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3.2 The selection of the design concept
Various methods of exchanging tools are commercially available in the manufacturing environment. Five conceptual designs were generated and considered for the development of the unit. A rotating carousel, a design that is widely implemented on CNC machines, was the emerging concept.
A graphical depiction of the structure and motion of the tool-changing concept is shown in Figure 3. The BT40 tool holders are suspended from the carousel using pull studs. Magnets are used to keep them in place during carousel rotation. The carousel, linear actuators, and gripper arm are capable of providing:
a) storage and appropriate selection of the required tools
b) translational motion to exchange the tools between the carousel and the manufacturing spindle
c) rotation to allow for spindles that may be at an angle
d) vertical motion to allow flexibility for different tool holders and for mounting the tool in the spindle and retrieving it.
3.3 The core structure of the unit
The tool changer was designed to be an autonomous module within a reconfigurable manufacturing system. It was therefore important to design a unit that would provide adequate stability during the tool-changing process. The tool changer would also need to be self-reliant, because it would be critical that it not depend on other structures for stability.
A structurally-autonomous module would offer the user the advantage of flexibility: the unit could easily be used on several different machines without concern about the structural surroundings. No hardware interface would be required between the tool changer and the machine it was interacting with, as the tool-changing unit would form a complete module in the RMS environment.
The final design used a 6mm-thick mild steel ‘rolled can’ to form the core of the structure.The ‘can’ would produce the structural strength of the machine as well as provide room for some of the inner mechanical workings of the unit. It was mounted on four adjustable legs that provided stability as well as the necessary height adjustment. The skeletal structure can be seen in Figure 4.
Figure 4: The skeletal structure of the tool changer (adapted from [6])
3.4 The tool transfer system
The tool transfer system is responsible for the required motions of the tools to and from the carousel and the spindle. Four degrees of freedom were needed from the tool-changing unit in order to transfer the tool effectively. The first required motion would be the rotation of the carousel. This would provide for the exchange of the different tools to and from the gripper position. The second would be the needed horizontal motion to move the tool away from the storage carousel and towards the spindle. The tool would also then need some form of vertical actuation to lift the tool into the spindle – the third degree of freedom.
A unique feature of this tool-changing unit is that it offers a fourth motion not commonly found in other tool-changing systems. This is the rotation of the tool about the horizontal axis so that the tool-changing unit can be used with a machine that uses a spindle at a nonorthogonal
fixed angle. This added flexibility increases the unit’s ability to interact with a greater range of machines.
In terms of the structural setup of the system, a thrust bearing was placed at the top of the core structure of the unit. The carousel was then mounted on the bearing and a geared 12V DC motor was used to drive the carousel. A DC motor was also used to drive the worm gear that would provide the rotation of the gripper arm.
For the horizontal and vertical actuation, two linear actuators manufactured by Festo were used. The actuation was delivered through electric ball screw spindle drives. DC servo motors were chosen to drive the spindles due to their small size and relatively high torque. Figure 5 shows the configuration of the two spindle drives and their directions of actuation.The larger horizontal drive delivers 300mm of translation, while the vertical drive provides the necessary 100mm to ensure adequate insertion of the tool into the spindle.