Technical Articles:

Optimizing Mold and Die Surface Finish With Small Pick Feed
Advanced ball nose cutters with specially developed nose geometry and tool coatings for high-speed and hard milling of die and mold steels.

Today's aggressive die and mold milling practices have resulted from developments in the fields of machine tool controls, spindles and drives, and advanced CAD/CAM software cutting strategies (two-axis, three-axis, three+two axis and five-axis). Optimized cutting tools are needed to support these advances. Desired results are to cut fully heat-treated materials with high process reliability at higher cutting speeds and feed velocities, and to achieve near net shape geometric parts accuracy and improved surface finish. These results are now attainable. The advances have led to a re-examination of the mold and die building process chain, and moldmakers realize that direct hard milling of fully hard steel cavities, cores or forms is a more flexible process, which requires a shorter production time cycle when compared to EDM processes.

The application of newly developed cutting tools with optimized geometry, tool coating and accuracy is a prerequisite to the full use of the advantages of high velocity and hard milling. The substantially higher cutting speeds employed lead to a significantly higher temperature at the cutting interface where the chip is formed. The high temperatures lead to a change in the plastic deformation and chip formation of the material being machined, making it possible to change the cutting geometry of the tool to correlate to the relatively lower overall cutting forces. These changes, in turn, influence the machining parameters and process.

Almost all of the thermal energy (heat) is removed with the chips, leaving the workpiece relatively cool. The cutting edge is exposed to higher heat, but new high performance tool coatings reduce the exposure. One new high performance tool coating has a very high temperature threshold, is extremely hard and has a very low coefficient of friction, based in part on a built-in solid lubricant. These features allow the tools to be used in high velocity milling, hard milling and dry milling. The advantages in performance add to the substantial increases in machining parameters that are now possible. Faster material removal in roughing and semifinishing is possible. In finishing operations, generally with small depth of cut and pick feeds, faster feedrates are employed, and higher part accuracy and far better surface finishes are achieved. The benefits are reduced machining time, significant reduction of manual benching and polishing time (arguably 25 to 40 percent of the entire processing time) and an equally significant reduction of throughput time.

Optimized Cutting Geometry of Ball Nose Cutting Tools
Since fine finishing in fully hard material can lead to such significant cost savings, it warrants closer attention. To finish mill with two-flute, two-effective ball nose tools is the obvious choice, whether milling in two-axis, three-axis or three+two axis mode. Ball nose tools with conventional cutting geometry have distinct disadvantages, especially in the tip gash area. This fact is particularly apparent when it is not possible to avoid cutting with the tip of the ball, either because the machine spindle cannot be adjusted from its fixed vertical or horizontal position, or because of part geometry and toolpath programming.

Figure 1 Optimized Tip Gash
• Minimized chisel edge at tool tip. • True radius form to center of tip. • Optimum finish with small pick feed. Sa = offset from cutting face/edge to center line. Sc = width of "chisel edge".

Conventional ball nose cutting tools, shank endmills and two-effective cutting inserts alike, are made with a minimum web thickness "Sa" as shown in the enlarged view of a conventional tool tip (Figure 1a). The two main cutting edges cut offset to the centerline of the tool. The resulting chisel edge forms a steep angle "a" with the main cutting edge. Sharp corners are formed where the two cutting edges meet, leading to early wear and cutting edge breakdown. The width of the chisel edge is generally smaller than the pick feed (line step-over) usually employed in finish milling. The finish achieved with the tool tip is greatly influenced by the early breakdown and wear in the tip area.

A European cutting tool manufacturer has touted a geometry with reduced chisel edge angle "a" (Figure 1b). The result is a longer chisel edge length "Sc" which generally overlaps the pick feed and is said to improve surface finish when tip milling. This approach seems a step in the right direction, but the true radius form of the cutting edge has been lost. The approach also does not eliminate the root cause of the early cutting edge breakdown - the main cutting edge and secondary chisel edge still form a sharp corner.

