Graphite Milling Shifts Into High Gear

Mold makers and injection molders are riding a wave of increasing demand for molded plastic and cast metal parts in consumer products. Yet efficient production requires parts that need little processing or finishing after molding or casting. The challenge for die/mold makers today is to satisfy the contradictory demands for intricacy and speed.

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Mold makers and injection molders are riding a wave of increasing demand for molded plastic and cast metal parts in consumer products. Yet efficient production requires parts that need little processing or finishing after molding or casting. The challenge for die/mold makers today is to satisfy the contradictory demands for intricacy and speed.

Many are turning to ram EDM machines for a solution. They know that a quality mold cavity requires a quality electrode. Several machine tool builders have addressed the previously lengthy, laborious and dirty job of graphite electrode milling. Now, rather than being the bottleneck in around-the-clock EDM sinking, a single graphite milling machine can often supply up to four EDM sinkers with accurate electrodes around the clock. In many cases, large full-form electrodes can be produced, eliminating the need to section and grind components. Delicate ribs down to 0.010-inch thickness can be produced, and finish quality eliminates bench polishing. Equally important, these machine are capable of reproducing dimensions, shape and finish to an unprecedented degree, making the multi-electrode strategy feasible.

Accurate machine tool capability requires accurate cutting tools, often the weak link in the process. For profile and cavity milling of molds and dies with conventional, high speed, and hard-milling methods, many users have turned to insert style ball nose tools They now have found that these tools work equally well on milling of graphite.

With these tools, increased feed rates on graphite from 15 ipm up to 50 ipm and depth of cut increases from 1/8 up to 1/4 inch using a 1/2-inch ball mill have been reported from the field. However, further gains in productivity did not seem out of reach. Indeed, test results from cutting tool manufacturers are only now beginning to show what is attainable with cutting tools designed for high speed, high accuracy milling of graphite.

For example, Millstar LLC a developer and marketer of cutting tool products, offers a new-generation profiling milling system which has surpassed the results noted above. Table I compares results of various cutting tool designs, including the new tools from Millstar. Column I presents original machining parameters; Column II presents the results using polycrystalline diamond (PCD) in 1995, and Column III represents the results using the latest tools from this manufacturer.

Users of the high speed graphite milling machine from Makino typically machine at 10,000 rpm with 1/2-inch diameter tools and as much as 15,000 rpm with 1/4-inch diameter tools. Users who do not have the benefit of high spindle speed have done well at 10,000 rpm or below. An Indiana automotive plant, milling an electrode for a large ribbed cover die, reported excellent results roughing a US-600 graphite electrode at 10,000 rpm with a 1/2-inch Millstar ball mill, at a 0.10-inch depth of cut and a 0.150-inch stepover. Finishing was done with a 1/4-inch Millstar ball mill at 10,800 rpm at 0.03 inch depth of cut and a 0.05-inch stepover. Both cuts were run at 200 ipm feed rate. Total cycle time reduction was reported at 65 percent, not an uncommon result, the company reports.

 

 

 

Millstar offers a range of toolholders and inserts for profiling in standard sizes from 1/4 to 1 1/4-inch diameter (6 mm to 32 mm). Inserts are available in various geometries and tool coatings (multi-layer CVD and PVD), designed for the speed and feed rates encountered in wet and dry machining of a broad range of materials.

Inserts for graphite milling are made of a proprietary material with high strength and shock resistance, and with a surface hardness the company says is equal to or greater than that of PCBN (polycrystalline cubic boron nitride). Unlike PCD (polycrystalline diamond) tools, these tools are not sensitive to shock, which often results in breakage of the costly PCD. This resistance to breakage makes them generally more economical than solid PCD cutters. An additional benefit is the two-flute two-effective design which allows for twice the feed per revolution versus single flute designs.