Oct.2023 07
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Precision and Characteristics of Slow Speed Wire Cutting Machines 
Introduction
WEDM, in Chinese is 线切割,it has few different speed which makes different accurancy, The slower the speed, the higher the precision
Details

Accuracy of Slow Speed Wire Cutting Machines

The accuracy of slow speed wire cutting depends on the machine model and the operator's skill level. Currently, in the machining industry, accuracy can reach 0.005mm.

Slow speed wire cutting utilizes a continuous moving fine metal wire as an electrode to perform pulse electrical discharge on workpieces, generating temperatures exceeding 6000 degrees Celsius. This process erodes the metal and cuts it to shape the workpiece, making it a type of CNC machining equipment.

The surface roughness of workpieces processed by slow speed wire cutting machines can typically achieve Ra=0.8μm or higher. Additionally, slow speed wire cutting machines have fewer roundness errors, straightness errors, and dimensional errors compared to fast wire cutting machines, making them widely applicable.

Characteristics of Slow Speed Wire Cutting Machines

Surface Quality

(1) Nanosecond Peak Current Pulse Power Technology

During electrical discharge machining, metal is removed through two mechanisms: melting and vaporization. A wider pulse width with longer action time tends to result in melting machining, which degrades the surface morphology, thickens the altered layer, increases internal stress, and may lead to cracks. Conversely, when the pulse width is small enough, and the action time is extremely short, it leads to vaporization machining. This reduces the altered layer thickness, improves surface quality, reduces internal stress, and prevents crack formation.

Advanced slow speed wire electrical discharge machining machines use pulse power sources with pulse widths of only a few tens of nanoseconds and peak currents of over 1,000 A, resulting in vaporization removal. This not only increases processing efficiency but also significantly enhances surface quality.

(2) Anti-Electrolysis (BS) Pulse Power

Slow speed wire electrical discharge machining employs water-based working fluids. Water possesses a certain level of conductivity, and even after deionization treatment to reduce its electrical conductivity, it retains some ions.

When the workpiece is connected as the positive electrode, under the influence of the electric field, OH- ions continuously accumulate on the workpiece's surface. This causes oxidation and corrosion of materials like iron, aluminum, copper, zinc, titanium, tungsten, and cobalt found in hard alloy materials. It results in the formation of a "softened layer" on the workpiece's surface.

Although measures have been taken to increase the resistivity (from several tens of kilohms per centimeter to several hundred kilohms per centimeter) and reduce ion concentration, they do not effectively eliminate the "softened layer" issue.

Anti-electrolysis power sources provide an effective solution to address the "softened layer" problem. These power sources use alternating pulses with an average voltage of zero, keeping OH- ions in the working fluid in an oscillating state without moving towards the workpiece and wire electrode. This prevents the oxidation of workpiece materials.

Utilizing an anti-electrolysis power source for electrical discharge wire cutting can control the altered layer thickness to below 1μm, prevent cobalt precipitation and dissolution in hard alloy molds, and ensure the longevity of hard alloy molds.

Cutting Accuracy of Slow Speed Wire Cutting Machines

(1) Multi-Pass Cutting Technology

Multi-pass cutting technology is the fundamental method to enhance the accuracy and surface quality of slow speed wire electrical discharge machining. It integrates design and manufacturing technology, CNC technology, intelligent technology, pulse power technology, precision transmission, and control technology.

Typically, it achieves high-quality surfaces through an initial cut, followed by a second cut to improve accuracy, and three or more additional cuts to enhance surface quality. In the past, achieving a high-quality surface required 7 to 9 passes, but now only 3 to 4 passes are needed.

(2) Continuous Optimization of Corner Machining Technology

Corner cutting can lead to corner collapse due to wire lag. To enhance corner cutting accuracy, researchers have implemented dynamic corner processing strategies. These strategies involve altering wire paths, adjusting cutting speeds for thin sheets, automatically regulating water pressure, and controlling machining energy.

By adopting comprehensive corner control strategies, shape errors at corners are reduced by 70% during rough machining, and a matching accuracy of 5μm can be achieved in a single cut.

(3) Techniques for Improving Straightness

High-precision finish circuits are deemed significant for thick workpiece machining.

(4) More Precise Machine Tool Structure

To ensure high-precision machining, many technical measures have been implemented to enhance the main machine's accuracy:

Control temperature: Water temperature cooling devices are used to maintain the internal temperature of the machine tool at the same level as the water temperature, reducing thermal deformation.

Use linear motors: These motors offer high response and precision positioning, achieving control equivalent to 0.1μm. They provide vibration-free feeding, noise-free operation, increased discharge frequency, stable discharge, and a roundness error of Ry5 μm in two cuts.

Use ceramic and polymer artificial granite components: These components possess 25 times the thermal inertia of cast iron, reducing the impact of temperature changes on cutting accuracy.

Adopt a fixed worktable and column moving structure: This design increases the worktable's load-bearing capacity and is unaffected by submerged machining and changes in workpiece weight.

Utilize submersion machining: This approach reduces workpiece thermal deformation.