Roll Forming Handbook / Edited by George T. Halmos

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tanks. The tanks are usually mounted on the top of the press to be as close to the valves as possible

(Figure3.12).

The press action is very quick. Sometimes, a0.004- to 0.008-sec opening of the valves is sufﬁcient.

Because of its size and thickness, the moving plate (head or ram) is heavy and its inertia greatly

contributes to the total capacityofthe press. The total press forceexerted is asum of the force generated

by the cylinders (or hoses) and the inertiaofthe heavy plate. The “impact tonnage”rating in the cylinder-

operated presses can be about twice as highasthe “holding”forcecreated by the air pressure. In the air-

hose operated presses, the increase owing to the impact is about 1.5. The inertia can be so signiﬁcant that

in some cases, the valves are even closed beforethe cutting/shearing action starts. The air pressure, the

valve opening time, the force of the lifting springs, and the elastic positive stops establish the lowest

position of the ram.

Smaller air presses (5 to 20 tons) can operate over 120 to 150 (or higher) strokes/min.Larger presses

are usually slower.

The advantages of the air presses are their lower price, highnumber of strokes per minute, fast

action, and accurate repeatabilityfor use with electronicmeasuring.The disadvantages are their high

noise, big impact and vibration, big bed areaathigher capacityrange, and large amount of relatively

expensiveair.

3.4 Hydraulic Presses

In hydraulic presses,one or morecylinders exertthe force required for the operation. The cylinder

can be located above the ram, pushing it down, or underneath of the base plate pulling the ram

(Figure3.13).

In most cases, the hydraulic ﬂuid pressure is generated by electrical motor driven pumps or

occasionally by air cylinders (Figure3.14).

Smaller Cframe press housing is used for punching,notching,orembossing narrow strips or

operations close to strip edges (Figure3.15)Inmost instances, for regular press work and larger tonnage,

four-posted presses are used.

The advantages of the hydraulic presses are their easy adjustabilityofforce,stroke,speed, and

accurate repeatabilitywhen used with electronic measuring system. The stroke can suit the application

and the press generates much less vibration and noise than amechanical or pneumatic press. Hydraulic

FIGURE 3.12 Large air press. (Cour tesy of Contour Roll, Inc.)

Roll Forming Handbook3 -12

presses cannot be overloaded and because they have fewer moving parts, they usually requireless

maintenance.

Foralong time, speed limitation has been the biggest disadvantage of the hydraulic presses. Hydraulic

component manufactures ﬁnally overcome the speed restriction by developing new valves permitting the

ﬂowoflarge volumeofhydraulic oil in ashorttime. This development enables the actuation of the

hydraulic presses intermittently with almost the same number of strokes per minute as the pneumatic

presses.

Shock loads in the system, created by the sudden breakthroughinpunching operations, can be critical.

Mechanical shock is developed by the sudden change in the compression or tension of the frame members.

Hydraulic shock is associated with the sudden decompression of the hydraulic system. The shock load can

be reduced by applying rake angle and staggering of punches. In critical cases, internal or external

decompression featuresare built into the hydraulic system.

Off-centric load can create problems by changing the parallelness of the ram and base plate. Linear

transducerscoupledwithservo controlledcylinders caneliminatethisproblem.Smallerpress

manufacturers mayhavetoresort to long and

rigid guides (gibs) or back gearing to control

parallelness.

Calculating the required cylinder diameter is

relatively simple.

Forexample, if 70 t(623 kN) forceisrequired,

and the applicable hydraulic pressure is 1800 psi

(12.4 MPa), then the minimum cylinder area will

be (using Equation 3.3)

¼ A ¼

In this case

A ¼

140; 000 lb

1800 lb= sq: in:

¼ 77: 78 sq: in:

or in metric:

A ¼

623; 000 kN

12: 41 kPa

¼ 50; 200 mm

where

A ¼ areainin.

(mm

)

F ¼ forceinlb(kN)

p ¼ hydraulic ﬂuid pressurepsi (kPa)

FIGURE 3.13 Different hydraulic press arrangements.

FIGURE 3.14 Air over oil hydraulic press. (Courtesy of

CompuRoll, Inc. With permission.)

Presses and Die Accelerators 3 -13

D ¼ diameter of the piston in inch (mm)

D 2

ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

77: 78 £ 4

¼ 9 : 95 in: < 10 in :

ð < 250 mmÞ

Note that this simple calculation should be used

as aguideline only.Itdoes not take hydraulic

losses, time delay, and other componentsinto

consideration. Foractual applications, refer to

thedataprovided by thehydraulic system

suppliers.

