Standing alone on the floor of a mold shop that makes 10,000 EDM electrodes per year is one of a new class of graphite-only machining centers. A complex network of overhead ducts runs throughout the shop to collect graphite dust from the many machine tools that cut graphite, but the new machine isn't connected. It has its own dust-collection system. What it is connected to is a DNC link that feeds vast amounts of 3D machining data to the company's CNC mills. With a spindle running at 10,000 rpm and with feedrates of over 300 ipm, the machine - a Roku-Roku Electrode Master - spits out electrodes four times faster than the conventional machining centers in the shop. This is the state of the graphite machining art.

The company is OAR Moldworks (Providence, RI), and the rest of its plan is devoted to the full range of moldmaking operations. As many as half of OAR's employees are engaged in making electrodes, however, because the toolchanger-equipped, round-the-clock CNC EDM machines produce work and consume electrodes at a prodigious rate. Conventional mills, grinders and a lathe or two are put into electrode-making service and each one, or sometimes each area, is fitted with a vacuum inlet that runs to the overhead ducts, which, in turn, collect in a single vacuum room dedicated to keeping graphite dust out of the shop.

3D CAD streamlines production of contoured molds, generating the enormous streams of positional data needed to machine complex electrodes. Ozzie Rosenholm of OAR Moldworks, an early adopter of productive technologies, has embraced solids modeling and untended CNC EDM. Now he's closed the technology loop with high-speed 3D electrode machining.

The Premier Electrode Material

Whenever there are ram-type EDM's making mold cavities or other tools, there are machine tools making graphite EDM electrodes. With its high resistance to erosion from EDM discharges and its easy machinability, graphite has long been the dominant material for EDM electrodes in the USA. But machining graphite creates a high volume of abrasive, noxious dust. Only recently has it made major inroads against copper alloys as an EDM electrode material in Europe and Japan, where EDM users had been willing to trade slower and more difficult electrode-making for a healthier environment for both machines and humans.

Graphite can be machined aggressively, constrained only by the required surface finish and its tendency to chip. But it's notorious for producing clouds of abrasive dust. Grinding is the worst offender, and it demands a powerful dust-collecting vacuum system with special vacuum shrouds.

Graphite wins the overall performance contest, however. The force required to cut most grades is low, and it cuts cleanly, without burrs. Thus, it has an advantage over copper in the machining of accurate ribs and fins, and deep, narrow pockets and slots. The development of ultra-fine-grain graphites allows machining of inch-tall vanes that are only 0.020 in. thick. These grades also have overcome much of the advantage that copper once had in producing fine surface finishes.

Coping with dust

"It's worth the trouble to cope with graphite dust," says Ozzie Rosenholm, OAR Moldwork's president, "because graphite is a better electrode material overall." OAR has found ways to live with graphite machining over the course of several decades. Early users of Swiss-made EDMs, and, later, Japanese-made ones, have used copper electrodes and considered the arguments for copper versus graphite for years. They still make some copper and copper-tungsten electrodes for special purposes, especially where they can cut the electrodes on wirecut EDMs. But graphite dominates at OAR largely because it cuts faster and lasts longer.

OAR has gone to extreme lengths to control graphite dust. The shop-wide, ducted dust-collection system has been mentioned. Grinders have dust-collection shrouds and even the benching stations have vacuum nozzles that can be tilted and turned to keep them close to the electrodes as they're hand-worked.

On a larger scale, Poco Graphite, a major graphite supplier, is like a laboratory for systematic dust collection. They have multi-spindle machine centers each producing up to 18 lb. of graphite dust per hour. "We haul it out with a forklift," say Larry Wingo, Poco's Machining Engineer. Poco's is the largest machine shop for graphite in the world. Aerospace, semiconductor, biomedical, and glass industry customers consume far more graphite than EDM electrode makers do, and Poco machines large quantities of it in its own shop, making such products as heat shields for spacecraft, artificial heart valves, glass-handling grippers and parts for experimental fusion reactors.

They've developed sophisticated vacuum systems to deal with the large amounts of dust, based on detailed engineering studies. Poco recommends vacuum systems with air velocities of 500 fpm. Of the dust-capturing end, and 2000 fpm in the ductwork, to keep the dust suspended in the lines. Poco's research has produced much information useful for the EDM electrode manufacturer. They've developed tables of cutting speeds and feeds, conducted cutting-tool tests, and they've produced extensive application data for the EDM process.

