Gears can be made using countless different manufacturing methods. As you might expect, the tooth profile needs to follow specific dimensions and constraints in order to transmit torque, prevent slippage and excessive undesired contact stress. Therefore, the manufacturing process you choose should meet the necessary tolerances and characteristics. This article will describe the EDM process in gear manufacturing.
Here are some terms you need to know before getting started:
Gear Blank: The gear material/block before the tooth profile is defined. It can be a cylindrical block, or other geometric shapes such as racks, cones, etc.
Workpiece: The material that will be cut to produce a specific geometry. Gear blanks can be workpieces, but not all workpieces are gear blanks, as the described manufacturing process can be used for other types of designs.
Knife: A tool used to cut a workpiece. In EDM, a tool is sometimes called an electrode.
What is EDM?
Electron discharge machining (EDM) is a manufacturing process that removes material from a workpiece by applying a series of electrical discharges between two electrodes separated by a dielectric bath.
Wire-cut EDM: In this type of EDM, a wire is used as an electrode and it is continuously fed from an automatic feeder with a spool during machining. Typically, the fluid is ionized water and the wire is brass or copper. We strongly recommend that you check the workpiece material and wire electrode material to determine the optimum process parameters. Because the continuous wire electrode cuts the workpiece, only the entire thickness of the workpiece can be cut.
Subsidence Electrical Discharge Machining: Also known as Tooling, Conventional EDM or Ram EDM. This type of EDM process can produce complex geometries that cannot be achieved with EDM. The electrode material is usually graphite or copper. Electrodes are machined to a specific design geometry before starting the EDM process. This geometry is basically the negative or mirror shape to be produced on the workpiece. After the electrode shape is prepared, the EDM process begins and the workpiece is damaged by sparks to form the correct geometry, a process called "electrode wear". Unlike wire EDM, Sinker EDM is not limited to fully cutting the workpiece (partial cutting is possible).
Drilling EDM: The third type of EDM. The process is used to drill holes, but Drill EDM is capable of making very small deep holes compared to traditional drilling methods. Another advantage is that drilled EDM does not require any deburring. In this process, the electrodes are tubular and the dielectric fluid is supplied through the electrodes themselves. EDM drilling uses an electrical current delivered to the workpiece through a tubular electrode. Similar to other EDM types, the electrode cuts/corrodes the workpiece. Due to the spark gap, the electrode is not in contact with the workpiece, so the deflection of the tube electrode is minimal compared to the deflection of the drill in conventional drilling processes.
Breakdown of EDM system components:
The figure shows a diagram with typical components of an EDM system.
EDM as a gear manufacturing method
Like every manufacturing process, EDM has limitations and advantages. If you don't have well-controlled processes and procedures, you can damage the surface of your part. This is especially important for gear manufacturing and tooth profiles, which include curvatures that can be challenging for CNC programs. The good news is that there are several high-quality 3D modeling and CAM software that are intuitive to use, produce smooth motion, and allow EDM machining of complex gear designs.
EDM machines have also seen advancements in recent years, improving surface finish, precision and ultimately material properties. These aspects are all important for gear fatigue. Tolerances can be as small as a thousandth of an inch, enabling EDM to generate complex geometries from small gears to large gears (gear diameters from fractions of an inch to over 20 inches in diameter).
As mentioned above, wire EDM only cuts perpendicular to the gear blank. This becomes a limitation for fabricating helical gears or more complex complex shapes, as shown in the figure. However, unlike Wire EDM, Sinker EDM synchronizes two axes. These two axes work simultaneously, producing radial or torsional motion, allowing the manufacture of helical, radial gears, and even internal full and partial cuts. In other words, the electrodes or cutters can be rotated. It is important to note that if the program and motion pattern are not designed properly, the electrodes may cut into areas of unwanted geometry, thereby eliminating the helical shape.
If your gear geometry is a custom and complex design, a good practice is to use different electrode sizes. This could be similar to sculpting or engraving, where at the end you fine-tune the details with smaller, more precise tools. Of course, this increases the working time, you now have effective control over the geometry.
Another consideration is planning: once you remove material on the first pass with an electrode that produces a high roughness value, the next electrode that is more detailed needs to be set up to make it precise. Even with timing on a CNC program, synchronizing multiple electrodes can be challenging. For this reason, some companies have been experimenting with individual electrodes containing geometries with multiple roughnesses, from coarse to fine. If you are going in this direction, there must be proper clearance between the electrodes and the different rough areas due to your machine power settings and values. This requires some knowledge of the EDM machine during the design phase. Fragmentation is another problem, especially if there are internal shapes in your design.
Regardless of the method you choose to clean debris during manufacturing, always remember that consistency in tooth profile quality and surface finish is critical during gear manufacturing. This safeguards the investment in cleaning methods to prevent unwanted EDM cuts due to debris coming into contact with the part.
Other important parameters are the electrode feed rate and any voltage and current related instrumentation. Any violent sparks or high electrical discharges will create undesired cutting forces resulting in gear pitch deviations or irregular shapes. Several studies have been developed to increase the speed of EDM gear manufacturing. A common finding is the correlation between pitch deviation and material removal rate. You don't want to rush through the cutting process and risk compromising the quality of your tooth profile.
Let's consider materials. If your gears are going to be made of mild steel, you may want to consider alternatives to dielectric fluids. Typically, deionized water is used for EDM, but it can cause some low-grade steels to rust. Some companies have developed their own in-house anti-electrolysis processes to prevent corrosion.
In addition to dielectric fluids, you should also consider gear geometry and design features to prevent corrosion. One of the advantages of EDM is the ability to cut small parts with tight tolerances. With sinker EDM, you can also cut internal shapes, giving you the opportunity to precisely work with tight gear profiles and new complex shapes and features. However, if these shapes include complex features such as offset edges, they may inadvertently have cracks that promote corrosion mechanisms. Before designing an EDM process for a gear, consider geometry, function, and possible failures.
With all the benefits of EDM for gear manufacturing, there are some cool applications. From micromachined gears for timepieces to stronger gears for racing cars, wire EDM and sinker EDM are slower than other machining methods, butstill valuable.