The sheave mechanism is applied on the machining center drum tool magazine

Abstract Based on the analysis of the working principle and structural characteristics of the sheave gear mechanism, the measures to reduce the impact noise of the external mesh sheave mechanism were put forward, and the mechanism was applied to the machining center drum tool magazine. effect.
Keywords groove wheel mechanism drum type tool magazine machining center
Classification number TH112.427

1 Overview

Machining center refers to a CNC machine with automatic tool changer. The automatic tool changer is usually composed of two parts: a magazine and a tool changer. There are also two parts that are integrated together to form a robot-free automatic tool changer. The magazine of the automatic tool changer is used to store the various tools required by the machining center's machining process. In the process of automatic tool change, the tool magazine is required to select the required tool according to the tool selection command issued by the numerical control device, and accurately stop at the tool change position so that the tool can be replaced automatically. The machining center has a wide variety of tool magazines. The common ones are drum-type, chain-type, and lattice-type. Among them, drum-type tool magazines have been widely used in machining centers due to their simple structure and easy control. There are usually two rotary drive methods for the drum type tool magazine, one is that the servo motor is driven by a deceleration device, and the other is that an ordinary motor is driven by an external meshing sheave mechanism after being decelerated. The former has a simple structure, fast rotation speed, and accurate positioning, but due to the selection of a servo motor, the electrical control is complicated and the manufacturing cost is high. The latter uses mechanical positioning and high reliability. Although the positioning accuracy is lower than the former, it can fully meet the requirements of machining center machines. Due to the choice of an ordinary motor, the electrical control is simple and the manufacturing cost is lower than the former. In the machining center of currently produced drum type tool magazines, a sheave mechanism is used to drive the indexing rotary motion of the tool magazine.

2 working principle and structural characteristics of sheave wheel mechanism

The Geneva wheel mechanism (also known as the Maltese mechanism) can convert the continuous motion of the master axis into the periodic intermittent motion of the slave axis, and is often used in various indexing mechanisms. There are three basic types of sheave mechanism: the outer meshing sheave mechanism, the inner meshing sheave mechanism and the spherical sheave mechanism. The machining center drum type tool magazine uses an external meshing sheave mechanism. The working principle of the outer meshing sheave mechanism is shown in FIG. 1, which is composed of a turning arm (active crank) 1, a sheave 2, a roller 3 and a locking plate 4.


Figure 1 Schematic diagram of external meshing sheave mechanism

The rotation axis of the arm of the external meshing sheave mechanism is parallel to the rotation axis of the sheave. Usually, the arm rotates at a constant speed. When the roller on the arm enters the groove, the groove wheel is pulled to perform the reverse rotation motion. When the roller is disengaged from the groove, the groove wheel is stationary and locked by the lock plate. Positioning. When there is only one roller, the arm rotates one revolution and the grooved wheel makes a turn-stop step motion.
The structural features of the outer meshing sheave mechanism are as follows:
(1) The momentary position of the roller into and out of the notch, the center line of the arm is perpendicular to the centerline of the groove, so that the angular velocity ω0 = 0 from the start of the grooved wheel to the moment of stopping, avoiding the rigid impact.
(2) The number z of the grooved pulley is 3 to 18.
(3) The angular acceleration ε0 at the start of rotation and stoppage of the grooved wheel is related to the number of grooves z. The less z is, the larger ε0 is, that is, there are different degrees of flexible impact.
(4) The positioning accuracy of the lock plate is low. When full accuracy is required, positioning devices must be attached.
(5) Simple structure, easy manufacture, reliable work, high mechanical efficiency.

3 Measures to reduce the impact of sheave wheel mechanism

Since the angular acceleration of the sheave wheel changes greatly and the angular acceleration direction is different between the first half and the second half of the indexing process, an impact occurs when there is a gap between the groove and the roller. In order to reduce the impact, the following measures should be taken:
(1) Reduce or eliminate the clearance between the roller and the groove.
(2) Eliminate the gap when the pin starts to enter the groove, and make the actual outer radius Ra of the sheave slightly larger than the nominal radius R of the sheave. 1 In the formula, rA is the radius of the roller, see Fig. 2, to eliminate the clearance â–³ between the roller and the two sides of the groove when the roller starts to enter the slot.


Figure 2 Schematic diagram of the roller entering the slot

(3) Let the sheave have an appropriate moment of inertia J2 such that |J2ε2|z, Mz is the load torque of the sheave.
(4) Increase the rotary damping of the sheave assembly.

4 Application Examples

Fig. 3 is a schematic diagram of a TH5640 vertical machining center drum tool magazine. The main components are: (1) magazine motor; (2) reducer; (3) bracket; (4) crank; (5) holder; (6) grooved wheel; (7) drum wheel; (8) lock Disk; (9) Roller; (10) Tool position detection switch; (11) Sliding bearing; (12) Rotary shaft.


Fig. 3 Structure of the tool magazine

The magazine can accommodate 18 tools, that is, the number of teeth of the sheave is 18. The rotary magazine of the magazine is driven by the magazine motor through the reducer to rotate the crank. When the crank rotates one revolution, the groove wheel rotates through 20°. The drum of the magazine rotates one tool position and is positioned by the lock plate. The knife position signal on the magazine is issued by two switches. A stopper is installed on the No. 1 location of the magazine, and a detection switch (not shown in Fig. 3) is installed on the bracket. After the machine is turned on, , The tool magazine will automatically turn to find the No. 1 tool position (reference tool position). When the check switch on the stopper of the No. 1 tool position triggers the tool magazine to stop turning, the current tool position of the tool magazine is No. 1 tool position. The library is in its initial state. The rest of the tool position signals are counted by the trigger detection tool switch 10 mounted on the crank to trigger the cumulative count, and the tool magazine rotates one tool position to trigger once per revolution. By controlling the rotation direction of the magazine motor, a two-way tool selection of the magazine can be realized.
In order to reduce the impact noise during tool selection and indexing in the tool magazine, some measures have been taken in the design.
(1) Make the actual outer radius of the sheave slightly larger than the nominal outer radius. The nominal radius of the sheave is R=150. cos10°=147.72, the actual outer radius 1 .
(2) By adjusting the pressing force of the two plane bearings, the rotation load of the sheave assembly is appropriately increased, and at the same time, the aluminum alloy material is used to manufacture the drum of the magazine, and the rotary shaft adopts a hollow structure to reduce the moment of inertia of the sheave assembly.
(3) The radial bearing of the rotary shaft is a plain bearing and PTFE is selected as the bearing material so that the clearance of the bearing is zero and the rotary damping of the sheave assembly is increased.
By taking the above measures, using the sheave