1/1/2000 | 8 MINUTE READ

The Key to High-Speed Mold Finishing

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To meet customer expectations and competitive pressures for higher tool quality, shorter turnaround times and lower cost, many moldmaking shops are upgrading their processes with capabilities for high-speed finish machining.

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Today, thousands of machine shops in the U.S. are involved in machining hard tooling used to mold, stamp or press a seemingly limitless range of consumer and industrial products. To meet customer expectations and competitive pressures for higher tool quality, shorter turnaround times and lower cost, many moldmaking shops are upgrading their processes with capabilities for high-speed finish machining. The major benefit of this approach is the achievement of machined surface finishes that are fine enough to dramatically reduce or even eliminate hand polishing or other manual finishing procedures at the bench. Such secondary operations typically account for as much as 25 to 30 percent of total manufacturing time.

High-speed finishing of molds is made possible in part by the spindle-speed capabilities of today's machine tools. Whether the equipment used is a small vertical machining center or the latest large-bed horizontal CNC, spindle speeds upwards of 10,000 rpm can now be routinely obtained. Such speeds are easily accommodated by today's secure tool-holding methods, which may incorporate precision collet chucks, or shrink-fit, mechanical or hydraulic chucks. 

An equally important requirement for the high-speed finishing of molds is the use of ballnose-style endmill cutters that are capable of running both at the higher machine rpms and at very high feedrates. The elevated machining speeds are essential for obtaining desired productivity results at the shallow depths of cut and small stepovers (cusp heights) needed to achieve fine surface finishes. 

High Performance at Lower Cost 

Both solid-carbide and small indexable-insert ballnose endmills are capable of the speeds and feeds required for high-speed finishing of molds. Moldmakers should keep in mind, however, that the indexable-insert tools offer important cost advantages - starting with a much lower replacement cost and no regrinding.

Solid-carbide endmills are very expensive. When they break, the entire tool must be replaced at new-tool cost. By contrast, a broken insert on an indexable-insert endmill can be replaced for just a few dollars. 

The indexable-insert cutters also eliminate the need for regrinding, which is labor-intensive and very costly. Regrinding also results in a loss of tool diameter size. In some moldmaking operations, that can mean the added cost of an extra finishing pass to meet critical dimensional requirements. 

Broad versatility is another cost advantage of the indexable-insert endmills. In many cases, only a simple change of insert grade or geometry is needed to equip the same cutter body for use in a variety of moldmaking applications and materials. 

For all of their cost advantages, however, today's indexable-insert ballnose endmills also provide the high speed and feed capabilities needed for productive high-speed finishing of molds. By using PVD-coated micrograin carbide inserts, the cost-saving indexable cutters are routinely run at cutting speeds of 10,000 sfpm and feeds of 300 ipm, while taking 0.010 inch axial and 0.010 inch radial depths of cut on molds or dies hardened to 55 Rockwell C. The operations typically produce surface finishes of 25 microinches or better.

Modular Copymilling Systems Do It All

Indexable-insert ballnose endmills for high-speed finishing of molds can be purchased as individual units from a standard endmill catalog, or assembled from components of available modular end-milling systems. The modular systems offer the advantage of broad application flexibility, based on a wide range of available insert grades and geometries and a large selection of shank types, styles and lengths. The tooling components can be combined to provide the best possible solution for particular moldmaking requirements.

Among the modular endmilling systems now available for moldmaking applications are two-piece, indexable-insert endmills specifically designed for copy-milling operations, which consist of steel shanks in different lengths and screw-in cutting heads in various styles using indexable inserts. 

As a whole, the two-piece modular endmilling systems can meet virtually any mold and die copymilling requirements - including high-speed finishing of molds. For example, cutting heads in the traditional 90-degree ballnose style are available in both a standard design for high-speed semifinish and finish contouring, and a helical-flute design for deeper cuts or sidewall milling. The latter design, in which inserts on the periphery of the cutter head are arranged in helical flutes, provides very smooth, vibration-free rough machining of hardened materials. 

Another important asset in the two-piece cutter line is a new 120-degree ballnose cutting head that can be used for both full 3-D cutting and high-speed semifinishing of vertical walls. Up to now, vertical walls have been a stumbling block to productive high-speed finishing of molds and dies. The conventional 90-degree ballnose endmills traditionally used to machine these features must be retracted after every downward stroke, interrupting the machining cycle and subjecting the cutter to rubbing and insert wear. By contrast, the 120-degree cutting head is designed with edges that extend beyond the tool shank and the stem of the cutting blade. This enables the cutter to take cuts on the back as well as the front of its edge, allowing it to remove metal in a consecutive series of downstrokes and upstrokes. The new machining technique not only provides an uninterrupted and faster machining cycle, but also eliminates cutter rubbing and insert wear, resulting in longer tool life and better surface finish. 

