on the assumption that the operator is not adjusting the roll gap to

suit the change in material thickness. This means that at the minimum thickness, the height of the section

will be 0.006 in. (0.15 mm) less than what is required, and an additional 0.050 in. (1.27 mm) will haveto

be added to dimension “ a ”ateach side. If the panel has eight cells (four at each side of the center line),

FIGURE 5.35 Maximum material thickness used for roll design.

FIGURE 5.36 Material thickness toleranceinﬂuences product dimensions (top roll position not changed).

Roll Forming Handbook5 -22

then the “ a ”dimension can ﬂuctuate ^ 0.200 in. (5.08 mm). However,adjusting the roll gap will reduce

this ﬂuctuation. This example also proves that by forming material with tight thickness tolerancethe

dimensional tolerance of the outside lip will be much tighter.

The above calculation provesthat tight material thickness tolerances are necessarytoroll form sections

to tight tolerances. In other words, if the material thickness tolerances are loose, then it is difﬁcult, if not

impossible, to manufactureproducts to tight sectional tolerances.

Achange in thickness also has other implications. The gaps between the rolls are designed to form the

product with the maximum material thickness (Figure 5.37b). Feeding thicker strip between the rolls

than the roll gap can break the rolls or yield unacceptable product. Reduced thickness can make

noticeable changes in the shape (Figure5.37a,b), especially if the roll gap is not reduced. Adjusting the

roll gap to the thinner material helps but angular changes will still occur.Areasonably large change in the

t : r ratio will also affect the springback.

Some rollsare designed to form products made from one speciﬁed thickness. Rolls for some older

building products havebeen designed to form materials from 0.018 to 0.108 in. (0.46 to 2.75 mm),

athickness ratio of 1:6. The large change in thickness results in changing in shape and coverage. At some

points, shims are used to correct the coverage. If the rolls are designed for the thickest material with

a t : r ¼ 2 ; then the same rolls will form the thinnest material with a r : t ¼ 12: The larger r : t ratio will

yield alarger springback and can adverselyaffect the web crippling resistance of the product.

The thickness tolerances for aluminum coils/strips are reasonably tight but the “standard” North

American steel tolerances are loose. The tolerancecan be morethan ^ 10% of the material thickness.

Steel mills are able to supply coilswithconsiderable better tolerances then the speciﬁed standard.

However,toguarantee tight product tolerances, rollformers, especially those in the automotive,

aerospace, appliance, furniture, and other industries are pressing the suppliers to provide coils with

so-called “half,”oroccasionally “quarter” thickness tolerances without extra charges. This means that

instead of, say, ^ 0.004-in (0.1-mm) thickness tolerance, the steel should have ^ 0.002-in. (0.05-mm) or

even ^ 0.001-in. (0.025-mm) tolerance. The thickness tolerance of acoil also depends on how close to the

edges can the measurements be taken.

5.4.5.2 Width

Width tolerancewillobviously inﬂuence some dimensions of the product. In most cases, the strip width

tolerancewill be added to the roll forming tolerance at the outside lip (the element from the strip edge to

the ﬁrst bend). If this dimension is critical and is tightly controlled, then the extra material maygather at

the center,creating waviness and other problems.

FIGURE 5.37 Change in thickness will change section dimensions (a). Adjusting roll gap restores height, but it does

not eliminate angular changes (b).

Roll Design 5 -23

Forbuilding products (rooﬁng, siding etc.), the

typical þ 3/16 in. 2 0in. ( þ 4.75 2 0mm) mill

tolerancedoes not represent anyproblem. All the

extra material is to increase the width of one or

both lips. Actually,for building products, it is

highly recommended to provide additional “run-

out” for an extra 0.5 to 1in. (12 to 25 mm) of

material (Figure5.38). This “runout”(additional

strip width space) in the rolls and in the cutoffdie

will permit the use of wider material when required. Wider material may be purchased as asubstitute in

case of material shortage or for other reasons.For example, if the blank size is 35.4 in. (900 mm), then, in

case of emergency,the morereadily available standard36-in. (914-mm) wide coils can be used. Suppliers

do not usually charge extra for this “runout” in the rollsand cut offdie.

