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.

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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

Mistake Consequence Fix Using wrong insert geometry Poor tool life, bad finish Match insert to material No through coolant Overheating, chip packing Use high-pressure coolant Excessive feed rates Tool breakage Follow manufacture feed specs Peck drilling aggressively Chip jamming, insert cracking Avoid pecking uless nesscessary Drilling too deep wit a 2xD Tool deflection, oval holes Use a longer U-drill or pilot

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 869 615 or .

Drilling wear and troubleshooting

Troubleshooting

• Indexable insert drill
• Exchangeable-tip drill
• Solid carbide drill

Indexable insert drill

Oversized holes

Rotating drill

1. Increase coolant flow, clean filter and clear coolant holes in drill
2. Try a tougher geometry on the peripheral side (keep center insert)

Non-rotating drill

1. Check alignment on lathe
2. Rotate drill 180 degrees
3. Try a tougher geometry on the peripheral side (keep center insert)

Undersized holes

Rotating drill

1. Increase coolant flow, clean filter and clear coolant holes in drill
2. Try a tougher geometry on the center side and a light cutting geometry on the peripheral side

Non-rotating drill

1. Stationary: Check alignment on lathe
2. Stationary: Rotate drill 180 degrees
3. Try a tougher geometry at the center and a light cutting geometry at the periphery

Pin in hole

Rotating drill

1. Increase coolant flow, clean filter and clear coolant holes in drill
2. Try a different geometry on the peripheral side and adjust feed rate within recommended cutting data
3. Shorten drill overhang

Non-rotating drill

1. Check alignment on lathe
2. Increase coolant flow, clean filter and clear coolant holes in drill
3. Shorten drill overhang
4. Try a different geometry on the peripheral side and adjust feed rate within recommended cutting data

Vibrations

1. Shorten drill overhang, improve the workpiece stability
2. Reduce cutting speed
3. Try a different geometry on the peripheral side and adjust feed rate within recommended cutting data

Insufficient machine torque

1. Reduce feed
2. Choose a light cutting geometry to lower the cutting force

Insufficient machine power

1. Reduce speed
2. Reduce feed
3. Choose a light cutting geometry to lower the cutting force

Hole not symmetrical

Hole widens at bottom (due to chip jam on center insert)

1. Increase coolant flow, clean filter and clear coolant holes in drill
2. Try a different geometry on the peripheral side and adjust feed rate within recommended cutting data
3. Shorten drill overhang

Poor tool life

1. Check cutting data recommendation
2. Increase coolant flow, clean filter and clear coolant holes in drill
3. Shorten drill overhang, improve the workpiece stability and check tool holding
4. Check tip/insert seat and screw for damage
5. See typical wear for specific remedies
6. Choose a more wear-resistant grade, if possible

Broken insert screws

1. Use torque wrench to fasten the screw together with Molykote

Bad surface finish

1. Important to have good chip control
2. Reduce feed (if it is important to keep vf, increase speed as well)
3. Increase coolant flow, clean filter and clear coolant holes in drill
4. Shorten drill overhang, improve workpiece stability

Chip jamming in the drill flutes

Caused by long chips

1. Check geometry and cutting data recommendations
2. Increase coolant flow, clean filter and clear coolant holes in drill
3. Reduce feed within recommended cutting data
4. Increase cutting speed within recommended cutting data

Deflection

• Hole tolerance out of range
• Bad surface finish – retraction mark
• Wear on tool body
• Insert breakage

CauseSolution Cutting forces too high1. Choose geometry with smaller corner radius
2. Reduce feed
3. Reduce feed at entrance Insufficient stability1. Increase stability
2. Choose a shorter tool body (4 × D -> 3 × D)

Chip forming problems

Long-chipping materials, e.g. stainless or low-carbon steel

ResultPossible causeSolution Bad/long chips – chip jammingWrong geometryChoose geometry -LM Bad surface finishCutting speed too lowIncrease cutting speed Insert or tool breakdownFeed too highDecrease feed

Short-chipping materials, e.g. normal steel

ResultPossible causeSolution Bad/long chips – chip jammingNon-rotating tool (lathe)Choose geometry for low feed (GR -> GM) Bad surface finishWrong geometryIncrease feed Insert or tool breakdown1. Cutting speed too low
2. Feed too lowIncrease cutting speed

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Exchangeable-tip drill

Out-of-hole tolerance

1. Check tip wear
2. Check run-out
3. Decrease feed
4. Check workpiece stability, tool holding and workpiece surface
5. Pilot/spot drill for longer drills
6. If non-rotating application, check alignment

