Theory Application of Roll Forming (2010)-Web

8/9/2019 Theory Application of Roll Forming (2010)-Web

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Theory &Application

ofRoll Forming


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TABLE OF CONTENTS

DEFINITION 1

CAPABILITIES 1

THE SHAPE 1

DESIGN RESTRICTIONS 1

TOLERANCES 4

SPRINGBACK AND END FLARE 5

THE FORMING MACHINES 5

Integral Drive Type 5

Universal Spindle Type 6

MACHINE ELEMENTS DETERMINING SECTION DEPTH 7

ROLL PITCH DIAMETER AND GEARING RATIO 8

FORMING MACHINE DRIVES 9

ROLL FORM TOOLING 10

REQUIRED AMOUNT OF FORMING STATIONS 10

ACCESSORY TOOLING 11

ORIENTATION 12

ROLL MATERIALS AND ROLL LIFE 12

SPLIT ROLLS AND COMBINATION TOOLING 13

THE CUTOFF MACHINE 14

THE COMPLETE LINE 16

NUMBER AND CALIBER OF OPERATORS 18

SETUP TIME 18

MACHINE COST AND LIFE 19

PRODUCTION RATE AND CONVERSION COST 19

PRODUCTION MANAGEMENT AND OPERATION 19


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DEFINITION

Fundamentally roll forming is defined as a continuous, high-volume, fabricating process inwhich a desired shape is formed from a flat strip of metal by passing it through a series ofmatching pairs of contoured rolls. Only bending takes place, the metal thickness does not

change except for a slight thinning at a bend radius.

CAPABILITIES

As a fabricating method, roll forming is used mainly for mass production of shapes providingthat shape has a uniform cross-section. Any material that can withstand bending to thedesired radius can be roll formed. This material might be hot rolled, mill finish, material, itmight be cold rolled, it might have a mirror or a high polished finish, or it might be of anyknown metal in wide use today. Likewise, it might be coated with another metal such asgalvanize, tin or copper, and it might be painted or plastic coated, the basic criteria beingonly that, whatsoever the material or the coating, it or they must be able to withstand thespecified bend radii.

The product range is almost limitless. The process has successfully made such products as1/8 diameter butt seam tube, filling it at the same time with a flux to create a welding rod;computer components out of .005 thick tin plate; or structural sections and pipe from thick plate, pipe as large as 48 diameter. In some instances, the type of section being thedeterminant, multiple sections can be made from a single strip or, for that matter, severalstrips can be fed simultaneously and combined into one composite section.

THE SHAPE

Roll forming may sound like the Answer to a Maidens Prayer but it, like every othermanufacturing process has some limitations, drawbacks, and restrictions that must beobserved in both machine operation and section design to provide a satisfactory application.

DESIGN RESTRICTIONS

For the most part end product design dictates shape configuration, but at the same timefamiliarity with the processing limitations goes a long way toward designing formanufacturing simplicity and thereby successful, continuous production of uniform pieceparts. Figure #1 shows some restrictions that should be considered:


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Theory & Application of Roll Forming

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Figure #1

a)

Blind bends for example, bends that cannot be reached by both a male and female

portion of a pair of driven rolls can be difficult to control.b)

Narrow slots require narrow rolls that present heat treating problems and are subjectto breakage particularly when excessive metal thickness variations are encountered.

c)

Flange lengths - the length of metal beyond the radius tangent point - should be atleast three times metal thickness.

d)

Modern installations often include piercing, notching, embossing and otheroperations either before or after the rolling operation. Prepunched holes and notchesshould be kept away from bend lines or edges and consideration should be given tothe possibility of slight distortion in their size and shape during forming. Obviouslytoo, rolls must be relieved to clear any embossed designs; therefore, embossingshould also be kept clear of bend lines for most effective forming.

e)

The press type cutoff machine, whether mechanical, hydraulic or pneumatic, becauseof its speed and accuracy, is the most popular type of cutoff used today with rollforming lines. If at all possible the shape should be designed so the cutoff die can beconstructed to develop minimum distortion of the cut end or, it this is impracticalthen the product assembly should compensate accordingly.


