Advances in Automation for Mold and Die - High-Performance Machining and EDM

The adoption of automation in the mold and die industry is motivated by many factors, including a shortage of skilled labor, the high cost of labor and machine tools, the need to reduce leadtimes, and the need for higher levels of accuracy and repeatability (i.e., higher quality). Automating the EDM process has long been at the heart of these efforts. More recent developments in automation are now providing significant benefits for high-performance machining (HPM) of electrodes and steel mold components. This article surveys recent advances in the application of automation to the mold and die industry.

A useful framework for describing the level of automation used in a mold and die operation has been developed. In this model there are three automation steps or phases through which mold and die shops progress.

• Phase 1: Increase of machine (EDM, HPM) utilization through off-line setups.
• Phase 2: Increased throughput through improved process robustness/capability and an increase in available machine hours.
• Phase 3: Increased throughput via system-wide integration and elimination of non-value steps (inspection) further process capability improvements.
• Each successive phase is layered on top of the previous phase.

Phase 1 automation has been achieved throughout the vast majority of the mold and die industry. The use of highly accurate palletizing for work pieces and electrodes on HPM and EDM machines is widely credited with reducing setup times on machines and improving overall tolerance levels. The use of these systems enables higher levels of throughput and significant improvements in accuracy and repeatability across components. Typically, within a Phase 1 environment, an off-line presetter is used to prepare jobs while the EDM or HPM is in production. This boosts overall capacity significantly. The standardization of these palletizing units also allows jobs to be targeted for a range of different machines within the shop. This flexibility can significantly increase throughput while reducing scheduling complexity.

Today, leading mold and die operations are progressing past Phase 1 and into Phase 2. Fundamentally, Phase 2 consists of two separate initiatives to boost overall through-put. The first initiative - and the more widely recognized of the two - is the use of robots to automatically load and unload electrodes or workpieces from the HPM or EDM. Originally, such robot machine systems were fairly simplistic, with poorly defined hand shaking. Now, a number of advances allow a significantly more integrated, and therefore productive, system to be created. When selecting a machine-robot combination look for the following features/capabilities:

• The ability to integrate off-line inspection data into the machine controller to adjust work offsets.
• The ability to easily reorganize the flow of work into the machine to account for changing priorities or emergency jobs. This can be achieved either through CNC software or separate job manage-ment software.
• The ability to alert the user as to the presence of a machine or robot fault. Particularly when running over the weekend, this should include a system that can place a telephone call to a pager to indicate a fault condition. Alternately, shops should consider using a web-based solution whereby a digital video feed is piped to a website. Shop personnel can then remotely access the website from anywhere in the world to visually confirm that the system is operational.

Phase 2 embodies a fundamental application of load/unload robot technology. Many of the advances within Phase 2 are related to simplifying the adoption of the robot technology itself, and providing the means by which the HPM or EDM is configured to optimize overall performance of the machine-robot system.

For example, some systems provide a completely integrated robotic solution that eases the adoption of this valuable technology.Figures 1 and 2 illustrate the integration of a robot with an HPM and EDM respectively. In this case, while the robot and machine are not a single unit, their performance is coordinated by close handshaking between the HPM CNC and the robot controller. These systems are, by definition, highly expandable.

While many shops have adopted loading systems within Phase 2, to achieve the maximum potential from an automation program, process robustness and capability must also be improved. It is relatively straightforward to load parts into a machine 24 hours per day, seven days per week. The difficulty comes in making high quality parts 24/7. A number of technical advances developed by HPM and EDM manufacturers help address this point.

In the realm of HPM, excellent repeatability from part-to-part is required to support an automated approach. To achieve this, a high degree of thermal stability is required. Some HPMs, for example, include not only glass scales, but a CNC-based thermal growth compensation system that corrects for thermal effects every servo update cycle (more than 1,000 times per second).

It also is important that an automated HPM or EDM be capable of providing a very high degree of accuracy when used in an unattended mode. This is true even if the parts to be produced are not exceptionally accurate. If, in HPM for example, the machine's dynamic tolerances (as measured by the ball bar or similar test) are not within a band of less than 50 to 60 percent of the desired part tolerance, inadequate "reserve tolerance" is left to accommodate drift in process parameters over extended production runs. Changes in ambient temperature, tool wear, electrode quality, spindle growth, etc. occurring over many hours can lead to inaccurate parts, and therefore, scrap. 3-D ball mill compensation, an HPM CNC feature that allows true surface normal adjustments for variations in ball mill diameters, is a highly enabling feature for automated operation. Contour optimization software, which defines specific contour tolerances (i.e. 0.0005") to be achieved by the machining process, also is critical to ensure that adequate "tolerance reserve" is left to accommodate "process drift." In high-performance EDM, features like adaptive control constraint (ACC) and adaptive control optimization (ACO) are vital.

In both HPM and EDM, every possible step must be undertaken to ensure that the machine can continue to produce quality parts without interruption. For HPM, laser tool setters are a critical technology for automation. By measuring tool diameters and lengths while the spindle is rotating, the negative effects of run-out are corrected for, and tools that have prematurely worn or broken can be replaced automatically with redundant tooling. In EDM, automatic wire threaders, and the ability to change dielectric filters without shutting down the machine provide similar benefits.

Today, most shops using automation solutions are in the process of establishing Phase 2 performance criterion. In its ultimate form, Phase 3 automation provides the most flexible approach to automating mold and die production by (a) providing improved process capability and (b) by more tightly integrating EDM, HPM and inspection elements.

Phase 3 solutions, which are only now becoming commercially practical (as opposed to technically) promise a significant leap in throughput and product quality.

A Phase 3 approach to automation requires the highest accuracy HPM and EDM machines, equipped with tool (as in Phase 2) and part measuring capabilities. Such a system may use a single robot to serve both the HPM and EDM, cutting down the investment required in automation. While this reduces the amount of hardware required, it significantly increases the complexity of the cell control system, particularly when a "chaotic" rather than "deterministic" work flow is desired. Some robots well suited to this approach are capable of swinging their end effector and a 330-pound workpiece through 300 degrees. This allows them easy access to one or more HPM and EDM machines.

While a Phase 3 approach also may include a CMM, it's possible to implement a solution without any distinct pre- or post-process measuring steps. This can be accomplished when working with HPM and EDM machines that provide a very high degree of accuracy. Referring back to the concept of "tolerance reserve," if the HPM machine, for example, is capable enough it can virtually guarantee that electrodes or hard steel parts are produced within required tolerance limits. This can eliminate the need for part inspection between the HPM and EDM processing steps. Likewise, a highly capable EDM can reduce the proportion of work that must be inspected before moving to subsequent process steps. The use of in-process part probing can provide further assurances as to overall part quality without the use of a dedicated CMM station.

Further flexibility can be provided in a Phase 3 implementation by using machine tools that are designed for handling a wide range of work piece materials and geometries. While traditional Phase 1 or Phase 2 implementations have focused on the production of electrodes on HPMs, and the use of these electrodes in EDM, the field of application in Phase 3 is expanding to encompass aluminum, and hard and soft steel as well. As compared to graphite or copper, the production of steel parts requires much more process capability when running unattended. For these materials, the importance of having machines with accuracy and capability to spare cannot be overstated. And, with respect to process design, it is critical that robust processes be built into the CAM-generated toolpath. When using a highly automated Phase 3 solution, the importance of high feedrates and low cycle times for HPM is reduced in favor of highly reliable processes. While an automated HPM/EDM solution can have between 2x and 4x the productive capacity of a conventional HPM or EDM, it will only achieve this potential if the process is able to continue without interruption for periods of between 16 to 72 hours, depending upon the time of week in question.

Figure 3 illustrates a five-axis HPM for use in a Phase 3 environment. The machine is designed to produce graphite, copper and hard steel components, and can access five sides of each part in one fixturing. Integrated with a robot capable of transferring work pieces of up to 165 pounds, the machine also is outfitted with a laser tool measurement system, a part measurement system and a range of accuracy enhancing features. While the capital investment for such a machine exceeds that for a three-axis HPM, it enables more complex parts to be made to a higher degree of accuracy with less operator intervention.

To conclude, the range of automation available to support HPM and EDM operations is quite broad. In adopting automation for mold and die operations, consider the current organizational culture in the shop and the current level of technical sophistication within the business. The best approach to automating HPM and EDM is to use an approach that begins with Phase 1 and proceeds step-wise through Phase 2 and into Phase 3. Such an incremental approach ensures that the necessary organizational learning takes place, and reduces risk and expense. Finally, it is critical when selecting HPM, EDM and tooling vendors that one selects a partner with experience across all three phases of the automation spectrum.