“For a lubricant, it's awfully abrasive...”

Graphite is used as a dry lubricant - so why is it abrasive? "Graphite used for lubricating is naturally formed material," says Poco's Laboratory Manager Bill Brixius,

"When it's formed naturally it has an ideal graphite structure that lets layers - planes - slip over each other. Graphites fabricated for EDM are synthetic. The process of making it varies, but, in general, it doesn't create the same structure as that of natural graphite. Synthetic graphite particles are crystalline. The mechanics of the two types are the same, but the layers don't slip in the synthetic."

Premium graphites are the most abrasive, says Brixius. Some of the cheaper, coarser grades do allow some slippage of planes and are not so abrasive.

Machined graphite dust contains an extremely fine fraction that gets through the smallest cracks and openings. Over time, it can get past way covers and seals, and do its damage to leadscrews and slideways.

Also, remember that graphite is electrically conductive, and, when it gets into computer and motor-control cabinets, it can lead to intermittent electrical shorts that raise havoc with delicate electronics.

“If it's so bad, it must be good...”

With such a litany of complaints against it, graphite must have some virtues much stronger than mere easy machining. And it has. "Graphite is superior for making complex, precise, contoured electrodes," says Rosenholm. "And these represent a growing trend in electrode making because they reflect a similar trend in mold making."

Mold design is gravitating away from simple prismatic shapes to complex contours. 3D CAD and 3D CNC milling have opened the doors - through which designers have stepped with delight. As in many other aspects of CNC machining, the capability to machine mathematically accurate, perfectly tangent contours has created its own demand.

Machining those contours can be slow, however, because they require the machine tool's CNC to process large amounts of positional data, and because contouring increments must be closely spaced to produce smooth contours. All of these factors put a premium on speed in the machine tool: high spindle speed to achieve surface smoothness along curved cutter paths, and high traverse rates and processing speed to keep up with the data flow - and to avoid accelerating and decelerating the machine beyond its physical limit. The limit is one imposed mostly by cutting tools, based on their ability, or inability, to withstand the shocks of rapid start-and-stop operation.

"Even our best conventional CNC machining centers are slow at precision contouring." Says Rosenholm, "and they're too slow for making electrodes". Machines made to cut mold steel are at opposite poles from the ideal for cutting graphite. They both need high accuracy and thermal stability, but, in the tradeoff between torque and speed, graphite needs only the speed. Thus, the recent development of specialized graphite-only machining centers, which employ all of the tricks of high-speed milling (look-ahead data processing: high spindle speeds: balanced wayloads) and precision machining. In a package that's far faster than comparable all-purpose machining centers.

Long-term thermal stability is a dominant requirement for electrode-making. Molders demand interchangeable mold components and extreme accuracy. This Roku-Roku addresses the issue with an oil-cooled spindle and balanced, bridge-type construction.

One week after his Roku-Roku was installed, Rosenholm paced between the new machine and the company's room of stock electrodes, many of which are repeat jobs that he was comparing with ones freshly produced by the new machining center. Holding up a contoured electrode that just came out of the machine, he reported "45 minutes. That job took 3-1/2 hours on our CNC Sharnoa." Another week of trials indicated that was the general relationship in machining time for the graphite-only machine versus OAR's all-purpose machining centers.

OAR's new machine is built around the configuration of Japanese and European jig border, with a bridge structure supporting the spindle and with oil cooling controlling spindle temperature. Thermal stability is the key to long-term accuracy, and making long runs of electrodes for multi-cavity molds requires that the accuracy held in the morning must still be held at the end of the day.

"No longer do we get mold prints that show tangent features without numerical values attached to them," says Rosenholm. "They used to jus say 'blend'. Now we have 3D positional data along the curves, and we have to produce miters (parting-line matchups in the mold halves), in the medical field and in some electronics component molds, within 0.0003-in. tolerances. The standard miter tolerance for many years was 0.003 in."

In one recent job for a razor manufacturer, OAR had to make several dozen EDM burns in one cavity, with different-shaped electrodes, to a total tolerance of 0.0002 in. the molds had to be interchangeable with the customer's molds in use all over the world.

Accuracy requirements in moldmaking have become extreme, and the requirement is complicated by the fact that replacements and additional molds have to be made to the same tolerances, months or even years later. The definition of moldmaking-machine accuracy, as in many other manufacturing fields, has expanded from one-time precision to absolute repeatability over long periods.

