Rotary Wire EDM in Modern Manufacturing

Transcription

1 Ann Mazakas DP Technology Corp. MA5439 While much is known of the strides made in multi-axis milling, less is known about the advances in multi-axis wire EDM technology. While wire EDM is widely used in the medical, aerospace, and injection-mold industries, the addition of rotary motion to a standard EDM machine is opening up new opportunities in other markets. As the technology for wire EDM advances to new levels, it is time to take a look at how this technology can replace traditional machining processes in EDM as well as milling and turning. This class will introduce a non-traditional approach to applying wire EDM technology to achieve part geometries that were previously difficult, if not impossible, to attain. Learning Objectives At the end of this class, you will be able to: Evaluate applications for EDM turning (Spin and Burn) Describe the part geometries that can be achieved with rotational wire EDM Compare indexed EDM (Turn and Burn) and simultaneous (Turn While Burn) Evaluate rotary spindles for wire EDM About the Speaker Ann Mazakas is manager of technical communications for DP Technology Corp., the developer of ESPRIT CAD/CAM software. Mazakas has an extensive background in CAD/CAM systems, design engineering, and metalworking. She is the author of numerous technical articles on issues facing the manufacturing industry. With a passion for metalworking, she has been a writer and speaker since ann.mazakas@dptechnology.com

2 While much is known of the strides made in multi-axis milling and mill-turn, less is known about the advances in multi-axis wire EDM technology. The advances in rotary technology for wire EDM and the software to run that technology have been accelerating in recent years. As the technology for wire EDM advances to new levels, it s time to take a look at how this technology can replace traditional processes in EDM as well as milling and turning. Traditional EDM Electrical Discharge Machining (EDM) feeds a thin single-strand of metal wire through a work piece that s submerged in a tank of dielectric fluid, usually deionized water. The wire, which is constantly fed from a spool, is held between upper and lower guides. Material removal is based on electrical discharges along the wire that erode the material. This is much different than traditional milling or turning, where material is removed by mechanical force from a cutting tool. EDM is used on materials that are electrically conductive and is often used on hardened metal since there is no need for heat treatment to soften and re-harden the workpiece. Wire control A traditional wire EDM has what s called XYUVZ control. The XY axes relate to the movement of the lower wire guide in the X and Y directions. The UV axes relate to the movement of the upper guide in the U and V directions. The Z-axis moves the upper guide up or down before XYUV movement starts. To watch a demonstration of how wire EDM works, follow this link to an excellent video on YouTube axis wire EDM The simplest form of wire EDM is 2-axis programming. In that case, a single profile is used as the reference geometry. As the wire moves along the profile, it can be held vertical to create what s called a land cut or the wire can be tilted to create a taper cut. The wire guides are kept as close as possible to the part to get the best intensity in the electric field and to get the best stability in the wire. dptechnology.com 2

3 4-axis wire EDM With 4-axis wire EDM, the upper and lower wire guides move independently along two different profiles. You don t need a specialized machine for 4-axis work since most EDMs are capable of both 2-axis and 4-axis cuts. The main difference is whether the wire guides follow one profile or two. You can see in the picture that the lower profile is a small circle and the upper profile is a large triangular shape. Producing odd shapes like this is quite typical of 4-axis EDM. When you have such different shapes, the synchronization of the wire between the two profiles is critical. Match lines between the profiles control the orientation of the wire at certain points along the path. When an EDM feature is created on a solid model in ESPRIT, the system automatically calculates the profiles and the match lines for you. Advantages of wire EDM A part that requires extremely high tolerances will naturally be cut on a wire EDM since wire is available in extremely small diameters, often smaller than the diameter of a human hair. Wire EDM is also the perfect solution for cutting delicate parts since there are no cutting forces involved. It s important to note that small, delicate parts can t be cut efficiently and with high quality using mechanical milling or turning methods since any tool pressure can distort or break the workpiece. And since the wire is being constantly fed through the workpiece, it s like you re constantly cutting with a new tool. EDM is even more advantageous when dealing with difficult-to-cut materials like titanium or Inconels or Hastelloys. These materials are very tough and springy, so when mechanical processes are used to cut them, chatter is a huge problem, causing high tool wear, poor surface finish and broken workpieces. Plus there s the benefit of recovering large pieces of material, especially expensive metals, that can be used for other purposes or recycled easily. Slugs of metal fall to the bottom of the tank where they are easily collected later. Applications for wire EDM Wire EDM is widely used in the medical, aerospace, and injection-mold industries to produce parts in complex shapes that can t be achieved with traditional cutting tools. A common appication is the production of extrusion dies and progressive dies for plastic injection molding. dptechnology.com 3

4 And EDM is usually the only way to produce extremely small holes, especially when those holes require deep drilling. Wire EDM is often used to cut multiple shapes from a single block of material. In many ways, rotary EDM is very much like rotary milling except that the rotary table is submersible. Like milling, these submersible tables allow the programming of continuous rotation and indexing. Like a lathe, an EDM rotary table can also be used as a turning spindle. Instead of the part being clamped into one position on the work table, it s mounted on the rotary table. The rotary table then allows the rotation and cutting of the part from virtually any angle without the need to unclamp and reposition the part between cutting operations. Advantages of rotar y EDM If you re cutting a multi-sided part, the clear advantage of rotary EDM is that you only need to clamp the part once to machine on all sides. The table rotates the part into position for you. It s especially fast when the rotary table has a hydraulic clamping system like a lathe. For complex parts that need machining on multiple sides this also means fewer setups since the part doesn t need to be moved between operations. The fact that you re not unclamping the part and moving it between operations also greatly improves the overall accuracy of the finished part. A big advantage is that a rotary table can be added to almost any kind of wire EDM, which immediately extends the capabilities of the shop without purchasing new machines. dptechnology.com 4 Rotary wire EDM Another example is this sliding assembly of interlocking gears.

5 Adding rotary functionality to your current EDM offerings also lets you expand business into new markets and opens up a whole new arena of design possibilities because now you can cut geometry that was never possible before. Applications for rotary wire EDM There seem to be new applications for rotary EDM every day as the technology becomes more widely known and the builders of rotary axes keep improving the types of tables available. Micro turning on an EDM is definitely becoming more popular because the absence of tool pressure allows for minimal diameters and high precision. Plus, notches or other flat features can also be cut in the same setup. In traditional machining, you d need a lathe with milling capability. A rotary axis also lets you cut multiple parts in one setup by indexing around the part. Rotary tables for EDM There are two basic types of rotary tables for wire EDM: stand-alone and integrated. You can also cut tiny slots through a cylindrical workpiece, which lets you cut on opposite sides at once. Cutting helical surfaces is also quite easy. This spring was created from a solid piece of tube stock using a simple rotation of the stock to cut the helical slot. Stand-alone rotary tables A stand-alone table can be positioned anywhere on the work table and can be moved from machine to machine. dptechnology.com 5

6 A stand-alone table is purchased with an external control that has basic functions that let you start and stop the rotation and enter the angular positioning. In this case, the operator is required to push a start button every time an index move is required. This type of table is simple to install and has no interface. Indexers Rotary tables are most commonly used for positioning, also called indexing. The axis rotates the part into position and then is locked during the cut. Rotary positioning only takes place between operations. However, you can achieve automatic sequencing with the NC program by installing an interface cable that connects to the machine control. In that case, you d need to have a connector for the interface cable installed on each machine where you want to use the rotary table. Tables that fall into the stand-alone category are indexers and rotary spindles that spin at high speeds for EDM turning. Integrated rotary tables An integrated rotary table is built directly into the machine to become an additional axis. This type of table allows for continuous rotation during the cut because the positioning is managed by the machine control. Integration of the rotary unit is done by the machine tool manufacturer because the components of the table must match the requirements of the machine control. One advantage is that you can rotate the part to cut a taper instead of tilting the wire. That way you keep the wire vertical and close to the workpiece, which is the ideal cutting condition for wire EDMs. Indexers can have either one or two rotary axes. A single axis will rotate the part. An indexer with two rotary axes can tilt and rotate the part, which gives you 5-axis positioning capability. To find out which rotary tables can be integrated into a particular machine, you can contact the EDM manufacturer. Many rotary tables are available as both standalone and integrated. Now let s discuss the specific functions of a rotary table. Continuous motion Another use for a rotary table is continuous rotation during the cut. With this type of table, you combine dptechnology.com 6

7 continuous rotary motion with the linear motion of the wire. This type of table must be integrated into the machine control as an additional axis to allow for constant feedback and monitoring of the servo speed and rotary positioning. This type of table is more flexible because it allows for continuous machining as well as indexing. In these images provided by AgieCharmilles you can see some design ideas that aren t possible with any other type of machining. High-speed spinners High-speed spinning units are used when no indexing is necessary. These require a simple variable-speed control. An AC drive motor is critical to the EDM process at high speeds up to 1500 rpm. There are also units that combine high-speed spinning with indexing. A high-speed spinner lets you do EDM turning, which is incredible for micro turning. Hollow axis A completely new type of rotary table was introduced by ITS-Technologies in In this unit, the part is mounted inside the opening of a hollow shaft. The advantage is that you can cut on all sides of a part without anything to obstruct the wire guides. dptechnology.com 7

