Troubleshooting Deep Hole Drilling in Titanium: Specific Problems and Solutions

I have drilled more titanium deep holes than I care to count, and I learned the hard way that titanium is not “tough steel.” It is a different animal. The thermal conductivity is roughly one-fifth that of steel. The chips are not chips – they are razor-edged bands that carry all the heat out of the cut. The work-hardening rate is aggressive enough to ruin a new gun-drill in seconds if you let the tool rub instead of cut. Every fix I learned on steel had to be re-learned for titanium. This article covers the specific failure modes I encounter in titanium deep hole drilling and the adjustments that actually work.

If you need a starting point for parameters before troubleshooting, I maintain a dedicated reference at titanium deep hole drilling parameters.

Notch Wear at the Depth-of-Cut Line

Notch wear is the failure mode that most commonly takes out my titanium gun-drills. It appears as a localized groove or notch on the flank and rake faces at the depth-of-cut line – the point where the cutting edge meets the uncut workpiece surface. In steel, notch wear is uncommon and usually a late-stage problem. In titanium, it can appear within the first few inches of cut and progress to catastrophic edge failure before you finish the first hole.

Why it happens. The depth-of-cut line is a stress concentration point. The work-hardened layer from the previous pass, the oxide scale from heat treatment, and the micro-chipping from interrupted cuts all converge on that single point on the cutting edge. Titanium’s low thermal conductivity means the heat generated at that point stays concentrated – I have measured edge temperatures over 1000 C at the notch location while the bulk of the tool remained cool. At those temperatures, the cobalt binder in the carbide diffuses into the chip, leaving a crater that grows rapidly.

How I reduce notch wear:

• Reduce cutting speed. Speed is the dominant variable for notch wear. I drop surface speed by 15 to 25 percent from my starting point when I see notch wear developing. For Ti-6Al-4V, that means running at 70 to 90 SFM instead of 100 to 120 SFM.
• Increase feed slightly. A heavier feed moves the stress concentration away from the depth-of-cut line by engaging a thicker chip. I increase feed by 5 to 10 percent, which spreads the thermal load across a wider edge area.
• Use a chamfered edge preparation. A sharp edge is more vulnerable to notch initiation. I specify a 0.002 to 0.005 inch chamfer or T-land on the cutting edge, which distributes the stress at the depth-of-cut line over a larger area.
• Apply a PVD TiAlN coating. Uncoated carbide is the baseline, but I have seen notch depth reduced by 40 to 60 percent with a TiAlN coating. The aluminum in the coating forms an aluminum oxide barrier at high temperature, which slows the diffusion wear mechanism.

Work Hardening and Why Feed Matters

Titanium work-hardens at a rate that makes stainless steel look forgiving. The alpha-beta microstructure of Ti-6Al-4V undergoes a strain-induced martensitic transformation under deformation. The hardened layer can reach 450 to 500 HV in the subsurface zone – well above the bulk hardness of 320 to 360 HV. That hardened skin destroys cutting edges if you let the tool dwell or rub.

The single biggest mistake I see in titanium drilling is backing off the feed to reduce load. In steel, reducing feed is often the right response to tool stress. In titanium, it is the wrong response. Lower feed means thinner chips, which means the cutting edge spends more time engaged with the work-hardened layer from the previous revolution. The edge rubs rather than cuts, generating more heat and more deformation, which creates a deeper hardened layer. The spiral accelerates until the edge fails.

How I handle work hardening:

• Maintain feed above 0.001 IPR. I never run a titanium gun-drill below 0.001 inches per revolution for diameters above 6 mm. For smaller drills, I adjust proportionally but keep the chip load high enough to cut below the hardened layer.
• Avoid dwell or spot-facing with the same tool. If I need a spot face, I use a separate carbide spot drill. Letting a gun-drill dwell on a titanium surface creates a hardened dimple that the drill cannot re-cut on re-entry.
• Use a positive rake geometry. A positive rake angle (6 to 10 degrees) shears the material cleanly instead of plowing it. Negative rake or zero rake tools push the work-hardened layer deeper rather than cutting it off.
• Replace reground tools at the first sign of edge rounding. Once the cutting edge radius exceeds 0.003 inch, the drill starts rubbing instead of cutting, and work hardening accelerates. I track edge radius with a comparator and retire regrinds when they hit that limit.

