Rolleri Bending Tips Cuprins :

1. Press Brake Components – page 1 2. Crowning system – page 4 3. Springback – page 5 4. Tool capacity – page 6 5. Tools material – page 7 6. Tools – Heating Treatments – page 8 7. Tools - Precision, length and interchangeability – page 9 8. Tools – Connection – page 11 9. Tools – Dimensions – page 12 10. Bending types – Air bending – page 13 11. Bending types – Bottoming – page 14 12. Bending types – Coining – page 15 13. Upper tool clamping – page 16 14. How to use the bending rule (bending table) – page 17 15. Calculation formula for the “V” and the bending radius – page 22 16. Punch Tip Radius – page 23

Distribuitor: S.C. SM TECH S.R.L.

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Bending Tipps

Lesson #1

PRESS BRAKE COMPONENTS Sheet metal bending involves pieces with a big ratio between area and volume. Most of the sheet metal

bent is less than 6 mm thick. Press brakes are the machines used to bend sheet metal, in most cases they are hydraulic machines with numeric control, which can bend different material thickness. They are very common in the metalworking field to work iron. Their working is based on the meeting of a higher pointed with a lower concave part and on the according deformation of the sheet metal which is between them.

In order to define a common lexicon for this handbook, let’s analyse briefly the press brake components.

In the draft below we have shown the components that we will analyse in the next chapters. Draft 1 highlights components of press brakes with Promecam connection, draft 2 press brakes with axial connection.

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SM TECH SRL BENDING TIPS

Bending Tipps

Lesson #1

Let’s define now the press brake components:

• MACHINE STRUCTURE is the frame that supports all mobile parts and it usually has a “C” shape in the back.

• NUMERIC CONTROL is a common feature of tool machines. Numeric control machines or NC machines are those tool machines whose working is driven by a computer integrated in the machine. This computer rules movements and functions of the machine according to a specific working programme. Thanks to an encoder, the numeric control measures the positions of its mobile parts and activates actuators, (motors, hydraulic pistons or other), which control the machine movements and position the tool to be worked in a specific point arbitrarily chosen. The movement of the machine measured by an encoder and controlled by a computer in order to position the machine in a certain point of the available stroke is called controlled axis or simply machine axis.

• HYDRAULIC CYLINDERS determine the movement of the upper beam (or rarely of the bench), thanks to a piston pushed by pumped oil. There are normally two cylinders which move on axes Y1 and Y2.

• BACK GAUGES are references for the positioning of the sheet metal. Their movement, ruled by the numeric control, follows the working programme put into by the operator.

• BENCH supports the press brake table where the operator mounts dies.

• UPPER BEAM is the upper part of the press brake where the operator mounts punches

• STROKE indicates the maximum allowed movement along the Y axis. This dimension is extremely important to verify the possibility to mount high punches. In such cases the operator has to check daylight, height of the die until the end of the vee, punch, sheet metal and machine stroke

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Bending Tipps

Lesson #1

• The main AXES of a press brake are highlighted in the image below. In this case the press brake has 6 axes (Y1-Y2-X-R-Z1-Z2), but press brakes with different features are in the market. During bending phases you shall check axes Y1 and Y2, because a difference of 0.05mm in a 3m machine would affect the bending by 2°. Therefore, you shall check the alignment of the piston before bending by setting “0”(zero) on the machine. In case of problems, you shall check the surface of the upper and of the lower beam, intermediates and tooling, as wear or deformation can modify the Y axis.

• INTERMEDIATES in press brakes with Promecam connection fix upper tools to the upper beam. They can bear 100 T/m and there are 5 intermediates every meter, so every intermediate can bear 20 T. In order to avoid bending problems, you shall preserve contact surfaces with the machine and with the tools. Intermediates can be provided with a wedge to make the crowning (see related chapter) and its regulation can be made by qualified technicians only. Therefore, you shall numerate the intermediates in case you need to remove them and in order to mount them in the same position. If you need, you can mount tools reversed without changing the bending axis, by mounting the clamp on the back of the intermediate.