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Referring now to Figure #2 an additional group of restrictions to contemplate include:

Figure #2

f)

Sections with wide flat areas that are exposed when assembled into the end productshould be viewed with caution. Despite advances in coil metal processing, no onehas made a perfectly flat coil of material. Imperfections such as loose edges orloose center (oil canning) tend to accentuate into such areas. Longitudinal ribs,perhaps high minimum, added on about 6 centers across the width are effectiveas a compensating measure.

g)

Sections with a wide flat surface along one edge can be improved with some edgeformation to remove the ripples that otherwise might be produced.

h)

Cold reduction and cold roll forming are not completely compatible, however, alimited amount can be accomplished. Metal thickness reductions create control

problems as the thickness and hardness of the strip varies.i)

Corner radii, control the amount of spring-back; if too large, the shape will not beheld uniformly as metal hardness varies; if too small, the result might beobjectionable pressure marks or the likelihood of metal fracture or cracking in the

bend areas.

j)

The minimum bend radius is largely determined by the ductility of the metal. It ishowever, a fact that sharper radii can be obtained by roll forming than by othermethods. Given a metal of sufficient ductility, absolute sharp corners can bedeveloped by creasing or actually reducing the metal thickness before forming up thecorner. Although this is performed when required, it is not a suggested designmainly because of the limited life of the creasing roll and the fact the section is

weakened by the reduction in area.

The various metal handbooks provide information giving bend properties of specificmaterials. It is advisable to adhere to these restrictions and to those imposed by metaltemper in all cases, noting that forming sharp corner involve added machine loads and mayaccelerate tool maintenance.


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Tolerances

a)

Tolerances on part dimensions are largely dependent on the tolerances of thematerial being formed, end flare and springback, notwithstanding. A dimensionaltolerance of 1/64 is commonly applied to cross-section dimensions and atolerance of 1 to 2 to angles. Given a specific set of conditions, closer tolerancescan be held, e.g., .005. But, whenever such tolerances are specified, it should berecognized they usually represent additional tryout time, more tooling expense andpossibly even the need for premium priced material that has special thickness andmechanical property controls.

The manufacturer is often asked to propose equipment to make a given shape frommore than one gauge of metal and in the interest of minimum tool cost, to do so inone (1) set of rolls. Remembering that rolls must be fitted for the maximum metalthickness and also that the only adjustment available to the operator is vertical to

bring the rolls closer together or farther apart. Figure #3 shoes the minimum andmaximum conditions that exist in a typical case. Note the change in the angle of thevertical leg, the change in overall height and the change in arc length at the bends.

Figure #3: Minimum / Maximum Relationship - Multiple Gauge Forming

Strip width variations also affect section tolerance. In the case of the hat sectionpictured here, width variation would be reflected as a variation in the length of thelegs. The part designer should be governed accordingly.

b)

The straightness of a formed section varies due to any one of several factors, stripthickness or hardness changes in a given coil, or for that matter, from coil to coil, rollpressure adjustments, lubricant or temperature changes and camber. The operatormust recognize the affect of these conditions and be able to compensate for them. A

straightening attachment on the exit end of every machine provides this facility.

c)

Straightness consists actually of three (3) considerations - camber, or deviation froma straight line in a vertical plane; sweep, deviation from a straight line in a horizontalplane; and twist, the order of magnitude, for camber and sweep being about 1/8 or in about 10 and twist about 5 to 15 in 10.


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Springback and End Flare

One of two elastic distortion phenomena that must be reckoned with by both the shape andtool designer is springback. It is defined as the general distortion of a part after its removalfrom the forming pressure. The amount varies with metal properties like yield point andelastic modulus. Usually by over-forming, the designer can compensate for a given set of

these conditions.

Closely related is a distortion commonly known as end flare which occurs, as its nameimplies, at the ends of a roll formed section or at any point along its length one might cutthough it, such as pierces or notches in the cross-section. The strains in roll forming aremuch more complex than in other types of bending. Residual stresses make themselvesparticularly apparent by a greater distortion at the ends of a part than at any point along itslength. Flare can be minimized by roll design procedures, but it cannot be completelyeliminated except by subjecting the metal to some amount of stretch forming or to a stressrelieving anneal.

The Forming Machines

The roll forming machine is the instrument needed to convert strip to a finished cross-section. Common to all makes is a fabricated base, roll spindles, spindle support housingsand a drive.

Figure #4: Integral Drive Type Roll Forming Stand

Integral Drive Type

The most popular forming machine is the integral drive type, one head of which is illustratedon Figure #4.

Several manufacturers make this style of machine in standard sizes of 1, 2, 2 and 3spindles to accommodate mild steel up to about .180 thick in widths to 24. Each can bemade with as many heads as desired, as few as two or three or as many as thirty-six,although generally twelve to fourteen meet most requirements.