If the working stroke is 1in. (25 mm), then

the volumeofthe 10-in. diameter (250-mm)

cylinder in 1-in. stroke will be 1in. £ 78.53

in.

¼ 0.34 gal, or in metric: 25 mm £ 50; 200 

¼ 1 ; 255; 000 mm

or 1.23 l.

When the cylinder returns, the same hydraulic

ﬂuid volume minus the volume takenbythe

cylinder rod will be required. If we assume that

the rod diameter is 3in., then this volume will be

7in.

.Therefore,77.78 þ 70.78 ¼ 148.5 in.

(0.643 gal, or approximately2.4 l) hydraulic

ﬂuid is required for one forward and return

cycle.

Assuming that the press has to be operated at 50

strokes/min, and that between each stroke, the

material (or the ﬂying die) must be moved. Let us

assume that the transfer of material takes 0.75 sec. In 50 cycles/min,the material movement requires atotal

of 50 £ 0.75 sec ¼ 37.5 sec. This leavesatotal of 22.5 sec for moving the cylinders forward and back

50 times. This equals to 0.45 sec/cycle. During this time, 0.643 gal (2.4 l) hydraulic oil has to be supplied

to the cylinder.This requires apump of 60 4 0.45 £ 0.643 ¼ 38.6 gal/min (146 l/min), if the pump is

supplying the hydraulic oil directly to the cylinder.During the material (die) transfer period, the cylinder is

not moving and the oil supplied by the pump is dumped back into the reservoir.

However,ifanaccumulator is added to the system, then the hydraulic ﬂuid supplied by the pump is

utilized all the time. During the cylinder action, the accumulator supplies alarge portion of the oil and

the pump recharges the accumulator during the time when cylinders are idle. Following the above

examples, it means that the required volumecan be supplied in 60 sec instead of 22.5 sec. Therefore, a

14.5 gal (55 l)/min pump will meet the requirements for 1-in. stroke.

Foractual applications, the stroke should be reduced to the minimum practical length. Forexample,

punching a0.060-in. or 1.5-mm thick material, the stroke may be only 0.160-in. or 4-mm long.This

means that 0.160 in. 4 1in. ¼ 0.16 times pump capacity:

0 : 16 £ 14: 5gal ¼ 2 : 32 gal per min ð 8 : 8lper minÞ is sufficient:

These calculations are agood approximation but do not take fast approach and slower punching and

other factors into consideration.

Forpractical purposes, the maximum stroke of the cylinder should be around 4to6in. (100 to

150 mm), depending on the die conﬁguration, to makepunch and die changes easy.Having alonger

cylinder does not change the previous ﬁguresbecause the working stroke willonly be the previously

mentioned 0.160 in. (4 mm).

FIGURE 3.15 Small hydrau lic “C”notch ingpress

(Courtesy of Delta Engineering Inc.)

Roll Forming Handbook3 -14

3.5 Information and Dimensions for Press/Die Purchasing

and Installation

Figure3.16 (Table 3.1 and Table 3.2) provide the information that press users should obtain from or

provide to suppliers when presses/dies are ordered. These data are in addition to the “basic information”

required for press installation such as foundation requirements, location and type of foundation bolts,

powerand air requirements (conduit, wire and air pipe sizes, locations), and others.

Height adjusting,leveling screws, and lateral adjustment should be used at the initial alignment of the

press. Oncethe press is properly aligned and installed, it should not be moved up or down or sidewaysto

suit the dies and product. All rolls, product pass-lines, cutoffand other dies should be made to ﬁt the

press accurately.When proﬁles (dies) are changed, past mistakesshould not be corrected by moving the

press or adjusting the die shut height, but by reworking the dies. These recommendations are obviously

not applicable to special cases such as moving presses to suit curved(swept) parts.

3.6 Rotaryand Other Cutting, Punching Equipment

3.6.1 Rotary Devices

Althoughrotary devicesare not called “presses,”they can perform manypress operations. The advantages

of the rotary devices are that they are considerably less expensivethan the presses, they can run at anyroll

forming speed, and can repeatedly produce the same pattern within tight tolerances. The disadvantages

are their limitation in handling thick material, and it is very difﬁcult to adjust the patterns althoughthey

are repeatedly uniform.

Rotarydevices usually consist of two working rolls. It is difﬁcult to synchronize the surface speed of the

rotary device with the speed of the roll forming mill. Therefore, in almost all cases, the rolls are rotated by

the strip,which is pulled by the rollforming mill. If the rotary device has to be driven because of the high

torque requirement, then usually one loop is ahead of the dev ice and one is behind it. The loop controller

will adjust the speed of the rotary device or the mill.

