Tips for machining Graphite

WHAT CHARACTERISTIC SHOULD A CUTTER POSSES TO MACHINE GRAPHITE?

One of the key properties of graphite is its brittle nature. The material is soft, abrasive, and brittle. Because of this, using the wrong cutter, feed or speed can result in chipping and breakout.

Both chipping and breakout, depending on the grade of graphite, can cause serious trouble to the electrode fabricator. Small ribs, thin walls, and sharp corners can all be jeopardized — causing rework, lost time, and lower productivity. So what should you do?

First, use only a high quality tungsten carbide to machine graphite. Yes high-speed steel could be used, but tool life is limited. The tool requires a sharp cutting edge to prevent chipping. I recommend good micro-grain C2 carbide be considered as a starting point. High-speed steel should be considered only as a last resort.

With the onslaught of high-speed machining, a good carbide may need to be combined with coatings.

But finding a good grade of carbide is only the first step. The geometric profile of the carbide (speaking of indexable cutters) is as important as the grade of it. To reduce chipping and breakout as a cutter exits the side or end of a workpiece requires a combination of sharp cutters, positive rake, and low feed rates. However, in today’s fast paced, globally competitive, tight lead-time industrial market, low feed rates are unacceptable, so how do we compensate for higher feed rates? Cutter geometry.

WHAT CUTTER GEOMETRY WORKS BEST?

Indexable carbide cutters are forging new inroads in this area. Button cutters have one of the most efficient shapes for roughing to allow high feed rates without chipping. The round insert with a positive rake cuts cleanly through the material. (Tip: gain as much time as you can on roughing the electrode down, so that the finish will allow for a fine step over to reduce bench work after. The more bench work required the greater the possibility of altering the CAD geometry.)

Button cutters have only one drawback — they are not centre cutting so the depth of cut (unless you can be sure the tool never feeds down on the work) is only 0.050 in. per pass. Using a ramp plunge technique you can go to 0.200 in. per pass. Forty-five degree cutters work very well allowing depth of cut to 0.250 in. per pass. But their application is limited when getting into tight places and developing profiles.

Toroid cutters have proven most effective allowing depth of cuts of 0.250 in. per pass as well as plunging capabilities depending on the rigidity of the set up. Again, the ramp plunge method of entry into the material is a preferred choice.

Using indexable cutters is a very efficient tooling method for cutters 1/2 in. diameter and up. When tooling with a diameter of less than 1/2 in. is required, I recommend using only a high-quality, solid, carbide end mill. Climb milling should be used to reduce wear on smaller cutters.

WHAT TYPE OF COATING SHOULD BE USED?

There are several types of coating that have proven very efficient in graphite: TiAIN, TiN, and PCD polycrystalline diamond. Of particular note here is the longer service life of PCD. PCD tools are rated up to eight times longer life. However greater care must be used in the handling, measuring, and storage of these tools. My recommended strategy for a shop that wants to take advantage of this technology is to use tungsten carbide for roughing and semi-finishing and then apply PCD enamels for finishing only, to maintain accuracy and longer tool life. PCD is very expensive, ranging from eight to 10 times the cost of the same cutter in tungsten carbide.

GRAPHITE IS UNFORGIVING

Graphite is unforgiving of surface defects, reflecting even the smallest flaws quite clearly in the workpiece. As such, a good surface finish on the electrode before die sinking reduces bench time on the tool after. A fine step over, and small chordal deviation used to optimize surface finish give good results.

Compressed air in an properly controlled environment is the best method to remove the graphite dust from the tool/work interface, in order to prevent premature failure and lessen tool wear. A strong vacuum system in a self-contained enclosure is a must to remove the dust from the work envelope.

MOUNTING ELECTRODES

One of the most important parts of electrode fabrication is the locating, rigidity and clamping methods. The least amount of vibration results in deep cutter marks, broken or chipped electrodes, and a lot of frustration. System 3R USA Inc. (Fairfield, NJ), Erowa Technology Inc. (Windsor, CT), and others have very efficient systems to support electrode fabrication. 3R’s system is to effectively reference the electrode during the machining process and to employ that same reference system on the EDM for sinking the cavity.

Most companies develop or adapt some of these fixtures holding devices to suit their own process. Productivity is directly connected to an efficient, operator friendly fixturing system and should not be overlooked. I have used 3R’s system and am impressed with its accuracy, quality, and with the company’s customer support. CPL

This is the second in a four part series written by certified tool & die journeyman Mark Benoit. Mark is employed at Waltech Engineering Ltd. (Wallaceburg, Ont.) and has been an instructor at St. Clair College for nine years.