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Bending Tips

Lesson #2

Eng. Marco Sala

CROWNING SYSTEM The crowning is a system which compensates the deformation of the press brake when bending. In fact, the elastic structure of the machine causes 0.15mm variation on the Y axis in the middle of the upper beam of a 3m press brake.

To compensate such deformation, the crowning creates an opposite force, so that the same force is applied along the sheet metal during bending operations. In this way, you avoid bending problems, such as curved profiles.

Intermediates with wedge create an opposite curve and represent a kind of mechanic crowning of the upper beam. The wedges have to be adjusted by skilled technicians, according to the press brake features, material to be bent and by using proper measuring tools such as comparators. This operation has to be carried out by sliding the comparator along the lower bench and measuring the height of the intermediate when the press brake is working.

Wedges are set up so that, starting from one side of the press brake, every intermediate goes down along the Y axis by some hundredths of mm; in the middle of the press brake, intermediates have to go up following the same proportions. To avoid any modification of the mechanical crowning, all intermediates should be numbered and kept always in the same order. This is possible by looking at the position of the screw in the intermediate, which reflects the wedge regulation and, therefore, helps identify any change in the crowning curve.

Modern press brakes are usually equipped with automatic crowning systems, such as a table with … supplied by a geared motor or a table with hydraulic cylinders with sensors connected to the CNC. Is this the case, the sensors monitor pressure changes and compensate the deformation.

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Bending Pills

Lesson #3

Eng. Marco Sala

SPRINGBACK Thin rolled materials present positive springback after bending, which is a kind of deformation, as the material elasticity isn’t cancelled after this kind of stress, even if over the critical point. For this reason, the angle realised during bending will become bigger when the sheet metal is no longer under pressure.

Springback depends on:

• Sheet metal yield point (directly proportional). The harder the sheet metal, the bigger the springback.

• Deformation speed (directly proportional)

• Bending type (see chapter on bending types)

• Sheet metal thickness (directly proportional)

• Bending radius. The lower the ratio between the bending radius and the sheet metal thickness, the lower the springback

• Bending along the rolling direction (lower springback), bending opposite the rolling direction (higher springback)

To prevent such phenomenon you can:

• Over bend, for example, you can bend mild steel 88° and obtain 90° thanks to springback.

• Increase the tonnage (coin)

• Bend for a longer time

Thin sheet metals have generally the following springback, which has to be compensated:

• Aluminium 1°

• Mild steel 2°

• Stainless steel 5°

Sometimes sheet metal present negative springback, therefore, it reduces the angle after bending. This phenomenon may be caused by the use of a punch with too small angle or radius or by higher tonnage during bending. In such cases you can see a mark all along the bending line.

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Bending Pills

Lesson #4

Eng. Marco Sala

TOOL CAPACITY Tool break or deformation are common problems when using tools with too high tonnage. Breaks are easy to be identified but deformation sometimes can’t be seen with the naked eye but it affects the bending quality. For example, a lower tool determines a wider angle in the bending.

A lot of CNC calculate automatically the tool capacity and stop the press brake in case you exceed the tonnage, but others do not, so the operator has to calculate the maximum tonnage to be used. Consider that the operator himself can bypass the CNC by inserting incorrect parameters of the tools mounted, for example, by increasing the length of the tool. In this case, the tools runs the risk to be severely damaged. In other cases, the CNC declares of being applying a few tons but actually they are much more, as the press brake also applies the weight of the upper beam.

Therefore, the press brake operator has to know the capacity of each tool, to keep them for 1 million bendings, and to be aware that the length of the tool involved in the bending is the only important parameter to calculate the maximum tonnage.