Chip Packing in Titanium – Different from Steel

Chip packing is dangerous in any material, but titanium chip packing is uniquely destructive because of two factors: the chip morphology and the material’s sensitivity to coolant starvation.

Steel chips, even when they pack, are relatively compressible and often break under their own weight as they fill the flute. Titanium chips are tough, sharp-edged, and incompressible. When a titanium chip packs in the flute, it forms a solid plug that locks the drill in the bore. The torque spikes instantly. I have seen a 12 mm gun-drill snap at the shank within one second of a titanium chip block.

The second factor makes it worse: titanium loses strength rapidly above 400 C. When coolant flow is restricted by chip packing, the temperature at the cutting edge rises, the workpiece material softens locally, and the chips become gummy instead of fracturing cleanly. The gummy chips pack tighter, which blocks more coolant, which raises the temperature further. It is a feedback loop that ends in a broken drill.

What I do to prevent titanium chip packing:

• Coolant pressure is non-negotiable. I run a minimum of 1000 PSI (70 bar) at the drill entry for diameters under 15 mm. For larger diameters, I target 800 to 1000 PSI. If the machine cannot deliver that pressure, I do not run titanium deep holes on that machine.
• Through-tool coolant is mandatory. Gun-drills and BTA tools must have through-tool coolant delivery. External flood coolant does not reach the cutting zone at depth. I verify flow at the tip before every setup using a flow meter at the drill point.
• I do not push feed to break chips in titanium. Unlike aluminum or steel, titanium chips do not respond well to feed-driven chip breaking. Instead, I adjust the chip breaker geometry on the drill tip. Most of my titanium gun-drills use a modified chip breaker with a 0.015 to 0.020 inch step.
• Peck cycles help, but only with fast retraction. I program a peck depth of 5 to 8 diameters with a rapid retraction at 200 to 300 IPM. Slow retraction lets packed chips settle and re-compact in the flute. Fast retraction pulls them out.

My full analysis of breakage patterns is in the gun drill breakage analysis guide, which covers how to read a broken titanium drill tip vs a steel drill tip.

Surface Finish Problems Specific to Titanium

Surface finish defects in titanium have a different fingerprint than in steel or aluminum. The low thermal conductivity means the bore surface is exposed to high temperature for longer, which changes the surface integrity in ways that show up as finish defects.

Burning and discoloration. A blue or brown discoloration on the bore wall is the most obvious surface defect in titanium. It appears as a dark band or spiral stripe along the bore. The cause is localized overheating from coolant starvation or excessive surface speed. I back speed down to 70 SFM or below and verify coolant flow before making any other change.

Tear marks and galling. These appear as irregular, torn patches on the bore surface, often at the exit end of the hole. Galling happens when the titanium welds to the guide pad and then tears away, leaving a rough patch on the bore. The fix is increasing coolant EP additive concentration and verifying that the guide pad clearance angle is at least 5 degrees. I also switch to a tungsten carbide grade with a higher cobalt content (10 to 12 percent) for the guide pads, which reduces the tendency for titanium to adhere.

Roughness spikes (intermittent high Ra). A bore that measures Ra 32 for most of its length but shows intermittent spikes to Ra 64 or higher points to chip re-cutting. The chip wraps around the drill shank and drags against the bore wall as it is evacuated. I fix this by increasing coolant flow rate to flush chips faster and, if needed, reducing the peck depth so the chip string is shorter.

Surface tearing from built-up edge. BUE in titanium is rarer than in aluminum, but it happens when surface speed drops below 50 SFM in Ti-6Al-4V. The welded titanium on the cutting edge tears rather than cuts, leaving a torn surface on the bore. The fix is increasing SFM to 80 to 100 to get above the BUE threshold.

Drill Breakage Patterns in Titanium

Titanium drill breakages look different from steel breakages, and reading the fracture correctly is essential for choosing the right fix.