8 This system has a range of 360 degrees and can rotate endlessly. The opening of the hollow shaft at the center of the system has a diameter of 115 mm. Because the workpiece continuously rotates, the wire can stay perpendicular the entire time. The unit can be purchased with a separate control or it can be integrated in the control of the machine. Parts are mounted on a face plate at the bottom of the shaft. Face plates can be completely customized to the shapes you re cutting. Robotics and rotary EDM A wire EDM with an integrated table can be totally automated by installing a hydraulic mounting system and a robot. In the system shown here, the robot grabs a workpiece from a rack of pre-processed parts and takes it to the wire EDM. The spindle on the rotary axis is equipped with a hydraulic system that will automatically clamp the part. The robot positions the part and it s loaded automatically. After the cut is finished, the robot grabs the part again and moves it to an inspection station. Things to consider - Capacity As part of your evaluation process you need to consider the load capacity of an axis. Unlike tables and axes that are built for milling machines, which are usually gear driven with heavy load capacities, EDM rotary tables are typically belt driven with very high accuracies so their weight and strength capacities are much lower. When determining the proper axis size, both the weight and most importantly the diameter of the largest part plus the fixture must be considered. The capacity of an axis is determined by the amount of force produced by the motor through the gearbox through the belt to start the axis turning and then the force needed to stop it at the commanded location. As an example, let s say the maximimum moment of inertia for an axis is 350 kgcm 2 and the maximum weight of your part plus fixture is 28 kg. The maximum diameter for that weight is 100 mm. I = (weight in kg/2) * R 2 in cm = kgcm 2 Max Dia = 350kgcm 2 /14kg = 25 = 5 cm = 100 mm If I use the same part weight with a larger diameter, the axis will not be able to handle it. Basically what happens is the excess diameter and weight will cause the axis to overshoot it s commanded position and then reverse, miss the position, keep hunting and eventually cause a servo alarm. So the first question you need to answer is what is the largest diameter and weight of your parts, including anything that attaches to the face plate to hold the part. Things to consider - Axis orientation Rotary tables can be mounted on the machine horizontally or vertically. This becomes extremely important when you program your parts because you need to instruct the machine which axis to rotate. If the rotary table has two axes of motion, you also need to instruct the machine which axis moves first and which axis moves second. When an axis is aligned horizontally, you can have either A rotation around the X axis or B rotation around the Y axis. When it s aligned vertically, you have C axis rotation around the Z axis. Other things to consider Other criteria you need to consider when selecting a rotary table are its ability to withstand being submerged for long periods and how much speed you need to rotate your parts. dptechnology.com 8

9 Accuracy also plays a critical factor. Positional accuracy is important for indexing and continuous motion and radial runout gains more importance for high-speed spinners. You also need to look at how you want to mount your workpieces on the rotary Turn Then Burn Indexed wire EDM operations are commonly referred to as Turn Then Burn because the part is first rotated into position and then the profile is burned. With an indexing operation, the rotary axis is used only for positioning between each machining cycle. Whenever a rotary move is required, the part is rotated and locked into position before cutting starts. The part is always held stationary during the cutting process. This process is simple because each operation is created like any other EDM operation, except that positioning moves are added between them. Defining the rotary axis in the CAM system If you re using a stand-alone rotary table, the position of the rotary axis can vary from machine to machine, so you must make sure to set it up correctly in the CAM system. In ESPRIT you only need to add a rotary axis to the machine setup and then choose the orientation of the axis and enter coordinates for the point at the center of the rotary table where the workpiece will table since there are different clamping options available. And the most important aspect comes down to the quality and warranty offered by the equipment manufacturer. be mounted. You can also add solid models of the table components for better visualization during simulation. As soon as a rotary axis is defined for an EDM in ESPRIT, functions for rotary EDM are enabled automatically. Work planes for indexing Work planes are very important to indexing operations, since the orientation of the work plane controls the rotary positioning of the part. When an operation is created, the wire is aligned parallel to the W axis of the work plane that s active at the time. The origin of the work plane sets the Z zero location of the operation. That means the locations of the XY and UV planes are measured from the location of the origin. If you ll be cutting multiple profiles, especially holes at different angles, you need to make sure your CAM system makes it easy to create the required work planes. dptechnology.com 9