CNC for Contoured Electrodes

Look-ahead machining and high-speed processing weren't created for machining electrodes, but graphite electrode making is one of its prime beneficiaries. Fanuc's parallel processing, 64-bit RISC-chip add-on is a current favorite for processing 3D data at high speeds, and for enabling look-ahead machining at high speeds.

The look-ahead capability is important to avoid breaking cutting tools, which tend to be small and delicate. The processor reads ahead in the program and anticipates abrupt changes in direction or speed. Equipped with a look-up table that relates feedrates to permissible accelerations and decelerations, the look-ahead CNC speeds or slows the machine when it's about to enter the work piece, or to enter or exit a sharp angle.

It also speeds us machining by anticipating when the machine will be cutting air, traversing between cuts. The Roku-Roku's feedrate when cutting is approximately 310 ipm; when it's cutting air, it can speed up to 780 ipm. The machine table will accelerate as fast as it can when it the tool leaves the cut, and then rapidly decelerate when it's about to re-enter. Thus, look-ahead capability lets the machine run a top speed all the time.

This function is internal to the CNC. To make the most of its data-processing capability, the CNC needs a stream of positioning data delivered to it at high speed. That data usually is fed from outside the CNC, in incremental chunks, while machining is underway. Large, digitized 3D programs can run upwards of 10 megabytes of data, and CNCs rarely are equipped with memory capacity to handle the large programs.

Direct links from the programming computer (DNC) are one way the data is transmitted, but it's becoming more common to "slave" an ordinary 486- or Pentium-based personal computer in the DNC link, between the host and the CNC. The relatively inexpensive PC acts as a buffer, downloading programs quickly from the host and storing them for incremental feed to the CNC.

The complex temperature-stabilizing, data handling and control capability used in the new graphite-only machining centers puts moldmaking on the leading edge of sophisticated manufacturing technology. The machines and their controls incorporate precision bearings and a lot of electronics - potentially exposed to that abrasive, conductive, insidious graphite dust. Rather than leave dust-handling to the chance that users will have advanced shop-wide systems, to which they'll hook their machines, the builders of these specialized machining centers have incorporated built-in dust-collection systems, backed up by extra-thorough attention to slidway and ballscrew protection. They're fully enclosed and self-contained, with high-volume dust vacuums that, as one expert puts it, could suck the dust off a prospector's mule.

Diamond Tools are a Trend

Graphite's abrasiveness is an issue in cutting-tool selection, as well as in machine protection. The forces required to cut it are unremarkable and high-speed-steel cutters do a fine job - for a short while. But, with the advent of so much 3D contouring, and the multiple-pass, fine-increment cutting that accompanies it, tool wear has emerged as an important issue. OAR's typical "stepover" between passes is now 0.002 in. Resolution between points is down to 0.0005 to 0.001 in. Approximately 75% of the electrodes they produce are machine finished; five years ago, the figure was 25%. All of these trends result in a multiplication of tool wear.

Carbide tools have been widely used since graphite's beginnings; coated tools have made some inroads; and the trend is pointing toward diamond-tipped tools, especially in 3D contouring, where tool wear is highest.

"Many coatings, such as TiN, improve cutting performance through their lubricity," says Poco's Larry Wingo. "In the case of graphite we're cutting a solid lubricant (at one physical level), so the improvement these coatings provide is smaller than it is when cutting metals. They do give some improvement, and multi-layered coatings sometimes give even more. But multi-layer coatings introduce another problem: the coating deposition process tends to produce radiused edges, and that leads to chipping when you're cutting graphite."

Chipping is a key factor that limits feed rates in cutting graphite. It's a particular problem where a cutter exits and causes breakout, and sharp tools all help.

Diamond produces great cutting performance and tool life in graphite, and sharp-edged polycrystalline diamond is the favored tool material. The recently introduced vapor-deposited diamond-film cutters perform nearly as well in terms of wear resistance but the, like the multi-layer coated carbides, often have radiused edges. Poco has found considerable variation in the amount of roundness that appears on vapor-deposited diamond-tipped cutters.

The combination of cutters that have long and accurate edge live; high-speed 3D contouring capability of new, graphite-only machining centers; and self-contained, high-performance vacuum systems is making electrode machining faster, cleaner and far more accurate than it was only a few years ago. And it's just in time. 3D CAD and untended EDMs are pushing electrode making from both sides: the demand is for better electrodes, produced in less time. Technology is the only way to address those demands, and the necessary electrode-making technologies are ready for action.