Fornarrower slit coils, the typical ^ 0.005-in. (0.013-mm) slitting tolerance is acceptable. However,

the rolldesigner must bear in mind that even this total 0.010 in. (0.25 mm) difference between the

maximum and minimum width will permit the strip to “wander” between the entryguides and between

the traps in the rolls, which havetobedesigned for the maximum width. This means that the 0.010-in.

strip width tolerancehas to be added to the normal roll forming tolerances. If this is not acceptable, then

atighter slit width toleranceisrequired. If only one edge of the product requires tighter tolerance, then

spring-loaded entryguides or tight entryrolls positioned at askewed angle, which forces the strip against

one side, can be used with the strips slit to “standard” width tolerance.

5.4.5.3 Camber

Camber is the deviation of the strip edge from astraight line in the horizontal plane. The inside concave

edge of the strip is shorter than the outside convex edge. During the roll forming process, the strip is

straightened. The shorter edge (marked“S” in Figure 5.39)willbeunder tension and the longer edge

(marked“L”)will be compressed. Depending on the conditions, the compressed edge may become wavy.

It has not been recognizedfor long time that camber is apossible contributor to product problems.

However,rollformers are now requesting tighter camber tolerances because rolls cannot be designed to

compensate for camber.Special devicessuch as automated camber correctors for narrow strips and

special levelers for wider strips can eliminate or minimize the effect of camber.

Excessive camber also creates problems when the strip is prepunched in along prepunching/notching

die. Because the punches are set in astraight line, the hole distance from the edges will varyaccording

to the camber in the strip (Figure5.40a). During roll forming, the strip is straightened; therefore,

FIGURE 5.38 Room for “runout” should be provided

in most panel tooling to allow the use of wider coils.

FIGURE 5.39 When astrip with excessive camber is straightened, the longer (outside) edge will be wavy.

Roll Forming Handbook5 -24

holes prepunched in astraight line will be on a

radius similar to the radius of the camber in the

strip (Figure 5.40b).

In the case of thin, narrowstrips, the guide in

the prepunching die may straighten the strip

during punching.Thicker,wider or higher

strength cambered strips may get stuck in the

prepunching die, which has aguide built to the

maximum strip width tolerance. If the strip guides

are made wider,then the tolerance on the hole

locations from the bend lines will be looser.

5.4.5.4 Cross-Bow (Crown)

As aresult of steel-making technology, some

thicker coils will haveacross-bow (Figure5.41).

This cross-bow(or crown) makes it difﬁcult to

feed in the end of the coil and can show up in

ﬁnished product having awide ﬂat surface. Roll

designer cannot compensate for this cross-bow,

but aﬂattener placed in front of the mill will

practically eliminate it.

5.4.5.5 Longitudinal Bow

Thick, higher strengthcoils arepermanently

curved in alongitudinal direction during the

coilingprocess (Figure5.42). The magnitude of

the longitudinal bowchanges within acoil, the

closer to the center,the moresevere the bowwill

be. To feed this type of coil into the mill, aﬂattener

is required. Aﬂattener will also be required when

FIGURE 5.40 Holes punched in astraight line in acambered strip will appear in acurved “scallop pattern”and

notches will be offbend lines after roll forming.

FIGURE 5.41 Cross-bowincoils may show up in some

ﬁnished products.

FIGURE 5.42 Permanent longitudinal bow.

Roll Design 5 -25

coil ends are welded together or the product is

prepunched.

The longitudinal bowisusually considered

during rolldesign. Aﬁnish product straightener

may compensate for the residual stresses left in the

product after strip ﬂattening and rollforming.

5.4.5.6 Twist

During roll design, it is assumedthatthe

incoming material does not havetwist. Rolls

are not designed to compensate for the twist in

the material.

