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Fully Committed to Zero Stock Machining

Article from Modern Machine Shop

Redoe Mold and Eifel Mold & Engineering represent two ends of the mold shop spectrum. Redoe is an international company with multiple mold-manufacturing facilities employing just over a hundred employees. Eifel is a 15-person, family-owned mold shop at a single location. Redoe specializes in large, complex molds for automotive lighting. Eifel is best known for molds for steering wheel components and interior instrument panels.

Both shops are fully committed to zero stock machining. This means having the dedication and discipline to routinely mill critical mold surfaces without leaving excess stock. The zero-stock condition of these surfaces precludes the opportunity to make up for tool wear, inaccuracies or uncontrolled process variables later. There is no second chance to correct or compensate with handwork at the bench.

Successful zero stock machining relies on a coordinated implementation of advances in four broad areas of manufacturing technology: super-rigid, super-accurate machine tools for high speed machining; workpiece fixturing that is flexible and highly repeatable; rigid holders for cutting tools with exceptionally accurate radius at the tip; and CAM software capable of producing tool paths for high-accuracy machining in the lights-out mode. Redoe and Eifel have in place the requisite hardware and software for zero stock machining. More importantly, both shops have a workforce that fully understands and embraces the no-room-for-error approach to mold machining that zero stock machining entails.

Common Ground

Both shops are engaged primarily in molds for the automotive industry, which is not surprising given their location in the greater Detroit area. Trends in automotive styling heavily influence their work, which represents two areas especially sensitive to consumer tastes.

A Small Shop Big On Zero Stock Machining

Eifel and Redoe are alike in their total embrace of zero stock machining. However, each shop has implemented this concept in its own way. The greatest difference lies in the machining strategy that works best for the inpidual shop.

Two five-axis VMCs from Hermle (Franklin, Wisconsin) represent Eifel’s main resources for zero stock machining. A Model C40 was installed in 2008; a slightly smaller Model C30 was installed in 2011. The new machine has high-pressure, through-spindle coolant delivery, a feature that has proven valuable for milling and drilling using the latest generation of coated carbide cutters with coolant holes. Otherwise, the machines are similar. Both feature a rotary table in a trunnion supported at both ends for the fourth and fifth axes.

Eifel’s reliance on 3+2 machining has implications for CAM programming, too. For all zero-stock machining, the shop uses PowerMill from Delcam. Gary Schulz, the shop’s CNC manager, says this software has features of particular value for this machining strategy. One, he says, is the variety of options for approaching the workpiece surface with the cutting tool. "When reaching into tight spaces at the bottom of a deep pocket, your approach angle may be limited, so it is best to have several choices. It’s all about protecting the cutting edge of the tool as it contacts the workpiece,” Mr. Schulz explains.

The other software capability that shines is visualizing the stock model. PowerMill displays material left behind by operations performed with the previous cutter. This allows the programmer to choose "rest machining” options for the next smaller cutter. "This feature helps us be sure that this cutter will not encounter unexpected material,” Mr. Schulz says.

Finally, advanced collision detection provides another layer of protection during 3+2 operations. Mr. Schulz says that when planning for 3+2, it is easy to think only of the workpiece surface and the tool tip, while neglecting to consider nearby geometry and the shape of the entire tool assembly. Collision detection, he explains, catches oversights that might otherwise lead to broken tools or spindle nose crashes.

Large Molds, Big Benefits

For a number of reasons, zero stock machining was embraced more readily by shops manufacturing smaller molds. Shops producing larger, complex molds initially perceived zero stock machining as risky, difficult and costly. At Redoe, Mr. Leene would not disagree with this assessment. However, advances in machining technology have significantly eased the risk and difficulty of zero stock machining on large workpieces.

He can point to two MCC 2013 six-axis HMCs from Makino (Mason, Ohio) on his shop floor as a case in point. These machines are the centerpiece of Redoe’s capability to finish large mold cores and cavities to a condition that requires little or no subsequent handwork. "These machines hold a tolerance of ± 0.001 to 0.002 inch all day long,” Mr. Leene attests.

Of course, Redoe adheres to the disciplines in the cutting tools and CAM programming capability required by zero stock machining. Cutting tools 3 mm and smaller in diameter are sourced from NS Tool, while larger cutters are supplied by OSG Tap and Die (Glendale Heights, Illinois). For programming, the shop uses CAM-Tool from CGS North America (Oldcastle, Ontario) for mold surfaces that require an optical-quality finish. The strength of this CAM product, Mr. Leene says, is the consistent chip load it maintains on high-speed finishing passes. All other programming for the five-axis machines is handled on the shop floor with Delcam’s PowerMill.

Article From: 5/21/2012 Modern Machine Shop, Mark Albert, Editor-in-Chief

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