Motors & Drives Information

Motion control board. Motion card. Motion controller. Servo card. These are some of the names, along with motion control card, that are used to refer to a very interesting class of electronic devices that promise to play an increasingly prominent role in machine tool technology. Essentially, a motion control card is a special purpose set of computer chips, or microprocessors, on an integrated circuit board designed to be mounted in an enclosure that connects it with other electronic and computer devices.

Motion control cards are a basic building block of many machine tool control units. But you have to understand something about the way computer numerical control (CNC) systems are constructed to see how motion control cards fit in and why they are important.

CNC Architecture

Everyone knows that the houses we live in are built in many styles and follow many floor plans, giving us both variety and comfort. CNCs are also built in many arrangements and configurations for the same reason. Just as different kinds of families have various likes and needs, different kinds of machine tools have various control requirements. Like the architecture of our houses, the "architecture" of CNCs follows many different patterns.

In fact, significant new patterns have been emerging lately, and not only are they changing the way machine tools are integrated with their control units, but also they are affecting the performance of the machine toolfor the better. It is necessary to keep up with these changes because they present the machine tool consumer with new choices and options that did not exist just a few years ago. Perhaps the most talked-about of these developments is the concept called "open architecture"building the CNC with key hardware and software components openly available from various vendors and sources as opposed to a "closed" system in which access to these components is limited or "closed" to the end user. In a closed or proprietary CNC, only the CNC builder can provide replacement parts, modify the software, or reconfigure the components.

The single most important development that makes open architecture viable for CNC is the personal computer, the same kind of high-powered, low-cost computer that is found in almost everybody's home or office these days. In fact, it's their widespread use in homes and offices that has made PCs so cheap and powerful. Intense competition for the huge market that home and office PC users represents pushed down prices while driving up functionality, power and processing speed.

In not much time, PCs became fast enough and powerful enough to lend themselves to CNC applications. This happened just a few years ago. At that point, it was feasible for CNC developers to use PCs as the core of a machine tool control unit. The hardware was readily available and inexpensive as an off-the-shelf item. Software could be written for these PCs to handle CNC functions, including the user interface. In fact, with no special hardware to design, engineer, test and build, developers could concentrate their energies on software, with products easily customized for special or unusual applications or even to emulate conventional proprietary controls widely used on popular types of machines such as machining centers and lathes.

However, the PCs designed for home or office use are not designed to be hooked up easily to a machine tool the way they are designed to be hooked up to a printer or a telephone communications line. Standard PCs aren't made to run a machine tool. They don't perform certain functions that a machine tool requires.

This is where the motion control card fits in. A motion control card is a special purpose computer that fills in the gap between the PC and the servo drives that move the machine's axes. Some additional hardware may also be required, such as a special computer card to handle electrical input/output functions if the motion control card doesn't include them.

What The Motion Control Card Does

So in a nutshell, the job of the motion control card is to maintain the machine tool's servo loop. It computes the paths for the machine tool axes and commands the servo motors to maintain those paths.

In simplified language, the motion control card pursues this sequence: It receives position commands issued by the PC softwarethe "blocks" of a G-code program. It sets the parameters in terms of speed, direction, and distance for the moves needed to follow that path and calculates a series of commanded positions for each axis along the desired path at the desired speed. The motion control card then adjusts the signals to the servo amplifiers accordingly, such that the servo motors follow that path. To make sure the path is followed, the motion control card repeatedly checks the actual position of the machine's axes against the commanded position and makes adjustments to keep the difference as small as possible. That step closes the loop.

Dr. Jacob Tal, president of Galil Motion Control Inc. (Mountain View California) uses a particularly apt comparison to explain the function of the motion control card. He likens it to the human brain and central nervous system. When a person's intellect decides to do something, let's say, pick up a cup, the brain and nervous system must respond by coordinating body movements to accomplish this task. They tell the muscles what to do. As the muscles in the arm and hand cause them to move toward the cup, the eye guides them to the right spot to grasp it.

