Common Deep Hole Drilling Problems and Solutions

Overview

Deep hole drilling brings its own set of challenges that standard drilling does not. The high length-to-diameter ratio (typically above 5:1), the reliance on high-pressure coolant for chip evacuation, and the single-lip cutting action of gun drills all create failure modes that are unique to this process. This guide covers the most common problems I encounter and their solutions.

I use this page as my starting point when something goes wrong. Most problems have simple causes that can be checked in minutes: coolant pressure, guide bushing condition, and tool condition. I always check these three things before changing process parameters.

Chip Clogging

Chip clogging is the most common problem in deep hole drilling. It accounts for roughly 60% of all troubleshooting calls I handle. The drill cannot eject chips fast enough, and they pack in the flute or bore, blocking coolant flow and increasing torque.

Root Causes

• Low coolant pressure (below 40 bar for gun drilling, below 15 bar for BTA)
• Coolant flow path obstruction in the drill or supply line
• Incorrect chip shape from improper feed rate
• Worn drill margins that reduce chip clearance

Solutions

I check coolant pressure first. For gun drilling, I need a minimum of 40 bar (580 psi) at the drill tip. For BTA, minimum 15 bar (220 psi). If the pressure reading is adequate but chip clogging persists, I check the flow rate instead. Flow rate is measured in liters per minute, and each drill size has a minimum flow requirement.

If pressure and flow are adequate, I check the chip shape. Long stringy chips indicate low feed rate. The feed needs to be high enough to break the chip into small segments. I increase feed in 10% increments until the chips form short crescents or commas.

For BTA drilling, a clogged coolant passage in the drill tube is common. Chips accumulate inside the tube over time. I remove the drill tube and flush it with compressed air or a flexible rod.

Tool Breakage

Tool breakage is the second most common problem and the most expensive. A broken gun drill can cost 100-1000 USD depending on diameter and length. A broken BTA head can cost 300-2000 USD. The part being drilled is often scrapped as well.

Root Causes

• Chip packing (see above) is the most common cause of tool breakage
• Worn guide bushing that allows the drill to vibrate at entry
• Misalignment between the spindle and the workpiece
• Incorrect peck cycle parameters in gun drilling
• Material hardness variation that shocks the cutting edge

Solutions

I start by inspecting the guide bushing. A bushing that is worn beyond 0.05 mm clearance will not guide the drill properly. I replace bushings at the first sign of wear. The bushing is cheap (50-200 USD) compared to the tool and part it protects.

Next, I check machine alignment with a test bar. The spindle centerline must align with the workpiece centerline within 0.02 mm per 100 mm. I check alignment at the guide bushing position and at the workpiece position.

For gun drilling, peck cycling parameters matter. I use a peck depth of 5-10 times the drill diameter. The peck retract distance should be at least 2 mm to clear chips from the flute.

Poor Surface Finish

Root Causes

• Worn cutting edge on the drill
• Incorrect feed rate (too high or too low)
• Coolant concentration below 5%
• Vibration from inadequate workpiece support
• Chip recutting from insufficient coolant flow

Solutions

I check the cutting edge condition first. A gun drill that is dull will burnish rather than cut, leaving a rough or glazed surface. I check under 10x magnification for edge rounding or chipping. If there is any visible wear, I replace or regrind the tool.

If the tool is sharp, I check the coolant concentration with a refractometer. The concentration should be 8-12% for oil-in-water emulsion in deep hole drilling. At concentrations below 5%, the coolant loses its lubricity and the surface finish degrades quickly.

Hole Deviation

Root Causes

• Machine misalignment (spindle not concentric with bore axis)
• Uneven material hardness across the bore cross-section
• Worn guide pads on BTA heads
• Incorrect drill point geometry on gun drills
• Entry surface not perpendicular to the bore axis

Solutions

I check alignment with a test bar first. This eliminates the most common cause. If alignment is within specification (0.02 mm per 100 mm), I check the material hardness.

For hole deviation caused by material hardness, I use a drill with a smaller point angle on the harder side to balance the cutting forces. A standard gun drill point angle is 30 degrees included. I grind the point to 25 degrees on the side facing the harder material.

On BTA heads, worn guide pads are the usual suspect. I measure pad thickness and replace if worn more than 0.10 mm.

Vibration and Chatter

Root Causes

• Inadequate workpiece support or clamping
• Spindle speed at resonance with the workpiece natural frequency
• Worn spindle bearings
• Excessive tool overhang beyond the support bushing

Solutions

I add steady rests or change the spindle speed to avoid resonance. Changing the speed by 10-15% is usually sufficient. If vibration persists, I check the tool overhang. The drill should be supported as close to the workpiece as practical.

For thin wall cylinders, the wall flexes under cutting forces. I fill the cylinder with a damping material or use a steady rest with rubber rollers.

Quick Troubleshooting Table

Key Takeaways

• Check coolant pressure, guide bushings, and tool condition before changing parameters.
• Chip clogging accounts for about 60% of all deep hole drilling problems.
• Maintain coolant concentration at 8-12% for optimal tool life and surface finish.
• A worn 50 USD guide bushing can cause breakage of a 500 USD tool.
• Use the troubleshooting table above as a starting point for any problem.