EDM/PLASMA

Replacing grinding with EDM on short-run production for steam turbine root form cutters enabled this shop to maintain wire-cut contour tolerances to 0.2 mil for more complex cutter designs.

Advanced Machining Corporation (Salisbury, NC) is an innovative tool and die shop that runs a mix of CNC (computer numerical control) EDM (electrical discharge machining) systems unattended, 20 hours each day. In addition, the company has successfully combined short-run production work and one-of-a-kind custom work, with business expanding at an impressive rate. The customer base includes clients in the automotive, aircraft, medical, and nuclear power industries.

According to Keith Felts, the company's co-owner, "A few years ago, most everything we did was one-of-a-kind, custom items. Now we're doing more repeat tooling and getting into short-run production and prototyping. Running more parts that are short- to medium-volume with our EDMs has meant the need to increase EDM capacity with automated machines that will run unattended, producing parts around the clock." Founded in 1986 by Felts and his partner, Bobby Miller, Advanced now runs eight EDM systems, four wire-cut and four-die-sinking. All are Agier and Eloxr machines from Agie Ltd. (Davidson, NC), including the Agie MondoT StarT die-sinking and an AgiecutT ClassicT wire-cut EDM systems.

The move into short-run production projects is changing Advanced's product mix. The shop continues to receive custom, one-of-a-kind jobs that tend to require a high level of expertise and labor input. At the same time, it is winning an increasing number of production jobs that demand attention only until the first successful production parts are made. This situation is attractive, Miller explains, because at this point the hard part is done: the work is programmed, and the setup is figured.

"By having a mix of the two," says Miller, "while we're doing the brain work on the really tough, complex one-of-a-kind parts, we've got production work out there keeping the machines running. This allows us to be more profitable. On the other hand, I really don't want us to head into doing only production work because we've invested in the skills that allow us to do the one-of-a-kind, complex jobs very well. We believe that doing them also enhances our reputation and our ability to take on the tough jobs. We want to maintain a mix of the two."

Precision Contouring Root Form Cutters

From the beginning, Advanced has done work in tough material such as Inconel, titanium, carbide, high-speed and stainless steels, and powdered metals. One particular job for a Winston-Salem, NC, unit of Westinghouse in the steam turbine industry involved manufacturing Christmas tree-shaped root form cutters from CPM 42 and HS 30 materials, previously ground by another shop. The cutter is 8" long with a 40-taper shank and a series of lobes: small and pointed at the top, and about 3" wide at the base, resembling a Christmas tree. The cutter is used to create grooves in the hub of a turbine, and turbine blades are then inserted into the finely cut grooves.

According to Felts, "With grinding, there was no way to provide any axial relief on the cutters. With EDM, we have no limits, no boundaries, and we can wire-cut and die-sink any electrode geometry into a cutter that we can design on our CAD (computer-aided design) system. The sinker puts all of the relief into the cutter." Manufacturing engineer Jim Cox of Westinghouse adds, "We've been able to double the cutting feed rate with the EDMed cutter, and cutter life is more than doubled."

Another interesting aspect of this job is that the customer is unable to inspect the slots made by the cutters; so the tolerances of the cutters' wire-cut contour must be to ñ0.0002". The sinkered reliefs are not quite as critical, but the completed cutter is what Felts calls "a gage with flutes." Felts explains, "We make the cutter so accurately that the customer is confident the slots are being cut well within their specifications. The only way to verify this is that everything fits; they know right away if there's a problem."

It actually took about nine months of collaborative research and development work for Advanced and Westinghouse to resolve the final form of the cutters. Testing included manipulating the cutter geometry, changing the relief design, adding helical geometry and positive rake angles, and more - all to improve cutting action. "The bottom line is that absolutely, positively, we're saving this customer money," says Felts. "The price per tool is slightly higher than it was with the grinding process, but the customer's tool life has doubled. Because of the way we've done our geometry, they can sharpen each tool more times before the form goes out of tolerance. Their feed rates are higher, and their cutting cycle times are shorter."

Reconditioning Inconel Hammer Dies

Another Westinghouse project involved a hammer die made from a 40 lb. block of Inconel. The dies are used in a forging-type operation in which stainless steel is pounded into turbine blades. They measure 8" ' 12" ' 3" thick with very large surface areas. The dies take enormous abuse; so much that they become deformed during their use in the forging operation. "Originally, Westinghouse wanted us to mill the dies back into shape on our machining centers," says Miller. "This was feasible, but Inconel is really tough stuff; it just eats up cutters and spits them out. We felt it would be more economical and more trouble-free to build graphite electrodes. Then we could just sinker the surfaces back into shape."

Some of the turbine blades are 6' long, and the hammer die strikes the blade from end to end. After the die wear and deformation reaches a certain level, Advanced restores the surface with EDM processing, making the die usable again. This is a big cost advantage to the customer, since in the past the worn dies could only be discarded.

Projects like the root form cutter and the hammer die have encouraged Advanced to install another die-sinking system. "We needed more capacity, and it was very important that we add a machine that had the automation level we've become accustomed to with the Agietrons," says Felts. "We needed the automatic electrode changing and the ability to process multiple workpieces while the machine runs unattended."

Miller adds, "Being able to run unattended makes a tremendous difference. It's really the key to profitability because you make money at night. The highest cost is personnel; our first-shift folks take care of programming, off-line setup, and getting everything else lined up so we can run two unmanned shifts. Our erosion time goal is 15 - 20 hours per day; and that's just about the maximum, since you're always going to have a certain amount of non-cutting time associated with setup, inspections, and those sort of things."

Regarding the Agie Mondo Star, Felts explains, "It has 85% of the features of a full-fledged CNC machine, but in reality it can do 99% of what most people use a CNC sinker for," says Felts. "The things that are missing are not needed; and if present they would add lots of cost without providing us with any usable benefit, for the most part. If you were to compare the Mondo Star to most manufacturers' full-blown CNC sinkers of the same size, the cost for their machines would be anywhere from 50% to 100% higher." Agie Ltd.

"Our erosion time goal is 15 - 20 hours per day; and that's just about the maximum."