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As you can see, each pair of spindles is carried in a separate gear-head and as per Figure #5,each has its own splash lubricated worm and gear train. Since the upper roll spindle must

be adjustable for rolls of different diameters, it is driven through a toggle or link type geararrangement from the bottom spindle permitting adjustment without sacrificing pitch linemesh of the gears. A micrometer dial on each adjusting screw indicates spindle adjustmentand parallel. The outboard housing is removable from the spindle as a complete assembly topermit roll changes.

Figure #5: Integral Drive Type Roll Forming Stand Solid Model

Operators View of Yoder M-Style Forming Machine

Universal Spindle Type


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For applications in which the limits of the self-contained gear-head construction have to beexceeded or in which additional versatility is desired, the spindle arrangement shown onFigure #7 is used.

Figure #7: Universal Spindle Type Roll Forming Stand

Here the gearbox is set back from the spindle housings and the spindles are driven throughuniversal couplings. This type machine has been made with spindles up to 15 diameter forheavy structural products and large pipe.

Machine Elements Determining Section Depth

The capability of any given machine with regard to height or depth of the section ismeasured mainly by the vertical distance available between spindles. This, together with the


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distance from the centerline of the bottom spindle to the top of the machine base and thehorizontal center distance between roll stands, establishes the maximum roll diameter.

Figure #8: Roll to Spindle Relationship

Let us look at the roll contour of the last roll stage for a simple channel as on Figure #8 andnote it relation to the other parts of the machine. In addition to the restrictions justmentioned, there must be clearance between the outside diameter of the bottom roll flangeand the top roll spacer, as well as clearance between the edge of the section and the top rollspacer. Note also the bottom roll flange diameter includes a lead-in or bell-mouthed contourto aid in threading from one pass to another.

Roll Pitch Diameter and Gearing Ratio

The roll pitch diameter is usually selected to engage a given profile at its widest and mostnearly horizontal area. This area is important as it provides the most effective and balancedtraction. The ratio between the pitch diameter of the top and bottom rolls must be the sameas the ratio of the gearing connecting the top and bottom spindles.

Some manufacturers provide machines that are able to accommodate either equal ratio orunequal ratio rolls. On the left of Figure #9 is a cross-section through a stage of equal pitchdiameter rolls. The drawing on the right shows the same roll stage in a machine fitted outfor unequal ratio rolls. The pitch diameter in the latter is well below the mid-point betweenspindles and as you see, gains enough space to allow forming a much deeper section with thesame vertical centers.


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Figure 9: Equal versus Unequal Gearing

In machines to which only equal ratio rolls can be applied, deep sections are formed by so

called floating the pitch line. This is not a good procedure but can be applied in specificinstances, particularly where light gauge and perhaps narrow shapes are concerned. Inprinciple, it involves a different pitch diameter in successive roll passes which, as each

bottom spindle is driven at the same RPM, can lead to roll fight between passes, excessivegear loads and excessive HP demands.

Forming Machine Drives

The OEM will determine the correct amount of horsepower required to form the section.The horsepower takes into account four critical items. They are:

1.

Material Thickness

2.

Total number of forming stands

3.

Desired line speed

4.

Yield strength of material

With these items in mind the proper DC or AC Variable Frequency Drive can be selected.Most drives today are digitally controlled, with some type of dynamic braking. Thiseliminates the old clutch and brake style drives. It also gives the operator the ability toincrease and decrease the line speed of the machine.

The larger, more complex lines, typically operate with a PLC to manage the functions ofeach piece of equipment. These lines can have multiple motor drives and color touch screencontrols for the individual functions.


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Roll Form Tooling

Tooling in a roll forming machine predominately consists of the driven forming rolls,together with the spacers that hold them laterally in alignment on the spindles. Theimportance of proper roll design, materials and manufacture cannot be emphasized enough.It is not my intent to discuss roll design in detail, this is the jurisdiction of knowledgeable,experienced engineers, but a few brief comments are certainly in order.

Required Amount of Forming Stations

The selection of the number of driven roll stages depends on the configuration of the shapeto be rolled, qualified to some degree by the characteristics of the material and the machine.Tool designing for roll forming is not an exact science. There are no definite rules that applyto a large variety of shapes. Generally speaking, it can be said the number of roll stages

needed to form a given shape will increase with depth or height of section, with the numberof bends and with increases in metal thickness. Moreover, the number of stages mightdecrease to some extent with an increase in the horizontal center distance between rolls, orwith an increase in roll pitch diameter.