By analogy, the PC host issuing position commands is the human intellect. The motion control card which reacts to the commands is the brain. The servo devices are the muscles and the encoder that measures and reports actual machine travel is the eye. Of course, picking up a cup is a perfectly natural task for a healthy human being and we do it with a smooth, easy motion. Ideally, motion control cards should be just as naturally suited for their tasks.

To do their job, the operations performed by a motion control card have to be done in real time at high frequency--thousands of time a second. Ordinary PC hardware and software are not optimized for these kinds of operations. However, the microprocessors and the software on the motion control card are optimized in this way. These systems are designed for the fast and repeated equation-solving routines involved in motion control.

To be clear, not all motion control cards are the same. There's no one motion control card that fits every situation and not all motion control cards are suitable for CNC applications. And not all motion control cards perform the same functions--some motion control cards rely on the PC for certain functions and others rely on additional devices such as an I/O card for certain functions. How these various functions are distributed and how the devices are structured to work together constitutes the CNC's architecture. No one way is necessarily best, just as it isn't necessarily best for all families to eat in the dining room instead of the dinette or kitchen.

Trends

Nevertheless, some trends have become evident. For example, high speed communication between the PC and the motion control card is critical. How fast this communication takes place ultimately affects the machine tool's performance because an information traffic jam between the motion control card and the PC slows the whole system down. Conventional CNC design struggles with this same issue as the "block transfer rate" of the internal "communications bus." Today, motion control cards are available that handle data communication at very high speeds. The PMAC motion control card from Delta Tau Data Systems, Inc. (Northridge, California), for example, achieves 7000 blocks per second transfer rates, using "dual-ported" (shared) memory technology.

Digital technology is also clearly the trend for motion control cards. The most advanced microprocessors used in motion control take advantage of digital signal processing. Digital signal processing allows these microprocessors to convert and process digital signals at very high speedsmany times faster than some of the fastest processors in the PC.

This speed reduces the servo cycle time--how long it takes the motion control card to process commands, measure the result, make adjustments, process a new command, and so on. Servo cycle time is directly related to a machine tool's accuracy. The faster the servo cycle time, the more often the CNC can compare actual and commanded position and the quicker it can make corrective adjustments, thus reducing this "following error" to a minimum.

Likewise, digital signal processing allows these processors to perform more complicated algorithms efficiently. These algorithms may embody more complex strategies for handling the acceleration and deceleration of programmed moves. For example, instead of ramping up to speed in a straight line (less complicated mathematics), the motion control card may ramp up along a smoother curve based on the dynamic conditions within the machine tool (reflected in more complicated mathematics). This fine-tuning of motion control reduces the tendency of the machine to lurch ahead from position to position, even at high speeds, for better surface finish, longer tool life, and less wear and tear on the machine tool. These improvements are especially welcome in the thrust toward high speed machining.

Better CNCs

Taken together, the advances in PC and motion control technology, which underlie the development of open-architecture CNCs, promise to give machine tool builders and users much greater flexibility. Builders will be able to design systems with greater functionality and more customized features and be able to deliver these systems sooner and at a lower cost to the buyer. Similarly, machine tool users will be able to maintain and upgrade their CNC machine tools at lower cost and with less downtime.

Perhaps the most exciting prospect is that open-architecture, PC-based CNCs will take machine tool performance to a higher level than that possible with conventionally structured CNCs.

Motion Control Card Tasks

The main job of the motion control card is to perform the time-intensive, high frequency tasks needed to keep each axis of the machine tool moving along the desired path.

1.Plan each move. Take a block of coordinate information (G-code statement) from the PC software and calculate the appropriate "equation of motion" to determine how long and how fast to move each axis to arrive at that programmed destination along the desired path.

2.Apply interpolation. Solve those equations of motion at small time intervals and generate the appropriate intermediate positions for each axis.

3.Close the servo loop. Compare readings from the encoders, which indicate actual axis position, with each of these intermediate positions, and issue new commands to the servos to drive the difference to zero. Do so for each motor.

