Diamond Tames Graphite’s Bite

Is the old adage, "The machine is only as good as the tool in it," correct? What is correct is that CNC machining centers are the high-tech machines capable of increasing graphite electrode production in your shop.

Their high rpm coupled with their multiaxis feeds permit the machining of the complex profiles required in today’s graphite electrodes. Add the CNC programmer’s knowledge and the correct tooling, and you have a winning combination for definitely increasing electrode production.

Using a specific endmill will either decrease, limit, or maximize the machining center’s ability to machine graphite electrodes. The endmill must be capable of performing under the maximum machine functions that a combination of the machine’s capabilities and part requirements will allow.

A CNC machining center that can mill a graphite electrode at 15,000 continuous rpm and 300 ipm will not do so if the endmill has one cutting edge (PCD-style endmill) and/or cannot withstand the high rpm (carbide endmill). The endmill must have a combination of multiflute geometries and extreme wear resistance. It is only then that the endmill will support the machine’s functions, and graphite-electrode production will increase.

Coating Technology

CVD diamond-coated endmills are coated with diamond through a chemical-vapor deposition (CVD) process. The first step in this process is a quality inspection for grind uniformity and cutting-edge chips, since the diamond coating will follow the endmill’s contours. The slightest imperfection on the cutting edge of the endmill will be evident in the cut taken on the graphite electrode by leaving a ridge or groove

The next step is the coating preparation process, which includes surface treatment and cleaning of the endmill. The surface treatment is a process that enables the diamond to adhere to the carbide substrate by creating voids within the surface of the substrate. Cleaning is extremely important. A speck of foreign matter on the cutting edge of the endmill will become coated, and the endmill will be scrapped in final inspection.

If this problem is overlooked, and the endmill is sent to a customer, he or she will immediately experience a depressed line in the graphite electrode caused by the coated speck on the endmill. The time involved in inspecting and preparing endmills assures that the endmill is ready for diamond coating.

Figure 1: A photomicrograph of the diamond coating on an endmill.

The actual CVD diamond coating is then accomplished by placing the prepped endmill in a unit called a reactor. A combination of intense heat and injected gases creates an atmosphere within the reactor that causes a chemical change which, in turn, begins a growth of continuous diamond film on the endmill. The result over a specific time period is a diamond coating of approximately 0.0006" thickness, which is now bonded to the carbide substrate (Figure 1).

The reason for the 0.0006" coating thickness is that it is optimum for retaining cutting-edge sharpness while still offering extreme wear resistance. The endmill’s cutting-edge sharpness is about that of a slightly honed cutting tool edge, which is ideal for graphite applications. Basically the cutting edge is no longer in an up-sharp, feathered-edge condition, which actually adds strength to the cutting edge, but still permits the endmill to perform thin-wall and finish-machining applications.

The high wear resistance is because the CVD diamond coating process provides a pure diamond film free of any metallic binder. The uniformity of the coating is within 10% of the coating thickness, or 0.00006" in variation. To compensate for the added coating thickness, the solid-carbide endmill flute diameter is initially ground undersize. The shank of the endmill is not coated.

Figure 2: CVD diamond-coated endmills from sp³.

CVD diamond coated endmills are manufactured in 2-, 3-, and 4-helical-flute geometries, and in both ball and square-end styles. The diameter range is from 0.093" to 0.500". Endmill lengths up to 6.000" are available. Special sizes, tapers, end configurations, etc. are usually manufactured on a quotational basis. The endmill sizes are governed by standard endmill tolerances (Figure 2).

Old, Proven Tool Geometries

As mentioned earlier, an endmill that will maximize the CNC machining center’s performance must have a combination of multiflute geometries and extreme wear resistance when machining graphite electrode materials.

Chipload per flute (cutting edge) is the basis for establishing machining feed rates by considering a combination of part finish requirements, profile configurations, available continuous rpm and other factors. Figure 3 provides recommended parameters for different endmill diameters.

Figure 3: The recommended starting parameters for endmilling graphite, carbon, and unfilled plastics with CVD diamond-coated tools.

In the following examples, let’s assume that a chipload per flute of 0.005" is required in a specific graphite electrode machining application.

Example #1, single-flute endmill: A chipload per flute of 0.005" on a single-flute cutter will yield a feed per revolution of 0.005" (0.005" X 1 flute = 0.005 ipr). If we carry this further by introducing a machine rpm of 7000, we will have a feed rate of 35 ipm (0.005 ipr X 7000 rpm = 35 ipm feed rate).

Example #2, 4-flute endmill: A chipload per flute of 0.005" on a 4-flute cutter will yield a feed per revolution of 0.020" (0.005" X 4 flutes = 0.020 ipr). If we again carry this further by introducing the same machine rpm of 7000, we will have a feed rate of 140 ipm (0.020 ipr X 7000 = 140 ipm feed rate).

The examples above readily illustrate the increased (4:1) feed rate opportunity by using a 4-flute endmill vs. a single-flute cutter. Obviously, a 2- or 3-flute endmill will have specific related feed-rate increases over a single-flute cutter.

