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Buying a Wire EDM: Speed, Accuracy and Finish

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What kind of surface finish can the purchaser of a wire EDM expect with today’s technology?

 

The two things every wire electrical discharge machine (EDM) user wants are speed and accuracy. Unfortunately, these objectives are usually incompatible. You don’t get speed with precision, and you can’t achieve high accuracy without also achieving a fine surface finish. Accuracy and surface finish go together. Speed and accuracy do not.

 

Cutting Speed, Accuracy and Surface Finish

EDM units from the early 1980s might achieve cutting speeds of 3 to 4 square inches per hour. With changes in machine design and power supplies, speeds of 17 square inches per hour became attainable in the 1990s. Today, with improved power supplies, working in conjunction with sophisticated adaptive controls, it is not uncommon to achieve 24, 37 and in some cases 45 square inches per hour.

 

The type of material and the height of the part being cut are critical as well. It is generally easier and faster to cut hardened tool steel than cold-rolled steel, for example. The harder material is the better. Typically, tool steels, carbide and special alloys have fewer impurities and lower porosity, making them easier to cut. Cold-rolled steel may contain impurities, so wire cutting is slower, and the surface finish is poorer. Although aluminum is easy to cut at higher speeds, the material is so soft that it is very difficult to get a good surface finish. Even a 30-microinch surface finish is difficult to achieve in aluminum. In contrast, it is possible to cut a 3-inch-thick carbide workpiece, with accuracies of ±0.0001 inch, and still produce a of 5-microinch Ra surface finish.

 

A typical wire EDM process consists of several passes, traveling at varying speeds. The first pass is generally a roughing pass designed to cut as quickly as possible, while accuracy and surface finish are less of a concern. Each subsequent skim cut travels at progressively faster speeds, takes less and less material while steadily improving dimensional accuracy and quality of the surface finish.

 

During the finish cuts, the tension on the wire is increased, the current is reduced, and the voltage gap narrowed, allowing the user to refine the spark and the distance the spark jumps from the wire to the part. The offset applied to the last finish pass might be as small as 3 microns. To achieve a 4- or 5-microinch Ra finish, as many as six or seven skim cuts might be necessary. Whereas the diameter of a cutting tool determines the offset in milling, the EDM controller applies a cutter comp based on the diameter of the wire. For example, if a 0.010-inch-diameter brass wire is used, the cutter comp will approach 0.005 inch plus a spark gap as the wire gets closer and closer to the part surface, and possibly finish at 0.0051 inch.

 

To achieve these close tolerances and super-fine surface finishes, every parameter must be properly set. The right type of EDM wire must be selected. The wire must have the right diameter and tensile strength. The power setting and tension of the wire must also be right. The condition of the deionized water and flushing arrangements must be optimized, as well.

 

Machine Accuracy

When attempting to hold ±0.0001-inch positional accuracy with wire EDM, the shop environment becomes a factor. For example, both steel and carbide have a thermal expansion coefficient of ~6.8 ppm per degree Fahrenheit. This means that, for every 2°F change in shop temperature, a 12-inch part could grow as much as 0.00016 inch, putting the operation over the 0.0001-inch tolerance it is trying to hold. To be successful under these conditions, a shop must be able to hold its ambient temperature within 1°F in either direction during an eight-hour period. Controlling the temperature of the dielectric solution to ±1°F also helps control the temperature of the machine and the workpiece.

 

The two most common machine designs use either ballscrews or linear-motion systems. In terms of machine accuracy, each design has pluses and minuses, which must be explored when choosing a wire EDM unit.

 

High-precision glass scales are used to negate the effects of pitch error or backlash on the linear feedback. On the best machines, high-resolution servodrives with fine increments are used to position the wire, thus improving surface finish and accuracy. Adaptive controls can compensate for thermal growth. High-speed circuitry in servomotors enables them to react instantaneously for finer control of the spark. High-peak power supplies can now put more electrical energy into the wire, greatly enhancing productivity.

 

If you need more information of EDM machine manufacturers, I sincerely recommend you to visit Excetek Technologies Co., Ltd. – the company specializes in manufacturing high-quality EDM machines. To get more details of EDM machining, welcome to check out their website and feel free to contact with Excetek!

 

Article Source: https://www.mmsonline.com/blog/post/buying-a-wire-edm-speed-accuracy-and-finish

How to Succeed with Smart EDM Operation

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The benefits of EDM have become clear to many mold manufacturers throughout the past several years. It allows for the super-precise machining of delicate, complex parts and hard-to-machine materials, and eliminates downstream operations such as polishing and deburring. Still, some moldmakers may not be getting the most from their EDMs. Several factors can contribute to the success of EDM or be detrimental to the process.

 

Choosing the Right Machine

First and foremost, machine selection is a huge part of operational efficiency in EDM. Depending on the application, moldmakers can choose from wire, drilling, sinker or some combination of the three.

 

  • Wire EDM:

Choose a machine with the latest cost-saving features. For example, some machines have an operational mode that minimizes wire consumption by slowing down the wire feed. The right wire EDM can also help manufacturers save on energy and consumables, with features that control and optimize dielectric fluid volume and pressure, pump and filter operation, and resin consumption.

 

  • Drilling EDM:

Moldmakers should look for machines that combat hole tapering—otherwise known as bullet nosing. To avoid this common problem, machines must not only use high-quality electrodes, but also feature a power supply with a low unload voltage—ideally less than 35 volts. This maintains a high speed, but reduces wear and tear on electrodes. The right type of dielectric fluid can also combat bullet nosing. Try tap water with the appropriate additives instead of de-ionized water.

 

These are ideal for cavity and rib machining, and as with wire EDM, selecting the correct machine is the key to productivity. Features such as c-axis, lateral servo, high-speed jump and state-of-the-art power-supply technology can reduce electrode wear and improve productivity. CNC sinkers with high-speed spindles can also perform the same job as an EDM drilling machine, but at a much smaller hole diameter and with more precision.

 

When it comes to purchasing any type of EDM, remember to consider its control technology. Many machines now have sophisticated software that incorporates 3-D modeling, which can significantly improve ease-of-use and operator efficiency.

 

Maintaining EDM Performance

Proper preventative maintenance is crucial in keeping EDM machines running as efficiently and economically as possible. Operators need to check and maintain consumables—such as power feeders, filters and resin—regularly to prevent unstable, inaccurate machining.

 

Cleanliness can also affect an EDM’s performance. Contaminants such as sludge, dirt and spray from dielectric fluid as well as burrs can compromise filter life, cause corrosion and reduce overall accuracy. Check and clean the worktable, lower arm and head, seal plate, conductivity sensor, fan cooler, filter pump and auto-threader to maintain peak performance. Also be sure to keep the machine well lubricated.

 

Check the power feed and ground cables regularly. Problems with them are the number-one cause of speed loss in EDM machines. The wire transport system, rollers, bearings and flush cups are other elements of the machine that are often the culprit of faulty performance.

 

Summary

With the pressures to compete on a bigger and more global scale, moldmakers have been forced to do more with less. Increasing productivity while lowering costs is a goal on every mold manufacturer’s list. Fortunately, the opportunities to succeed and grow with new EDM technologies—and several tried-and-true best practices—have never been greater.

 

OSCAMRAX has been totally dedicated to design and manufacturing of electrical discharge machines and CNC sinker EDM. If you are interested in learning further details about EDM, welcome to browse our website or contact with us directly!

 

 

Article Source: http://www.moldmakingtechnology.com/articles/how-to-succeed-with-smart-edm-operation