In order to clarify this concept, look at the following example:

Ø Maximum tool capacity = 60 T/m

Ø Length of the sheet metal to be bent = 200 mm

Ø Max. tonnage to be applied to this sheet metal piece (60/1000)x200 =12 T

As you can see, the length of the tool mounted on the press brake is not important, but only the surface that actually works!

A simple rule: a tool with max. capacity 60 T/m can bear max. 0,6 T/cm

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Bending Tips

Lesson # 5

Eng. Marco Sala

GENERAL INFORMATION ABOUT TOOLS – MATERIAL

During bending operations, a tool has to support different forces; a tool mounted on CNC machines may be used for 3,000 work cycles a day and therefore about 1,000,000 a year. The strength and resistance of a tool are of prior importance to support such forces and require high quality and a specific heat treatment. There are two different types of high temperature heat treatment: induction hardening and quenching. The description “hardened tool” or “quenched tool” depends on the treatment used.

From the analysis of such forces, depending on some factors like type and thickness of the sheetmetal, the profile to be realised and the shape of the tools, we can choose the most suitable type of tool steel and heat treatment. C45, 42CrMo4 are the most common types of tool steel, whose features will be explained as follows.

42CrMo4 - Tool steel to be quenched and hardened made of chromium and molybdenum. Its homogeneous molecular structure can easily support the stress caused by induction hardening, making it particular suitable to be hardened.

It is normally supplied after annealing (with max. strength 82 Kg/mm2) or quenching (with max. strength 90 ÷ 115 Kg/mm2 ). Thanks to its high resistance after quenching, it is used for tools that have to support high tonnage.

C45

Tool steel with max. strength 60 ÷ 75 Kg /mm². It is normally supplied after quenching, which increases the max. strength to 80 ÷ 85 Kg/mm² and makes it suitable to support specific forces. This tool steel can be easily induction hardened and guarantee high resistance of the hardened surface.

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Bending Tips

Lesson # 6

Eng. Marco Sala

GENERAL INFORMATION ABOUT TOOLS – HEATING TREATMENTS

In order to improve the mechanical features of tools, they undergo heating treatments like quenching and hardening.

Quenching: it’s a heating treatment consisting in heating and subsequently cooling steel to decrease the internal stress of the material. During the heating process martensite is produced, which has got a very hard structure and high ultimate tensile strength but low resilience. For this reason the material can be easily broken after one hit; to avoid such phenomena, steel is then tempered by controlled cooling. Cooling speed during tempering is fundamental to determine the residual stress of the steel, as the slower the cooling phase, the weaker the residual stress. Steel types which can undergo this treatment contain 0,4-0,6% carbon and, therefore, are called quenched and tempered steel types.

Hardening: This treatment has the aim to increase the material hardness and it consists in the steel heating to a certain temperature and in the subsequent fast cooling. The hardness reached thereafter by the tools is measured by the Rockwell test, which determines hardness by measuring the depth of penetration of a conical indenter (HRc) or of a spherical indenter (HRb) in the material.

Induction hardening: it’s the most common heating treatment for press brake tools and, as a surface treatment, it affects the external layer of the tool. This type of hardening takes advantage of the principle of electromagnetic induction: by placing a conductive material (a coil) into a strong alternating magnetic field, the coil is heated and then rapidly cooled by a flow of coolant. Induction hardening creates very hard surfaces, resistant to wear and fatigue with no changes in core toughness.

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Bending Tips

Lesson # 7

Eng. Marco Sala

GENERAL INFORMATION ABOUT TOOLS

PRECISION, LENGTH AND INTERCHANGEABILITY

In the past press brake tools were produced in one piece only, i.e. in the same length of the press brake or of the profile to be realised. Such tools were planed, as hardening and grinding would have deformed the tools due to their length. As tools couldn’t be ground, their precision was quite low, about 0.1mm per meter.

Thanks to new technologies, press brake tool precision has been improved. Today, tools are produced in shorter pieces hardened and ground, which can guarantee higher precision (0.01mm per tool) and have better mechanical properties than in the past. Press brake tool length varies according to their standard, e.g. Promecam tools are 835mm long.

