Roll Forming Handbook / Edited by George T. Halmos

Подождите немного. Документ загружается.

TABLE 10.5 (Continued)

Problem Possible Causes Check/Eliminate/Prevent

Material Coil had wavycenter:

Roll forming will not remove waviness, but ribs can

dissipate (appearancewill be somewhat better)

Center waviness

(continued)

If material thickness is reduced,the product will be more

prone to center waviness

Waviness maynot showupinshort products (about 10 ft

or 3m), but longer products will displaymore

pronounced waviness

Identical waviness will appear morepronounced in high

luster (shiny) surface, and will be less visible when the

surfaceisdull

Corrective leveling of the coil before roll forming makes

the edges longer.Ifcorrectly applied, in manycases,

waviness of the center part will be minimized or will

disappear

Tension leveling of the coil will not help,frequently it

makes the appearanceofthe product worse

The simplest way to prove or disprovethe cause of

waviness is to roll form productfromthe “proof coil”in

the same setup

If center waviness is dominant at one side of the product,

then it couldhavebeen caused by camber in the coil.

Flip the coil over and run afew pieces. If waviness

appears at the other side, then it is caused by the

camber in the coil (or waviness at one side only).

If the waviness appears at the same side, then it is

caused by other factors

Quality of slitting can inﬂuencecenter waviness

Low strength material often has ahigher tendency to be

wavythan high strength material

The steel making technology,metallurgical properties, and

residual stresses in the coil have strong inﬂuenceon

center waviness

Product Design The wider the ﬂat surfacebetween the bend lines, the more

prone it is to waviness (high ﬂat width:thickness ratio)

Large radii at bend lines will reduce and very sharp bend

line will increase the tendency of waviness

Holes and notches will reduce the buckling strength of the

ﬂat section, henceincrease waviness

Stiffening embossment and increasing buckling resistance

will diminish the waviness

Equipment and tooling Bent shafts can induce waviness

Misaligned equipment, misaligned rolls and side-rolls will

prompt waviness

Incorrect setup,nontracking bend lines can cause

waviness

Excessivepressure on the ﬂat section will reducethe

thickness of the material and create waviness

Herringbone and ripple

effect [Figure10.38(h)

and (i)]

Setup Check setup,especially the tracking of the bend lines from

pass to pass

Material In borderline cases, the thinner the material, the more

likely the product will have ripples

( Continued)

Roll Forming Handbook10-44

TABLE 10.5 (Continued)

Problem Possible Causes Check/Eliminate/Prevent

The relation between the moment of inertia of the cross-

section and the contraction of panel width can effect

herring bone defect

Herringbone and ripple

effect (continued)

Equipment Misaligned shafts

Refurbished or modiﬁed mill where the shafts maybe

parallel but not exactly perpendiculartothe direction of

the strip travel create ripples, waviness, and other

problems in the ﬁnished proﬁles

Coil break [Figure

10.38(m)]

Uncoiler Uncoilers with narrow,overtightened leaves will “break”

the coilsfromedge to edge, in repeated straight lines.

Coil breaks can be eliminated by using at least four wide

leavesinthe uncoiler and less pressure

To prevent coil break, the large hydraulic pressure required

to hold large diametercoilsshould be reducedwhen the

coil has only afew wraps close to the inside core

Recoiling Incorrectrecoiling by the coil supplier can also result in

coil breaks

Almost all coil breaks can be eliminated with ﬂatteners

installed after the uncoiler but the best is to prevent

them

Cracked material

(at bend line)

[Figure10.38(n)]

Material has higher strength

than the tools are

designed for

Yield strength of the material is higher (and elongation

lower) than the rolls are designed for

Material work hardens quickly

Textured embossment of coilsresults in work hardening.