Vibration

1. Shorten drill overhang, improve workpiece stability, check tool holding
2. Reduce cutting speed
3. Adjust feed/rev

Insufficient machine power or torque

1. Reduce speed
2. Reduce feed
3. Choose a light cutting geometry to lower the cutting force

Hole not cylindrical

Hole widens at entrance

1. Check run-out
2. Adjust feed
3. Shorten drill overhang and check tool holding
4. Make a pilot hole with a short drill for longer drills

Poor tool life

Check cutting data recommendation

1. Increase coolant flow, clean filter and clear coolant holes in drill
2. Shorten drill overhang, improve workpiece stability and check tool holding
3. Check tip/insert seat and screw for damage
4. See typical wear for specific remedies
5. Choose a more wear-resistant grade, if possible

Bad surface finish

1. Important to have good chip control
2. Reduce feed (if it is important to keep vf, also increase speed)
3. Increase coolant flow, clean filter and clear coolant holes in drill
4. Shorten drill overhang, improve the workpiece stability

Chip jamming in the drill flutes

1. Adjust cutting data for improved chip control
2. Increase coolant flow, clean filter, clear coolant holes in drill, check coolant concentration
3. Problems with chip jamming can cause extreme drill body wear
4. Remove any workpiece material stuck on the drill body to avoid chip jamming

Entrance chip

"Needles" on the periphery

1. Risk of excessive wear if needles can be seen on the start-chip periphery
2. Probable cause – imbalance due to: Run-outInclined entranceFeed too highUnstable/weak conditionsCorner breakage/wear

Chip control – optimization

1. A scratch mark on the chips, as seen above, is a sign of chip jamming, which influences hole quality negatively. To improve hole quality, the recommendation is to reduce feed and, if possible, increase speed

Solid carbide drill

Entrance chip – solid carbide/exchangeable-tip drills

"Needles" on the periphery

1. Risk of excessive wear if needles can be seen on the start-chip periphery
2. Probable cause – imbalance due to: Run-outInclined entranceFeed too highUnstable/weak conditionsCorner breakage/wear

Chip control – optimization

1. A scratch mark on the chips, as seen above, is a sign of chip jamming, which influences hole quality negatively. To improve hole quality, the recommendation is to reduce feed and, if possible, increase speed

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Drilling wear types

• Indexable insert drills
• Exchangeable-tip drill
• Solid carbide drill

Indexable insert drills

Flank wear

Flank wear is the preferable wear type when balanced. Flank wear can result in

• Poor surface finish
• Hole tolerance out of range
• Power increase

CauseAction 1. Cutting speed too high (vc)1. Decrease cutting speed (vc) 2. Insufficient wear resistance in grade2. Select a more wear-resistant grade

Crater wear

Central insert

Peripheral insert

Crater wear can result in

• Weak cutting edge that can cause edge breakage and generate bad chips
• Poor surface finish
• Power increase

CauseAction Central insert: abrasive chips (workpiece material)Central insert: reduce feed Peripheral insert: diffusion wear caused by high temperature (worse for PVD)Peripheral insert:
1. Decrease cutting speed
2. Select a more wear-resistant grade (often MT-CVD) Select a more positive geometry for better chip formation

Plastic deformation

Depression

Impression

Edge depression or impression can cause poor chip control, poor surface finish and the hole to be out of tolerance.

CauseAction Cutting temperature too high, combined with high pressure (feed and/or workpiece hardness)Decrease feed (fn)
Select grade with better resistance against PD (hot hardness)
Decrease cutting speed (vc) A final result of excessive flank and/or crater wearSelect grade with better resistance against PD (hot hardness)
Decrease cutting speed (vc)

Chipping in cutting zone

Chipping in cutting zone can result in

• Exaggerated flank wear
• Poor surface finish

CauseAction Unstable conditionsImprove stability (tool overhang, fixating) Irregular surfaceReduce feed at entrance. Choose tougher geometry Insufficient toughness of gradeSelect a tougher grade Insert geometry too weakSelect a stronger geometry Insufficient cutting fluidIncrease cutting fluid Sand inclusions (cast iron)Choose a stronger geometry, reduce feed

Breakage

Breakage can result in

• Tool breakdown
• Destroyed workpiece

CauseAction Insufficient stabilityImprove stability (shorten tool overhang, better workpiece fixturing, etc.) Intermittent cuttingReduce feed, choose tougher geometry (-GR or -GT) Insufficient cutting fluidIncrease cutting fluid Feed too high or cutting speed too high/lowAdjust cutting data Grade too brittle (P-insert)Choose a tougher grade Insert worn outDetermine safe tool life on peripheral insert

Built-up edge (BUE)

Built-up edge can result in

• Poor surface finish and edge frittering when BUE is ripped away by chips
• Chipping of cutting edge