To punch, notch, or emboss the lead end of each coil,the pull-throughrotary device must be

temporarily driven. In small, light gauge applications, the lead end of the strip can be hand-cranked

until it is engaged between the rolls in the mill. If larger torque is required, the rolls of the rotary device

can be motor-driven at aslow speed. Throughanoverriding clutch, once the end of the strip is caught

by the rolls of the mill and is pulled at afaster speed, the rotary device runs freely.

POST

SLEEVE

BASE

HEAD (RAM)

TOP DIE RAIL

BOTTOM DIE RAIL

FIGURE 3.16 Press data and information required for die design.

Presses and Die Accelerators 3 -15

Most rotary deviceshaveseparate stands, because the punching,notching,orembossing forcesare

larger than the forces required to form the section in the mill. Furthermore,the stands havetobemore

precise and they are usually not driven by the drive train of the mill.

Rotarydevicesare usually mounted at, or close to,the entryside of the mill. With afew

exceptions, the male punch and the female die must work together in aprecise position. To obtain

TABLE 3.1 Typical Press Data (see Figure 3.16)

Roll Forming Handbook3 -16

this preciseposition, sometimes within 0.001 in. (0.025 mm) accuracy,the upper and lowerrolls are

geared to each other with zerobacklash gears. The “mating” position of the tworolls, at least in

one position, must be accurately marked to ensurethat wheneverthe tworolls are taken apartfor

maintenance, rework, or other purposes, they will be assembled back together again exactly in the

same position.

The rotarypunching and notching processes, the most frequently used rotaryoperations, are described

in Chapter 4.

TABLE 3.2 Typical Press Information for Die Speciﬁcations (see Figure 3.16)

Presses and Die Accelerators 3 -17

3.6.2 Cutoffwith RotaryDevices

Several methods are used to cut strips or products to length withrotating devices:

Scoring:Shortpieces can be cut at highspeed to uniform length with rotary scoring. Asharp

rotating die mounted in the full length of the roll cuts into the strip at each revolution. The

opposite roll can haveaﬂat surface. The distancebetween the tool and the smooth roll will dictate

the reduced metal thickness, which still holds the strip together.This thickness is sufﬁcient to

provide continuity to roll forming, but when it is subjected to up and down or sideways bending

by deﬂectors at the end of the mill, the parts break off(Figure 3.17). The disadvantage of this

process is that the brokenedges can be sharp.

Rotarycutting and cold working:Byapply ing this method, the author had designed and built

equipment to cut 2in. (50 mm) pieces wi th both ends ﬂattened (Figure 3.18). The line speed was

750 ft/min (230 m/min), producing 4500 pc/min.

Foranother application, the author designed and used arotary “propeller” type of device

(Figure 3.19). This device cut two separate strands into 1-in. long pieces, running at 750 ft/min

(230 m/min), thus producing about 18,000 pc/min (equivalent to 300 pc/sec or over amillion

pieces per hour). The speed of forming was reasonably uniform and the rpm of the propeller

see detail B

scoring

insert

strip

detail A

Top

Deflector

Bottom

Deflector

see detail A

Scoring

rolls

strip

Roll Forming Mill

Deflectors Broken off

parts

detail B

FIGURE 3.17 Shallow products can be scored at the entryend of the mill (A) and broken offatthe exit end (B).

FIGURE 3.18 Rotarycutter cuts and ﬂattens both ends of the productinone operation at arate of 24,000 pc/min

Roll Forming Handbook3 -18

cutting blades were mechanically governed by the rpm of the forming rolls. This arrangement

provided close tolerances on the 1in. long pieces.

It was relatively easy to apply this process to rigid products, such as highstrength, low

elongation material having across-section of 0.001 in.

(0.65 mm

Cutting with arc motion:Flying cuts can be made with blades moving in an arc motion (Figure3.20

and Figure 4.103). The rotation of the top and bottom blades is tied together with the zero

backlash gears or linkages. The back and forward movement can be accomplished with adie

accelerator or with linkages. The center of the top blade holding-arm is in aﬁxedposition, whereas

the center of the bottom blade holding-arm can be moved up and down or both centers can be

movedupand down.

Before cutting ,the bottom center is in the upper position. When the signal is given for cutting,

the twoblades swing for ward, overlap each other,and cut through the material. When the cut is

ﬁnished, the center of the bottom blade holding-arm movesdown, both blades swing back to the

starting position without touching the material, and then the center of the bottom blade holding-

arm moves up to the starting home position. One disadvantage of the arc movement is the

restricted depth of the cut. Therefore,this system is used only to precut strips. The arc motion can

be substituted with abetter pattern by using proper linkage systems.

