Down to the wire
- January 11, 2002
Controlling consumable costs in wire EDM
by Jim Barnes
Wire is one of the most visible costs in EDM. As a primary consumable in the process, most owners are interested in reducing their usage of wire. As with any exercise in cost reduction, though, it is important not to confuse price with cost. Wire consumption does not exist in isolation. Saving money on wire to lower costs in one part of an operation can have unexpected consequences by driving them up in another.
"There's usually a price that's paid for reducing wire consumption–accuracy, productivity or something else,” notes Todd Romeiser, managing director, ONA EDM USA. "A lot of people go for the low initial-cost outlay on wire, without thinking about their total costs.”
Many owners need to study their real costs more thoroughly, according to Gisbert Ledvon, business development manager for GF AgieCharmilles LLC. Conventional hourly operating-cost analyses can be misleading. Users are asking themselves the wrong questions. Rather than ask themselves how much wire their machine uses per hour, "they should be asking themselves how much it costs to make a part and how many parts they can make per hour,” he says.
Advances in EDM technology over the past few years have dramatically changed the cost equation for wire.
"If you want to save wire, you're better off getting a newer machine,” claims Romeiser. For one thing, older machines often have DC power supplies, meaning slower cuts and more wear on the wire. "Between an older (DC) power supply and a newer (AC) power supply, you are saving roughly 20 to 30 per cent in wire usage,” he says.
That isn't the only advantage to new machines. Mechanical improvements to Makino's newer machines have been mated with refinements to the controls to better control wire feeding, positioning and usage. One guiding objective is to reduce wire consumption, explains Jeff Kiszonas, EDM product manager, Makino.
A number of relatively recent technologies can contribute to reducing wire waste and machine downtime, notes Ledvon. Automatic wire changing, now a third-generation technology at GF AgieCharmilles, is a good example, he says. It makes the process of going from one type or diameter of wire to another fairly bullet-proof, and supports unattended machining.
Wire preparation is another concern, adds Ledvon, noting that GF AgieCharmilles supplies a wire-preparation table for this specific purpose. Users have to bear in mind that costs associated with machine downtime due to wire breaks and quality problems can quickly surpass wire costs, he notes. "Proper wire preparation ensures that the machine will run efficiently, without missing a thread or missing the next hole.”
Quality and consumption
Compensation for wire wear is another well-established control function that helps reduce wire consumption. Instead of increasing the wire unspooling rate to compensate for wear, the machine's U/V axis is used to adjust for wire taper. "At the end of the day, you are still cutting a straight part–even though you have more wear on your wire,” notes Ledvon.
Compensation can go beyond taper. Makino machines incorporate a feature called BellyWizard. "We go a step further during the rough pass. That's where a belly or bow would occur in the wire during the roughing, and that's because of wire lag,” says Kiszonas. The technology also compensates for changes in the wire caused by eroded wear and wire lag. The result could be the elimination of one or more finishing passes, reducing wire consumption significantly.
Your choices in wire can also impact your wire usage and dramatically impact your cost per part. "It's kind of disheartening, actually,” says Romeiser of the misconceptions many users have about wire costs.
Brass wire should not be an automatic choice, despite its low cost. "There are a number of factors that should enter into the process of deciding what type of wire to use,” notes Dave Lynch, consumable sales manager, Sodick Inc. He cites several considerations:
- Performance: can be defined as a combination of cutting speed, surface finish and accuracy. Cutting speeds with coated wires are typically 20 to 25 per cent faster than brass wire. Brass and coated wires can achieve different surface finishes and accuracies based on workpiece material, dimensions and geometry.
- Workpiece details: thin parts cannot tolerate high speed cutting with coated wires, while cutting where poor flushing conditions exist (such as with tall workpieces) are well-suited for coated wires.
- Compatibility: all machines can use brass wire. Most machines can use coated brass wires, while only one brand uses coated copper wires.
- Availability: Most users achieve actual cutting hours of 2,000 to 2,500 cutting hours per year. Using coated wires will increase actual linear distance cut, increasing billing potential.
The machine builder is a good source for information if you want to avoid problems, notes Ledvon. "You save a couple of bucks on the wire, and then spend hundreds of dollars replacing your guides and power contacts because the surface of the wire was too abrasive.”
"Many people use 10 thou (thousandths of an inch) brass wire,” says Kiszonas. However, he says, "There are applications where it may be more cost-effective to use a coated wire that costs two or three times as much.” He cites the example of one customer who cut close to 30 hours out of an 80-hour job by switching to coated wire from brass.
Zinc-coated wire could give you about a 15 per cent speed increase in cutting speed, according to Kiszonas. With diffusion-annealed wire, you might see a 25 per cent speed increase.
Wire thickness is another issue. "If you go from a 10 thou to a 12 thou wire, you will see about a 20 per cent cycle time decrease, just by the diameter of the wire,” says Kiszonas. He says he can get the same accuracy and finish with 12 thou wire in 20 per cent less time with about 30 to 40 per cent less wire than if we used 10 thou. "Since we are going faster without increasing the wire feed rate, we are saving wire, too.”
"Using thinner wires, say six thou instead of 10 thou, will cut material usage, but cost you in time on top of other costs–like installing new guides and wire changeover time,” notes Romeiser. "Users want to know what gives them the best bang for the buck in cutting speed and wire usage. The answer is 12 thou brass for many applications… You are cutting 20 per cent faster and the cost difference is maybe 15 per cent, so it's pretty cost-effective.”
There is no one right answer for all users. "The proof is in the pudding,” says Romeiser. "Do test cuts.”
Wire usage and costs can be a thorny issue. "I think a lot of shops out there just don't really know how much they spend in wire and they don't know about the alternatives out there,” says Kiszonas.
"The ones that understand EDM, they track things very closely. They can see to the penny what one machine has to offer over another.”
Jim Barnes is a Toronto-based writer with 30 years of experience in technical journalism.