4.Regulate motor commutation (optional). Calculate the level of current applied to each phase of the servo motor to produce desired torque. Do so for the motor at each axis.

5.Maintain the current loop (optional). Compare desired current levels with actual levels and modulate current by adjusting the power transistor on/off times to drive the difference to zero. Do so for each servo motor.

A motion control card must perform all of its tasks at high speed and with extreme reliability. Safety features allow a motion control card to bring a machine tool to a safe condition in the event of an error, or if the PC "crashes" and stops functioning. MMS

# posted by Medical Information : 9:37 PM  0 Comments

Machines that move need a means of measuring movement. Since the machine tools, inspection machines, material handling equipment and the like have themselves evolved from basic rudimentary manual machines to highly sophisticated automated pieces, so have the internal measuring mechanisms. The most common type of measurement component today is the encoder.

Encoders can be generally categorized into optical (photoelectric), magnetic encoders, and mechanical contact types. Photoelectric encoders in particular—due to their high accuracy, high reliability and relatively low cost, play a significant role in machine tool technology.

There are two basic types of encoders: rotary and linear. While the technical principles behind them are similar, their specific applications most often are not. And while the basic principle of operation developed many years ago is still the basis of today's encoders, a revision of even that technology, highlighted for especially high accuracy needs, is now available.

The Basic Principle

Most of today's linear and rotary encoders operate on the principle of the photo-electrical scanning of very fine gratings.

The so-called scanning unit in an encoder consists of a light source, a condenser lens for collimating the light beam, the scanning reticle with the index gratings, and silicon photovoltaic cells. When the scale is moved relative to the scanning unit, the lines of the scale coincide alternately with the lines or spaces in the index grating. The periodic fluctuation of light intensity is converted by photovoltaic cells into electrical signals. These signals result form the averaging of a large number of lines. The output signals are two sinusoidal signals that are then interpolated or digitized as necessary.

Rotary Encoders

In various sectors of machine technology, angular positions and angular motions need to be transduced into electrical signals, either for display, automation or numerical control. Rotary encoders are used for this purpose of measurement of rotational movement drives. They are also often used in measuring linear movements, for example when used with spindles and especially with recirculating ballscrews.

# posted by Medical Information : 9:36 PM  0 Comments

Mike Province, the vice president of Clarich Mold (Westchester, Illinois) had planned to take a vacation over the Christmas holiday but that was not going to happen. He needed to burn one more mold on his CNC ram-type (sinker) EDM. The electrode, about 4 inches wide by 6 inches long, had complex geometry and deep ribs. Instead of using those four vacation days, Mike got stuck at the shop "babysitting" the EDM process—constantly adjusting the cutting conditions and the flushing ports. Almost a year later that same job came through the door again, only this time, Mr. Province burned the job on his new EDM with linear motors. Instead of spending four days burning the mold, he spent only 15 hours. The new machine completed the job faster because it did not require any special flushing setups.

Adapting linear motors on machine tools has created a lot of interest in the last few years. This interest was apparent at Chicago's International Manufacturing Technology Show in September 2000. Several major machine builders including Cincinnati Machine, Mazak and Sodick unveiled various types of machines with linear motor drive units. Other manufacturers of servomotors or control units such as Fanuc, Mitsubishi Electric, Yaskawa and Siemens also displayed linear motor technology.

Understanding Linear Motors

Linear motors are not a new invention. Rather, they are an innovative adaptation of an existing technology. It is, in fact, the very same technology that propels roller coasters and bullet trains to record speeds in Japan. What makes the current linear motor units so intriguing is their entry into mainstream machining operations. Many industry observers believe that linear motors will have a dramatic impact on the design of machine tool axis motion, outmoding the present technology much like CNC did to control systems a few years ago.

This sample shows the precision that can be held with linear motor EDMs. The webs on this piece are only 0.005 inch wide and would be distorted if auxiliary flushing had to be used. This part would be impossible to machine on either a non-linear motor EDM or on a machining center.

