Poco Benefits

Issue 151

Graphite Machining Tips

 SFM FEEDRATE

Many concerned machinists have asked, “What is the proper surface feet per minute to use when cutting graphite?” There are many resources available when determining the proper feed and speed rate to cut graphite. Some of these are supplied by OEM’s of graphite machining centers while others are supplied by manufacturers of cutting tools. One source available to those involved in machining graphite is the POCO EDM Technical Manual. In this manual is a chapter devoted to machining graphite and contains guidelines and recommendations on SFM ranges for HSS, Tungsten Carbide and Diamond cutters. Many shops cut outside these ranges; however because of tooling or spindle limitations there are many that do not have the ability to run at the feedrate thought to be required to generate enough heat to machine graphite. This is not a problem as some may think.

Graphite is not cut by the same mechanisms as steel,aluminum and other metals. In this case, metal chips are formed by a ductile deformation process. This occurs when cutting at the proper feed and speed rates to create a tiny spot of heat softened metal just in front of the tool’s cutting edge. This tiny spot makes the metal easier to cut and reduces the power required for the cut. Cutting metal too slow does not generate the proper cutting conditions to create the heat softened zone for metal removal. On the other hand, cutting metal with higher feed and speed rates than required will generate too much heat and affects the cutting tool. Both of these situations will reduce the life of the cutter.

So what happens when cutting graphite? Does it soften if we get enough energy into the cut? Not really. Graphite does have certain conditions under which it softens, but these conditions cannot be met under normal atmospheric pressure or oxygen content present in the typical machine shop. The temperature required to “soften” graphite is over 3000° C. At this temperature, the cutting edge of the tool would soften and become ineffective in the cut. Also at this temperature, the graphite would react with the oxygen and oxidize when it came into contact with the shop’s air. For this reason,common machining practices cannot develop the temperature required to soften the graphite. Instead, graphite is removed by a fracturing process. With this, the surface of the material is broken into tiny pieces with being no larger than a few microns. This is exactly where the dust comes from. The greater the chip load on the cutting edge, the larger the dust particles will be.

So what does the brittle fracture mean for surface footage? It means that graphite is not sensitive to cutting at exactly the“right” SFM feedrate. Graphite can be cut on an older knee mill or on the latest high speed CNC machining centers. So don’t worry if your machine is not able to move at the speeds of the newest technology on the market today. There are still methods of machining the detail in your graphite electrodes with success.

Chip Load

 Chip load is a factor that should be considered when determining the effect that cutting graphite has on the material surface finish and the life of the cutting tool. A roughing chip load should be kept in the range of 0.003” – 0.005” per tooth(inch revolution for lathes). Significantly larger chip loads causes undue stress on the cutter and can damage the surface of the graphite. In particular,the brittle fracture means of cutting graphite generates microscopic cracks in the surface of the material. As long as the chip load is consistent with the guidelines, there is little chance of affecting the strength of the part and reduces the potential of any microscopic cracks. If roughing at chip loads greater than what is recommended (for example, 0.010” per tooth), then you should leave approximately 0.020” of material surface to finish. This allows for a finishing machining pass to remove any microcracking created during the roughing passes.  A suggested chip load for a finishing pass would be 0.001” – 0.003”. In addition to affecting the surface integrity of the graphite, it should be noted that the heavier the chip load the larger the dust particles. Conversely, the smaller chip loads create a very fine dust so care should be taken in order to capture this. Remember,graphite dust is electrically conductive so keep it away from any electronics or circuitry in the shop.   

Controlling Graphite Dust

There are two methods for controlling graphite dust during the machining process. The first method is to use a high air velocity system with a dust collector. Conventional machining centers such as mills and grinders can be equipped with a fixed or portable dust collection unit.Portable dust collection systems may be preferred in shops that machine minimal amounts of graphite. This “shop vac” system is mounted on wheels and easily moved from one machine to another as needed. While this is typically considered to be the most economical approach, special consideration should be taken when determining the filter to use. Most standard shop vac systems do not have the HEPA filter required to capture the fine submicron particles that are generated when machining graphite.

Most shops machining larger amounts of graphite prefer a fixed system with vacuum heads located on the machine. These heads are connected to vacuum ducts leading to a central collection unit or “hopper” generally located outside the building. The recommended speed for capturing dust is an air velocity of approximately 500 feet per minute at the vacuum head and 2,000 feet per minute in the vacuum duct. This maintains a positive air stream from the head to the collection unit and keeps the dust from settling inside the duct work. The air velocity is determined by the amount of graphite dust in the air, which relates to the rate of material removed during the machining process. Poco’s machining operations recommend Industrial Ventilation: A Manual of Recommended Practice by the American Conference of Governmental Industrial Hygienists, as a source for information on designing dust collection systems. Graphite machining centers on the market today have built in dust collection systems and eliminate the concern of machining dust in the shop.

The second method of dust collection is the wet graphite machining process. Directed streams or curtains of fluid around the spindle head capture the dust before it becomes airborne and escapes into the shop’s atmosphere. These systems include a filtration system to remove the graphite sludge from the fluid that is circulated back through the system. When using the wet method of machining, extra care should be taken to ensure a rust inhibitor is added to the fluid to protect the machine tool from rusting. Also,it should be noted that graphite is inherently porous and fluids may penetrate the porosity and introduce impurities into the material that may cause problems in some applications. After machining with coolant, the material should be “baked out” in a convection oven (not a microwave) at a temperature of around 400° F for a sufficient amount of time to remove the liquid. Most electrodes can be baked out in one hour or less.