The Application of Ratchet Mechanism in the Drum-type Tool Magazine on the Machining Center
(The Hengli Wanjun Hardware Processing Plant in Dongguan specializes in large-scale CNC machining, large-scale computerized milling processing, large-scale aluminum plate processing, large-scale precision parts machining, machinery equipment panel processing, precision sand casting, and large-scale mold processing.)
1 Overview
A machining center refers to a numerically controlled machine tool with an automatic tool changer. The automatic tool changer usually consists of two parts: the tool magazine and the robot arm for changing tools. There are also designs where these two parts are integrated into a tool changer without a robot arm. The tool magazine in an automatic tool changer stores various tools required by the machine tool during processing. In the process of automatic tool change, the tool magazine needs to select the required tool according to the tool selection instruction issued by the numerical control device and accurately stop at the tool changing position for automatic tool replacement. There are many types of tool magazines for machining centers, common ones include drum type, chain type, and grid box type. Among them, the drum-type tool magazine is widely used in machining centers due to its simple structure and convenient control. The rotary drive method of the drum-type tool magazine usually has two types: one is driven by a servo motor through a deceleration device; the other is driven by a common motor through an external meshing ratchet mechanism after deceleration. The former has a simple structure, fast rotation speed, and accurate positioning, but since it uses a servo motor, the electrical control is complex and the manufacturing cost is high. The latter uses mechanical indexing positioning, which is reliable, although the positioning accuracy is lower than the former, but it can fully meet the requirements of the machining center machine tool. Since it uses a common motor, the electrical control is simple, and the manufacturing cost is lower than the former. In the currently produced drum-type tool magazine machining centers, many use ratchet mechanisms to drive the indexing rotational motion of the tool magazine.
2 Working Principle and Structural Characteristics of the Ratchet Mechanism
The ratchet mechanism (also known as the Maltese cross mechanism) can convert the uniform continuous motion of the driving shaft into the periodic intermittent motion of the driven shaft, and is commonly used in various indexing transfer mechanisms. There are three basic types of ratchet mechanisms: external meshing ratchet mechanism, internal meshing ratchet mechanism, and spherical ratchet mechanism. The drum-type tool magazine of the machining center adopts the external meshing ratchet mechanism. The working principle of the external meshing ratchet mechanism is shown in Figure 1. It consists of a rotating arm (driving crank) 1, ratchet wheel 2, roller 3, and locking disc 4.
Figure 1 - Schematic diagram of the principle of the external meshing ratchet mechanism
The axis of rotation of the rotating arm in the external meshing ratchet mechanism is parallel to the axis of rotation of the ratchet wheel. Usually, the rotating arm rotates at a constant speed. When the roller on the rotating arm enters the groove, it drives the ratchet wheel to make a reverse transfer movement. When the roller exits the groove, the ratchet wheel stops and is positioned by the locking disc. When there is only one roller, the rotating arm rotates one revolution, and the ratchet wheel makes one step movement of turning and stopping.
The structural characteristics of the external meshing ratchet mechanism are as follows:
(1) The instantaneous position when the roller enters and exits the groove opening is perpendicular to the centerline of the groove from the centerline of the rotating arm, making the angular velocity 0=0 at the start and stop instant of the ratchet wheel's transfer, avoiding rigid impact.
(2) The number of teeth z on the ratchet wheel ranges from 3 to 18.
(3) The angular acceleration ε0 at the start and stop of the ratchet wheel's transfer is related to the number of grooves z. The fewer the z, the greater the ε0, meaning there are different levels of flexible impact.
(4) The positioning accuracy of the locking disc is low. When high accuracy is required, additional positioning devices must be added.
(5) Simple structure, easy to manufacture, reliable operation, high mechanical efficiency.
3 Measures to Reduce Motion Impact of the Ratchet Mechanism
Since the angular acceleration of the ratchet changes significantly and the direction of angular acceleration in the first half and second half of the transfer process is different, impact will occur when there is a gap between the groove and the roller. To reduce the impact, the following measures should be taken:
(1) Reduce or eliminate the gap between the roller and the groove.
(2) Eliminate the gap when the pin starts to enter the groove. The actual outer radius Ra of the ratchet wheel should be slightly larger than the nominal outer radius R of the ratchet wheel. Take the form where rA is the radius of the roller, as shown in Figure 2, to eliminate the gap △ between the roller and the top of both sides of the groove when the roller starts to enter the groove.
Figure 2 - Schematic diagram when the roller starts to enter the groove
(3) Allow the ratchet wheel to have an appropriate moment of inertia J2, so that |J2ε2|z,Mz is the load torque of the ratchet wheel.
(4) Increase the rotational damping of the ratchet wheel assembly.
4 Application Example
Figure 3 shows the structural schematic diagram of the drum-type tool magazine of the TH5640 vertical machining center. The main components are: (1) tool magazine motor; (2) reducer; (3) bracket; (4) crank; (5) tool holder; (6) ratchet wheel; (7) drum; (8) locking disc; (9) roller; (10) tool position detection switch; (11) sliding bearing; (12) rotating shaft, etc.
Figure 3 - Simplified diagram of the tool magazine structure
This tool magazine can accommodate 18 tools, i.e., the number of teeth on the ratchet wheel is 18. The rotation and tool selection of the tool magazine are driven by the tool magazine motor through the reducer, causing the crank to rotate. For each full rotation of the crank, the ratchet wheel turns 20 degrees, and the drum of the tool magazine rotates one tool position and is positioned by the locking disc. The tool position signal on the tool magazine is jointly issued by two switches. A baffle is installed on the No. 1 tool position of the tool magazine, and a detection switch is installed on the bracket (not shown in Figure 3). After the machine is turned on, the tool magazine automatically rotates to find the No. 1 tool position (reference tool position). When the baffle on the No. 1 tool position triggers the detection switch on the bracket, the tool magazine stops rotating, and the current tool position of the tool magazine is set to No. 1, placing the tool magazine in the initial state. The remaining tool position signals are cumulatively counted by the baffle installed on the crank triggering the tool position detection switch 10 once for each full rotation of the crank, i.e., each time the tool magazine rotates one tool position. By controlling the rotation direction of the tool magazine motor, bidirectional tool selection of the tool magazine is realized.
To reduce the impact noise during the tool selection and positioning process of the tool magazine, the following measures were taken in the design.
(1) Make the actual outer radius of the ratchet wheel slightly larger than the nominal outer radius. The nominal outer radius of the ratchet wheel R=150 . cos10=147.72, the actual outer radius.
(2) By adjusting the clamping force of the two plain bearings, appropriately increase the rotational load of the ratchet wheel assembly. At the same time, use aluminum alloy material to manufacture the drum of the tool magazine, and adopt a hollow structure for the rotating shaft to reduce the moment of inertia of the ratchet wheel assembly.
(3) Use sliding bearings as radial support bearings for the rotating shaft and select polytetrafluoroethylene as the bearing material to make the bearing clearance zero and increase the rotational damping of the ratchet wheel assembly.
Through the above measures, the inherent disadvantages of the ratchet mechanism structure are overcome while utilizing its advantages, achieving good practical results.