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Putting control in your bending operations.

Putting control into your bending operations

Press-brake operations can benefit greatly from systems and procedures that reduce setup time. As lot sizes become increasingly smaller, the need to eliminate setup time becomes critical. This article looks at some state-of-the-art equipment, control systems, software, and tooling that are helping to make modern bending operations more efficient.

There may be more controversy surrounding the press brake than any other commonly used machine tool. Nearly everyone has a favorite type or style. Mechanical, hydraulic, precision hydraulic, bed up, ram down, mechanical control, hydraulic control, electric control, CNC, DNC, front gages, back gages, air bending, bottom bending, wiping dies, crown control, shimming, rotating dies, die-clamping systems, bend up, bend down, single operator, multiple operators, no operator--you name it, somebody will tell you it's the only way to make a good part.

So who's right; and who's wrong? How can you sift through the claims and counter-claims and identify the best machine for your operation?

What it all boils down to is that every machine, every control, every gaging system, every feeding system, and every tooling system has strenghts and weaknesses. If you're making gutters, your needs are very different from the shop making precision electronic hardware. If you're dealing with rapidly changing parts in small batch sizes, your needs are very different from manufacturers who run the same part for days or weeks at a time.

Your product mix, your operator skill level, the consistency of the material you're bending, the type of equipment you have on hand, and, of course, your budget for new capital equipment will all go a long way toward determining the best solutions to your bending problems.

In an effort to make your job somewhat easier, we asked a fairly long list of suppliers to send us particulars on what they were offering in the way of CNC systems, computer-assisted programming, automatic back-and front-gaging systems, and robotic or related manipulative devices for blank holding and feeding. Many supplied us with comprehensive data on these and related bending technologies. Many did not.

With no attempt to act as a filter or judge regarding the merits of any or all of this equipment, we prepared this article to present a summary of what is now available to help in your precision-bending operations, based on data from the individual suppliers identified.

Cincinnati's Autoform

Cincinnati Incorporated, Cincinnati, OH, says its adaptive-control software enables the Autoform [R] CNC Forming Center to automatically compensate for variations in material strength and thickness during the bend cycle to produce the desired air-bend angle. The Force/Displacement Graph illustrates the sequence of events that take place during an air bend cycle. The adaptive Autoform control monitors and processes data as the ram proceeds through the cycle, to calculate an optimal reversal point for the desired bend angle.

The reversal position is based on a predetermined relationship between upper-die position, material thickness, material strength, die geometry, desired bend angle, and other factors--in conjunction with realtime measured force and displacement data. Accurate bends are produced by calculating a unique reversal position for each bend.

Measuring material strength and thickness, and calculating the reversal point to produce the required bend angle, result in first bends that are highly accurate, thus reducing setup time. Often first bends are accurate enough that parts can be used without further adjustments and will repeat consistently throughout a production run. Where greater accuracy is needed, a first-bend correction can be made. This is done by measuring the actual bend angle and entering the result into the control.

New part programs are made rapidly by simply selecting the adaptive-angle reversal mode, inputting specific bend sequences, and recalling a predetermined adaptive file for the desired tool set. Subsequent use of the part program requires only simple recall of the program and adaptive file from internal memory.

Increased accuracy

As stated earlier, variations in material are compensated for by adaptive technology. This results in improved accuracy and consistency during a production run. Adaptive air-bend tolerance will approach [+ or -]0.5 [degrees] while traditional air-bend tolerance on gage material generally runs [+ or -]1.5 [degrees].

The relationship between upper-die position, material thickness, material strength, die geometry, desired bend angle, and other factors must be developed for each specific set of dies. The teach mode consists of 12 bends--four different angles in three thicknesses. The operator must measure the thickness and bend angle for each sample and enter the data into the control. This is done once, and subsequent use of the die set requires only that the operator enter the teach-mode data into the control by specifying the adaptive file name.

A single teach mode for mild steel yields accurate results for mild steel, stainless steel, and HSLA steel. For best first-bend results, aluminum requires a separate teach mode. After completion of the teach mode and verification that all bends are correct, the data are saved in the internal memory of the Autoform control.

Curing deflection

Deflection in the press-brake ram and bed causes bend-angle variations along the length of the bend. For accurate forming applications, Autoform must be equipped with Auto Crown [R], which compensates for the effect of bed and ram deflection. Adaptive technology benefits cross-sectional accuracy, while Auto Crown benefits longitudinal accuracy. The combined features provide total part quality control.

Six-axis back gage

Cincinnati says that complex shapes can be formed efficiently in one handling of the Autoform press brake with its programmable six-axis back gage. Independent programming of each back-gage finger permits virtually any part configuration and bending sequence to be formed.

