Page 60 - MetalForming May 2012

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MetalForming/May 2012
www.metalformingmagazine.com
Tooling Technology
Cutting Tool Selection Critical
Successful hard milling of die and tooling components requires careful selection of machine tool, spindle, CNC control and CAD/CAM system. But even more important is selecting a high-quality cutting tool during having a clear understanding of how to effec- tively implement the tool into the machining process. In fact, most man- ufacturers that fail at hard milling do so because they skimp on their cutting tools, not realizing that the right choice can save money in the long run.
The recommended hard-milling cutting-tool material is cubic boron nitride (CBN), the second hardest material on the planet. It proves ideal for rough- and finish-machining of hard steels to 70 Rockwell, as well as for abrasive materials such as grey cast irons. CBN is harder and more abrasive resistant than the commonly used car- bide and ceramic inserts. And, while CBN costs nearly seven times more than carbide, manufacturers will ben-
efit from its adequate balance of strength, toughness, thermal conduc- tivity and chemical inertness that ulti- mately provides optimum tool life and finishing capabilities.
To take the sting out of the high upfront cost of CBN, die shops should seek to purchase CBN from suppliers as a solid insert—available in various geometries including square and tri- angular designs, and grades where each corner is utilized. This practice will reduce the price per edge compared to the purchase of tipped inserts, which do not allow for the application of every corner of the product. For example, Secomax CBN products (from Seco) reportedly can outperform ceramics by more than 30 times when machining pearlitic cast iron.
In terms of cutter bodies, the best designs for optimizing surface finish when hard milling tooling components for stamping are shell milling cutters, six- pocketed milling cutters, cassette milling cutters and adjustable milling cutters.
Considerations for Hard Milling
For manufacturers considering hard milling for the first time, or looking to get back into it, there are several vari- ables to consider when selecting the right cutting tools, and ultimately to move forward with conducting a CBN milling test.
First, determine whether the hard- milling operation will involve rough- or finish-machining, as this will help guide the selection of the right insert and cutter for the job. The approach angle also influences cutter selection, so it’s important to decide whether square-shoulder or face milling is the optimum choice.
Another consideration is evaluating cutter geometry and setup rigidity, as this helps determine whether a posi- tive or negative cutter works best in a particular application. And, manu- facturers should establish whether or not a wiper insert is necessary. Wiper inserts sit slightly higher than the other inserts and wipe down the part
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