A titanium breakage typically shows a shear lip with a dark heat-affected zone on one side of the fracture. The dark zone is thermal discoloration from the temperature spike that preceded the break. In steel, the break surface is usually clean. In titanium, the dark zone tells you the tool was hot before it broke – which nearly always means coolant starvation from chip packing.

Torsional twist-off with flute packing. The break is at the tip, 10 to 30 mm from the cutting edge. The flute is packed solid with compressed titanium chips. The fracture surface shows a flat torsional shear face with a dark center. The root cause is always chip evacuation failure. I address this by increasing coolant pressure and verifying peck parameters.

Spiral fatigue fracture. A 45-degree spiral along the drill body, often with beach marks visible at 10x magnification. In titanium, spiral fractures are more common than in steel because the higher cutting forces put more bending stress on the drill body. I add steady rest support at intervals of 25 to 30 diameters (tighter than the 30 to 40 I use for steel).

Catastrophic edge failure with no chip packing. The tip is shattered but the flute is clear. This is impact-related – a hard spot in the titanium billet or a variation in alpha-case thickness. I request material certification and microhardness testing when I see this pattern. A surface-hardened layer deeper than 0.010 inch requires a pre-drill operation to remove it before the gun-drill enters.

Coolant Pressure and Type for Titanium

I consider coolant the most important variable in titanium deep hole drilling, more important than speed, feed, or tool geometry. Get the coolant wrong and nothing else matters.

Pressure requirements. My minimum is 1000 PSI (70 bar) for gun-drills under 15 mm diameter. Below that pressure, chip evacuation is unreliable in deep holes. For BTA drilling titanium, I run 400 to 600 PSI at the cutting head, with a flow rate of 80 to 120 gallons per minute depending on hole diameter. I measure pressure at the drill tip, not at the pump. A 30 percent pressure drop across the coolant system is common, and you want to know the pressure where it matters.

Coolant type. I use sulfur-chlorinated oil for titanium. The chlorine extreme-pressure additive forms a sacrificial iron chloride film on the cutting edge that prevents titanium from welding to the carbide. Straight sulfurized oil does not perform as well because the sulfur reaction layer is less stable at the high edge temperatures in titanium drilling. My preferred viscosity is 40 to 60 SUS at 100 F.

Filtration. Titanium chips are abrasive to pump seals and coolant lines. I run 10-micron absolute filtration to keep the coolant clean. Chips that recirculate through the coolant system score the bore surface and accelerate pump wear. I check filter pressure drop daily and change elements when the drop exceeds 15 PSI.

Temperature control. I keep coolant temperature below 120 F (49 C). Above that temperature, the chlorine EP additive breaks down, and the coolant viscosity drops below what is needed for effective chip evacuation. I use a heat exchanger on the coolant return line and monitor temperature at the tank.

Problem vs Cause vs Fix Reference Table

Parameter Adjustment Table

Key Takeaways

• Titanium is not steel with a different name. Notch wear at the depth-of-cut line, aggressive work hardening, and incompressible chip packing are failure modes that require specific responses – many of them opposite to what works in steel.
• Notch wear is driven by high edge temperature and cobalt diffusion. Reduce speed, not feed. A TiAlN coating and chamfered edge preparation extend tool life significantly.
• Work hardening gets worse when you lower the feed. Maintain chip load above 0.001 IPR, use positive rake, and never let the tool dwell on the workpiece surface. Retire reground tools once the edge radius exceeds 0.003 inch.
• Chip packing in titanium forms solid, incompressible plugs that break drills instantly. Coolant pressure of 1000 PSI or higher is non-negotiable. Peck cycles help, but only with aggressive retraction speeds.
• Surface finish defects in titanium – burning, galling, and roughness spikes – each have distinct root causes tied to coolant delivery, guide pad geometry, or speed selection. Diagnose the specific pattern before changing parameters.
• Coolant selection and condition are critical. Use sulfur-chlorinated oil at 1000+ PSI, filter to 10 microns, and keep temperature below 120 F. Measure pressure at the drill tip, not the pump.
• For detailed parameter recommendations, see the titanium deep hole drilling parameters guide. For reading broken drills, see the gun drill breakage analysis article.