Contents

• Introduction to Indexable Drills (U-Drills)
• Understand What U-Drills Are Best For
• Select the Right Insert Geometry & Grade
• Mind the Coolant Supply
• Optimize Feeds & Speeds
• Correct Entry and Exit
• Watch the Depth-to-Diameter Ratio
• Chip Control is Key
• On CNC Lathes: Use U-Drills as Boring Tools
• Secure Tool Holding & Setup
• Monitor Tool Wear
• Bonus: Common U-Drill Mistakes to Avoid
• Final Thoughts

Introduction to Indexable Drills (U-Drills)

Indexable drills, commonly known as U-drills, are powerful tools for high-performance drilling and rough boring operations. Whether you're working on a CNC lathe or a machining centre, these tools offer speed, repeatability, and cost-efficiency - but only when used correctly.

Here’s a comprehensive guide on best practices, tips, and tricks to get the most from your U-drills and avoid common pitfalls.

Understand What U-Drills Are Best For

U-drills use replaceable carbide inserts to drill holes rapidly and can also double as a rough boring tool. They're ideal for:

• Drilling large holes (typically 12 mm and above)

• Centreline and off-centre boring (in turning operations)

• High-speed production environments

• Soft steels to exotic alloys

Select the Right Insert Geometry & Grade

Inserts are not one-size-fits-all. Choose based on:

Material:

• Steel: General-purpose or wear-resistant carbide with TiAlN coating

• Stainless: Tougher grades with honed edges and sharp geometry

• Aluminium: Uncoated or polished inserts with high rake

Application:

• Center inserts typically handle more radial force

• Periphery inserts manage higher cutting speeds

Tip: for best performance use the Korloy King Drill, it has excellent performance on all materials, for general or economic performance select the Mammut u-drill.

Mind the Coolant Supply

Coolant is critical for chip evacuation and tool life:

• Use high-pressure through-coolant (ideally 20 bar+), although low pressure through coolant is suitable with machining care

• Ensure ports are clear and the U-drill is properly sealed

• For materials like titanium or stainless steel, use synthetic or semi-synthetic coolant for better heat control

Optimize Feeds & Speeds

Don’t apply traditional drilling values. U-drills operate more like turning tools:

• Speed: Based on the periphery insert (outer diameter)

• Feed: Use manufacturer tables, but as a rule of thumb:

• Steel: 0.08–0.2 mm/rev per insert

• Stainless: 0.06–0.15 mm/rev

• Aluminium: 0.1–0.25 mm/rev

• Reduce feed when breaking into cavities or exiting angled surfaces

Tip: Start conservatively and dial in based on surface finish and chip formation.

Correct Entry and Exit

Avoid crashing or damaging inserts:

• Ensure your starting surface is flat

• If unavoidable, ramp in slowly or pre-spot with a flat-bottom drill

• Avoid drilling through angled surfaces unless using a reduced feed and pilot

Watch the Depth-to-Diameter Ratio

Standard U-drills are most stable up to 3xD (depth = 3× diameter). For deeper holes:

• Consider a pilot drill first

• Use peck drilling cautiously - U-drills aren't ideal for it

• Use a spade drill 

Chip Control is Key

Poor chip evacuation is the #1 cause of tool failure with U-drills.

• Ensure coolant pressure is adequate

• Use correct chipbreaker geometry

• Watch chips - long stringers or compressed “bird nests” are a red flag

Tip: If chips are wrapping or clogging, reduce feed or switch to a sharper geometry.

On CNC Lathes: Use U-Drills as Boring Tools

One huge advantage on lathes is their versatility:

• Offset the U-drill on the X-axis and use it for rough boring

• Great for ID profiling, saving a tool station

• Use in combination with internal turning holders for finishing passes

Secure Tool Holding & Setup

• Ensure U-drill is perfectly perpendicular to the workpiece

• Use shortest possible overhang to reduce vibration

• Check insert clamping torque - loose inserts = catastrophic failure

Monitor Tool Wear

Even though inserts are replaceable, poor wear monitoring leads to bad holes:

• Replace inserts as a pair (center and periphery)
• Use wear indicators or set tool life in your CNC program
• Pay attention to chatter, surface finish, and chip colour - they all reveal wear issues

Bonus: Common U-Drill Mistakes to Avoid

Final Thoughts

U-drills are production workhorses - when used correctly. By dialling in your feeds, maintaining coolant flow, choosing the right inserts, and following best practices for entry/exit, you can dramatically boost tool life, reduce cycle times, and improve hole quality.

Whether on a CNC lathe or a vertical machining centre, mastering U-drills gives you a serious edge in speed and productivity.

Got questions about vice alignment tools or setup accessories? Please contact our technical team on 01924 869 615 or email sales@cutwel.net.