X-axis gage data can be entered into the control through either a standard or independent mode of gage operation. For ease of first-time setup, the control automatically transfers programmed left-gage "R" or "Z" axes coordinates into the right-gage program to achieve back gage symmetry, unless manually overridden by the operator.

Alternately, each back gage finger can be positioned left-to-right and up-and-down using a convenient hand-held operator control, supplied as standard. A teach mode allows these position settings to be transferred directly into program memory.

CNC press brakes

Data in this section have been condensed from, "Precision Hydraulic Press Brakes and the Evolution of the Multiaxes CNC Controlled Hydraulic Press Brake," a paper by Jose Pacheco, director of engineering, A Dias Ramos-Maquinas-Ferramentas Lda, and Stephen A Lazinsky, Comeq Inc, Baltimore, OH. In the section on CNC press brakes, Pacheco and Lazinsky point out that the use of numerical control on press brakes has increased tremendously in recent years.

It started with retrofit of a one-axis control for front-to-back travel of the back gage (X axis), and progressed with addition of control for ram depth to change the bend angle (Y axis). Today, users of press brakes want to control as many machine functions as possible; computer-controlled functions include: * Regulation of the stroke (amount of ram travel used for a given job). * Bending speed adjustment (control of oil flow). * Tonnage limitation (control of hydraulic pressure). * Timer control (dwell time or time the material remains under load). * Memory for library of tooling available. * Graphics. * Control review of part to be formed and bend sequence. * Communication with computers used in manufacturing.

To fulfill these customer requirements, ADIRA factory engineers studied possible machine designs. First attempts resulted in very complex and expensive machines. This was not acceptable; so research continued.

Further research resulted in design advances both in the operation of the hydraulic circuit used on hydraulic press brakes, and in the CNC unit available. This solution offered optimum use of the CNC with tremendous computer-power integrated with the press brake. This new system overcame many of the error-causing variables previously encountered and ensured excellent repeatability.

After several attempts, each more and more satisfactory, engineers finally arrived at the best coordination of CNC and hydraulic press brake from the different versions they had developed. Following is a brief, basic description of how this precision hydraulic press brake operates.

Each hydraulic cylinder is connected to a holding block to keep the ram in proper position when the machine is idle, as well as to a servo valve. The hydraulic pump feeds both servo valves. A pressure-regulator block permits flow to the tank when idling and in case of overload.

On each end of the ram there is a high-resolution linear encoder measuring distance between the bed and ram. This also provides measurement of the actual distance between upper and lower die.

The CNC monitors all critical parts, including pressure regulator block, servo valves, holding blocks, and linear encoders. The computer controls the distance between bed and ram at all times.

If the ram tilts for any reason (for example, if there is a change in the gibbing or an off-center load), the CNC will immediately control the pilot stage in the servo valves, which in turn will amplify the CNC information to correct the tilting error by changing oil flow to the cylinders, bringing the ram back to level.

To assure proper bend angle, the CNC must identify the corresponding position of the ram. Once the control identifies this position, it will automatically decelerate the ram according to machine characteristics, thus ensuring accurate stops.

The design of the machine provides repeatability (stopping of the ram) to within [+ or -]0.0002" to [+ or -]0.004". The stopping point (bending depth adjustment) is independent of the bending length, hydraulic pressure, oil temperature, and load location, because measurement is between upper and lower dies. In this system, adjustment of bending depth (bending angle) under load is maintained. The control allows changes on ram position in increments of 0.0004".

The hydraulics and powerful multiprocessor CNC and its software on this new CNC precision hydraulic press brake allow it to: * Accept preprogramming of all tooling sets available to the operator. * Accept programming of the bending angle as a function of material type, thickness, and tensile strength; and the tooling used on the jobs. * Accept programming for ram tilt to form conical and tapered sections. * Automatically calculate tonnage and limit hydraulic pressure to ensure full protection against overload. * Accept programming of proper bending speed; for instance, on pieces such as a door with very long flanges turned toward the operator, it is important to slow the speed to improve machine safety and prevent whip-up of the part. * Accept programming of return speed up to the pinch point. For instance, as in the previous example, to avoid a dangerous release of the workpiece when the ram is returning. * Accept programming of the dwell time to compensate for spring back. * Accept optimizing of stroke length (short when there is no need for workpiece removal, and long to take out big workpieces). * Supervise safety functions of the machine as specified by the user. * Set crowning of the bed.

The CNC machine can be interfaced with thickness gages for automatic correction in the stroke when there are thickness deviations in the material being bent. By replacing the CNC with a Direct Computer Numerical Control (DNC) with graphics, it can be linked to a personal or central computer, which programs, does the work preparation, and sends all the programs for several CNC-controlled machine tools. The unit can even work with CAD/CAM systems.