Linear motors are rather simple. On a CNC ram EDM, two series of magnetic plates are mounted on the Z-axis quill, along with fixed magnetic coils on each side of the axis. The basic components of a linear motor are shown in Figure 1. A linear scale with very fine resolution is mounted to the Z-axis quill in order to detect axis movement location. As the control signals the Z axis destination, electrical current is introduced into the copper coils, which are adjacent to the magnet plates. The resulting difference in polarity between the plates propels them in opposite directions. Because the one set of plates is fixed, the other set is driven rapidly along its path. The more current that is introduced, the faster the moving axis will travel. The Z axis can travel more than 1,400 ipm, or nearly 22 times faster than a traditional ballscrew equipped EDM.

No Backlash

With conventional motors, before any movement is realized, the electrical motion (rotation) must be converted into mechanical (linear) motion through the use of belts, gearboxes or ballscrews. All of these conversions introduce issues of mass, inertia, backlash, lag-time, overshoot, friction and heat. Even in the case of direct-drive systems (where the motor-shaft is mounted directly to the ballscrew), it must first overcome the mass, inertia and friction of the ballscrew mechanism before it encounters the mass, inertia and friction of the table and the workpiece weight. Then of course, to stop this motion, the same amount of time and energy is required. With linear motors, most of these issues are reduced or eliminated because no conversion from rotational to linear motion takes place.

Although a linear drive produces less torque at low speed than conventional drive systems do, EDM machines don't have high-torque, high-load requirements as chip cutting machines do. Therefore, this characteristic is not an issue for EDM. Because EDM is a non-contact machining process, it is a perfect fit for linear motor technology. Unlike machining centers that take advantage of the table speeds of linear motors, EDM uses linear technology's speed for the Z axis (ram stroke) in order to create its own flushing capability. EDM also uses the speed of linear motors to react to changes in the spark gap. Linear motor EDMs will excel in difficult-to-flush applications.

Figure 2 compares a typical rotary ballscrew system to a typical linear motor system. In the ballscrew system, position and velocity are controlled by encoder signals. The ballscrew transfers the rotary movement into linear movement. During machining, the control needs to maintain the desired spark gap between the electrode and the workpiece, which is a distance of 3 to 50 microns. This oscillation is repeated up to 500 times per second. The inevitable mechanical twist and "backlash" resulting from the ballscrew's movement will ultimately reduce machining accuracy.

Backlash is a phenomenon that occurs in machine tools when the rotary motion of the ballscrew is converted into linear motion for the bed or table. The conversion is generally carried out via a geared feed mechanism, which requires a certain amount of clearance between the driving and driven gears, as shown in Figure 3. Although the clearance does not cause problems when gears are driven at a constant speed or direction, when the drive direction is reversed, there is a short delay before the driving gear teeth mesh with the driven teeth. This lag time, known as backlash, directly affects machining accuracy. As the driving mechanism wears over time, the gear tolerances degrade, which also contributes to a reduction in machining quality.

On the linear motor system, the motor is in effect the only moving part, so the travel distance measured by the linear scale is sent directly to the motor. This allows for a simpler control mechanism without the effects of backlash. Because the electrode is directly connected to the motor assembly, the movement of the electrode and the motor are in unison. This direct connection lets the system operate at very high speeds without mechanical vibration.

Therefore, voltage feedback of the spark gap is precisely followed up with an extremely fast response rate, resulting in faster machining speeds and improved machining accuracy.

Applying Linear Motors

Certain considerations have to be addressed by machine designers when creating a linear motor ram EDM. These include designing and building an extremely rigid yet lightweight head, balancing magnetic forces to eliminate distortion and counter balancing the head for accurate high speed machining. See Figure 4.