When an asymmetrical section is roll formed,

the ﬁnished product will haveatwist after

leaving the last pass, even if the starting material

is ﬂat. In some most unusual cases, rollformers

may minimize this twist by twisting the strip in

the opposite direction beforeitenters the mill

(Figure5.43). This pretwisting is only applicable to certain thickness:width ratios, strength, and when

space is available for twisting.Itmust create apermanent twist in the strip but should not create a

problemwhen the end of the strip is fed into the line. Trying to twist relatively thin and wide material

may create unwanted permanent edge elongation.

Amorecommon method to compensate for the anticipated twist is to “twist” the product in

the opposite direction in the mill, from pass to pass. This principle is used when forming “Z” sections

(Figure5.44). In other cases, the “countertwisting” is accomplished just in the last one or two passes.

The most frequently used method to eliminate the twist is to apply an adjustable “straightener” after the

last pass.

5.4.5.7 Waviness

Starting material with wavy edges will most likely

yield products with wavy edges. Panels and other

ﬂat products made out of center-wavymaterial

will be center-wavy. Rollscannottakeout

waviness, but incorrect roll design or incorrect

setup or equipment can inducewaviness in the

ﬁnished product.

Coil center-waviness or edge-waviness may be

corrected by passing the coil throughaspecial

“corrective” leveler ahead of the roll forming

mill. These ﬂexible levelers can “elongate” certain

parts of the coil,thus eliminating waviness in the

strip or making either the edges or the center

wavy if required.

It is importanttoknowthat tension-leveled

coils, which haveexcellent ﬂatness tolerance as

sheets, have considerable but balanced internal

stresses. The balanceisbrokenbythe bend

lines during roll forming and it is possible

that apreviously ﬂat segment of the strip

becomeswavy. Again, roll designers cannot

FIGURE5.4 3 Tw istinthe ﬁnishedproduct (a)

occasionally can be reduced by twisting the strip in the

opposite direction (b).

FIGURE 5.44 Forming a“Z” section with the web

gradually twisted in the opposite direction will eliminate

twist in the ﬁnished product.

Roll Forming Handbook5 -26

design rolls to compensate for this waviness. However,asmentioned above, incorrect design can

create waviness.

5.4.5.8 Surface

The surface appearance requirements of the ﬁnished products and the surfaceofthe starting material

will inﬂuence the roll design. Hot-rolled, hot-rolled pickled and oiled (HRPO) and cold-rolled steels

are the least sensitivetosurface scratches and requirethe least amount of precaution. If low melting

temperature metallic coatingsuch as zinc, aluminum, or tin has been applied to the base metal, then

forming requires more precaution to minimize the “pick-up”ofthe coatingmaterial on the rolls. The

roll designer should aim to minimize the speed differential between the rollsand the formed material,

and the roll former must apply good lubricant. For further details, see Chapter 6“Materials” and

Chapter 7“Lubrication.”Prepainted material does not usually require special consideration but it is

prudent to minimize the surfacespeed differential wherever possible. Using the same rolls for forming

both galvanized steel and prepainted steel is nor recommended.Zinc pick-up on the rollswillscratch

the prepainted surface.

Hot-rolled steel withscale (not pickled) must be formed with rollsmade fromD2tool steel because of

the highly abrasivenature of the iron oxide. Other tool materials will wear very fast. In extreme cases,

tooling made from 01, L6, or similar material mayberendered useless after 50 to 100 hours of roll

forming hot rolled steel with scale.

Some laminates can be relatively thick, for example, 0.010 in. (0.25 mm) or more. The designer must

provide sufﬁcient roll gap for the total material thickness including the laminate. If the customer speciﬁes

0.024 ^ 0.003-in. steel with 0.010-in. thick laminate, then the roll gap should be

0 : 024 þ : 003 in: ð toleranceÞþ0 : 010 in: ð laminateÞ¼0 : 037 in: ð 0 : 61 þ 0 : 075 þ 0 : 254 ¼ 0 : 94 mmÞ :

Operations with extreme roll pressureonprepainted or laminated material should be avoided.

Glossy and other surfacessensitive to scratches should not be formed with highpressureorwithhigh

surface speed differential. To minimize the rollpressure, the forming of prepainted, laminated and high

luster surface material has to be accomplished in morepasses. Frequently,more side-rollpasses with free

running rollswillhelp eliminate surfacescratches or other blemishes.

