In mold processing, medium-speed wire EDM is generally the final process, so it requires high processing accuracy. If the machine tool's mechanical accuracy is poor, it will directly affect the processing quality of the product and mold, ultimately leading to the loss of customers and market share. Therefore, when selecting a medium-speed wire EDM machine tool, it is very important to judge its mechanical accuracy, structure, and quality.
Below, we will elaborate on the transmission structure, guide rail structure, and drive method of the worktable of the medium-speed wire EDM machine.
1. Lead screw and its mounting structure
While the precision of the lead screw is important, even the best lead screw will be useless if the installation structure is not reasonable.
First, observe the type of lead screw to determine whether it is a ball screw, a triangular thread screw, or a trapezoidal thread screw. On wire EDM machines, ball screws are superior to triangular and trapezoidal thread screws, and the screw diameter should be as large as possible to increase rigidity. It is best to use such components from well-known companies.
Secondly, the installation structure of the ball screw is important. Ball screws are precision transmission components, so in order to fully utilize their precision, the assembly structure of the ball screw must be of high quality. It must be fixed at both ends and supported by the shaft. For large-stroke machine tools, the ball screw must also be pre-stretched to reduce the deflection of the ball screw, thereby improving the motion accuracy of the machine tool table.
2. Gear transmission structure
(1) First, it is important to understand the number of gears in the gearbox. The more gears involved in the transmission, the greater the transmission resistance, the easier the tooth surface wears, and the easier it is to generate gear backlash, which leads to a larger systematic error of the machine tool table. If the more gears there are, the more likely they are to generate random errors if they are not assembled properly. Therefore, the fewer transmission gears the better. Using a pair of gears to transmit the transmission has the smallest error. Therefore, it is very important to understand the gear transmission structure of the table movement when selecting a model.
(2) Carefully observe the working environment and lubrication performance of the worktable transmission gearbox. Some early machine tools from certain manufacturers had unreasonable designs that allowed cutting fluid to easily enter the gearbox, causing the gears to work in a state of prolonged immersion in cutting fluid. This design is highly unreasonable, as it can increase gear wear or even cause the stepper motor to burn out due to water ingress. Therefore, users must carefully observe the position of the worktable gearbox when selecting a machine to ensure that cutting fluid cannot enter, as this is crucial for maintaining the machine tool's accuracy.
3. Rigidity and overall structure of the guide rail
(1) The guide rail is the key to ensuring the accuracy of the worktable movement, and users should pay close attention to it when selecting a model. First, observe the size of the cross-section of the guide rail. Under the same conditions, the thicker the cross-section, the better the rigidity and the less likely it is to deform during processing. Secondly, inquire with the manufacturer about the material and heat treatment process of the guide rail. Generally speaking, to ensure its strength and minimize deformation, high-carbon alloy steel and integral quenching or high-frequency quenching processes are preferred.
(2) Directly observe the guide rail structure. The common guide rail structures on the market are as follows:
Steel-lined ball bearing rolling guide;
Steel-lined roller type rolling guide;
Linear rolling guide;
Flexible bearing cast iron guide rail.
The difference between the first and second types lies in the rolling elements of the guide rail: one uses balls, and the other uses rollers. Balls have point contact with the guide rail surface, while rollers have line contact. Therefore, the wear resistance and load-bearing capacity of the roller type are significantly superior to those of the ball type. Elastic bearing guide rails, being a simple type, have been phased out. Users should prioritize the third type when selecting a guide rail; currently, medium-speed wire EDM completely favors the third type.
4. Driving method
Medium-speed wire EDM machines are generally driven by stepper motors. Stepper motors are further divided into reactive and hybrid types to achieve different step angles. The most common type on the market is the reactive stepper motor, which comes in different drive modes such as three-phase three-step, three-phase six-step, five-phase ten-step, and five-phase double ten-step. Three-phase motors have a large step angle and only three-phase windings. With increasing market demand and the rising requirements of the Ministry of Machinery Industry for the overall level of the machine tool industry, five-phase ten-step motors will gradually replace three-phase six-step motors. This is because five-phase ten-step motors not only further refine the step angle of three-phase six-step motors but also increase the motor windings to five phases, resulting in smoother transmission and greater force. In recent years, there has been further progress; major domestic medium-speed wire EDM manufacturers have begun using five-phase double ten-step drive modes, which further enhances drive performance.
In conclusion, to ensure long-term reliability and accuracy of the machine tool, users should choose the following structure when selecting a medium-speed wire EDM machine:
(1) Select a precision ball screw with the largest possible diameter, and ensure that both ends have fixed bearing support structures;
(2) The number of transmission gears on the worktable should be small, preferably a pair of gears, and the position and structure of the gearbox should ensure that it is absolutely not immersed in cutting fluid;
(3) Use linear guides with a large cross-sectional area or a large width-to-thickness ratio whenever possible;
(4) Select a five-phase double ten-step stepper motor drive mode. If conditions permit, it would be even better to use an AC servo drive.
5. What issues should be considered when purchasing large-scale wire EDM machines?
Large-scale medium-speed wire EDM machines have a wide processing range and strong load-bearing capacity, making them unmatched by foreign machines when processing large molds. Therefore, they can generate considerable benefits. However, if the machine malfunctions, especially if there are problems with the quality of the machine's internal memory, the losses can be severe. Therefore, the selection of large-scale medium-speed wire EDM machines must be done with extreme care.
Due to its large stroke and high load capacity, the selection of a machine tool should focus on its structure and rigidity to ensure reliable accuracy when machining large workpieces. Structurally, it differs from small and medium-sized wire EDM machines in the following significant ways:
(1) The worktable must adopt a fully supported structure to improve the rigidity of the machine tool and ensure that it will not deform under heavy load.
(2) Imported linear guides are used to ensure good sensitivity and motion accuracy even under heavy loads.
(3) The diameter of the ball screw must be increased compared to that of a small machine tool.
(4) If the user has sufficient resources, it is best to use an AC servo motor to drive the machine tool's worktable. Its features are accurate precision, fast movement speed, high reliability, and convenient operation.
