In the next two installments, we've prepared pure, practical information for you: the key to rotary axis positioning accuracy in five-axis machining, mainly discussing the following two points:
1. Discuss the full closed-loop and semi-closed-loop control modes of the rotary axis of the machine tool rotary table;
2. Discuss the role of two high-precision positioning rotary axes in 5- axis machining on machine tools;
The rise of globalization and the internationalization of markets have led to increasingly diversified customer demands. In the end market, users expect more diverse and personalized products. In the manufacturing industry, these changes are no secret. To compete with suppliers worldwide, manufacturers must offer innovative products, superior quality, and robust functionality. They also need to meet requirements for small batch sizes, tight geometric tolerances, and higher surface quality for visible and functional surfaces. Simultaneously, manufacturers must adapt to shorter time-to-market and shorter product lifecycles.
In production, increasing the flexibility of product specifications necessitates the use of 5- axis machining technology. A versatile tooling system allows for multi-faceted and complete machining, enhancing automation, flexibility, and machine tool utilization. Because 5- axis technology allows for the extensive use of standard cutting tools and enables changes in tool orientation along the milling path, it is suitable for machining complex geometries.
5- axis machining
In 5- axis machining, all five machine tool axes move relative to each other and are interpolated synchronously (three linear axes and two rotary axes).
3+2 processing
If the rotary axis is moved to a fixed position before machining and remains in that position throughout the machining process, this is 3+2 axis machining.
Even if environmental and processing conditions change, machine tools must maintain high precision in machining. Therefore, the requirements for measurement accuracy and long-term consistency must be met in position information processing. This is especially true in 5- axis machining, where rotary axis positioning errors significantly affect machining accuracy, thus directly impacting workpiece precision.
Figure 1 : Typical workpieces machined on 5 axes
Depending on cost structure, customer requirements, and machine tool processing conditions, specific rotary axes can be driven by torque motors or by servo motors and mechanical transmission systems. For rotary axes using mechanical transmissions, encoder selection is particularly crucial, and this article focuses on this topic. For rotary axes composed of servo motors and gear transmission systems, the simplest method to measure their position is using a motor encoder and the transmission ratio. This is the position feedback control method of semi-closed-loop control.
semi-closed-loop control
In semi-closed-loop control, errors in mechanical transmission components are not considered. These errors are not included in the feedback control loop. Periodic loads in semi-closed-loop control cause heating of the transmission components, which is a significant reason for larger positioning errors.
Full closed-loop control
If the angle encoder is directly mounted on the rotating shaft of the rotary table for position feedback, this is a fully closed-loop control. In fully closed-loop control, almost all mechanical transmission errors are contained within the position control loop.
In the discussion below, we will see the significant impact of the gear system in the measurement chain on the positioning of the rotating shaft and compare the effect of this method with that of using an angle encoder directly on the rotating shaft.
First, let's understand the configuration of the machine tool under test:
The following measurements are from a high-end 5- axis vertical machining center configured with one linear axis and two rotary axes for the workpiece, and two linear axes for the cutting tool (the machine structure is similar to Figure 2 ). The machine's travel range is approximately 600 mm x 600 mm x 500 mm . The main focus of the test was the C- axis rotary axis of the worktable , which is driven by a servo motor and a worm gear.
Figure 2 : Schematic diagram of a 5- axis machining center
The design and manufacture of the reference encoder are used to determine the positioning accuracy of the rotary table. Figure 3 is a schematic diagram of its structural design. The reference encoder has an optical scanning grating [1] and four non-contact reading heads [2] . The scanning grating is located at the center of the rotary table and is mounted and fixed with an adapter [3] , which drives the rotary table to rotate during measurement. The reading heads are distributed in the mounting base [4] and fixed to the machine tool spindle with a clamping system [5] .
Figure 3 : Schematic diagram of the reference encoder
Figure 4 shows the reference encoder installed inside the machine tool. During measurement, align the rotation axes of the following components: the rotary table, the reference encoder's grid drum, and the spindle. The accuracy of the reference encoder is verified by a calibrated measuring machine to ensure a system accuracy of ±0.5" . This accuracy requirement can be achieved within a larger installation tolerance range and a working tolerance range of ±1.0 mm radially and 0.4 mm axial runout . Perform multiple measurements on the machine tool to verify the repeatability and quality of the reference encoder.
Figure 4 : The reference encoder is located on the rotary table of the machine tool.
The advantage of this reference encoder is its large allowable installation tolerances (especially ±0.2 mm eccentricity), which greatly simplifies installation and facilitates practical use. The nominal system accuracy of this encoder refers to the accuracy of the entire reference encoder system and is unaffected by external environmental factors. Measurements can be performed at any position of the rotary axis, and extremely small angular steps are also possible. A fixed number of measurement positions or equal spacing between measurement positions is not specified. There is no need to associate the reference encoder with the machine tool.
Although direct-drive systems for torque motors are widely used, a significant proportion of machine tool rotary axes still employ a combination of servo motors and mechanical transmissions. The main reasons for this include the complexity of machining and the cost structure of the machine tool. For servo motors, there are two methods for determining the angular position of the rotary axis: two position feedback modes, namely full closed-loop ( CL ) and semi-closed-loop ( SCL ), as shown in Figures 5 and 6 , respectively .
Unlike full closed-loop control, semi-closed-loop control has multiple error sources due to the numerous components between the encoder's position measurement point and the corresponding rotary table. These include geometric errors, elastic errors in mechanical transmission components, temperature effects, and wear. The dynamic effects of machining forces and vibrations also influence position measurement. However, in full closed-loop control, positioning accuracy is largely unaffected by these main error sources because the angle encoder measures these errors at the origin position and accounts for them in the position control loop.
So what conclusions did we draw? Stay tuned for the next installment!
To be continued!
