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Fig. 3 - The slots on these workpieces could have been made with slitting saws, but wire EDM had the advantage of producing no burrs. the large one is about one inch in diameter.
Fig. 1 - Inside corners with a small radius, thin walls, straight walls -- a perfect candidate for wire EDM.
Fig. 4. - The tip of this medical device was produced with wire EDM rather than with a formed grinding wheel. The close-up shows the tip's complex contour.
Fig. 2. - These parts were wire-cut in the hardened state. Cutting a stack of parts at one time makes wire EDM very productive. Each part is slightly larger than a postage stamp.
Lavelle Machine installed its first electrical discharge machine (EDM), a Fanuc wire unit from Methods EDM (Sudbury, Massachusetts), about five years ago. This shop in Westford, Massachusetts, quickly discovered that this new wire machine was not just a way to bring secondary operations in house, but a process that would expand the capabilities that could be offered to customers. Soon the shop was using wire EDM in many ways it had not originally imagined.
In 1998, realizing the value of in-house EDM capabilities, Lavelle Machine added three more Fanuc auto-threader/submersible wire EDM units along with two Workman model ram EDM units from Hansvedt Industries, plus a HoleMaster small-hole EDM, also from Methods EDM. As remarkable as the growth of EDM at this shop is, more remarkable is how EDM changed the shop's approach to manufacturing.
"EDM taught us to look at producing parts from a totally new perspective," says Ed Lavelle, Jr., company vice president. "It has allowed us to produce parts with greater consistency, repeatability, and efficiency."
And yet, the shop does not see itself turning into an EDM specialty shop, going after exotic or extreme applications. "We're not trying to take EDM to a new level," Mr. Lavelle explains. "We're using EDM to take our shop to a new level."
For this user, ordinary EDM is having extraordinary results. It's a good example of what EDM can do, and many other shops can see how valuable wire, ram and small-hole EDM can be for the machine shop. Even a small sample of typical parts from Lavelle Machine makes this clear.
Small Corner Radii
The need to produce openings, slots or other workpiece features with corners having small radii is common. If you think of the wire electrode as a kind of cutter with a very small diameter, you can imagine its ability to cut very small radii.
"We frequently use wire diameters of 0.004 or 0.006 [inch], allowing us to produce corner radii of 0.0035 to 0.0040 with no special effort to obtain these results on a consistent basis," says Mr. Lavelle. Although the most commonly used wire diameters within the industry are 0.010 and 0.012 inch, most of the wire machines produced today can easily handle wire of much smaller diameter. (EDMing with wire sizes below 0.004 inch diameter will require special guides supplied by the various machine manufacturers.)
The workpiece shown in Figure 1 is a good example of how Lavelle Machine uses wire EDM to its advantage for sharp internal corners and other benefits. Although the workpiece would appear to have a very simple contour, the sharp internal corners of this 6061-T6 aluminum part would be virtually impossible to make with any other method of machining. Using a wire diameter of 0.004 inch allowed an internal corner radii of approximately 0.0032 inch (one half the diameter of the wire plus the expected overburn). Moreover, the straightness and consistency of the wall thickness was a critical requirement.
Mr. Lavelle also points out that cutting with wire EDM produces no significant pressure on the workpiece. "Actually, what we have to pay attention to is the high pressure flushing available on today's wire machines," he says. "Enhanced flushing is one of the major improvements made to wire EDM machines and it contributes to faster cutting speeds, but the high pressures have to be used carefully on delicate parts." In any case, flushing pressures can be controlled manually or automatically by the machine's control.
The shop produces this type of part in quantity, utilizing automation to run the machines in the unattended mode whenever possible. Blank plates are stacked together in various thickness, depending on the required geometry. The plates are held together by various means (welding, pinning or fastening with screws). The small-hole EDM is used to produce wire start holes in exact locations, with diameters from 0.015 to 0.080 inch. Typically, wire start holes are located inside and outside the specified contours.
By nesting multiple contours, material consumption can be maximized and unattended operation extended as far as possible. When the entire array of parts is completed, the wire path is retraced with the power supply turned off until the wire reaches the holding tabs left between the contours. Power is turned on and the wire cuts through, allowing finished parts to drop free.
"Cutting nested parts from stacks of plates is a strategy we employ as often as we can," reports Pete Smith, who is in charge of EDM/technical sales at Lavelle Machine. "It's perfect for production jobs but has its place even in prototype work." Figure 2 shows a good example.
Prototypes In A Hurry
These two pieces were part of an R&D project Lavelle Machine produced for a medical industry customer. Mr. Lavelle explains: "We're not sure what the intended application was, but the customer e-mailed us the geometry as a DFX file, which we imported directly to our CAM software to produce the wire path. We didn't have to touch the geometry to do the programming."
The customer supplied the blank workpieces, made of 17-4PH stainless steel. These were stacked and cut to produce six sets of parts. Turnaround time was two days.
"Another big plus for EDM," notes Mr. Smith, "is cutting workpieces in a hardened condition." These workpieces had been heat treated to 42-44 Rockwell C and were wire-cut as is. "There was no distortion to affect the intricate geometry or the flatness of the part," Mr. Smith adds. "When we look at how to produce a part, EDM gives us some options for machining after heat treating or when working with prehardened materials. Often, we can eliminate operations for correcting geometry changes that occur in heat treat."
Another operation that wire-cutting eliminates is deburring. Figure 3 shows a couple of workpieces where burr-free cutting was a definite plus. These 316 stainless components are part of a motor housing assembly. The slots allow these parts to be compressed as other components grow in size due to heat generated by the motor.