Take any shape to a roll designer and quiz him as to the number of passes it requires.Applying what I will call the Seat of the Pants Theory, more likely than not he will lookout the window, look at the ceiling, scratch his head, put a couple of lines on paper and thuscome up with an answer.

There is another theory that relates the overall forming length to the height of sections of a40 to 1 ratio. Another that relates center distance between roll stands to metal movement

around the arc of bend and in turn to the resulting strain along the strip edge. Still anotherrelates roll diameter, transition or roll contact distance, metal movement around the arc of

bend and again, the resulting edge strain. Common to all is the basic premise that the metalcannot be unduly stretched in its progression from the flat to the finished part.

Each of these theories, there may be more, have some logical basis. Each has its place in thescheme of things; but none tell the whole story. The engineer must be able to visualize theshape the metal will take at each stage. Or, in other words, visualizing the formingprogression is termed a flower layout.

Figure #11 shows the progression (flower) of a typical C section, involving purebending in all stages; Figure #12, the flower for a hat section, where both bending and

drawing are considered.

The cross-section of Figure #13 also illustrates the combination of drawing and bending. Itpoints up too, the demand for drawing metal into and forming, the center tongue beforework on the edges is started.


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Flower Drawing of Strut Section

Figure #11: Forming Progression -C Channel

Figure #12: Forming Progression -Hat Section

Figure #13: Forming Progression -Center Tongue

Accessory Tooling

It is sometimes necessary to mount rolls on vertical axes, between driven roll stages, to exertside pressure to a shape when it is need for forming or guiding.

Likewise, when cut-to-length strips are fed through a roll forming machine, interstageguiding devices are used to guide the lead end in its progression from roll to roll.


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These items, together with straightening guides or rolls, are considered accessory tooling andare mounted on standard fixtures available from all manufacturers for the purpose.

Orientation

Shape orientation, its position relative to the roll axis, is to be considered as an importantelement of roll design. It can affect machine cost, tool cost, part quality and also the overallefficiency of the operation. A particular orientation may receive preference because of:

1)

The limitations of the forming machine as to the number of roll stages and rolldiameter.

2)

The limitations of the cutoff machine as to the die space and stroke.

3)

A desire to retain the finished or exposed surface of the section in a position visible tothe operator as it is being formed.

4)

A desire to position the cutoff burr in a particular direction.

5)

A desire to tool similar sections in combination or sectioned rolls.6)

A desire to minimize, control or eliminate:a)

Scratching and gallingb)

Blind bendsc)

Trapped coolantd)

Springback

7)

The requirement of other in-line operations, e.g., coiling, prepunching,postpunching.

8)

Tool cost economies or ease of setup and operation.

9)

The need to position a laminated section most conveniently for the application of the

core of cover strips involved.

Roll Materials and Roll Life

Rolls are usually made from tool steel, the grade being dependent on the expected productionand finish of the piece part. For general duty applications where a smooth finish strip is to beformed or when shapes are formed from hot rolled, unpickled, steel, a High Chrome-HighCarbon tool steel (AISI D-2) with about 1.5% carbon and 12% chrome is to be suggested. Thisgenerally heat treats to 60-63 Rockwell C and represents about 2 to 2 times the life of thepreviously used oil hardening tool steel. This tool steel has good wear quality and for theaverage run of light gauge cold formed sections could be expected to roll several, on the orderof 3 to 5 million feet or more before regrinding. Generally speaking, 4 to 5 regrinds can bemade before the rolls are scrapped.

For specific industry and product applications, other materials may be used. For light gauge,pre-painted or galvanized wide products, such as building panels or metal roofing, chromedcoated alloy steel may be used, due to the liberal tolerances of the application and the lightforming duties involved. Additionally, if there is a non-magnetic or temperature sensitiveenvironment for the roll tooling, such as inline welding or heat treating, other materials may berequired, specifically aluminum bronze (non-magnetic) or AISI H13 tool steel (heat resistance


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compared to D2). Finally, for extremely high speed, high wear applications, most commonlytube mills, tungsten carbide tooling may be used. Carbide tooling may offer an order ofmagnitude improvement in tooling wear, but it is substantially more brittle than toolset, socase must be taken in the handling of the tooling pieces.

Split Rolls and Combination Tooling

Rolls are split where necessary to facilitate machining, grinding or for change changes, andalso to provide for easy replacement of a roll piece that might be subject to extreme wear. Ingeneral, no roll pieces exceed 6 wide, meaning that wide shape rolls are made up insections, sometimes even of different materials, according to the service requirement. Suchwould be the case for a wide panel in which flats predominate, the flat roll pieces being a softmachine steel, the forming or working portions of tool steel.