Segmented tools have several pros:

• Modular length, so that customer can buy tools to compose the total length they require

• Easy to be handle, therefore operators save a lot of time in equipping the press brake

• Savings, as only worn or damaged tools have to be replaced and not the total machine length

• Possibility of stage bending, whereby the operator can equip the press brake with different sets of tools and realizing a profile without changing the tools

Precise tools and their perfect interchangeability and alignment are fundamental to take advantage of the modern press brake potential and to guarantee high quality bending and repeatability.

Thanks to grinding machines, press brake tools can be produced with a radius in the die V and on the punch tip. The radius enables bending uniformity without marking the sheetmetal, as the press brake CNC knows exactly the contact point between tools and sheetmetal and therefore can calculate precise bending parameters.

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Bending Tips

Lesson # 7

Eng. Marco Sala

Since the company was set up, Rolleri SpA has been improving its quality system, which represents the pillar of the company development and service.

This allows Rolleri SpA to:

• Supply products that always meet customers’ needs

• Improve customer satisfaction

Thanks to its continuous efforts to improve its quality system, Rolleri SpA obtained the ISO9001 certification in May 1999.

Rolleri SpA always pays high attention to the production process of its tools, which are made of certified steel by high-tech machines. At Rolleri every single tool produced is traceable, i.e. people can check the steel lot whereby the tool has been produced, the dimensions checked by our operator, when it was produced and the resources involved in its production process. To trace its story, every tool produced since May 2002 is marked with an identification number.

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Bending tips

Lesson 8

Ing. Marco Sala

GENERAL INFORMATIONS ABOUT THE TOOLS – CONNECTION

In the past the choice of the pressbrake machine defined the choice of the typology of the tools. Every pressbrake manufacturer commercialized tools whose connection was expressly studied for their own pressbrake machines. This system allowed to fidelize the customer to the manufacturer and to induce the customer to the purchase of new pressbrakes from the same manufacture, in order to exploit the tools he already had.

However, the foundation of companies specialized in the pressbrake tools manufacture, has leaded in the time the pressbrake manufacturers to leave this market mentality in order to comply with customer’s wishes. The spreading of high quality tools at a more reasonable price has allowed to proceed towards the standardization of the same inducing the manufacturers to conform to.

In particular, the tools produced by the french society Promecam have established themselves as the new standard of master for the bending sheet. The Promecam connection, usually mounted on press brake machines equipped with intermetiate pieces, forecasts for punches a connection’s height of 30mm with a base of 14mm, the tool’s tip at is at a half of this base giving the possibility to turn the punch with double clamp. The standard dies have a base that can be both of 60mm and 90mm.

In the pressbrake tooling market axial connections have established themselves, especially during the bending of high thicknesses and when a tonnage over 100Ton/m (limit of promecam connections) becomes necessary. These tools, usually mounted on the top beam, give the possibility to reach higher tonnage. On the other side, they can not exploit the crowning system oft the promecam intermediates.

It becomes essential to know the standard connection mounted on its own pressbrake machine, in order to buy the correct punches and dies that can be mounted on the pressbrake machine.

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Bending Tips

Lezione #9

Eng. Marco Sala

GENERAL INFORMATION ABOUT TOOLS - DIMENSIONS The features of the profile to be realized and of the press brake deeply influence tool profiles. For this reason there are different shapes to make different jobs.