Linear Motors For Wire EDMs, Too While CNC ram EDMs have benefited the most from linear motor technology, their wire EDM counterparts have also posted impressive gains in surface finish, machining time and accuracy from the application of this technology. The improved servo response and sensitivity that linear motors provide result in quicker and more accurate wire alignments and touch-off routines. The precision fit of the above part was attained with only one skim pass and a 18 rms surface finish. The superior response and virtually vibration-free movement of linear motors improves the corner accuracy, positioning accuracy and roundness capabilities versus traditional ballscrew machines. The faster servo response and increased sensitivity of linear motors results in fewer wire breaks and lost time during rethreading operations. Because linear technology is combined with a higher level of control technology, more power can be put into the wire without breaking. Cutting speeds on a linear motor driven system increase by 20 percent when compared to a ballscrew model. The combination of high speed roughing and improved discharge frequencies has been found to consistently deliver a surface roughness of 18 rms after only one skim pass. Eliminating multiple skim passes can reduce machining time by as much as 60 percent on certain wire EDM jobs. Wire EDMs lend themselves to linear motors for another reason. Because the X and Y axes on a wire machine do not move with the same high speed as the Z axis on a CNC ram machine, less heat is generated. Cooling lines are therefore not required on wire EDMs with linear motors.

Linear motors do generate a great deal of heat, which, if not dissipated, can reduce the life of the linear motor's magnets. To overcome this problem, some sort of system must be added throughout the magnets to circulate chilled coolant in order to maintain optimum operating temperatures.

Because the Z-axis column must travel at high speeds, a conventional head counterbalancing weight system is inadequate. The counter weight was eliminated on Sodick's design and replaced by an air cylinder system that could offset the forces generated by the high speeds and high acceleration rates at which the Z axis travels.

Similarly, the "ramp speed," or acceleration, of a linear motor system from command to actual position is more than 30 percent faster than a rotary system given the same command. A linear motor control, such as that developed by Sodick, must be able to process information and change the spark gap faster than a traditional ballscrew-driven unit, which has to send data through the NC unit to a motor driver, an encoder and down to a ballscrew. The linear control sends gap detection data directly to the linear motor, which responds at a much faster rate to the command.

When set in the automatic jump cycle mode, a linear motor EDM can accelerate up to 1.22 G and reach a Z-axis speed of 36 m/min. (1,440 ipm). The maximum servo speed of ballscrew models is about 1.5 m/min. (60 ipm) with 0.05 G acceleration. With linear motors, the combination of speed and acceleration results in machining accuracy.

Natural Flushing

This combination of speed and accuracy results in the ability to machine shapes up to (but not limited to) 4 inches deep without auxiliary flushing, as seen in Figure 5. Because the electrode is moving so fast, it creates its own natural flushing currents within the spark gap. Yet at this speed, motion is smooth enough to allow a nickel to be balanced upright on the chuck while moving at 1,400 ipm. The high-speed jump of the Z axis creates a uniform and minimum spark gap while reducing electrode wear and improving cutting accuracy. See Figure 6. The need for flushing ports and forced flushing operations is eliminated.

By creating its own natural flushing for large electrodes, a linear motor EDM produces an even, uniform surface before the finishing process takes place. Because there is less material to remove, time needed for the finishing pass is reduced. Likewise, polishing time is reduced.

Linear motor ram EDMs are proving to be easier to operate than ballscrew driven EDMs because dealing with auxiliary flushing is eliminated. Adding just the right amount of flushing is something many EDM operators learn only after years of experience. Too much or too little flushing can create uneven surfaces. With linear motor EDMs, it is unnecessary to determine where to drill flush holes through the electrode, where to set up auxiliary flush ports or how to set the right amount of flushing pressure.

Eliminating the need for flushing is ideal for unattended operations that use automatic electrode changers and robots. With only one main moving part, a linear motor EDM requires less maintenance and less downtime.

Beyond EDM

In addition to EDM, linear motors are used in other industrial equipment such as the automated assembly systems for production of semi-conductors, biotechnology-dispensing devices, machining centers and grinding machines. With any sort of linear motor technology, fewer parts are required in the design. In semiconductor assembly and in grinding systems, positioning speed and accuracy are very important.