Button Cutters for Heavier Metal Removal

The term "high-speed machining" in mold and die applications can actually mean two different things. Primarily, it means finishing operations at very high spindle speeds and super-high feedrates that provide machined surface finishes fine enough to dramatically reduce, or even eliminate, secondary hand-finishing at the bench. In addition, however, the term can refer to heavy roughing cuts taken at accelerated metal removal rates. 

In this context, "button" cutters with round inserts have emerged as an important new moldmaking tool. Such cutters typically feature very thick, round inserts designed with patented integrated anvils for extra strength and security. Combined with special positive insert geometry, they are capable of very high metal removal rates.

Button heads offered in modular two-piece endmills for mold and die operations make use of 0.500 inch and 0.750 inch diameter round inserts for facing, ramping and plunging. In machining deep, enclosed cavities, these tools can be run at up to four times the metal removal rates of other ballnose endmills. They also provide longer insert life than the ballnose cutters, which are sometimes subject to severe rubbing and chipping in these applications. 

High-Performance Finishing

In addition to modular endmilling systems specifically designed for mold-making and other copymilling applications, general indexable-insert endmilling systems are available that include components for high-speed/high-feed finishing of molds and dies. Using one such system, ballnose endmills in diameters from 0.250 inch to 0.750 inch can easily be assembled by screwing a single precision-ground indexable carbide insert (with a choice of two or three effective cutting edges) into a reusable steel or heavy-metal tool shank. The insert clamping mechanism ensures rigid and secure insert retention.

Ballnose endmills created from the modular endmilling system offer performance and productivity in high-speed finishing of molds and dies. For example, using a balanced high-speed-steel shank, these tools can be safely run at cutting speeds up to 12,000 sfpm. 

The modular ballnose endmills also are very accurate. Precision ballnose inserts, for instance, achieve dimensional tolerances ranging from +0.0002 inch to 0.0006 inch - easily matching results obtained with any solid-carbide cutters. 

In addition to their high-performance capabilities, modular ballnose endmills can satisfy a broad range of particular application requirements and operating preferences from roughing to semi-finishing and finishing.

The modular endmilling system also provides a wide selection of shanks, ranging from stubby cylindrical styles to long styles for added reach. At the very high machining speeds obtained with finishing ballnose endmills, the shorter, more stable shank design provides added performance reliability. If a longer reach is needed, heavy-metal shanks are available in lengths to meet most reach requirements, while also providing the stability needed for good machining performance. 

The Sharp Edge Solution

To provide the high-performance results demanded in today's moldmaking, many indexable-insert ballnose endmills use PVD-coated, precision-ground, micrograin- carbide inserts. Experience shows that such inserts perform best with sharp edges, which represents a departure from the conventional wisdom that extra-strong edges are required to stand up to hardened materials at very high feedrates. 

The micrograin-carbide inserts used for high-speed finishing of molds are designed with a free-cutting insert geometry that allows the high cutting speeds and high feedrates needed to obtain ultra-fine surface finishes at acceptable productivity rates. The ultra-sharp edges complement the free-cutting insert geometry by effectively penetrating the hardened work material at the small axial and ultra-small radial depths of cut required to minimize stepovers (cusp heights) and achieve the desired finish quality.

A current application of this tooling shows the results that can be obtained using micrograin-carbide inserts with an ultra-sharp edge. To finish-mill a mold made from D-12 die steel hardened to 55 Rockwell C, a ballnose endmill was assembled from a modular endmilling system using a precision 90-degree ballnose insert in a very tough PVD-coated milling grade with medium-duty geometry. Run at a 0.020 inch axial doc, 0.010 inch stepover (radial doc) and 260 ipm feed, the endmill produces a 15-microinch surface finish with 90-minute tool life. 

PCBN-tipped inserts are also available as an alternative to the PVD-coated micrograin carbides for high-speed finishing of molds. The real value of PCBN for high-speed finishing of hardened materials lies in the future, when requirements for still higher cutting data can be expected. In the meantime, the PVD-coated micrograins with ultra-sharp edges offer proven capabilities to meet today's requirements as well as or better than PCBN, and at significantly lower cost. 

Summing Up 

Beyond all of the details, a single compelling reason makes today's modular indexable-insert endmilling systems the best place to look for the right solution to your moldmaking needs. Whether your requirements are for high-speed finishing or heavier metal removal, you'll find the right shanks and insert grades and geometries to do the job productively and to the highest quality standards. In any moldmaking application, indexable-insert endmills are a good bet to out-perform the latest solid-carbide tools by providing the best possible combination of economy, versatility, metal-removal rates, machine utilization and reliability.