Profile

It is important to evaluate the overall dimensions of the sheet metal during bending to avoid collision and to work easier with a press brake. For example, gooseneck punches are suitable to realize U-shape profiles and pillar dies to make tight counterbending. In order to choose the most suitable tools, you should simulate all bending phases on the graph paper around a tool. Another important aspect is that the tool shape influences the maximum tonnage, therefore a gooseneck punch can bear fewer tonnes than a straight punch made of the same tool steel. Angle

Standard tools have from 26° to 90° and the angle choice depends on the profile to be realized. Therefore, tools with narrower angle than the angle to be realized less springback have to be chosen and mounted. If you have to bend stainless steel at 90° with 5° springback, you should mount tools with 85° or less. As we will see in the next lessons, the tool choice influences the profile and in case of dies, it also influences the maximum tonnage a die can bear (the lower the angle, the lower the maximum tonnage). Radius on the tip of the punch

The radius on the tip of the punch must always be lower than the internal radius of the profile and we recommend the following ratio: radius on the punch = internal radius of the profile x 2/3. In case a too small radius is used, an evident mark will be visible in the internal side of the profile.

Useful height

The useful height of a tool has to be considered accurately to avoid collision during bending operations. Punch useful height can be calculated as follows: total height – tang height, whereas die useful height coincides with the total height. During the tool choice you have to check that the press brake stroke enables the bending and that the daylight is bigger than the sum of punch height + die height + sheet metal thickness.

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Bending Tips

Lesson #10

BENDING TYPES – Air bending (Partial bending)

Air bending is the most frequently used type of

bending thanks to the good improvement of

new press brakes, which can manage better the

sheet metal springback. This type of bending is

used when air is between sheet metal and die

vee. The name “partial bending” comes from

the fact that sheet metal gets in contact with

upper and lower tools in three points only

during the bending procedure.

Advantages:

• Air bending enables to realise a big angle range with tools with acute angles. For example, you can use punch and die at 30° to realise profiles with any angle from 30° to 180°

• Air bending is faster than other bending types. Springback is managed by going further deep with the punch tip in the die vee and making a closer angle, instead of increasing the bending force or the machine stand-by.

• The necessary bending force is lower than in other bending types thanks to the possibility to choose a wider die vee.

• The sheet metal is less marked by the friction with tools

• Tools undergo less wear

• Air bending enables to use a low tonnage press brake. Costs are lower and therefore air bending is a cheap bending type

Disadvantages:

• The angle precision by air bending is lower then by other bending types. The tolerance is ¾ of a degree (45')

• The bending angle isn't very precise. Actually the result on the tip is an ellipse

• As the sheet metal isn't yielded, springback is higher than by other bending types

• In case there are holes near the bending line, they will be deformed

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Bending Tips

Lesson #11

BENDING TYPES – Bottoming

During bottoming the punch reaches the bottom of the die vee and pushes the sheetmetal against the vee.

This bending type is suitable for precise profiles, as its accuracy is higher than air bending.

During bottoming sheet metal is pressed between upper and lower tools,

therefore internal radius is more accurate, sheet metal reaches its yield point

and subsequently springback is lower.

Tool choice is fundamental for bottoming, as you have to identify the best

angle for both punch and die and the expected springback to get the

required profile angle. Punch and die must have the same angle to get good

bending result.

Advantages:

• Good precision with low necessary tonnage

• Good bending repetition in case of big production series

• Low springback

• In case there are holes near the bending line, they are pressed between tools during bottoming,

therefore they won't be deformed as during air bending

• Tolerance is abou t half a degree

Disadvantages:

• Angle correction by further movement downwards of the punch is impossible, as punch is already

in the vee bottom

• Bottoming can be used for 88° and 90° bendings only

• Tool sets dedicated to a specific profile

• Aesthetics of the profile isn't very good

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Bending Tips

Lesson #12

BENDING TYPES – COINING

People may think that such a name is strange. Actually, coining means “creating metallic coins”, whereby

every single piece is identical to each other in form and dimension. For this reason, “coining” can be used in

the bending process to indicate a method to obtain consistently very

accurate results. To coin you need four to five times the tonnage required

for air bending, therefore you need heavy duty press brake and tools.

