Randy McEachernClick image to enlargeBy Randy McEachern

Chip control is critical in drilling operations

 

To better manage your chips in drilling operations, first, review the chip formation. You should drill one or two holes, stop the process, review the chips, check the hole size and finish, then make any required changes to the drilling process. Understanding different characteristics of indexable insert drills, solid carbide drills, and exchangeable tip drills is important to ensure good chip control and a reliable drilling process.

Managing chips with Indexable insert drills

  • Most indexable drills have only one insert working at the nominal diameter, therefore is considered by some to be and unbalanced cutting action. Each flute will produce a different shape chip, usually a cone shape from the central insert, and C's or 6's from peripheral
  • Improved chip evacuation is initially achieved by improving chip formation. When examining chips on indexable insert drills it's important to identify there will be differing chips produced from the peripheral and central inserts.
  • Long chips may cause chip jamming in the drill flutes. Also, the surface finish may be affected and the insert or tool may be at risk. The goal is to have short chips in a C or 6 shape.
  • You can shorten chips by selecting the right insert geometry for the specific material being drilled, and adjusting cutting data accordingly.
  • Geometries designed for long chipping materials with gummy characteristics, such as low carbon steels and stainless steels, are usually designed with a deep chip breaker, sharper cutting edge, and intended to run at low to medium feed rates (0.002 – 0.006 in. depending on diameter of drill). Use caution not to over feed this type of geometry which will produce a long straight chip and can wrap around the tool.
  • Other geometries suitable for a range of materials may have a medium depth chip breaker, very strong reinforced cutting edge and will function in a broader feed range from low to high feed rates (0.002 – 0.013 in.) depending on drill diameter). These will offer good chip control in most materials, and are the first choice for unstable conditions and interrupted cuts.
  • Feed rate is the most significant factor in achieving good chip formation, and speed can be used to optimize smaller changes to the chip. Speed is a also an important factor to determine tool life.
  • It can be difficult to eliminate the continuous pigtail style chip from the centre insert. The best case is when the drill achieves a cone on the centre insert and 6 or 9 shape on the periphery insert.

Solid carbide & exchangeable tip drills;

  • With solid carbide or exchangeable tip drills a symmetrical tool provides a balanced cutting action and produces similar chips from both flutes. You will have slightly different chip shapes than indexable drills.
  • Many solid carbide drills and exchangeable tip drills may have dedicated geometries for specific materials, and others will have an optimized geometry designed to work well in a range of materials. It's important to use correct cutting data for the drill selected in your operation.
  • The goal is to achieve a chip with a 6 or C shape for good evacuation and hole quality. There will always be some pig tails at the start. If the chip is unacceptable, increase the feed remaining within the recommended range for the drill.
  • Oncee you have the correct size and shape, take a closer look at chip condition.
  • Scartch marks on the chips influence the hole quality negatively. Check the feed rate; the probable cause is too high a feed. When possible, change to a more suitable geometry.
  • The starting chip for solid carbide and exchangeable tip drills can give insight to problems with the operation. If the chip has needles on the periphery, there may be an issue with unbalance in the drilling process. Causes: runout, incline entrance, high feed, or unstable or weak conditions, which will lead to corner breakage or wear.
  • Hydraulic chucks are the first choice for reducing runout, eliminating vibration, and providing balanced cutting action.

Summary:

  • Use the shortest drill possible for good stability.
  • Select the correct geometry for your material and application.
  • Review the chips and optimze feed rate for good chip control.
  • Reduce runout and vibrations by using proper drill holders (ISO 9766) or
  • hydraulic chucks.

Randy McEachern is product and applications specialist for holemaking and tooling systems, Sandvik Coromant Canada Inc., Mississauga, ON

 

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