Roll forming these embossed material usually does not

cause cracking but additional operation such as curving

maycrack it

Note: Hydrogen embrittlement, caused by galvanizing or

aging of steel, does not contribute to edge cracking

Similarly,the direction of shearing (rough breakthrough)

which is critical in brakeforming does not appear to

inﬂuenceroll forming

Incorrect tool setup Check roll setup against setup chart

Check roll alignment (and shaft shoulder alignment)

Check tracking of bend lines. If rolls are not properly

aligned, abend line can be straightened out, then bent

again. Bending back and forth can crack the material

Check roll gap.Hard“pinching”ofthe bend line can

harden material

Scoring roll cuts too deep in the material or is on the

wrong side of the strip (should always be inside of

the bend line)

Section formed from high strength material was

excessively overbent and then “opened” up

Material Deep embossing, such as creating“woodimitation”doors,

often requires drawing quality steel. Owing to material

or die problem, the embossment maycrack but the

crack is not at the bend lines. This is adrawing

(embossing)and not aroll forming problem

Dimples, louvers,and other bent and stretched shapes may

crack because of the mechanicaland metallurgical

properties of the material

( Continued)

Equipment Installation, Roll Setup, Maintenance, and Troubleshooting 10-45

TABLE 10.5 (Continued)

Problem Possible Causes Check/Eliminate/Prevent

Equipment and tooling Misaligned male and female rotary embossingtooling can

create undue high stresses resulting in cracks

Cracked material

(at bend line)

(continued)

Embossing with pressdie: incorrect tool setup can crack

the material

Sharp corners of punched angular holes can initiate cracks

because of high stresses during roll forming, especially

at or close to the bend lines, or at the outside ﬁber of

curved(swept) products

Cracked coating

[Figure 10.38(o)]

Material Quality or thickness of coating is changed; roll form

product from other coil(possibly from the already

proven“proof coil”)

Material is substantially thicker than before; the thicker

the material, the higher the stresses will be at the

outside ﬁbers

Thick laminate also increases the total thickness of the

product. It will havethe same effect as substantially

thicker material described above

Material is very hard. Even if the material does not crack,

the coating may showhairline cracks

Tooling If anyorall of the rolls have been recut to asharp radius,

or asharp radius is createdbyaside-roll stand or

straightener

Setup Incorrect alignment —bend has been straightened and

rebent

Painted or laminated coating cracks at sharp bend lines.

Cracking often can be eliminated by preheating the coil

just beforeitenters into the mill using electrical or gas

heaters, hot water “bath,”and so on

If prepainted material has been stored outdoor (or

transported) at averylow temperature, and is processed

without warming it up,itmay crack

Peeled, scraped off

coating, and ﬁnes

[Figure 10.38(p)]

Coating quality When zinc, paint, or laminate is peeling at the bend lines

or anyother parts of the product, it is caused by poor

quality coating. It cannot be eliminated by roll forming.

Solution: change the coil

Crude test: scratch with acoin the paint of the coil that

createsproblem, and with the same force, scratch the

paint of agood coil.Comparethe amount you can

scrape offand the scratch marks

This method is not concrete proof for good or bad quality

coating,but can provide an indication of quality when

comparing coatings

Tooling When scraped offpaint (or laminate), powder or ﬁbers are

collected after apass, then check the roll setup and gaps,

check the mill and product design again. If scraped off

coating still appears, check side-rollpasses, entryguide,

and possible placeswhich mayscratch the paint offthe

product

Check product for scratch marks (see below)

Check slit edges (see “slitting” below)

Check rolls for pick-up (roll surfaces should be smooth)

If rolls have been recut, rough roll surfacemay crate

problem

( Continued)

Roll Forming Handbook10-46

TABLE 10.5 (Continued)

Problem Possible Causes Check/Eliminate/Prevent

Peeled, scraped off

coating, and ﬁnes

Toonarrow entryguide or trapping on the rolls can “shave

off” coating or precut material

(continued) Check strip width. It must not be wider than the speciﬁed

maximum

Slitting Bad slitting frequently leaves metal ﬁnes or ﬁne, cracked,

pulverized material at the slit edges which will be

deposited on the mill. After cleaning the mill bed and

tooling, clean the edges of the strip beforeitenters the

mill with a“rag.”Contact the slitter if the rag picks up

undesirableﬁbers or ﬁnes

Lubrication Check the lubricant used in forming

Check the tank and the ﬁlter in the recirculating system

In critical cases, install open cell foam blocks, pushed

against the top and bottom rolls to removeﬁnes

Surface scratch marks

(straight) [Figure

10.38(r)]

Entryguide, side-roll stand,

straightener,cutoffdie

Long straig ht scratch mark on the ﬁnished product. It is

relatively easy to trace it back to asharp edge or corner

causing this defect

Surface scratch marks

(arc) [Figure 10.38(s)]