Electronic hydraulic positioning

Essay International Inc, Elk Grove IL, is the US representative for Darley EHP CNC press brakes and related controls with DNC software. The CNC press brake (EHP-standard version) initially was developed to eliminate technical limitations of conventional mechanical or hydraulic press brakes. One of the greatest disadvantages of a conventional hydraulic press brake is its mechanical depth stops, which do not allow accurate adjustment of the press brake under pressure. This leads to excessive test bending and wasted setup time.

By using a microcomputer in combination with a qualified hydraulic system, Darley developed a press brake using principles that would provide the optimum in reliability, accuracy, and ease of operation. The Darley EHP bed acts as the reference point for depth adjustment, as well as for parallelism during ram movement. Ram movement is measured continuously, checked, and, if necessary, corrected in relation to the bed.

For that reason, the EHP is equipped on both ends of the ram with a very accurate electronic measuring system. This system continuously measures position and movement on both left and right ends of the ram. Position readings are loaded into the microcomputer, which uses them for controlling the hydraulics and checking feedback.

Continuous computer control of hydraulic valves and feedback results in high accuracy during movement of the press ram. Repeatability of [+ or -]0.0004" for depth positioning of the press ram is attainable 100 percent of the time. Only tonnage needed to produce the desired bend is developed, regulated by electronic control of hydraulic proportional flow valves.

Graphical controller

The optional EHP unit DA 58 is a very sophisticated multiple-axis, multiple-function control unit. The high-resolution screen provides clear display of all information. Using graphics, the DA 58 shows the operator the exact bending sequence in full color. Features of the control include: * 12" high-resolution RGB screen. * Real-time bend sequence. * Multiaxes control, plus auxiliary functions. * Large working memory, plus 3.5" floppy disk drive. * Graphic tool programming with memory. * Blank length calculation. * RS 232 C serial interface with two-way communication. * Priority selection for absolute dimensions. * Production-time calculation. * Programming codes ISO/R 1058-1969 (DIN 66025).

An optional software package (Pro-File) in ROM for CAD/CAM is available with the DA 58P. Pro-File software also is available for use on personal computers; and the PC can be connected to the DA 58, providing full graphics display.

The Pro-File software package allows you to create the product drawing and also the tool shape on the computer screen. If the product is designed, all necessary parameters will be calculated and transmitted to the CNC unit on the press brake. The software package has: clear instructions, high calculation speed, user friendliness, and extensive abilities to design specific tooling and machine shapes.

Chasing rainbows

Niagara Machine & Tool Works, Buffalo, NY, calls its Rainbow the ultimate in press-brake technology. Niagara's Rainbow Line uses Cybelec's DNC press-brake-system control with CRT graphics display and a number of operator-friendly features to provide high productivity with minimal operator training. In addition to fully programmable Y-axis stroke control, all Rainbow press brakes feature complete electronic regulation and synchronization of the two cylinders ([Y.sub.1] au [Y.sub.2]).

Parallelism is monitored directly by a computer-controlled servo valve for each cylinder that maintains [Y.sub.1] and [Y.sub.2] ram level to within [+ or -]0.0004" ([+ or -]0.01 mm). Bidirectional, linear encoders at each end of the ram provide continuous position feedback. Both cylinders are totally synchronized without use of a steel tape.

Accuracy and repeatability within [+ or -]0.0004" ([+ or -]0.01 mm) of the bottom-stop setting of the ram are assured by this latest press-brake-control system. With this degree of machine accuracy available, springback of the formed material becomes the primary factor controlling angular tolerances.

Different degrees of springback result from variations in ductility, hardness, and internal stresses that occur even within one sheet of material. Springback also will change from the different grain direction encountered when forming two adjacent edges of the same blank. Other factors that affect the bend angle obtained are variations in tensile strength and material thickness.

Cybelec DNC

Features of this multiaxis control include the following: * Separate or simultaneous positioning of two to eight digital axes and up to four analog axes; the DNC can simultaneously control the press brake and a feeder robot. * 12" monochrome green CRT display; graphic and alpha-numeric display with double-height characters. * Graphic simulation for program test. * CAD/CAM functions. * Overall blank-length calculation. * Direct programming of bending angle, with calculation of penetration, according to material and tool data. * Calculation of bending force with correction possibility. * Sequences and products may be inserted, copied, skipped, and deleted with automatic justification of the program. * Relative and absolute programming. * Inch-metric programming. * Display of product quantity required and number already produced. * Display of product numbers stored in each cassette with indication of the free sequences. * Product search according to criteria; e.g., drawing number, sheet thickness, tools. * Subprogram for big-radius bending according to desired radius and number of bends. * Direct production control by the DNC. * RS 232 or 20 mA data link to connect the DNC to a personal computer, central computer, or digital micro-cassette unit.

