It’s only natural that designers and toolmakers should ask about the precision of electrical discharge machining, more commonly known as EDM. Knowing the capabilities of the process informs design decisions and influences costs. Whether the parts being made are surgical instruments, extrusion dies, or aerospace components, it’s important to know what tolerances are achievable.
Unfortunately, there’s no simple answer. While accuracies of +/- 0.00004” (0.001mm) are sometimes quoted for wire EDM, +/- 0.0002” (0.005mm) is more widely accepted. The reality is that EDM accuracy depends on a combination of workpiece characteristics, the surface finish desired, and the time available for machining. The only way to fully appreciate the potential precision of EDM is by understanding the process.
EDM is a non-contact machining process that uses electricity to remove material. The tool is separated from the workpiece by an insulating fluid, (water, in the case of wire EDM,) and an electrical potential is applied. The fluid breaks down and a spark strikes the workpiece. This blasts a particle from the surface, which is carried away by the movement of the fluid. Unlike conventional machining processes, material hardness has no impact on cutting speed.
Wire EDM is used for cutting complex 2D shapes or profiles from electrically conductive materials. Much like the carpenter’s fretsaw, wire EDM can cut in all directions. Most wire EDM machines hold the wire vertically and move the workpiece in x and y. Many machines offer the ability to incline the wire, allowing production of draft angles on tooling and conical features.
The wire used for EDM ranges in diameter from 0.004” (0.100mm) to 0.013” (0.33mm) with 0.010” (0.254mm) being the most common. When it cuts sparking occurs over the forward 180 degrees, so the kerf width is significantly wider. Actual width depends on the power supplied but is typically about 1/3rd larger than the wire diameter. In practical terms then, a 0.10” wire cuts a path around 0.13” wide.
Speed and precision are controlled by managing cutting parameters – power and frequency – and controlling the rate of fluid flushing. Lower power and speed improve accuracy. This is because flushing moves the wire slightly, and flushing pressure is higher at higher feed rates. In addition, the wire bends slightly in a phenomenon known as barreling and at higher speeds the effect is more pronounced.
Tolerances of +/- 0.001” are achievable during a relatively quick roughing pass. Cutting the power and reducing flushing pressure for a skim achieves around +/- 0.0005”. Following up with a second skimming pass improves the precision of EDM, to around +/- 0.0002”.
Wire EDM is capable of very high precision, even in very hard materials. However, getting there requires several passes. Many products and applications don’t need such accuracy, and so may be machined by EDM in much less time.
There are several reasons why EDM is a good machining solution: