Di-Spark – Automated EDM Machining – Automation aspects of precision EDM machining
Continuing our series of electrical discharge machining articles (extracts from our EDM manufacturing guide, ‘The EDM Handbook’), this snippet examines automated EDM machining.
Di-Spark: The Automated EDM Machining Company
12. AUTOMATION: AUTOMATED EDM MACHINING
One of the most appealing features that EDM presents to manufacturing is unattended machining – or automated EDM machining. The ability for a machine to run unmanned for long periods goes a long way in helping to justify its purchase.
Automatic Wire Feed (AWF) in Automated EDM Machining
The most common item used in wire EDM to support unattended and overnight operations is automatic wire feed (AWF). This is a device that will automatically rethread the wire in the event of an accidental wire break or in the case of a work-piece with multiple apertures, like those often found in a press tools.
For example, multiple programs can be written and linked, and the wire machine will move to the first aperture, thread the wire, erode the first detail, cut the wire, move to the next aperture, rethread the wire and continue machining far into the night.
AWF is becoming increasingly popular but it has not always been well received. Automatic wire feed devices were typically very complex and precise mechanisms. When first introduced, they were a source of frustration to machine operators due to poor reliability and a high degree of maintenance. Over the years, many refinements have been made and modern systems are fast, reliable, and give good overall performance. They are most useful for jobs with multiple apertures or in production applications where multiple parts are fixed and movement and re-threading is required from one part to another.
AWF is the first step toward total automation of wire EDM and more and more machines are being sold with this feature.
There are several types of threading devices. These feed the wire through the lower guide to a retrieval system of drive belts or pinch rollers. The spent wire is then fed into a waste bin or take-up spool. In the event of a mis-feed or blocked wire-start hole, machines can be programmed to retry for a specific number of times. If the feed is still unsuccessfully after the programmed number of attempts have been completed, the machine will move on to the next position, leaving only that detail unfinished instead of the rest of the part.
Automated Wire EDM
One of the problems presented by wire EDM is the removal of the slug that is often left when cutting a part. If a slug is allowed to fall free, then it can pinch and break the wire. Worst-case scenarios can have it become jammed in the lower nozzle, resulting in broken nozzles or damage to the work-piece, tooling, or the lower control arm.
Today’s higher cutting speeds have allowed pocketing routines to be used, effectively eliminating the slug altogether, but this is used only on small details that do not consume a lot of time.
For certain applications, optional slug removers can be used. When the work-piece has been completely roughed and skimmed (except for strategic tags that prevent the slugs from falling free), the wire is automatically cut and the lower arm traverses away from the cutting area. A piston or punch then knocks the slug free from its tag to fall clear of the eroded aperture. On some machines, the slug drops away from the work-piece and it is then swept away from the machining envelope by a mechanical arm. Other machines use the lower arm itself to push the slug out of the work area.
If automatic slug removal is to be used successfully, it must be carefully planned into the EDM process. Firstly the work-piece must lend itself to automatic slug removal. The shape of the slug must allow for strategically placed tags to balance the slug and prevent it from tipping and wedging it in the work-piece. Obviously, if the slug is too large or heavy, it will have to be removed manually.
In pursuit of greater automation, pallets and pallet changers have been in use for some time. There are many types of pallet systems. These allow work-pieces to be pre-set away from the machine tool.
Wire EDM systems equipped with automatic handling systems are becoming more prevalent. Most of these units require the work-pieces to be mounted on pallets. The pallets are then stacked into the handling system that in turn uses a robotic arm to load and unload them between the wire EDM machine and the handling system. Such systems are produced by most of the traditional EDM tooling manufacturers. They are usually free-standing devices that can be moved to the appropriate machine and operated by commands from the machine’s control.
Automated Spark EDM
For unattended running of spark erosion machines automatic electrode changers are most commonly used, these often being an integral part of the machine tool. Like conventional machining centres they allow multiple electrodes to be used and changed as needed. However unlike conventional machine tools it is not only electrodes that can be changed but work-pieces also if the electrodes are fixed to the machine to table. As in wire EDM automatic handling systems can be integrated with spark erosion machines. A typical application may be to use the handling system to load and unload palletised work-pieces and the automatic tool changer to load and unload electrodes allowing many hours of uninterrupted machining.
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