Pick-up on roll, chipped roll,

crackedroll

Arc(circular) types of scratch marks on products on the

nonhorizontal legs or webs are most frequently caused

by “pick-ups”onthe rolls. The pick-up is usually zinc or

other metallic material transferred and fused to the roll

by combination of pressure and friction

clean the rolls

prevent pick-up by applying properlubricant and assure

proper gap between rolls

Polished rolls have lesser tendency to “pick-up”foreign

material than rolls with rough surface

Chrome plating of the rolls are most often unnecessarily

recommended. In some cases, it helps to eliminate or

minimize pick-up,but in most cases, the polished,

smooth surfaceofthe plated rolls is moreimportant

than the chromeonthe surface

If the marks are caused by chipped (or cracked) rolls, then

agood solution is to replacethe chipped (or cracked)

rolls. Aquick, short-term solution is to grind offthe

sharp edges around the chipped roll edge

Surface scratch or mark

(repeated) [Figure

10.38(t)]

Same as listed under “Arc”

type scratch marks

It appears at intervals of approximately threetimes the

roll diameter

Bent shaft It is usually easy to traceback to the roll or to other

rotating componentOther rotating tools, side or

straightener roll, secondary

operation rolls, and so on

Shinymarks

[Figure10.38(q)]

Material Some material, such as Galvalum-coated steel, dull

stainless steel or some dull painted surfaces willshow

shinypressure marks along the bend lines, even when

minimum roll pressure is applied. These marks are due

to the surfacecharacteristicsofthe strip (metal or

coating) and usually cannot be eliminated

Roll setup To omuch roll pressure (or insufﬁcient roll gap) will create

shinymarks in anarrow strip in the full length of the

product. This is the most frequent cause of shinymarks.

The location of large pressure can be easily traced

back to the pass causing it while the productisstill in

the mill

( Continued)

Equipment Installation, Roll Setup, Maintenance, and Troubleshooting 10-47

TABLE 10.5 (Continued)

Problem Possible Causes Check/Eliminate/Prevent

ShinyMarks (continued) If the shinymarks are intermittent at the interval of about

threetimes the roll diameter,then most probably the

shaft is bent or the boreinthe roll is offcenter

Nonrotating rolls If anondriven shaft or aside-roll is stuck (not rotating),

it can create long shinymarks

Supporting tables, guides,

and other components

Entrytables, entryguides, side-roll stands, straighteners,

cutoffdie, and other components rubbing against a

sensitivesurfacecan cause shinysurfacestreaks.

Relieving the pressure usually eliminates these marks

Stop and startFrequently,when the line is stopped and is restarted,

due to the inertia of the strip and coil,the rolls will

slip momentarily on the product. This slippage leaves

narrow,shinymarks acrossthe strip

Uncoiler If the uncoiler brakeisnot tight, aloop will develop

between the uncoiler and the mill whenever the line

stops. Everytime when the line starts and the loop

“straightens,”the inertia of the coil will momentarily

stop or reduce the speed of the strip engaged in the mill.

This momentary change in speed will makethe rolls slip

on the material and will create ashinymark across the

strip at the contact surfaces with the rolls

Gears The teeth of the gears, which are squeezed closer than the

permissible minimum, grind into each other,generating

noise and vibration. The gearboxvibration can be

transmitted onto the products as narrow,shinymarks

across the strip (called “gear chatter”)

Location of prepunched

holes, cutouts (in

longitudinal direction)

[Figure 10.38b(v)]

Feeder or pick-up Fluctuation in the feeding distancebetween repeated hole

patterns will show up in the ﬁnished product

Tool setup The accuracy of tool setup of adjustable dies and separately

installed die sets has an obvious effect on the hole

pattern of the ﬁnished product

If the distancebetween the die sets is incorrectly set, then

it will be consistently show up in the ﬁnished product;

thus makes it easy to track back the sourceofthe

problem

Roll setup Afrequent reason of change in the hole, notch and other

cutout locations is the roll setup.Too high pressure

exerted on the material will makeitthinner and longer.