In coining punch and die must have the same angle required by the profile,

so, in case of a 90° bending, you have to use 90° punch and die and forget

about the springback. The die vee is smaller for coining than for bottoming and air bending and it’s five

times the sheet metal thickness.

We recommend you not to coin over 2mm thick sheet metal to prevent any damage to your press brake,

tools or sheet metal.

Advantages:

• Consistent result

• Very tight tolerance in the angle (1/4 of a degree)

• Possibility to bend sheet metal with big tolerance in the thickness

• The punch tip penetrates the material with high tonnage and eliminates the sheet metal springback

• Possibility to obtain very tight radii (half the sheet metal thickness)

Disadvantages:

• Press brake and tools wear quickly

• Sheet metal doesn’t look good after bending

• For any angle and internal radius of a profile you need the related punch and die

• You can’t coin over 2mm thick sheet metal

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Bending Tips

Lesson # 13

UPPER TOOL CLAMPING Every tool can be clamped into a press brake in a different way according to press brake connection. We can identify two different categories:

Tools clamped directly into the upper beam

Punches are inserted in a slot exactly the same as the punch tang and fixed directly into the upper beam. Such tool type is called “axial”, as the punch tip is along the same axis going through the middle of the tang (see picture on the right). The punch is fixed by clamps to the front side of the upper beam either manually, with two screws per clamp, or automatically with a pneumatic or hydraulic clamping system. The clamp length depends on the press brake type. Such tool type has got high capacity and therefore it’s suitable to bend thick sheet metal. The most well-known tools of this type are Wila, Trumpf, Beyeler, Gasparini, Colgar, LVD, Colly, Axial, American e Newton.

Tools clamped through intermediates and adapters

Punches are clamped to an intermediate thanks to a clamp, which is screwed to the press brake upper beam. Intermediates are never removed unless you need extra space to realize particular profiles. Standard Promecam intermediates are

100mm high and 150mm long and they’re mounted at 70mm distance to each other for a total quantity of 5 intermediates per meter. Intermediates with wedge are used to create manual crowning of the press brake.

If you need to mount tools with a different tang from the press brake slot, you can use adapters. They have the same tang of the press brake slot and their lower part gives the possibility to clamp tools with a different tang. Adapters are more and more used for old press brakes with old tang types to mount cheaper tools with different tangs, which can be easily found on the market. In case you decide to mount an adapter, you have to check if the daylight of the press brake is big enough.

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Bending Tips

Lesson #14

Eng. Marco Sala

BENDING RULE

The bending rule is a very useful tool for anyone designing or realizing an object made of sheet metal bent

with a press brake. The bending rule in a form of a table is usually stuck on the press brake or simply

included in its instructions of use (see image below).

Bending rules with an internal sliding table showing the same data have recently become more popular,

since they are easier to be used (see image below).

In this lesson we are going to explain the use of the bending rule with the sliding table and in the next

lessons we’ll investigate the formulas that lie behind this table.

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Bending Tips

Lesson #14

Eng. Marco Sala

The bending rule sets some parameters of air bending:

• Die vee opening

• Dimension of the shortest side that can be bent (minimum internal edge)

• Necessary tonnage per meter

• Internal radius of the bending

The correct use of the bending rule can reduce trials and errors during the bending process. The best way

to use it is starting from the upper part and going down analyzing every parameter given for the selected

die.

IMPORTANT: suppose to bend mild steel at 90° in air

Step 1 – Select the sheet metal thickness

In the first row you can find a box showing the sheet metal thickness. If you slide the

internal table you can select from 0.5 from 25 mm.

Step 2 – Choose the die vee (V)

Once you’ve selected the sheet metal thickness, the

bending rule shows five numbers in the second row.

They represent five possible die vee dimensions in

millimeters that can be easily found in the market.

These five vee dimensions guarantee the best ratio

between internal radius of the bending and necessary tonnage without stressing the sheet metal too much.

