Figure 3. Presetting of WEDM tooling |
Figure 4. Presetting of EDM Electrodes |
Before we can automate an EDM process, certain steps need to be taken.
The EDM machine must be CNC controlled and be available with some type of robotic interface. This can be accomplished by simple M functions or through the use of RS232 serial or parallel interface. Depending on the type of automation selected, automated electrode changing may be required. Some robots will load and unload both electrodes and workpieces while others are designed exclusively for workpiece changing only. Auto wire feeding is a must for automation of WEDM machines. Drop tanks may be required if the robot does not have the capability to be programmed to move over such obstructions. Access to machines with enclosed safety gates may require a pneumatically operated door.
The EDM machine must be equipped with a tooling system featuring automatic chucks for unmanned operation (see Figure 2). This tooling must provide for absolute repeatability, the ability to operate in an EDM environment, and reliability. One used to be taught that an electrode or workpiece must not be unclamped between machining operations in order to maintain fixed reference positions. Today, technological developments have made this rule obsolete. It is now possible to clamp and reclamp workpieces and electrodes repeatedly in different machines without losing this fixed reference position. For Erowa, this was realized by the development of a spring steel centering system.
One side of the workpiece or electrode is prepared for clamping on a pallet. The workpiece or electrode is then bolted securely to the pallet. It can then be machined from five sides in a multi-axis machine, or the pallet can be transferred from one machine to another without losing its fixed reference position. Positioning errors between reclamping are +/- 1 micron (0.000078").
Spring steel centering is based on the concept that the three main functions of the tooling are separate from each other:
- Radial positioning
- Axial positioning
- Clamping
Radial positioning is accomplished as the angled prisms of the chuck are mated with the punched holes in the centering plate. Axial positioning occurs as the pallet supporting feet contact the ground flats on the chuck. Clamping is done through the use of a spigot held in position by mechanically activated spring-loaded clamping balls. The spring steel centering plate is slightly flexed when the pallet is clamped. The flexing compensates for any inaccuracies in the centering plate. The positional accuracy and repeatability of the pallet, however, amount to less than +/- 1 micron, even in automatic operations. The reliability of this type of clamping makes it highly suitable for automatic operations.
During the load and unload functions of this tooling, compressed air is blown through the chucks to clean off any mating surfaces. This insures a clean connection between chuck and pallet. The chuck backpressure is also monitored to insure correct seating of the pallet. This is critical with unattended operation.
Figure 2. Ram chuck and electrode pallet |
The final prerequisite to automation is the presetting of the workpiece and/or electrode (see Figures 3 & 4). Presetting allows a programmer to start from a fixed reference position. Workpieces and electrodes are clamped on to a pallet or palletized fixture outside the machining center. If necessary, the workpiece-offset data can be registered on a preset station or CMM and then transferred or postprocessed into the CNC program. A workpiece can be clamped to a pallet and moved through the entire shop through a number of separate machining operations without the need to repalletize or realign the workpiece at each step, as long as each machine is equipped with a suitable chuck. The workpiece can be safely moved from machine to machine at a repeatability of under 78 millionths of an inch. Imagine what this would mean when having to interrupt a job for higher priority work. No re-alignment would be necessary.