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Slide 003: Drilling, Boring, and Hole Operations on the Mill

Slide Visual

Drilling, Boring, and Hole Operations on the Mill

Slide Overview

This slide covers hole-making operations on the milling machine including center drilling, through-drilling, reaming, boring, and bolt circle layout—operations that take advantage of the mill's precise XY positioning capability.

Instruction Notes

Advantages of Drilling on the Mill

The milling machine is often preferred over a drill press for precision hole work because: - Precise XY positioning using handwheels, dial collars, or DRO - The workpiece can be securely clamped in a precision vise - Multiple holes can be located relative to each other with high accuracy - Boring operations can follow drilling for precision diameters - The rigid spindle and table produce straighter holes

Drilling Procedure

  1. Layout: Calculate hole positions relative to a reference edge. Edge-find to establish coordinates.
  2. Center drill: Position the spindle over the hole location. Install a #2 center drill. Set RPM for the center drill diameter. Center-drill each location.
  3. Pilot drill (if needed): For holes over 1/2", drill a pilot hole first (approximately 50% of final diameter) to reduce forces on the finish drill.
  4. Final drill: Install the specified drill bit. Set RPM for the drill diameter and material. Peck drill: advance 1-2× drill diameter per peck, retract to clear chips. Use cutting fluid.
  5. Deburr: Countersink each hole to remove the entrance burr.

Reaming

Reaming produces a more accurate and smoother hole than drilling alone. A reamer follows a drilled hole (typically drilled 1/64" undersize) and removes a thin layer to achieve ±0.0005" diameter accuracy with excellent surface finish.

Key rules: - Drill the hole 0.010-0.015" undersize for the reamer - Run at 50% of drilling RPM (reamers cut on their full circumference—lower speed prevents chatter) - Feed continuously—never stop or reverse a reamer in the hole - Use ample cutting fluid

Boring on the Mill

For hole diameters that do not match standard drill or reamer sizes, boring is used:

  1. Drill a starting hole (approximately 1/8" undersize).
  2. Install a boring head in the spindle. The boring head holds a single-point boring bar.
  3. Adjust the boring bar to the desired radius using the graduated dial on the boring head.
  4. Lower the quill to bore the hole. Take light cuts: 0.010-0.020" per pass.
  5. Measure with telescoping gauges and micrometer or bore gauge.
  6. Adjust and take the finishing pass.

Bolt Circle Layout

A bolt circle is a pattern of evenly spaced holes on a circle of specified diameter (e.g., 4 holes on a 3" bolt circle).

Calculation: For N holes on a circle of radius R: - Hole 1: X = R × cos(0°), Y = R × sin(0°) - Hole 2: X = R × cos(360°/N), Y = R × sin(360°/N) - Continue for each hole

Example — 4 holes on a 3" diameter (1.5" radius) bolt circle: | Hole | Angle | X Position | Y Position | |------|-------|-----------|-----------| | 1 | 0° | 1.500" | 0.000" | | 2 | 90° | 0.000" | 1.500" | | 3 | 180° | -1.500" | 0.000" | | 4 | 270° | 0.000" | -1.500" |

All positions are relative to the bolt circle center. Use the DRO or dial collars to position the table for each hole.

Key Talking Points

  • Center drill EVERY hole—skipping this step causes drill walking and broken drills
  • Reaming is not drilling—it requires lower speed and continuous feed
  • Boring is the most accurate way to produce a precision hole on the mill
  • Bolt circles require trigonometry or a DRO with bolt circle function

Learning Objectives (Concept Check)

  • Can students perform the complete drill-ream sequence?
  • Can students set up and use a boring head?
  • Can students calculate bolt circle positions?

Last Updated: 2026-03-19