10 In ESPRIT it s easy to create new work planes by selecting the faces of solid models or any other geometry that defines a plane. There are also three predefined work planes for plane orientations that are machined most often. Part definition for indexing In most CAM systems, machining operations are based on features. For EDM operations, ESPRIT will automatically recognize EDM features from solid models or 3D geometry. Depending on the geometry of the part, the system will automatically create a 2-axis or 4-axis feature. The system then associates the active work plane as a property of the feature. In this example, the work plane is set to ZXY and faces are selected for machining, shown in blue. EDM feature recognition is activated and the system automatically finds the locations of the XY and UV planes based on the origin of the work plane and the selected geometry. An excellent use for this type of indexed movement is to produce multiple parts from a single setup. This example shows how multiple pin gates used in plastic injection molding are cut all at once by first cutting a profile in one direction and then rotating the stock to cut a profile in a second direction. After the profiles are cut, a final operation can be performed to separate all the pins from the block. This example shows how indexing can be used to cut holes in a dome that each have a different orientation. This type of indexing is only possible when the rotary table has two axes of motion. This is a good application for a hollow axis table. The user only needs to select the type of part being machined. In this case it s an open profile with the wire located on the right. The resulting feature contains all the information for machining, so the user only needs to apply an operation and the system automatically loads the machining data from the feature. dptechnology.com 10

11 Turn While Burn Synchronized rotary motion is commonly referred to as Turn While Burn. Rotation of the part occurs during the burn at the same time as the linear motion of the wire. In this example, the A axis tilts the spindle at the same time the C axis rotates the hollow shaft. This lets you create finely detailed 5-axis work without a mill. The wire guides are kept vertical and close to the part to improve the burn and to improve flushing of the metal particles that are being burned away. A fully integrated rotary axis is required for synchronized motion. You also need a 5-axis machining function for EDM. In ESPRIT, creating a Turn While Burn operation is just like creating a 4-axis operation except that rotary motion is enabled throughout the cut. Part definition for synchronized rotation Unlike standard or indexed EDM operations, Turnwhile-Burn functionality requires a multi-axis feature that isn t restricted to a single work plane. However, the active work plane is still important because it sets the orientation of the wire at the lead-in position. ESPRIT has a specialized multi-axis feature called a Ruled feature that includes the top and bottom profiles of the surfaces to be machined plus the synchronization lines between the two profiles. The surfaces contained in the feature must be ruled, meaning that a straight wire traveling along the upper and lower profiles can maintain contact with the surface at all times. Turn While Burn offers a lot of possibilities as far as geometry is concerned. You can use Turn While Burn to create a spring from a solid piece of tube stock. Or, instead of using tube stock, you could create a custom spring by first contouring a taper or almost any shape inside a solid cylinder. Then you could cut the helix for the spring. Conceivably you could create a spring with almost any internal or external shape. Turn While Burn also lets you cut slots through cylindrical stock. This example shows fairly large slots, but the typical application is to cut small intricate slots that cannot be machined any other way. An advantage is that the walls of the slot can be tapered. dptechnology.com 11

12 Spin and Burn EDM turning, often called Spin and Burn, is similar to lathe or Swiss-style turning except that the wire doesn t exert mechanical pressure on the workpiece. The advantages of Spin-and-Burn are high accuracy with zero part stress. This allows for the production of symmetrical parts with minimal diameters and tiny details. For slow-rotation cutting applications, a standard rotary table is typically adequate. However, lathetype applications require a specialized spindle called a high-speed spinner. High-speed spinning units are capable of speeds ranging from 0 to 1500 RPM. Typically, the minimum speed for EDM turning is about 800 RPM or higher. To maintain rigidity of the wire during the high-speed spinning process, the wire must stay vertical at all times while the workpiece spins. Rough turning In EDM turning, getting the workpiece to the proper net shape before creating a finishing operation requires a specialized roughing process. Since the material used for EDM turning operations is typically expensive, the roughing operation in ESPRIT uses balanced indexing moves. This removes the stock in pieces large enough to allow for easy recovery and recycling. The first two indexing cuts are created on opposite sides. This removes large pieces of material on both sides and aids in balancing the workpiece. The same technique is used for the rest of the indexing moves. The workpiece is rotated to the next angular position for the cut and then the part is rotated 180 degrees for a balanced cut on the opposite side. The user has full control over how many indexed moves are created to achieve the desired net shape. Finish turning A finish EDM turning operation is the same as a lathe contouring operation except that a wire feeds into the material instead of a tool. A 2-axis profile is cut while the workpiece spins at the specified spindle RPM. dptechnology.com 12