In the Japanese machine tool market, linear motor ram EDMs are reportedly taking back work that had been moved to machining centers. This trend is attributed to the increased machining speeds seen on linear motor EDMs, the elimination of flushing and the fact that no more operator intervention is required after the process begins. With linear motor technology, ram EDMs have fewer variables and are easier to operate, while the linear motor wire EDMs attain higher accuracy and better finishes with fewer skim passes.

Although linear motor technology in the machine tool industry is relatively new, it is an emerging development that will not only increase production but also raise the level of accuracy, thereby attracting new applications for EDMs.

# posted by Medical Information : 9:34 PM  0 Comments

Linear motors account for part of the reason why DaimlerChrysler’s Stuttgart, Germany, manufacturing facility has been able to nearly double the productivity of machining centers producing automotive cylinder heads. The company replaced more conventional machines with horizontal machining centers featuring GE Fanuc (Charlottesville, Virginia) linear motors for X, Y, Z motion, with control coming from a GE Fanuc 161 CNC fast enough to keep up motor position through high speed moves. The high feed rate and high acceleration the linear motors make possible help reduce cycle times. DaimlerChrysler manufacturing personnel Thomas Brandstetter and Ingolf Kurschner analyzed the productivity improvement. They say the linear-motor-equipped machining centers now produce at a rate that would have required 11 traditional machines.

An operator enters information into one of GE Fanuc's Series 16i CNC interfaces at one of six Ingersoll high speed HVM 600 horizontal machining centers at DaimlerChrysler, Stuttgart

Ingersoll Milling Machine Company (Rockford, Illinois) supplied the six HVM600 linear motor horizontal machining centers for cylinder head production to DaimlerChrysler. The machines have traveled distances of 630 mm by 630 mm by 600 mm (X, Y, Z axes). A hydrostatically mounted main spindle attains outputs of 37.5 kW at speeds of up to 20,000 rpm. Workpieces are clamped to pallets measuring 630 by 630 mm2 on an NC turntable with a load capacity of 1,400 kg.

Four cylinder heads are secured simultaneously in a clamping device. In one continuous work process, the cylinder heads are extensively machined. End faces, screw contact faces and threads are milled. A total of 130 core holes must be drilled and the threads milled on each of four cylinder heads. For example, the main spindle requires 1.2 seconds to mill an M6 thread to a depth of 14.1 mm at a rotary frequency of 20,000 rpm and 700 mm/min. feed rate. The HSC machining center mills screw contact faces of 24 mm diameter with a diamond-coated face mill cutter at speeds of 20,000 rpm (corresponds to 150 m/min. cutting speed) and 5,200 mm/min. feed rate. Forty of these have to be machined on every cylinder head.

Before a toolchange takes place, all similar parts of the four cylinders are machined. During the toolchange, a laser beam verifies that the proper tool is being substituted. With its 32-bit RISC processor and high-cycle frequencies, the CNC system can process NC programming very quickly. Furthermore, it contains the well-developed look-ahead functions that are required for high speed feed.

In conventional machining centers, 11 machines would have been required to produce the same quantity of components that DaimlerChrysler is producing on its six high speed HVM 600 horizontal machining centers with GE Fanuc Series 16i CNCs and linear motors.

The high speeds of these six machines allow Daimler Chrysler to produce 300 four-valve cylinder heads for four-cylinder engines daily. In conventional machining centers, 11 machines would have been necessary to produce the same quantity of components, thus requiring more personnel, larger factory floor space, more tools and equipment, maintenance and investment, machine interlinking, lifting gear and additional measuring devices.

The high acceleration power of the hydrostatically mounted main spindle enables the machines to achieve short cutting times. The spindle requires only 1.5 seconds to accelerate to a speed of 20,000 rpm. The linear motors’ high acceleration power and feed force also ensure minimum non-machining time. In rapid motion, they accelerate in the Z axis at 1.5 g (14.5 m/s2) and in the X and Y axes at 1 g (9.8 m/s2) to 76 m/min.