Flexible bender

Fagor America Inc, Elk Grove, IL, sends details about what it calls a flexible bender; a new concept in sheet-metal processing. Data presented here are condensed from a paper by Felix Remirez Salinas of Fagor.

The recently developed Fagor flexible bender solves problems associated with conventional press brakes Briefly, these can be listed: * Bending is carried out in the open. This can lead to various undesirable effects. For instance, lifting the sheet to be bent can cause undue stress and deformation, resulting in buckling and sometimes even corrugation of the parts. Likewise, heavy weight in the blank can produce overbending in the area of contact with the bending die, which will be reflected in the final part. * Bending, even in NC press brakes, requires frequent change of dies in accordance with the configuration of bends to be made, which gives rise to significant preparation and downtime. * Part quality depends on the operator and his particular physical and psychical state. * For relatively wide parts, handling of the part can require two or more operators during loading, positioning, and raising operations for bending and removal. * Operations are repeated time and again, each wasting a small amount of time, which when added up, represents a significant loss in productivity. * Even on press brakes with NC stops, part accuracy depends, to a large degree, on the operator. * With many press brakes there is a definite lack of safety and ergonomic features.

The Fagor bender enables two parallel sides of parts with complex forms to be bent automatically without need for human intervention, and with high quality. The whole process is controlled by computer. This ensures the following: * No need for labor. * High quality parts; the computer calculates the right parameters, in accordance with the type of material and the thickness, to obtain a certain angle. * Any type of bend between 30 [degrees] and 120 [degrees], in either direction, up or down, can be obtained automatically without changing dies. * Two sides can be bent automatically in a computer-controlled sequential process.

How it works

The Fagor flexible bender consists of the following elements:

A robot-controlled blank manipulator, which takes charge of loading the length of flange to be bent to an accuracy of [+ or -]0.1 mm. Likewise it incorporates a 180 [degrees] rotation mechanism that enables two sides of the part to be folded automatically. When this rotation is carried out, a pneumatic holder descends onto the part to ensure that there is no sliding between the sheet and the rotator.

The grippers of the manipulator enable it to work with sides that have already been bent, even when prior bending results in a complex shape. These grippers allow a maximum part height of 85 mm (depending on the model chosen).

The bender itself. Two (patented) opposite pivoting mechanisms are mounted on a rigid shaped frame. Each of these mechanisms bends the sheet up or down. Control of these mechanisms enables different angles to be bent automatically without the need to change dies.

As is well known, there are three types of bend tooling. The wiper die gives great accuracy and squareness, but produces lines and scratches on the sheet surface. Open bending has problems as were outlined in disadvantages of conventional press brakes earlier. The tangential bend ensures extra care of the material at the cost of a certain loss of squareness. The advantage is that it permits bending of prefinished and delicate materials.

The Fagor flexible bender produces a semitangential bend. Bending movements are carried out hydraulically. Precise brushless motors move the pivoting system.

A hierarchical control system. This is based on the combination of a computer, a programmable control, and two pieces of precision positioning equipment. The link between the computer and the PLC is through an RS 232 serial connection. The computer does not just control parameters of the machine, but also incorporates a suitable production control. It's easy to use and does not require skilled personnel.

Peripherals. These include the automatic blank loader and the part remover.

Special applications

The Fagor flexible parallel-side bender has been specially designed for the metal-furniture, electronics, and electric-furniture sectors, where statistics indicate that 75 percent to 85 percent of parts have only two parallel bent sides. For those cases in which it is necessary to bend four sides of the part, Fagor has designed a more complex machine.

In the latter case, the rotating system is different in that it is capable of dealing with angles of 90 [degrees], 180 [degrees], and 270 [degrees]. The bending die also is different in that it is expandable so as to be able to bend both the long and the short sides of the part. [Graph Omitted]

PHOTO : The 135-ton Cincinnati press brake has 10 ft clearance between housings, and is equipped

PHOTO : with an Adaptive Autoform [R] CNC system for state-of-the-art forming.

PHOTO : To achieve a high level of plate manipulation, Darley offers this PVS-02 plate support

PHOTO : system. The two sliding support arms can be provided with optional suction pads.

PHOTO : The Niagara Series HBR is available in 100- to 350-ton capacities. HBR press brakes can be

PHOTO : equipped with optional automatic crowing to compensate for bed and ram deflection.

PHOTO : The Fagor flexible parallel side bender is specially designed for the metal furniture,

PHOTO : electronics, and electric furniture industries.
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Article Details
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Author:Dobbins, Donald B.
Publication:Tooling & Production
Date:Feb 1, 1990
Previous Article:DNC - implementation of a basic concept.
Next Article:MEs in the '90s: pulling together on the shop floor.

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