Additionally, a helical, multiflute, CVD diamond-coated endmill provides more machining stability in the cut than a single-cutting-edge tool, i.e. PCD (polycrystalline diamond) endmill. The increased cutting stability comes from the multiple cutting edges simultaneously removing material while utilizing the cutting action of helical-flute tooling geometries.

Depending on the feed direction, it is possible to have three cutting edges of a 4-flute endmill in a cut at the same time. The endmill is in a "balanced cut" configuration which causes improved finishes, reduced tool pressure, etc. Additionally, depending on the feed direction, radial tool pressures are reduced and replaced by axial tool pressures which are transferred accordingly into the spindle.

Extreme Wear Resistance

Tool life is an important issue. Graphite, as we all know, is abrasive. As soon as an endmill starts to cut into a block of graphite, tool wear begins. The amount of tool wear is directly related to the graphite grade, machining parameters, and to the cutting tool material.

If the endmill immediately loses its cutting-edge sharpness (the actual feathered edge only), there is not a noticeable difference in machining. The endmill is now in a honed edge state caused by the abrasiveness of the graphite. If it continues cutting in a honed edge state, tool life will also continue.

However, if the endmill experiences increased wear, a rapid decrease in tool life will also be experienced. The endmill’s cutting edges are now dulled beyond that required to efficiently machine the material. This, in turn, causes a variety of tool pressure machining problems (i.e., part-finish loss, breakout, chipping, part-tolerance loss, etc.).

All of these problems add up to remachining and/or scrapping of the electrode. Either way, the CNC machining center must be stopped and the endmill changed. During this tool-change downtime mode, the tool must be replaced, touching off accomplished, and the previous cut picked up (for remachining) somewhere in the worn tool’s cut path. Cutting air or recutting an area is certainly not adding to increasing graphite electrode production. Operator efficiency can also affect downtime.

Tool life is an extremely important key to keeping your graphite machining center not just running, but efficiently machining graphite.

If we consider all of the multiflute endmill materials and coatings on the market today, CVD diamond is far superior in tool life to any other. Documented testing and constant use in actual graphite machining facilities have proven that CVD diamond-coated endmills have a tool life increase of up to 50:1 over carbide endmills.

Graphite electrode manufacturing facilities have taken advantage of the CVD diamond-coated endmill’s extreme tool life increase by running their CNC machining centers in a lights-out environment, to find a large, completely machined, finished electrode within tolerance upon their return. Other graphite-electrode producers have used the same CVD diamond-coated endmill on multiple electrodes without losing size. And in another documented graphite electrode application, a CVD diamond-coated endmill was run for 153 hours, and was then set aside for roughing applications only.

A major U.S. manufacturer of graphite electrode materials has found that using a specific manufacturer’s CVD diamond-coated endmills has improved his tool life 33:1 as compared to carbide endmills.

All of this extensive documentation and related constant use of CVD diamond-coated endmills proves that they are maximizing the performance of graphite-electrode machining centers through extreme tool life improvements and multiflute geometries.

Putting CVD Diamond to Work

CVD diamond-coated endmills offer the two absolute necessities for maximizing graphite-electrode production on your CNC machining center by offering multiflute geometries and extreme wear resistance.

It is the extreme wear resistance of the CVD diamond-coated endmill that will permit your machining center to operate at any rpm that you desire. Attempting to maximize the use of your graphite machining center at high rpm with carbide endmills cannot be accomplished due to their lack of tool life.

PCD endmills have excellent wear resistance in graphite machining at high rpm, but do not have the multiple helical-flute geometries to perform at the feed rates of CVD diamond-coated endmills. In documented testing and daily applications, we have yet to find a CNC machining center used on machining graphite electrodes that has an rpm capability beyond the wear resistance limitations of CVD diamond-coated endmills.

If we use the previous example of a 4-flute CVD diamond-coated endmill operating at 0.020 ipr (0.005" chip load/flute), we can obviously calculate a variety of feed rates per minute, depending on the desired rpm. As the rpm increases, the feed rate per minute also increases.

Maintaining a specific feed per revolution suitable for the machining application and increasing the rpm will directly reduce the time required to machine a graphite electrode. Basically, the endmill is retaining the feed per revolution necessary for part finish requirements while operating at an increased feed/minute due to the rpm increase.

Thin-wall sections will require increased rpm and reduced feed per revolution to limit cutting pressures, while roughing passes can be done at both increased rpm and higher feed per revolution. The important point is that the highest possible rpm be used with feed per revolution variations suitable for the endmill size and for the specific electrode profiles being machined. By using a combination of feed rates per revolution and rpm, maximum machining parameters can be constantly maintained.

CVD diamond-coated endmills are the highest technology cutting tools available for increasing graphite electrode production. CNC machining centers are the highest technology machine tools available for increasing graphite-electrode production.

Your graphite-electrode production will definitely increase by using CVD diamond endmills to support the maximum possible machining parameters of your graphite machining center.

About the Author
Ed Kwasnick is vice president of sales at sp³ Inc., Mountain View, CA.