Double Margin Drill Bits: What They Are and When to Use Them

Double Margin Drill Bits: Benefits, Design, and Applications

Double margin drill bits are those drills that have two margins instead of one on each land, which makes the drill make more contact with the hole while drilling. The added stability results in round and straight holes with an improved finish. That is why a machinist uses a double margin drill in case of tight tolerances.

Whether you're choosing a CNC drill bit for aluminum aerospace parts, cross-drilled hydraulic manifolds, or straight flute drills for cast iron, understanding the second margin helps you pick the right tool the first time. This guide explains what double margin drill bits are, how they compare with standard drills, and exactly when to use them.

What Are Double Margin Drill Bits?

A double margin drill bit is a twist or straight flute drill with two full-diameter bearing surfaces, called margins, on each land. A standard drill has one margin along the leading edge of each flute. A double margin design adds a second margin near the heel, the trailing side of the land.

Each part has a clear job:

• Primary margin: sits right behind the cutting edge and sizes the hole.

• Secondary margin: touches the hole wall farther around the circumference and stabilizes the drill.

• Body clearance: the relieved area between the margins that keeps friction low.

A two-flute drill gets four contact points with the bore instead of two, so it guides itself like a reamer while cutting.

Key Benefits of a Double Margin Drill

The second margin delivers six practical gains:

1. Straighter holes: extra guidance reduces drift, especially in deeper holes.

2. Better roundness: four-point contact resists lobed, out-of-round holes.

3. Smoother finish: the secondary margin burnishes the hole wall, which can remove a reaming pass in some jobs.

4. Stability in interrupted cuts: the drill stays centered when it breaks into a cross hole or exits at an angle.

5. Less chatter: less vibration results in longer life and clean exit points.

6. Fewer operations: closer to final size for shorter cycle times.

When to Use Double Margin Drill Bits

Use double margin drill bits when hole quality is more important than lowest tool cost.

• Tight tolerances: holes with tight requirements on diameter, position or roundness.

• Cross holes: intersecting holes drilled on hydraulic manifolds and valve bodies.

• Interrupted cuts: drilling in slots, holes or irregular cast surfaces.

• Stacked materials: multi-layer plates in aerospace assemblies, where the drill exits and re-enters.

• Angled surfaces: entries or exits that would push a single margin drill off center.

• Pre-ream or ream-replacement holes: when you want a near-finished bore straight off the drill.

When a Single Margin Drill Is the Better Choice

A double margin drill isn't always the right call. Stick with a single margin design for:

• General-purpose holes with open tolerances.

• Heat-sensitive or work-hardening materials, unless speeds and coolant are well controlled.

• High-volume, cost-driven jobs where standard hole quality is enough.

Double Margin vs. Single Margin Drills

The core trade-off is precision versus friction. A double margin drill gives better hole quality, while a single margin drill runs cooler and costs less.

Straight Flute Drills with Double Margins

Straight flute drills have flutes that run parallel to the tool axis, with no helix. Because they don't pull themselves into the work, they resist grabbing in brass, cast iron, hardened steels, and thin sheets.

Adding a double margin to straight flute drills pairs that neutral cutting action with extra guidance, a common setup for cross holes and interrupted cuts. Straight flutes don't lift chips well, so use through-coolant in deeper holes and favor short-chipping materials.

Baucor Double Margin Drill Bits: Sizes, Materials, and Coatings

Baucor manufactures double margin straight flute drills in standard and custom sizes, from micro tools to large-diameter drills.

How to Choose the Right CNC Drill Bit

Start from the hole, not the catalog. Follow these steps to choose a CNC drill bit:

1. Define the hole: diameter, tolerance, depth-to-diameter ratio, and surface finish.

2. Match the material: aluminum, steel, cast iron, titanium, or composites all need different geometry.

3. Pick the flute style: helical for chip evacuation, straight flute drills for neutral cutting.

4. Choose the margin: double margin for straightness, roundness, and interrupted cuts.

5. Plan for coolant: through-coolant for deep holes and tough alloys.

6. Go custom when standard tools fall short: Baucor's drills design guide and design your own drill form help you specify one.

Conclusion: Are Double Margin Drill Bits Right for Your Job?

Double margin drill bits earn their place wherever straightness, roundness, and finish can't be left to chance. They shine in cross holes, interrupted cuts, stacked materials, and tight-tolerance bores, and they can sometimes eliminate a remaining step.

Get a Custom Double Margin Drill Built for Your Part

Baucor engineers design and manufacture custom double margin drills, straight flute drills, and step tools matched to your material, tolerance, and machine. Send your part print or hole specs, and we'll recommend the right geometry with a fast quote.

Request a Quote or ask an engineer today to drill straighter, rounder holes in fewer operations.