Tips for Machining Large-Diameter Holes

Helical interpolation with indexable high-feed tooling

Hole making is both a time- and tool-consuming process for many shops. Helical interpolation with indexable tooling is a great way to tackle this challenge quickly – and it's an especially advantageous approach when creating larger-diameter holes.

Although you can helix with a variety of tool types, high-feed takes its benefits to another level. Here, we'll machine a 4" diameter hole in 2.5" thick 4140 PH steel on a 40-taper machining center. Total cycle time? Just two and a half minutes, running somewhat conservatively.

Benefits of high-feed helical interpolation

Helical interpolation offers a crazy-fast way to machine holes, but there are benefits beyond your cycle times.

  • Low tool pressures
  • Very easy to program, especially on newer CAM packages
  • Create many hole sizes with a single tool by simply updating your program
  • Utilizes low horsepower, so both small- and large-diameter holes can be made quickly and easily on even a 40-taper machine
  • Eliminates the need for time-consuming starter holes and step cuts

While high-feed tooling will generate a scalloped wall finish, it provides the shortest cycle times by far, and it works very well in more challenging long-reach scenarios.

Learn more about our indexable high-feed tooling.

Cut large holes fast in 4140 PH steel

Tips for hole helixing

  • Choose a cutter that can handle your hole size without leaving material at the center, and that will leave the desired wall finish
  • The insert must cover the hole's center line, or you'll need to drill a start hole first
  • Recommended minimum 1/2 insert IC for center overlap
  • Maximum hole size = 2xD (for single pass)
  • Position the tool at the 3 o’clock position over the hole to be machined, .100” above the part to start, with the OD of tool at the ID of finished hole size
  • Execute G02 (CCW arc), with I-value = interpolation radius, Z-value = pitch; repeat the number of times necessary to reach bottom
  • Add a straight G02 pass (no Z-value) for flat-bottom hole
  • 0.5 to 1.0 degree ramp angles are typically recommended, depending on hole size – more important is to specify a depth per hole revolution (pitch) that will allow good chip evacuation
  • Feed rate does not typically need to be adjusted, unless chip evacuation is an issue
  • Excellent air blast or high-pressure coolant is necessary for chip evacuation, especially on VMCs

Have questions about implementing this approach?

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