CauseAction Unfavorable temperature (cutting speed)Increase/decrease cutting speed (high/low temperature)
Select a coated grade Cutting geometry too negativeSelect a more positive geometry Sticky materialIncrease oil mixture and volume/pressure in cutting fluid Oil mixture in cutting fluid too lowIncrease oil mixture and volume/pressure in cutting fluid

Exchangeable-tip drill

Typical wear in different materials for -PM geometry

Unalloyed steel / CMC01.1

• Margin/periphery wear starts as notch and develops along margin width and into flute
• Continuously growing wear on main edge

Low-alloy steel / CMC02.2

• Continuously growing wear on main edge/margin close to corner

Flank wear

Flank wear on main edge

Flank wear on circular land

Normal and preferable wear type when balanced

CauseAction Cutting speed too highDecrease cutting speed Percentage of oil in cutting fluid flow too lowIncrease percentage of oil in cutting fluid
(check with oil distributor to be sure not to exceed recommended percentages of oil) Insufficient cutting fluid flowIncrease cutting fluid flow Total indicator run-out too large (if wear on margin)Check radial run-out (if wear on margin)

Plastic deformation

CauseAction Cutting speed and/or feed too highDecrease cutting speed and/or feed Insufficient cutting fluid flowIncrease cutting fluid flow

Chipping

Chipping on periphery

Chipping on main edge

Chipping is a very common wear type when drilling into a pre-drilled hole. If the point angle is smaller on the pre-drilled hole, stability will be poor and the corners can be damaged. This can also happen if tolerances on point angles do not match. This can be avoided with custom-made drills or with flat bottom holes made by milling.

CauseAction Unstable conditionsCheck set-up Total indicator run-out too largeCheck radial run-out Feed too highDecrease feed Insufficient cutting fluid flow (thermal cracking)Check cutting fluid supply

Built-up edge

CauseAction Unsuitable cutting speed1. Increase cutting speed when BUE in center
2. Decrease cutting speed when BUE in periphery Percentage of oil in the cutting fluid too lowIncrease percentage of oil in cutting fluid (check with oil distributor to be sure not to exceed recommended percentages of oil)

If you cannot fully avoid the BUE zone, calculate a speed that locates the BUE in the strongest part of the drill (= 50% of diameter).

The use of external cutting fluid can influence chip evacuation negatively.

Increase cutting speed to move BUE
towards BUE center of the drill

Decrease cutting speed to move
to periphery or eliminate BUE

Solid carbide drill

Flank wear

Flank wear on main edge

Flank wear on circular land

Preferable wear type when balanced

CauseAction Total indicator run-out too largeCheck radial run-out Cutting speed too highDecrease cutting speed Feed too lowIncrease feed Grade too softUse a harder grade Insufficient cutting fluidIncrease cutting fluid pressure

Flank wear on chisel edge

CauseAction Total indicator run-out too largeCheck radial run-out Cutting speed too lowIncrease cutting speed Feed too highDecrease feed

Flank wear on chisel edge will also influence hole quality due to bad centering.

Chipping

Chipping on periphery

Chipping on main edge

Chipping is a very common wear type when drilling into a pre-drilled hole. If the point angle is smaller on the pre-drilled hole, stability will be poor and the corners can be damaged. This can also happen if tolerances on point angles do not match. This can be avoided with custom-made drills or with flat bottom holes made by milling.

CauseAction Unstable conditionsCheck set-up Total indicator run-out too largeCheck radial run-out Insufficient cutting fluid (thermal cracking)Check cutting fluid supply Maximum allowed wear exceededAdjust cutting data

Drill breakage

CauseAction Total indicator run-out too largeCheck radial run-out Unstable conditionsCheck set-up Insufficient spindle powerCheck cutting data Chip jammingCheck cutting fluid supply Feed too highDecrease feed Excessive wearCheck wear more frequently

Built-up edge

CauseAction Cutting speed and edge temperature too low1. Increase cutting speed when BUE in center
2. Decrease cutting speed when BUE in periphery Negative land too largeSharper cutting edge No coatingCoating on the edge

If you cannot fully avoid the BUE zone, calculate a speed that locates the BUE in the strongest part of the drill (= 50% of diameter).

The use of external cutting fluid can influence chip evacuation negatively.

Increase cutting speed to move
BUE towards center of the drill

Decrease cutting speed to move
BUE to periphery or eliminate BUE

Typical wear

Regardless of material

• Always flank wear
• Chipping is seldom seen Process security

Unalloyed steel / CMC01.1

• Margin/periphery wear Wear starts as notchGrows towards corner

Low-alloy steel / CMC02.2

• Corner wear

High-alloy steel / CMC03.11

• Significant flank wear
• Small damages on corner

Corner design

Caution!

• Do not misjudge the reinforcement as wear

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