The distancebetween holes, cutouts (in the longitudinal

direction) will increase

Thin material can stretch between operations and between

passes, especially if the incremental increase in roll

diameters are relatively large

Location of prepunched

holes, cutouts (across

the direction of

rolling) [Figure

10.38(x) and (y)]

Precut strip or coil Change in the bend line location changes hole location

Amodiﬁcation in the method of guiding and locating

precut strips or coilsinthe punching die can inﬂuence

the hole locations in the ﬁnished product

Entryguide If the entryguide position is moved, the holes will be at

incorrect distance from the bend lines

( Continued)

Roll Forming Handbook10-48

TABLE 10.5 (Continued)

Problem Possible Causes Check/Eliminate/Prevent

Location of prepunched

holes, cutouts (across

the direction of rolling)

(continued)

Camber in the strip Camber in the strip has an inﬂuenceonthe hole and

notch locations. The longer the hole pattern per hit

and the larger the camber,the moresigniﬁcantthe effect

Strip width To onarrow strip can “wander” back and forth between

guides in the prepunching die, the mill entryguides,

and between the traps in the rolls. The distancethe strip

wanders inﬂuences the location of holes from the bend

line and edges

Setup (cross-section) As aresult of incorrect setup,the section can be out of

tolerance(e.g., too shallow) and the bend lines will be

at differentposition. This will inﬂuencethe holes to

bend line distances

Material thickness The bend line of thicker material takes morematerial than

athinner one. Therefore, the hole location tolerance

across the strip should include the difference between

the arc lengths of different thicknesses at each bend line.

Theoretically,for each thickness, aseparate punching

pattern is required, but in most cases, only one die is

built. As aresult, an unexpectedchange in the thickness

can change the hole locations in the ﬁnished product

Location of postpunched

holes and notches

[Figure10.38(z)(26)]

Method of cutting to length The distanceofholes from the cut edges is afunction

of the cut-to-length accuracy (for additional details,

see “Length Tolerance” in this table and Chapter 3)

Punches in the cutoffdie The distancebetween the holes punched at the front and

the tail end of the product depends on the length

accuracy of the cutoff(see above)

Separate die sets Holespunched by adjustable die sets or individual

punches installed in the presswill be inﬂuenced by the

accuracy of the setup

Embossments, louvers,

lances

Wrong location (not within

tolerance)

See comments above in “Location of Pre- and

Postpunched Holes and Notches”

Tooshallow Punches are worn

Punches have been reground and installed without

compensation for their reduced length

Press stroke is incorrect (if stroke is adjustable)

Press tonnage is not sufﬁcient (insufﬁcient air or hydraulic

pressure etc.)

Yield of material is too high (press tonnage is not

sufﬁcient) or springback is excessive

Material is too thick (press tonnage is not sufﬁcient)

Embossments, louvers,

lances

Depth is correct after the pressoperation but the

protrusions are “squashed” back in the mill because the

roll spaceistoo small

The height of protrusions can be reduced by side-rolls,

guides, or by the too small opening in the cutoffdie

Protrusion height can be reduced by the method of

ﬁnished product handling (bundle weight), shipping, or

assembly equipment

Toodeep Replacement punches are too long,orshims under

punches (if any) are too thick

Press stroke or die stops are incorrectly set

( Continued)

Equipment Installation, Roll Setup, Maintenance, and Troubleshooting 10-49

TABLE 10.5 (Continued)

Problem Possible Causes Check/Eliminate/Prevent

Embossments, louvers,

lances (continued)

Cracked Properties of the material are different.Test the setup with

apiecefromthe “proof coil”

Punch penetratesdeeper than speciﬁed

Punch and die are “slightly” misaligned

Press stroke is too long

Section deformed by the

cutoffdie [Figure 10.38

(aa and ab)]

Section The roll formed section is not within the speciﬁed

tolerances. Check setup or material

AlignmentCutoffpress/cutoffdie is not aligned with the mill

Cutoffdie Loose or damaged cutoffdie component(s)

Incorrectly removed or reassembled cutoffdie

Section stuck in the

cutoffdie

Section The roll formed section is not within the speciﬁed

tolerances. Check setup

FlareCut ends with unusual high ﬂare(caused by setup

problems or higher strength of the material) can be

caught in the die

Waviness Wavy section maynot pass throughthe die clearance

Camber,bow,ortwistDies made for straight section maynot haveenough

clearancetopass throughproducts with camber,

bow,ortwist

Material Springback deforms sections.Material strength maybe

higher than speciﬁed

Material is thicker than speciﬁed

Burr Heavy burr created by slitting, incorrect entryguide, roll

setting or by punching can stop the product from

passing throughthe die

Die Loose die segments, blades and inserts, or incorrectly

reassembled die may have “steps”hindering the passage

of the product

After cutting, the die does not open completely.Check

springs and other lifting devices

Pick-up pins or other moving parts do not function

properly

The die is not ﬂying exactly parallel with the direction

of the product travel (not parallel to the center line).