Among these five vee dimensions the best one is in the middle (i.e. in case you want to bend 3mm sheet

metal the best die is with V 25mm). Now you have to check which die is available and choose accordingly

(in case die with V 25mm is not available you can choose die with V 20mm). We always recommend

designers are informed about the die availability in their production departments and indicate the selected

die in the product drawing.

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Bending Tips

Lesson #14

Eng. Marco Sala

Step 3 – Check the minimum internal edge (B)

Once you’ve chosen the die vee, check the minimum internal edge for it,

which you can find in the third row. It’s expressed in millimeters and

represents the shortest side that can be bent at 90° in such vee because it

can lay on the die before bending (for example, the minimum internal edge

is 16.5mm for V 25mm). If the internal edge of the profile is shorter than

this, the sheet metal will slip into the vee and subsequently the bending will

be deformed. Product designers have to take this parameter into

consideration and should avoid holes and other details in the area

between the bending axis and the end of the minimum internal edge

on both sides of the bending.

If the internal edge of the profile is not long enough you have to

choose a smaller die vee to enable the sheet metal to lay on the die

before bending.

Example: the internal edge of the profile on the left is 78 mm.

Step 4 – Check the internal radius

Once you’ve chosen the vee die and checked the minimum internal edge,

you have to check the internal radius of the bending, which you can find on

the fourth row. The internal radius depends on the die (for example die

with V25 determines a 3.2mm internal radius) and not on the sheet metal

thickness. For this reason the choice of the vee is fundamental to influence

the profile features in terms of internal radius and, therefore, of sheet

metal development. Thanks to the use of the bending rule, product designers can dimension drawings

properly and calculate the right development. For example, if the drawings imposes 2mm internal radius,

the operator has to choose a die with vee 16mm.

Step 5 – Necessary tonnage

Once you’ve chosen the vee die, checked the minimum internal edge and the internal

radius, follow the arrow and find the necessary tonnage to bend 1 meter mild steel of

the selected thickness. For example, you need 23 T to bend 3mm mild steel in a die

with vee 25mm. In case your profile is 2m long, you will need 46 T (23 T x 2).

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Bending Tips

Lesson #14

Eng. Marco Sala

The same tonnage is required for bottoming. In case you want to calculate the necessary tonnage for other

type of sheet metal, divide the tonnage you’ve found in the bending rule by 420 (the average tensile

strength of mild steel is 420 N/mm²) and multiply it by the tensile strength of the sheet metal you want to

bend. Product designers have to calculate if the available press brakes are powerful enough to bend their

profile and, in case they aren’t, change the selected die. Consider that the bigger the die vee and the lower

the necessary tonnage. Furthermore, product designers have to check tool capability to avoid any damage

during bending.

Correction factors in case you don’t bend a 90° angle

If you don’t bend a 90° angle, you have to use correction factors listed on the bending rule, as the smaller

the angle and the longer the minimum internal edge. This is caused by the fact that you have to push the

sheet metal further down into the die vee.

To find the right minimum internal edge, multiply the value

(B) on the third row of the table by:

• B x 1,6 for a 30° angle

• B x 1,1 for a 60° angle

• B x 0,9 for a 120° angle

• B x 0,7 for a 150° angle

Example: in case of a 30° bending with a die vee 25 mm, the minimum internal edge is = 26,4 mm (16,5 x

1,6 = 26,4 mm).

Correction factors to bend stainless steel or aluminium

In case you bend stainless steel or aluminium, the different springback determines different internal radii

even if you use the same die vee. For this reason you have to use correction factors to find the right

internal radius of the profile.

• R x 0,8 for aluminium

• R x 1,4 for stainless steel

Example: in case you bend stainless steel in a die with vee 25 mm the internal radius is 4.48mm (3,2 x 1,4)

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Bending Tips

Lesson #14

Eng. Marco Sala

Differences between rule with sliding table and fixed table

Although they show the same values and can be used in the same way, the fixed table has a different

structure.