Closely related to the subject of splitting rolls is the use of combination tooling, to enable amanufacturer to produce several shapes with a minimum of tooling outlay. Note thesimilarity of sections #1 and #2 on Figure #14. Combination tooling was provided for

theses. The basic set being that to the right of line XX with separate roll sets for the portionsto the left of this line.

Figure #14: Combination Tooling Arrangement

Looking at Section #3, splitting a set of channel rolls along line XX as shown, allows theforming of numerous widths simply by adding spacers at the split.


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The Complete Line

The complete roll forming line can become one of two approaches to the processing of aparticular part; It can be arranged to produce that part

1.

From precut lengths, or

2.

From coiled strip.

Some parts demand the cut length approach; however, the most efficient, productive,consistent, and reliable arrangement is the coil fed line. It most often consists of a coil reel,roll forming machine, cutoff machine and run-out table. This is the basic concept applyingto the majority of installations in the world. A more complex arrangement might beprovided whereby auxiliary operations, such as prenotching, punching, embossing, marking,trimming, seam welding, spot welding, curving and coiling are performed continuously, withthe result that a minimum of subsequent operating procedures might be involved to providethe finished NET product.

Figure #16: 26-Stand Refrigerator Panel Machine

Figure #16 shows a 26-pass machine for producing a refrigerator panel from precut,trimmed, prepunched lengths.

Figure #17 is a typical basic, coil fed line using a coil reel, a 11-stand forming mill and a 4-post type pneumatic cutoff press.

Figure #18 shows a prenotch, form and cut line. This, you see, includes not only the cutoffmachine after the rolling machine but also another cutoff, in this case, termed a prenotchpress, and a small stock straightener ahead of it. Although used most generally fordeveloping a contoured end formation that cannot be developed after the shape is formed,this system can be used to prepunch many repetitive pattern combinations.


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Figure 17: Typical Coil Run Line with Cutoff Press

Figure 18: Entry end of Typical Prenotch Line with Servo Feed


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Number and Caliber of Operators

A roll forming line of average proportions, usually is operated by a single man. Dependingon the number of machines in the plant, this man may or may not be capable of making aroll setup.

As lines increase in complexity or as the shape gets longer, wider or heavier, depending uponthe degree of automation, a helper may be added to the picture to assist in coil loading orpart handling.

Sometimes, a roll forming machine is operator-less and simply functions as a conveyorbetween operations. An example of this is the typical refrigerator panel line wherein the flatsheet is automatically conveyed from the trim and notch press to the forming machine andalso from the forming machine to the tangent bender.

It is often assumed that a roll forming machine operator must be a Jack of all trades. Thismay be far-fetched, although on some troubleshooting occasions, such attributes might help.The operator should be a good mechanic capable of accepting and utilizing such training as

is made available. They do not have to be a tool-maker, as this phase or work has alreadybeen accomplished for them.

Setup Time

Qualified to include the time consumed to strip, clean and store one set of rolls and to load,and adjust another set; setup time must be predicated on a well organized and readilyavailable roll storage facility, an experienced conscientious operator, and crane or hoistservice readily available when needed.

The 1 and 2 spindle mills that are changed over by stripping the rolls from the ends ofthe shafts can be setup in about 15 minutes per pass, both driven and idle. The 2 and 3mills require about 20 minutes per pass.

Additionally, if a cutoff is involved, about 15 minutes is needed to remove and replace thedie, excluding any internal die changes. Length changes, which involve a mechanicalmethod of length control, may require repositioning the cutoff target and, roughly, 5 to 10minutes of time. If a system has an electronic method of length control, changing productlengths and or hole patterns, can be accomplished on the fly, so halting the roll formingoperation may not be necessary.

Typically, a roll change in a large, wide (6 diameter x 60 wide) mill wherein roll andspindle assemblies are removed through the top of the housing, and replaced with pre-

assembled rolls and spindles, requires something on the order of 30 minutes per pass.

In the situation where rafted roll formers are employed, the roll stand and tooling is mountedon a plate, typically referred to as a raft or cassette, the complete raft unit is removed anda second set of rafts with tooling in place are mounted on the machine. Though this setuprequires greater capital investment, the changeover time is much faster and the operator doesnot need to physically remove or adjust the roll tooling.


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Theory Application of Roll Forming (2010)-Web

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