Forexample, aproduct would traditionally be formed as shown in Figure 5.45a. However,the speed

differential between parts of the rolls and the product will mar the productsurface. Therefore,itis

recommended to change the product orientation as shown in Figure 5.45b. In this way,more passes will

be requiredbut the speed differential between the strip and the top rollwill be negligible. In both cases,

the application of several nondriven side-rolls can minimize the surface mark problem.

Apolished, highluster rollsurface, occasionally different rollmaterial (bronze, plastic) as well as good

lubrication can help to eliminate or minimize surface blemishes or scratches.

FIGURE 5.45 Critical, highluster surface of aproductcan inﬂuence productorientation.

Roll Design 5 -27

The surfaceofembossed materials usually does not cause difﬁcultyinroll design. The material can be

roll formed as usual but extreme pressure must be avoided.

When products withsensitivesurfacesare roll formed, special precaution must be takenatall

componentsofthe roll forming line. Entryguides, side-roll stands, straighteners, cutoffdies, and other

guides may leave unacceptable scratch marks unless they are properly designed and are made from the

right material.

5.5 Roll Forming Mill

5.5.1 Inﬂuence of Roll Forming Mill on Roll Design

Rolls are designed to form the material and they must match the rollforming mill.

The roll designer must haveall the relevant equipment dimensions and other data which can inﬂuence

roll design. Table 5.1 (Sheet 1and Sheet 2) show atypical questionnairefor “standard”mills. Missing or

misinterpreting anydata or dimensions can be very costly.

5.5.2 Type of Mills

(a) Cantilever (Overhanging) Mill.This type of mill limits the formable strip width to about the

length of the shortshafts. The rolls can be slightly extended beyond the shaft length but this

method is not recommended. The further the forming forces are from the supportbearings, the

larger the shaft deﬂection will be.

If only one edge of widestrip is to be formed, then the opposite edge should be properly

guided. In this case, it is more practical for all the material used for forming to be taken from the

driveside and locate the sheet guide at the operator side of the rolls(Figure5.46).

(b) Duplex Mill.Two cantilever mills facing each other,forming bothedges of thestrip

simultaneously are called aduplex mill (Figure 5.47). Adjusting one or both mills in-and-out

enables the forming of an inﬁnitive number of widths within the limits of the mill. The roll

designer should take the following into consideration:

Maximum roll length is limited.

The forming pressure at the end of the rolls(away from the bearings) may bend the shafts

The material for forming should be taken from the drive side only (material cannot be taken

for the strip center,orinother words, the guide plane must be at the center of the strip).

Forwider sections, supports are required between the rolls, both underneath and above the

strip (Figure5.48a,b). Asection is considered to be wide when the width:thickness ratio is

over 60 to 140 (depending on the yield strength of the material and other factors).

To eliminate marks on the product, the end of the rolls shall not havesharp corners(Figure

5.49a,b).

Asymmetrical forming may drive one edge faster than the other.Proper guidance in the mill

as well as at entryand exit is required.

Proper guidance as well as additional devices mayberequired when shortprecut sections

are formed. Under normal conditions, the minimum product length should not be less than

twicethe horizontal rolldistanceplus 1to2in. (25 to 50 mm). This willensurethat the

product is engaged in at least in two passes at each side, at anytime.

If the width and the length dimensions of the precut blanks are similar,orthe length is

shorter than the width, then special deviceswillberequired to avoid “skewing.” One side

driving faster than the other may cause jamming and stop the product in the mill.

In the case of asymmetrical forming,fully formed angles will provide better guidance than

partially formed ones (Figure5.50)because it is more difﬁcult to guide an edge formed at

10 to 15 to 208 than the one formed at 908 .

Roll Forming Handbook5 -28

DELTA ENGINEERING INC.

INFORMATION

FOR

DESIGN &SUPPLY OF ROLLS

No.