The press or rails are at an angle to the center line

Postpunchingdie Punch is broken or not lifted up

Punch creates excessiveburr

Punch does not push the slug out of the hole

Product is deformed in the die

Press Press is not positioned properly in the line

Press centerline is not parallel with the travel of the

product

Press stroke is not correct

Press does not have the requiredforce. It shears through

the product partially instead of cutting or punching it.

Check pneumatic, hydraulic pressure, clutch, slippage,

and so on

( Continued)

Roll Forming Handbook10-50

plant, equipment, tooling,and the work force. It is easier to discuss potential problems at this stage then

to discoverthem later during production.

10.4.8.2 Material

To ensure good material qualityfor manufacturing, it is recommended to establish and use the following

standards or procedures, which are speciﬁcally designed to meet the company’srequirements.

TABLE 10.5 (Continued)

Problem Possible Causes Check/Eliminate/Prevent

Section stuck in the

cutoffdie

(continued)

Die accelerator and presscycle Die speed is not synchronized with the speed of the

product. During cutting, the slowdie holds back the

product, which maybuckle or crumple in or ahead of

the die. Pick-up holes can be elongated and material

wrinkled

The die is stopped and pulled back into the direction of

home position by the die accelerator beforethe cutting

blade or punches are disengaged

Press cycle is incorrect. If it is too slow, the

cutting/punching is not completed before the end of the

forward travel of the die

Straightener The operator tries to compensate for excessively cambered,

bowed, or twisted product by forcing it in the opposite

direction while one end of the product is in the cutoff

die. The forces created by friction can be higher than the

force pulling back the cutoffdie. The section traveling at

roll forming speed pushes the die out of the press,

actuating the emergency stop

Welded strip joint Incorrectly welded strip end joint can get stuck in the die.

The weld can be thicker than the thickness of the

material, or it is cracked

Cutoffburr Cutoffdie Cutoffdie insert is dull, requireresharpening

Material thickness or strength is not within the speciﬁed

range

Cutoffdie components are loose, damaged,orare

incorrectly reassembled (possibly incorrect shim used

after sharpening)

Prepunching/cutoff

mismatch or other

problems [Figure

10.38(ac, ad, and ae)]

Preand postpunch mismatch

See comments at “Location

of reprepunched holes and

cutouts in longitudinal

direction and across the

direction of rolling”aswell

as in “Location of

postpunched holes and

notches”inthis table

Re-prepunched holes

damaged by cutoffdie

pick-up

Same comments as above

Cutoffdie accelerator or other related problems

Hole or notch deformed

by the punch [Figure

10.38(af)]

Punch was not retracted

in time

Press timing

Die accelerator problem

Punch clearance

Inadequate or off-center stripping force

Equipment Installation, Roll Setup, Maintenance, and Troubleshooting 10-51

Material standards

Standardordering speciﬁcations

Incoming material inspection (spot checks)

Procedure to identify everyfull and partially used coil, with indication of the original coil data

fromthe supplier

Procedure to keep (and replenish) “proof coils”

Procedure to investigate and to followupmaterial-related production and product problems

10.4.8.3 Equipment and Tooling

Specifying,selecting,and procuring inefﬁcient, low qualityequipment, and tooling is arecipe for

troubles, headaches, and inefﬁcient production for manyyears to come. This does not necessarily mean

buying the most expensiveequipment, but buying the best within the available budget. Good equipment

and tooling will savesubstantial amount of time and money.

After the roll forming line and the tooling are installed, working properly and are producing acceptable

qualityand quantityofproducts, aproper maintenanceschedule should be prepared. The maintenance

work should be carried out as speciﬁed and at the scheduled time.

10.4.8.4 Work Force

The best equipment and tooling alone will not manufactureproducts without knowledgeable operators,

maintenancepersonnel, and supervisors. Other personnel involved in the manufacturing, such as the

purchase agent, production and inventorycontrol personnel, engineers, and manufacturing management

can also greatly contribute to,orinthe case of lack of knowledge greatly retardthe productivityof

the plant.

Continuous hands-on training of the line operators and maintenancepersonnel, and providing regular

updating courses for all other personnel are the key elements of successful manufacturing.