Lavelle Machine produces these parts by the thousands. According to Mr. Lavelle, the parts are turned from tube stock on CNC lathes, but the slots are wire-cut. "Cutting the slots with slitting saws was an obvious possibility but having to remove the burrs from the inside of the part made this unattractive, even though the slitting saws are very fast," he says. "What makes wire EDM a preferred alternative is unattended operation."
The parts are mounted on fixtures in six rows, each row three parts high. Slots are wire-cut from both directions prior to moving down the line to the next row. Multiple rows as well as stacked parts allow for longer unattended machine time. This type of multiple fixturing has proved to be more economical than a "chip cutting" operation.
Mr. Smith points out that automation is the key. "When we run this part, we set up a fixture block, hit the cycle-start button, and walk away. When we come back, we reload the fixture and start over." Submerged cutting is essential in this approach, he says. "The wire enters the parts from the side, makes a U turn, and finishes the cut on the way out. Because the parts are cylindrical, cutting conditions are changing constantly as the wire passes through. Efficient flushing would very difficult to maintain without submerged cutting."
The Fanuc OC machines that produce this type of part operate with the workpieces totally submerged. The recycling water system on these machines uses ordinary tap water that has been deionized to remove conductivity. The water continually circulates through the filters and the machine-controlled deionizing unit. The advantages of a submerged cutting machine are temperature stabilization and closely controlled flushing even in difficult cases where the workpiece has varying thickness along the wire path.
Burr-free cutting is especially valuable in the production of surgical instruments and other items for the medical industry. A burr left behind by manufacturing has the potential to be detached and to contaminate a medical procedure. The regulations governing medical devices are very strict regarding this issue. Figure 4 shows another unusual workpiece, manufactured by Lavelle Machine, in which burr-free cutting was important.
The part is a sheath for a device that injects soft tissue anchors. These anchors are like miniature staples that clip tissue together to repair tears or cuts. The anchors are absorbed into the body as it heals. The tips of the sheath are pointed to hold tissue in place as the anchor is inserted.
"We could have ground the contour that forms these tips using a formed and dressed grinding wheel," explains Mr. Lavelle. "Grinding would be faster than EDM but deburring would have taken up the time saved. With EDM, the points need only a brief electro-polishing step to round off edges so that the points grip tissue without severing it.
Just as burr-free cutting favors wire EDM, chipless cutting favors ram EDM in some instances. The titanium bone screws shown in Figure 5 are a case in point. The outstanding feature of these screws is the unusual thread form, designed for compatibility with the structure of living bone. The shop produces a number of different sizes of these screws. The thread form varies according to the size of the screw. Precision ground form tools cut these threads on the shop's Citizen screw machines. Less remarkable but no less important is the precisely formed hex-shaped cavity in the top of each screw. This is where EDM comes in.
"This is another part we produce in quantity," notes Mr. Smith. "We looked at drilling a blind hole and broaching the hex shape but could not be sure that all broaching chips could be removed from the bottom of the hole, where broaching would tend to lodge them. Because bone screws are implanted in the body, the presence of any sort of chip is unacceptable."
Ram EDM was the answer. "Surprisingly, we found this approach to be very productive, even compared to broaching," remarks Mr. Lavelle.
The shapes are EDMed on a Hansvedt Workman ram unit using tellurium copper electrodes, one for roughing and one for finishing. The screws are mounted in fixtures (literally screwed into place) to be EDMed. All roughing burns are completed, then all finishing burns are completed to finish the batch.
"Titanium is a difficult material to machine conventionally," says Mr. Smith, "but for EDM, workpiece hardness is not an obstacle. As long as the material conducts electricity, we can EDM it."
Small-Hole Enhances Wire And Ram
Mr. Lavelle feels that small-hole EDM completes the EDM triad because the shop relies on this ability to make small, deep straight holes. For wire EDM work, the small-hole machine is used regularly to make start holes, as has been mentioned. Small-hole EDM is also very valuable for drilling flush holes in the electrodes used in ram EDM. In fact, EDMing contoured blind pockets is a good example of how wire, ram, and small-hole EDM work together. Blind pockets can even be formed on the interior of hollow workpieces, a practice Lavelle Machine often follows to complete entire workpieces in one piece.
To make deep contoured pockets, an electrode can be cut with wire EDM. Lavelle Machine generally uses copper for long, slender electrodes because of the superior strength and resistance to wear of copper. Although this material can be difficult to machine conventionally, it can be readily cut with wire EDM. In most cases, a roughing and a finishing electrode are required. The correctly sized electrode for each operation can be produced on the wire EDM by making an offset change and producing a larger or smaller electrode as required.
The electrodes used to make deep pockets require flushing holes, which are frequently formed down the entire length of the electrode using the small-hole EDM. While the pocket is being EDMed, dielectric fluid is forced through the hole under pressure. As the fluid exits from the bottom of the electrode, the fluid carries away ash and debris as it flows up and out of the gap between the electrode and the walls of the pocket. By varying the location of the flush hole from roughing to finishing electrode, any spike or mark left by the hole on the bottom of the pocket is eliminated easily.
According to Mr. Smith, the shop is capable of burning small flush holes through material as thick as 7.0 inches with the HoleMaster small-hole EDM. The ability to make holes this deep while maintaining straightness through the electrode is critical, especially on electrodes with a small cross section.
EDM In The Mainstream
"Now that we've had EDM in our shop for five years, we consider it an indispensable complement to our expertise in other machining processes," claims Mr. Lavelle. "It's never a last resort. We've made it a habit to ask ourselves, how could we EDM this part? It's part of our initial planning routine." Sometimes EDM, whether ram, wire or small hole, isn't the right choice. "Even so, we're more creative in our thinking for having considered EDM," he says.blog comments powered by Disqus