Customer:

Date :

Product:

Information Needed to Suit

Specific Mill

Helps to Know No Spec

The following information is required and/or will be

helpful to design rolls:

MILL

1.1 Make, Type



1.2 Shaft Diameter



1.3 Key Size



1.4 Maximum Roll Space



1.5 Pass Line Above Center of Bottom Shaft



1.6 Clearance Below Center of Bottom Shaft



1.7 Max. Vertical Center Distance Between Shafts



1.8 Min. Vertical Center Distance Between Shafts



1.9 Horizontal Center Distance Between Shafts



1.10 RPM Ratio Between Top &Bottom Shafts



1:1 or

1.11 Number of Available Passes



1.12 Will Product Straightener Be Used ?Yes No

1.13 Is Entry Guide Available? Yes No

1.14 Is Coolant /Lubricant Applied? Yes No

1.15 Is any other process incorporated in the line?

(If yes, provide information)

Yes No

1.16 Direction of Line

(flow of material viewed from operator side)

From Left to Right

From Right to Left

Note: For other equipment in

the line, see other Tables

ROLLS

2.1 Material (Type)

PROVIDE INFORMATION IF AVAILABLE

NO SPEC

2.2 Is Splitting required (if Yes, provide details)

2.3 Is Side-Roll recommended

2.4 Is Side-Roll prohibited (space)

INFO NEEDED INFO USEFUL

MATERIAL TO BE FORMED

3.1 Material Type, Physical Properties



3.2 Coating (Type, Thickness)



3.3 Nominal Thickness “t”



Page

3.6 Blank (Width) Tolerance .005 or :

3.7 Continuous Feed (Coil, Strip) Yes or Sheet

3.8 If product is pre-cut, provide Length(s) :

3.9 Additional information on Material :

3.4 Max. Thickness

“t”

(incl. +Tolerance &Coating )



3.5 Minimum Thickness (at minus -Tolerance)



TABLE 5.1

( Continued)

Roll Design 5 -29

DELTA ENGINEERING INC.

INFORMATION

FOR

DESIGN &SUPPLY OF ROLLS

No. Page of

Customer:

Date :

Product:

PRODUCT

4.1 Product Drawing(s) attached :

Note: Specify all Tolerances on drawings

4.2 Are cut-outs, notches, embossing etc. on

product before roll forming …………………………… Yes

4.3 Is orientation of product specified

(Position of shape at last pass) ……………………….. Yes

If Yes, e.g. for other in-line process or inspection,

please indicate it on drawing, mark also painted side etc.

4.4 Application of the product :

ROLL DRAWING AND OTHER REQUIREMENTS

5.1 Set Up Drawings supplied …………………………… Yes No

5.2 Additional Drawings required ………….….. Flower Diagram Roll Drawings

Note: Normally one set of drawing is supplied

Additional sets of roll drawings or flower diagrams are available at extra charge

ADDITIONAL COMMENTS

NONE Additional sheet(s) with comments attached

DRAWING NO. REV. NO. DATE OF LAST REVISION

Form CompletedBy:(printname) ___________________________ Date:________________________

Contact Name (ifdifferent from above) :______________________

TEL: _____________________________ FAX :_____________________ Email: ___________________

Address :_______________________________________________________________________________

{{

TABLE 5.1 Continued

FIGURE 5.46 Forming narrowand wide section in acantilever mill.

Roll Forming Handbook5 -30

In more complexcases, short

precut blanks areguidedthrough

the mill with special devices, chains,

and so on (Figure 4.50).

Long entryand exit guides are recom-

mended especially for precutsections.

Guides for precut sections should be

longer than the width of the blank, and

also at least 3to4times the horizontal

distance between passes.

The horizontal center of most duplex

mills is short. The little spaceleft between the rolls makes the installation and adjustment

of side-rolls or guides difﬁcult.

FIGURE 5.48 In wide duplex mills ( W ¼ 75 to 200 t), supports are required at the bottom and top of the strip.

FIGURE 5.47 Duplex mill. (Courtesy of Dahlstrom Industries.)

FIGURE 5.49 Large radius at the end of the cantilevered

rolls will eliminate marks.

Roll Design 5 -31