To understand the need for controlling the wire from the spool to the scrap bin or scrap roll, we look at each component and its importance in the wire EDM transport system. We’ll look at where we started and then where we are today.
At the heart of this system are the wire guides; the upper guide and the lower guide. The first type of guide I used was a series of rollers and round die guides that would anneal the wire and draw it through a sizing guide to maintain a consistent diameter. This was an extraordinarily complex process and required a lot of maintenance, the result was a soft, but a very precise wire.
The Round Carbide
This system was easy to use, but took EDMers backward in their quest for a more precise cutting process. It took a round carbide with a V ground in the outside diameter and mounted in a horizontal plane on the upper z-axis. It gave users a long-wearing guide that was quick to thread and easy to maintain. On the lower z-axis, the system featured a large quarter section of a 3″ diameter round sapphire with a similar V ground in it. It was also mounted in the horizontal plane, but 90 degrees from the upper V to change the direction of the wire feed, towards the back of the machine for wire disposal. The downside to this guide system was the need for constant and precise tension on the wire to keep it in the V. It didn’t allow for cut tapers larger than half of a degree, and sharp corners were near impossible.
The Split V Guide
The split V guide system is still popular in many of today’s machines. With simple maintenance and reliable auto wire threading, it’s easy to see why. The first version of this system used two square sapphire blocks: one with a polished flat side that was pressed against the other, and the V side, which consisted of two pieces glued together with a chamfer on the seam to make a perfect V. The solid side was called the pressure foot, and it pushed the wire firmly into the V guide to create a light tension at the guide in both the upper and lower heads on the Z axis. This precise guide system increased cutting speed around the corners of the part being machined. Better yet, it made a precision point for the start and end point angle of the wire, giving manufacturers the ability to create angle cuts for more than just simple die relief on die blocks.
The Split Round Guide
Splitting a round guide instead of a V gave users greater angles of wire EDMing—which let a soft wire make more precision-angle cuts. When manufacturers use diamond guides instead of sapphire guides, the guide life increases exponentially. It’s still one of the most popular and widely used versions of the split guide today.
The Donut-Shaped Guide
Along the same timeline, manufacturers started using a donut-shaped guide with a hole slightly larger than the diameter of the wire EDM. It’s widely used across most manufacturers and allows for wire threading and angle cutting like the V guide system. While the donut guide has a short lifespan and requires more maintenance and monitoring then the split guide system, it’s much easier to replace (and costs less, too).
The Wire Transport System (1970s)
Now we will look at the overall wire transport system from the wire spool to the scrap bin starting with the simple systems used circa 1970.
Starting with a shaft that holds the spool of wire in a horizontal position, the wire then is fed across pre-guide rollers and under a small micro switch used for wire break detection. It then heads to a feed roller that changes the direction of the wire from horizontal to vertical. Next, it’s sent down through the upper head where electrical current is applied to the wire, then through the workpiece where the EDM cutting is done, and onto the lower guide system where again power is applied to the wire. At the exit of the lower head, we have a roller that changes the direction from vertical back to horizontal to exit the cutting area. If the wire goes into a scrap bin, it is pinched between rollers to keep tension on the wire from the source spool. If the scrap wire goes to a take up spool, the take-up spool will have a take-up motor that rotates in a counterclockwise direction, opposing the source spool and keeping the wire taut while it travels along the programmed path maintaining accuracy and corner definition. If the end point of the wire is a scrap bin, then a drive motor and pinch roller would pull the wire into this bin creating the tension in the transport system.
The Wire Transport System Today
Starting at the wire source spool, back tension motors with breaks are a popular option because they keeps the wire spool form unwinding in case the wire has run out, or broken before the tension roller. It also backs up to respool wire during the wire cut and rethread operation. Next, the wire will usually go through a series of idler spools too keep the changing speed of the wire from causing the wire to jump off the rollers. Then the wire wraps around a tension roller attached to a servo motor or a power clutch to create tension in the wire guide area. Next, in the path between the tension roller and the upper guide, machines can have a wire annealing system or wire cutter for the auto wire threading process. It adds a few more tension and feed rollers to the system, all of which require periodic maintenance. Now, the wire enters the upper head, where the electrical current is applied through an energizer plate. This is another maintenance-heavy item; with some advanced systems it is a simple indexable plate that requires the operator to index at pre-determined intervals. As the wire passes through the guides and through the material being machined, the advanced tension system monitors the tension and adjusts for the change in cutting speed and geometry. After leaving the lower head, the spent wire is on its way to the scrap bin. In some advanced systems, the wire is cleaned and dried before entering the pinch rollers to ensure the tension is precise and constant.