Machining Method for Hole: Drilling, Core Drilling, and Reaming

Hole machining is a fundamental operation in mold manufacturing. Common methods include drilling, core drilling, reaming, boring, grinding, and lapping. This article covers the first three: drilling, core drilling, and reaming.

1. Drilling (Creating the Initial Hole)

Drilling is the process of making a hole in solid material, as shown in the video. There are two common approaches:

• Rotating drill, stationary workpiece – Typical on drill presses and milling machines. If the drill deflects, the hole centerline can shift.

• Rotating workpiece, stationary drill – Typical on lathes. If deflection occurs, the hole may become oversized or out‑of‑round.

Centerline deviation is the most serious error because it is difficult to correct later. To prevent drill deflection, use the following measures:

1) Pre‑machine the end face so the drill starts on a flat surface.

2) Use a smaller feed rate for small, deep holes to reduce axial force.

3) Use a drill bushing to guide the drill and ensure accurate positioning.

4) Pre‑drill with a short, rigid center drill (point angle 90°–100°).

5) Maintain proper sharpening, clamping, cutting parameters, chip removal, and cooling.

2. Core Drilling (Hole Enlargement)

Core drilling enlarges an existing hole that has been drilled, cast, or forged. Core drills have high rigidity and multiple cutting edges, allowing them to:

• Remove material with shallow cutting depth

• Evacuate chips easily

• Correct minor centerline deviations from the pre‑drilled hole

This method is often used as a preparatory step before reaming, or as a final operation for holes with moderate accuracy requirements.

Typical parameters:

• Allowance: about 1/8 of the hole diameter

• Feed rate: 0.4–2 mm (high productivity)

Note: Core drilling is generally not used for holes smaller than 10 mm or larger than 100 mm. Boring is preferred for large diameters.

3. Reaming (High‑Precision Finishing)

Reaming is a finishing process for non‑hardened workpieces. Reamers are precise, rigid tools that remove very little material, operate at low speeds, and provide excellent chip evacuation and cooling. As a result, reaming produces high‑quality holes with tight tolerances and smooth surfaces.

Reamer types:

• Fixed reamers – High rigidity and precision, but limited to standard hole sizes. Ideal for dowel pin holes and standard‑size holes.

• Floating (adjustable) reamers – Allow diameter adjustment, suitable for non‑standard holes.

Important notes:

• Reaming uses the existing hole as a reference and cannot correct positional errors — accuracy must be ensured by pre‑reaming operations.

• Best suited for small holes. Not recommended for short, deep, or interrupted holes.

Allowance:

• Rough reaming: 0.15–0.35 mm

• Finish reaming: 0.04–0.15 mm (smaller for small holes, larger for large holes)

Common issue – Hole shrinkage:

Shrinkage can occur if the allowance is too small and the cutting edge is dull, allowing elastic recovery of the metal. A significant temperature rise during reaming followed by cooling can also cause shrinkage.