A new entry in the die/mold finish milling market is an innovative cutting geometry introduced by a specialty manufacturer of cutting tools for the die/mold industry. This insert tool employs an optimized geometry, which eliminates a chisel point at the tool tip (Figure 1c). The two active cutting edges cut exactly on the centerline of the tool all the way to the center point. This new geometry eliminates the weak points in the tools in Figures 1a and 1b and can be manufactured with extreme accuracy. All insert radii are precision ground to an accuracy of ± 0.00025 in./± 0.007 mm. Thus the workpiece is milled very accurately, and optimum surface finishes and long tool life are achieved. The inserts have an additional distinct advantage in mold milling - the inserts are ground with a radius that continues past 180 degrees to 230 degrees. These inserts can undercut and can be used to up-mill steep or vertical walls.

Rather than listing charts or tables with results achieved in laboratory conditions, following are a couple of typical "real world" results achieved by users of these inserts.

Glass Mold
A glass moldmaker in the eastern U.S. used to machine glass molds with conventional carbide end mills and conventional milling machines. A typical mold cavity for a rectangular glass baking dish with steep sides and rounded corners (10" x 93/4" x 31/8"/254 x 248 x 80 mm) took 22 hours to mill and an average of seven hours to polish. After investing in a modern mold milling machine with higher spindle speed and switching to ball nose roughing/finishing insert ball mills, total machining time was reduced by 68 percent to seven hours and polishing time was reduced by an average of 36 percent to four and a half hours.

Switching to the new insert did not shorten finish machining time further, but the finish improved drastically. Currently this company spends an average of 45 minutes of polishing per cavity or 3.4 percent of the original polishing time. Needless to say that the production manager is very happy with these achievements. He comments, "We finish an entire cavity with one insert and we then use the same insert to semi-finish the next cavity. There is plenty of tool life left in the insert, even though we run at very, very aggressive cutting parameters."

Stamping Die for Automobile Roof
The die shop in the Ohio stamping plant of a major U.S. automobile manufacturer recently switched to one-inch (25.4 mm) diameter ball insert tools to finish machine the entire cavity of a huge automobile roof die on their rather conventional equipment. Like so many customers who have found a more competitive way to machine, this user was reluctant to release comparative machining data.

Increases documented at initial testing of the new tools were quite substantial: Flame hardening cast steel (SAE -0050A) was cut with cutting speed increased from 500 sfm to 1,567 sfm (152 m/min. to 478 m/min.). The feedrate was increased from a slow 35 ipm to 144 ipm (889 mm/min. to 3657 mm/min.), a four-fold increase. The die manufacturing team decided to use some of the time gained to improve the surface finish. The pick feed was reduced from 0.050" to 0.030" (1.29 mm to 0.76 mm). This change reduced the cusp height and a far better than expected finish was achieved. The theoretical peak to valley height in pick feed direction improved from 0.00063" to 0.00022" (0.0159 mm to 0.0057 mm). It was immediately apparent that the reduction in manual surface polishing time would be quite substantial. Present results: machining time was cut in half. Tooling costs also were reduced, due to the long tool life of these inserts. The reduction in polishing time? "Substantial, terrific," says Bob Berlocker, the application specialist at Millstar distributor J&L/Strong Tool (North Canton, OH). "They just will not let me tell anyone. They want to keep competitors in the dark."

References
Lux, Stefan, Bellach, Switzerland, Chisel edge overlaps step-over, Swiss Precision Manufacturing (in German), Supplement to Werkstatt und Betrieb, vol. 5/99, a Carl Hanser Verlag, Munich publication. Millstar LLC, Bloomfield, Connecticut, USA, e-mail: info@millstartool.com. MMT

For more information contact Wolfried H. Mielert, co-managing partner in cutting tool companies Galaxy Technologies LLC, Izar Tool LLC and Millstar LLC (Warren, Michigan) or Ron Field, manager, product application and development, Millstar LLC (Warren, Michigan) at (860) 769-7999.

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