13 Profiles for EDM turning For turning operations on a conventional lathe as well as on an EDM, ESPRIT has features that recognize the true turned profile of a solid model or selected faces of a solid. This is useful when the solid model contains flats or other geometry that must be machined using another method. ESPRIT calculates the shape of the true turned profile by rotating the solid model or solid faces around the U axis of the active work plane. Part features that cannot be turned, such as flats, are ignored. For parts with extremely small diameters, EDM turning has two advantages. There is absolutely no mechanical pressure on the part and you can cut fine details with a small diameter wire. You can see that the wire can get in to cut the tiny grooves and fillets on this pin. When working with tough or difficult-to-machine materials, EDM roughing may be preferable to roughing on a standard lathe. In this example, the EDM rough turning operation includes only 4 indexing moves. The material is cut away in large chunks instead of making multiple passes and lots of chips on a lathe. Afterwards, the workpiece can be moved to a traditional lathe or mill for additional processing or another EDM process can be applied. dptechnology.com 13

14 CAM software for rotary wire EDM CAM software developers often overlook rotary wire EDM because it is an emerging technology. However, adopting a wait-and-see attitude toward the development of rotary EDM solutions until the technology becomes mainstream is a missed business opportunity for the software developer and a disservice to the manufacturing industry. Without proper control of the rotary motion directly through the NC program, the functionality of the rotary axis is seriously hampered. This is particularly a hindrance when trying to synchronize the rotary axis with the standard axes of movement. With properly engineered and implemented CAM support for rotary EDM, machinists can take advantage of the full capabilities of an additional rotational axis with full confidence that the NC program will be reliable and correct. The convergence of rotary EDM technology There has never been a better time to invest in multiaxis EDM technology. It is becoming more common to find EDM machines with built-in ports that allow the easy installation of a rotary table. It is also more common to find vendors that supply these rotary tables. The technology to easily produce NC programs for rotary EDM is advancing. CAM software has been developed specifically to take advantage of rotary machine technology and to output the NC code for these specialized processes. The demand for complex part geometries and micro parts is growing. Manufacturers who can prove their capability to produce such parts can capture that demand. About DP Technology DP Technology is a leading developer and supplier of computer-aided manufacturing (CAM) software for a full range of machine tool applications. ESPRIT, DP Technology s flagship product, is a powerful, high-performance, full-spectrum programming system for milling, turning, wire EDM, and multitasking machine tools. ESPRIT embodies DP Technology s passion for excellence and its vision of technology s potential. The opinions, solutions, and advice in this article are from the experience of Ann Mazakas and is not intended as an endorsement of any product. The accuracy of the information in this article is based on Ann Mazakas knowledge at the time of writing. Unattended machining with fewer setups allows U.S. shops to compete with cheap labor costs abroad. These developments in 5-axis wire EDM open up exciting new opportunities for cutting complex shapes unattended, cost-effectively and precisely in one setup. For many manufacturers, there is great potential for adding 5-axis EDM technology to their current capabilities. In many cases, it is a better alternative to machining processes that use traditional tooling. As stated earlier, imagination is the biggest limitation to what can be cut with rotary EDM. Special thanks to the following people for contributing their expertise to this paper: Peter Knowles, President, Hirschmann Engineering USA, Inc. James Kim, Wire Applications Engineer, GF AgieCharmilles Jörg Springmann, Project Engineering, Managing Director, ITS-Technologies GmbH & Co.KG Thomas Kennedy, Applications Engineer, EDM Group, Makino Die/Mold Technologies Peter Fonda, Development Engineer, IMS-Mechatronics Laboratory, University of California, Davis About the author Ann Mazakas is Manager of Technical Communications at DP Technology Corp. Ann has an extensive background in CAD/CAM systems, design engineering, and metalworking. She is the author of numerous technical articles on issues facing the manufacturing industry. With a passion for metalworking, she has been a writer and speaker since DP Technology Corp Avenida Acaso Camarillo, CA USA Tel: Outside the US: esprit@dptechnology.com Copyright 2011 DP Technology Corp. All rights reserved. ESPRIT is a registered trademark of DP Technology Corp. dptechnology.com 14