Even at these high speeds, the linear motors position the spindles in the entire work area to a degree of accuracy of approximately 5 microns. To further increase accuracy, glass scales are installed in all axes, and pretensioned roller guides direct the carriages in each axis.

After optimizing some of the para-meters of mechanical engineering, the HSC machining centers with GE Fanuc’s linear motors have proven very successful at shortening production times and increasing accuracy. Cutting figures and productivity levels has exceeded what the experienced machininsts at DaimlerChrysler had envisioned, as the machining centers now achieve uptime levels of more than 95 percent. In addition, the enclosed design of the linear motors protects the equipment.

# posted by Medical Information : 9:34 PM  0 Comments

Rob Grant, president of Pressway, Inc. (Roseville, Michigan), a stamping supplier, was looking to sharpen the company's competitive edge. He was looking for greater part quality and the ability to hold tighter tolerances, increased uptime with shorter setups and changeovers, longer die and tool life, more efficient diagnostics to reduce downtime, improved customer service, an expansion of part runability and more.

"Today's manufacturer not only has to be able to produce good parts efficiently," says Mr. Grant, "they have to also be able to handle data and use information about their products and processes effectively." To accomplish all this, the company decided to make major investments in new presses, coil handling, feed lines and scrap removal, plus network communications.

A critical element to Pressway's additional capacities and capabilities is a full line of press feed and coil handling equipment provided by Dallas Industries, Inc. (Troy, Michigan). Of the ten press lines at the facility, all are outfitted with Dallas' precision servofeeds, along with threading tables, power straighteners, peeler/threader/debender units and coil reels. And eight of those lines are also equipped with Pressmaster servofeed controllers from Dallas, control devices that assure synchronous speeds of stock feed and press operation regardless of any fluctuations that may occur. The Pressmaster units, linked with the press controls and networked to management information systems, provide a continuous flow of operational data that can be accessed and used by management, setup personnel and line operators.

For Mr. Grant, information handling, along with the enhanced quality output and improved productivity, is what Pressway needed to surpass its competition. The 30-year-old company had a lot of old and add-on equipment. To keep its business, it needed the technologically advanced equipment. Dallas lines met those specifications.

A few of the examples that Mr. Grant highlights from this equipment are the coil reels that have a 72-inch coil OD capacity and 20,000-lb capacities that provide for longer run times and reduced downtime for coil replacement. Also, on many of the lines, a coil car is included that allows the next coil of material to be prestaged and prepped for quick loading on to the reel when the currently running coil is depleted. In addition, the lines are equipped with threading tables that help align stock edges more accurately, resulting in faster changeovers and quicker setups.

Mr. Grant also cites the safety factor of the new lines and, specifically the single unit threader/debender/coil restrictor mechanism. The adjustable height threading table provides easy, accurate guidance into the feed, allowing the stock feed and its path to be kept aligned and under control, and coil stock is prevented from unwinding prematurely.

Another critical area to Pressway is quality. The precision of the new CNC servofeeds provides feed increments of 0.001 inch and repeatability to the same 0.001-inch tolerance, meaning material is consistently positioned correctly in the dies. Not only are the stampings produced more accurately, but the precision also lessens the risk of mis-hits and helps lengthen the life of tools, punches and dies.

Also contributing to quality improvements at Pressway are the power straighteners. The electronically controlled units can be adjusted to provide stock that is flat to approximately ±0.001 inch, depending on the material. "The straighter the material going in, the flatter the part coming out," says Mr. Grant.

At Pressway, according to Mr. Grant, the key element of the new press lines is the control offered. Most of the presses at the facility are CNC controlled and equipped with electronic sensor systems for monitoring operations, which in turn are linked to the lines corresponding to the feed system's Pressmaster controller. Changeovers and setups are now accomplished by merely entering data at the press control, either through an onboard stored part number program or by downloading from the external PC source located in the Pressway offices. The program contains operational parameters such as cycle times, feed lengths and so on. While the press is reset, the controller also relays the necessary program commands to the feed system control to automatically make adjustments such as roller speed, and it also synchronizes the feed cycle to achieve the infeed length. Other than the manual, mechanical operations of changing the dies (which Pressway has upgraded with several devices to make this as easy, accurate and fast as possible) and loading new stock if required, the setup is complete.