In the fixed bending table the first column shows the sheet metal thickness (s) and the first three rows on

the top the die vee (V), the minimum internal edge (B) and the internal radius (Ri). If you cross the sheet

metal thickness with the selected die, you can find the necessary tonnage per meter.

Knowing how the bending rule works is fundamental for designers to design their products properly and to

avoid bending problems to press brake operators.

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21

Bending Tips

Lesson #15

Eng. Marco Sala

CALCULATION FORMULA FOR THE “V” AND THE BENDING RADIUS In case you do not have the bending ruler, you can use formulas to find out the groove width and the radius of your profile.

Given that you are bending in the air, the width of the V would be calculated

by: V = t x 8 (t = thickness)

Choosing a die with a smaller V is best to get an inferior radius, this will decrease the springback but will increase the necessary tons and is likely to enervate excessively the material. There is no contraindication if you choose to expand the Vee over the recommended ratio.

It is important to consider that the die vee affects the bending radius. In fact the inner

radius on the piece of sheetmetal will have to be: r = V/8 The values derived from the equation r = V/8 apply to mild steel, while the same values have to be multiplied by the following correction factors to find the inner radius for stainless steel and aluminum :

• Stainless steel r = (V/8) x 1,4

• Aluminum r = (V/8) x 0,8

Therefore, the measurements in the design can affect the choice of the die and can force you not to use the ideal Vee. For example, in case you are required radius 2 for 1mm mild steel, the choice of the Vee will not be V = 8 but instead, V = 16 which will satisfy the designer 's requirement .

The rule “V = t x 8” enables a correct relation between the radius and the thickness, avoids sheet metal stress and enables good control of the springback. Keep in mind that:

Ø If the bending radius is too small, it can cause cracks and reduce the resistance of the sheetmetal to the mechanical stress. Normally the bending radius has to be greater or equal to the sheetmetal thickness

Ø The minimum bending radius acceptable varies depending on the nature and the state of the material, as well as the thickness.

Ø The minimum radius is greater for hard materials with high tensile strength.

Ø If you bend very thick sheetmetal you should use wider vee and therefore increase the inner radius.

As seen in the ruler, the choice of the V also affects the tons required to bend the sheet metal,so the wider the V , the lower the necessary tons.

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Pillole di Piegatura

Lezione N.16

Ing. Marco Sala

PUNCH TIP RADIUS

The previous chapter has explained how the internal radius of the product is determined by the die vee; however there are cases where the punch tip influences directly the final bending result. Press brake operators usually pay little attention to the punch tip radius and, for this reason, in most workshops there are punches with 0.8mm radius only, which are standard and therefore cheaper.

The internal radius of the bent product is influenced by the punch tip radius only when the latter is bigger than the internal radius deriving from the vee chosen. For example:

1 mm mild steel to be bent

Die chosen V = 8 mm, which determines 1mm internal radius of the bent product.

The internal radius of the bent product will be:

r = 1 mm in case the punch tip radius is smaller than 1mm

or

r = punch tip radius in case the punch tip radius is bigger than 1mm

Hence, the punch tip radius has to be chosen according to the outcome required. The ideal punch tip radius can be calculated with the following equation:

(R determined by the vee) x (2/3) = ideal punch tip radius

Thanks to this ratio, the punch surface is proportional to the required tonnage to bend the sheet metal; the punch tip doesn’t penetrate into the sheet metal and aesthetic problems or sheet metal cracks in the internal part of the radius are avoided. Furthermore, if the tip radius of a punch with acute angle is too small, the sheet metal may be almost cut by it and the angle may get smaller than required.

The above mentioned equation is difficult to be respected due to the lack of different bending tip radii in most workshops; however press brake operators should use the closest tip radius to the ideal one. Especially for thick sheet metal, which require high tonnage to be bent, operators have to use big punch tip radii to avoid sheet metal deformations, dimensions or aesthetic problems or cracks.

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