Contents

• Introduction to Machining Holes
• Why Use a Milling Cutter to Machine a Hole?
• Tools & Equipment Needed for Miiling a hole
• Recommended Milling Strategies for Holes
• Tool Selection Tips for Hole Milling
• Expert Tips for Better Hole Milling
• Sample CAM Strategy: Milling a Ø30 mm Hole with a Ø10 mm End Mill (CNC)
• Common Mistakes to Avoid
• When Not to Mill a Hole
• Summary of Milling a Hole

Introduction to Machining Holes

When, Why, and How to Mill Holes with Precision and Efficiency

Drilling is the go-to method for producing round holes, but it’s not always the best choice. In many advanced machining scenarios, especially when dealing with large diameters, tight tolerances, or non-standard geometries, using a milling cutter instead of a drill can offer superior control, surface finish, and flexibility.

In this expert’s guide, we’ll explore when and why to machine holes with an end mill, how to set it up, what strategies to use, and common pitfalls to avoid. Whether you’re programming CNCs or running a manual mill, this guide will level up your hole-making techniques.

Why Use a Milling Cutter to Machine a Hole?

Here’s when milling a hole beats traditional drilling:

Tools & Equipment Needed for Milling a hole

•    CNC or manual milling machine

•    Center-cutting end mill (2 or 4 flute, carbide or HSS)

•    CAM software (for CNC users)

•    Clamping or workholding system with minimal vibration

•    Edge finder or probing system for accurate location

•    Optional: Helical interpolation-capable machine/controller

Recommended Milling Strategies for Holes

1. Circular Interpolation (Helical Milling)

The most common and effective strategy for cutting holes using an end mill.

How it works:

•    The tool spirals down into the material while tracing a circular path.

•    Ideal for through-holes and holes larger than the tool diameter.

CNC Example:

•    Tool: Ø10 mm carbide end mill

•    Desired hole: Ø25 mm

•    Strategy: 2.5 mm stepover, 0.5 mm depth per pass with coolant

Pro Tip: Use a ramping spiral entry to reduce tool wear and load.

2. Peck Milling (Plunge and Pocket)

Useful when your machine doesn’t support helical interpolation.

How it works:

•    Peck into the material in Z-axis.

•    Use circular pocket milling to clear out the hole in layers.

Best for:

•    Shallow pockets and blind holes

•    Machining softer materials like aluminium or plastics

3. Ramp Entry & 2D Contour

When working with limited toolpaths or on manual mills, use a ramping motion to enter the material and mill the profile in steps.

Manual Mill Strategy:

•    Center punch the hole

•    Plunge slowly into the edge

•    Mill in small arcs, increasing depth per pass

Tool Selection Tips for Hole Milling

Expert Tips for Better Hole Milling

1.    Start with a Pilot Hole or Spot Drill

•    Especially helpful on sloped or uneven surfaces to avoid tool deflection.

2.    Use Climb Milling Where Possible

•    Produces a better finish and reduces cutting forces.

3.    Watch Tool Engagement

•    Don’t overload the end mill by using a massive stepover—start small (10-20% of tool diameter).

4.    Use Proper Feeds and Speeds

•    Adjust for radial engagement and depth of cut.

•    Use CAM software or a machinist’s calculator.

5.    Coolant is Key

•    Keep the chips clear to avoid recutting and reduce tool heat.

6.    Check Hole Quality

•    Verify size and roundness with a bore gauge, micrometer, or CMM.

Sample CAM Strategy: Milling a Ø30 mm Hole with a Ø10 mm End Mill (CNC)

Common Mistakes to Avoid

•    Using a non-center-cutting end mill – won’t plunge properly.

•    Forcing the cutter into the material – always ramp or helical mill.

•    Ignoring chip evacuation – packed chips kill tools fast.

•    Wrong feeds and speeds – leads to chatter or tool breakage.

•    Excessive stepover – creates poor finish and overcuts.

When Not to Mill a Hole

While milling offers flexibility, it may not be ideal if:

•    You need many deep holes quickly — drilling is faster.

•    The hole diameter is very small (under 5 mm).

•    You don’t have access to CNC or proper interpolation capability.

•    Material is very hard or abrasive (use a drill with coolant-through instead).

Summary of Milling a Hole

Milling a hole instead of drilling gives you the power to:

•    Produce custom-sized or oversized holes

•    Improve location and roundness accuracy

•    Achieve superior surface finish

•    Minimise tool changes in production

It's a versatile skill every machinist should master. With the right tooling, strategy, and setup, milled holes can often outperform traditional drilled ones in quality and precision. For more information or to make an enquiry please contact our expert sales team on 01924 869 610 or email sales@cutwel.net.