Things have definitely improved for the company. "We've recently moved into a new facility, more than doubling our size. The new presses, the larger capacity reels, the accuracy and reliability of the feeds, the straighter stock, and the efficient scrap handling have helped increase our productivity providing 10 percent to 20 percent faster run times," says Mr. Grant. "Also, an important factor is the data and information handling—we can make intelligent decisions regarding scheduling, maintenance requirements, continuous quality improvements and customer relations."

# posted by Medical Information : 9:31 PM  0 Comments

The traditional machining center has axis motors that push in one direction or the other along X, Y and Z. In the 1990s, all of us who attend machine tool trade shows learned that this wasn’t the only way to design the machine. A variety of builders brought out "hexapod" and/or "parallel kinematic" machines in which a daddy-longlegs arrangement of linear-motion members used CNC interpolation to achieve precisely the same X-Y-Z motion as a standard machine.

These two machining centers might be thought of as cousins to the hexapod (top left), but only in the sense that they use resultant motion to define at least one linear axis. In the Genius 500 horizontal machining center (top right images), the linear motors that move up and down in Y also produce the X-axis motion whenever they move separately, causing the coupler between them to shift. With no motor pushing in X, more of the machine’s force is directed downward instead of from side to side. In the S-500A "relative motion" vertical machining center (below), the table and the tool move toward one another simultaneously. The feed rate and acceleration result from the sum of these two movements.

These novel machining centers make for attention-grabbing live demonstrations at trade shows. In fact, these eye-catching machines began to appear at trade shows not long after the use of attractive female models at these shows began to decline—almost as if some different means of capturing attendees’ attention had to be invented. However, real-life users of these machines remain uncommon to say the least. While a machine tool buyer may notice a trade show booth because of the complex motion of one these machines, that buyer is still more likely to spend money on a machine that has a more standard design.

But now, an important development may go unnoticed beneath the presence of these more elaborate machines. Some machining centers have begun to appear that use the same fundamental idea—resultant motion—in a less elaborate way. That is, these machines use the resultant motion of different elements moving in different directions to achieve the motion along X, Y or Z, but they do so within machining center designs that are considerably more like standard machines.

Here are just two examples. One is the line of "relative motion" machining centers from Olympic Seiki (represented in the United States by Vigor Machinery Company). With these machines, it is not the tool motion or table motion alone that provides the machine’s traverse; it’s the tool and table together. In the X and Z axes, the ballscrew simultaneously moves both table and tool in opposite directions. The feed rate of the tool relative to the part is the sum of the traverse rates of both elements. Ditto for the acceleration. On smaller machines in this line, the resultant acceleration is 2G. In addition to speed, another benefit may be stability. With mated elements moving in symmetry, says the company, the design of this machine favors dynamic balance.

Another machine design taking advantage of resultant motion is the "Genius 500" horizontal machining center from Cross Hüller. On this machine, the X-axis motion—that is, the side-to-side motion—comes from elements that move up and down along the Y direction. The mechanism for this is an inverted V-shaped coupler that carries the spindle. This inverted V straddles between two sets of linear motors that run up and down. When the linear motors move together at the same speed, the result is pure Y-axis motion. But when the linear motors move differently, the difference causes the coupler to pivot, providing the motion in X.

As a result, there is no need for a motor to push along the X axis on this machine. Therein lies a benefit of the design. Whether the motion is X or Y, the force of the axis motors goes along the direction of gravity, where the machine is well-supported (by the floor). Thus the machine can move the tool rapidly throughout the X-Y plane, with none of the motion of the axis motors directly producing sideways forces.

# posted by Medical Information : 9:17 PM  0 Comments