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Millstar | Tech Article


2011-09-01 - Source: Millstar (Warren, Michigan)

Most of the tool types that are familiar to mold and die makers using conventional machining techniques are also applied in high speed machining: ball nose, bull nose and toroid, for example. However, high speed processing capabilities may encourage shops to consider departing from conventional cutter wisdom in some cases such as applying face mills to cut molds and dies.

Face Mills

Face mills can bring a major metal removal rate for roughing along with high accuracy for finishing flats. It depends on sufficient torque capability from the machine tool, but depending on the number of cutters, surface speed and chip load can be maintained at lower rpm.

  • In materials up to 42 HrC, use positive rake
  • For harder materials, use neutral or negative rake
  • PVD and CVD insert coatings
  • Rough Milling (round inserts)
  • Finishing Flats (square, triangle or rectangle inserts)

Ball Nose End Mills

Ball nose cutters are the workhorse of the mold and die industry. Whether solid body or indexable insert designs, they are applied in roughing, semi-finishing and finishing operations on cores and cavities. In general, application differences between an insert cutter and a solid carbide tool are marginal. Inserts cost less but solid tends to be stiffer. Inserts have effective size limits ranging from 0.25 inch diameter to 1.25 inches in diameter.

  • Use in materials up to 62 HrC
  • PVD coatings available
  • 6 to 10 percent cobalt content with micro grain carbide
  • Specify ground inserts over pressed
  • Use for cores and cavities
  • Helical hole boring
  • Roughing slots or channels

When cutting slots or channels in hard metals, conventional milling techniques suggest a bull nose end mill. "At high speed," says Millstar manager Ron Field, "we've found that roughing the slot or channel with a ball nose mill–then coming in to finish size and shape with a bull nose or square corner end mill–is actually a faster process than using the bull nose or end mill alone at conventional speeds."

Bull Nose End Mills

Bull nose cutters are applied in milling flats and sides. Their distinguishing feature is a radius on the edges that allows the cutter better heat distribution when end milling.

Even a radius of 0.5 mm provides significant strength over a square shoulder end mill.

  • Use in materials up to 54 HrC
  • PVD coating available
  • Larger diameters can be used with small corner radii
  • Use in cores and cavities
  • Semi-finishing and finishing
  • Straight sides and corner radii

Toroid Bull Nose End Mills

Toroid bull nose end mills blend ball nose and bull nose cutter design into a versatile core and cavity cutting tool. No area in the toroid cutter's active zone has zero cut speed. Step-over can be larger without creating a cusp. Finish surfaces tend to be smooth because there is no rubbing from a dead center area.

  • Use in materials up 62 HrC
  • PVD coatings available
  • Larger radius edge gives better heat displacement
  • Use for roughing, semi-finishing and finishing
  • Can be used for helical hole boring

High speed machining gives shops an opportunity to look closely at their cutting tool strategies.–GCK