Cutting with parallel linkages:Another system, patented long ago,utilizes parallel-moving plates,

acting like connecting rods (Figure3.21). The twoconnectingrods shown in the ﬁgure represent

the die plates to which the cutting dies (blades) are attached. This system eliminates the angular

ﬂotation of the blades described in the previous section; thus, it can be used to cut ﬁnished

products, such as tubes, at highspeed. However,owing to the rotary motion, the speed of the die

plates in the horizontal direction changes during engagement.

products

CUT

cutting blades

stationary

blade

FIGURE 3.19 Rotarypropeller type of cutoffequipment.

CUT

Home position

115432

Home position

FIGURE 3.20 Arc motion ﬂying shear.

Presses and Die Accelerators 3 -19

Rotarycutoffwith up and downmovement:The simple rotary cutter mentioned in the ﬁrst partof

this section cuts all pieces to the same length at everyrevolution (or cutting two to threeormore

pieces per revolution if two, three, or more blades are installed in the rotating body).

The simple principle of this modiﬁed system is that after cutting,the tworolls are separated

at adistancejust slightly morethan the material thickness (Figure3.22). However,the upper and

lowerrolls remain connected together all the time with zerobacklash gears. In the upper

position, the rotating blade is not in touch with the material and is not cutting.When acut is

required, asignal movesthe top rolldown, and the blades overlap each other at one point

during rotation. The length of the cut piececan be adjusted by slightly reducing or increasing

the rpm of the two rolls while they are not in contact with the material. Just beforecutting,the

rpm of the rolls is changed to meet with the speed of the strip.The top roll moves down,

completes the shearing,and then moves up again. Proper programming allowschanging the cut

lengths without stopping the process.

3.6.3 Flame, Plasma Jet, Laser,orOther Cutting Methods

Cutting the product by melting is rarely used because of the slow cutting speeds. If this method is applied,

it requires good speed matches. Nonmatching or ﬂuctuating speed will result in bad cuts or incorrect

cutting pattern.

3.7 Flying Die Accelerators

3.7.1 Pneumatic Die Accelerators

Accelerating aﬂying die from its standing (home) position to the speed of the strip by apneumatic

cylinder is arelatively inexpensive method (Figure 3.23). However,the accuracy of the pneumatic die

accelerators is relatively loose.

In most cases, alength or awhole location-sensing device provides the signal for the operation. The

signal activates asolenoid valve that supplies air to the die accelerator cylinder.The cylinder accelerates

the die, which slides on the die rails. When the die startstomove, it actuates the press.

12341

CUTHome position

Home position

FIGURE 3.21 Schematic representation of aparallel linkage ﬂying shear.

down

cut

FIGURE 3.22 Rotar ycutter combined with up and down movement.

Roll Forming Handbook3 -20

Foroptimum performance, the ﬂying die should travel at the speed of the strip when the tool engages

the material, at about the center of the press. Once the tool is engaged, the die will travel with the speed of

the product(strip) until the operation is completed and the tool (cutoffblade) is disengaged. After that

point, the cylinder will decelerate the ﬂying die and, after amomentarystop,will return it to the home

position.

In reality,the die acceleration is not linear and it wil lnot be zero(constant speed) when the die speed

reaches the speed of the strip.Moreover, the speed of the die is also inﬂuenced by the changing friction

between the die rails and the slides, and by the ﬂuctuation in the pressureand volume of the compressed

air.Furthermore, the speed of the ﬂying die is not measured accurately.Inmost cases, it is adjusted by eye

to match the speed of the strip.These factors, superimposed on other factors such as ﬂuctuating strip

speed, press stroke speed, accuracy of the length (position) of measuring device, and others, reduce the

tolerance.

The actual tolerances can varygreatly,but in most cases, ^ 0.25 in. ( ^ 6mm) can be achieved for

products running at about 120 to 180 ft/min (36 to 55 m/min).

The areaofthe die accelerator air cylinder is determined by dividing the calculated die accelerating

forcebythe available air pressure:

¼ A ¼

; hence D

4 F

and D ø 1 : 13

ﬃﬃﬃ

ð 3 : 4 Þ

Example

Let us say that the required force to accelerate the die is 800 lb (3560 N), and the available air pressure

is 75 psi (5.17 bar ¼ 0.517 MPa).

The required cylinder diameter willbe

D ¼ 1 : 13

ﬃﬃﬃ

¼ 3 : 69 in:; so select a4- in: diameter cylinder:

In metric, the same result will be 93.7 mm, so select a100-mm diameter pneumatic cylinder.

FIGURE 3.23 Air cylinder die accelerator.

Presses and Die Accelerators 3 -21