Hole defects in deep hole drilling are the symptom language of the process. A bell mouth at the entry tells a different story than a barrel-shaped bore or a spiral mark pattern. Learning to read these defect signatures is the key to fast, accurate troubleshooting.
Defect Identification Guide
Quick Reference
| Defect | Visual Signature | Typical Location | Severity | Primary Cause |
|---|
| Bell mouth | Entry wider than rest of hole | Entry only | Functional | Worn bushing, high entry feed |
| Barber pole | Diagonal banding on bore surface | Full length | Functional | Guide pad burnishing variation |
| Spiral marks | Helical groove on bore | Full length | Cosmetic to functional | Feed-rate interaction with tool geometry |
| Barrel shape | Middle wider than ends | Mid-hole | Functional | Coolant pressure variation |
| Taper | Hole diameter changes along length | Full length | Functional | Tool or coolant temperature drift |
| Rifling | Helical scoring lines | Full length | Functional | Chip scoring |
| Oversize | Diameter above tolerance | Full length or partial | Functional | Worn guide pads, excessive runout |
| Undersize | Diameter below tolerance | Full length or partial | Functional | Built-up edge, incorrect pad size |
| Lobing | Polygonal cross-section | Full length | Functional | Vibration, harmonic interaction |
| Exit burr | Raised material at exit | Exit only | Cosmetic to functional | Feed at breakthrough |
Bell Mouth
Cause and Correction
| Cause | Mechanism | Diagnostic Check | Fix |
|---|
| Worn starting bushing | Drill oscillates in oversized bushing bore | Measure bushing bore diameter | Replace bushing (limit: 0.02 mm oversize) |
| Entry feed rate too high | Drill tip deflects on initial contact | Check feed rate at entry | Reduce entry feed to 30–50% of production |
| Bushing clearance excessive | Incorrect bushing size for drill diameter | Verify bushing-to-drill clearance | Use correct bushing (drill Ø + 0.01–0.02 mm) |
| Bushing alignment off | Drill enters at angle | Check bushing concentricity to spindle | Realign within 0.01 mm TIR |
| Workpiece face not square | Uneven initial contact | Check face runout | Face workpiece within 0.05 mm over 25 mm |
Bell Mouth Measurement
| Measurement Method | Tool | Acceptance Criteria |
|---|
| Entry diameter | Bore gauge or inside micrometer | Within print tolerance |
| Entry diameter vs bore diameter | Step gauge or air gauge | < 0.02 mm difference |
| Entry roundness | Roundness gauge | < 0.01 mm |
| Entry depth affected | Depth measurement at entry | Typically 2–5 mm |
Tip: A small bell mouth (under 0.02 mm oversize at entry) is acceptable for most applications. Bell mouth beyond 0.05 mm requires correction. The most common fix is replacing the starting bushing — it solves 70% of bell mouth problems.
Barber Pole (Diagonal Banding)
Cause and Correction
| Cause | Mechanism | Diagnostic Check | Fix |
|---|
| Guide pad pressure variation | One pad applies more force than others | Check guide pad wear pattern | Inspect and replace pads |
| Drill misalignment | Head not centered in bore | Measure bore roundness | Realign drill head |
| Coolant pressure fluctuation | Uneven lubrication on pads | Observe pressure gauge during cut | Stabilize coolant pressure |
| Worn guide pads | Pads no longer provide consistent burnishing | Measure pad thickness | Replace guide pads |
| Material hardness variation | Uneven resistance to pad burnishing | Check material hardness across section | Normalize material |
Spiral Marks (Helical Grooves)
Cause and Correction
| Spiral Pitch | Cause | Fix |
|---|
| Equal to feed per revolution | Feed rate interacts with tool geometry | Reduce feed by 20% |
| Multiple of feed per revolution | Harmonic interaction | Change spindle speed by 20% |
| Varying pitch | Coolant pressure fluctuation | Stabilize pressure, check pump |
| Increasing pitch along hole | Worn guide pads | Replace pads |
| Decreasing pitch toward exit | Chip accumulation at depth | Improve chip evacuation |
Taper (Diameter Change Along Length)
Types of Taper
| Taper Type | Diameter Profile | Cause | Fix |
|---|
| Entry larger than exit | Decreasing diameter | Tool cooling during cut — drill contracts | Allow thermal stabilization before cutting |
| Entry smaller than exit | Increasing diameter | Tool heating during cut — drill expands | Stabilize coolant temperature |
| Entry and exit smaller than middle | Barrel shape | Coolant pressure variation at mid-depth | Check coolant pressure profile |
| Entry and exit larger than middle | Hourglass shape | Rare — usually indicates guide pad issue | Check pad condition and alignment |
Taper Troubleshooting
| Observed Taper | Most Likely Cause | Corrective Action |
|---|
| Decreasing diameter (entry > exit) | Spindle warming up, thermal growth | Complete warm-up cycle before production |
| Increasing diameter (exit > entry) | Coolant heating up, tool expanding | Check chiller, stabilize coolant temp |
| Barrel (middle larger) | Coolant pressure drop at mid-depth | Increase coolant pressure or flow |
| Step change at specific depth | Material change or obstruction | Investigate at that depth |
Oversize and Undersize Holes
Dimensional Defect Diagnosis
| Defect | Cause | Diagnostic | Fix |
|---|
| Oversize (consistent) | Worn guide pads | Measure pad thickness | Replace guide pads |
| Oversize (entry only) | Bell mouth — bushing worn | Measure bushing bore | Replace bushing |
| Oversize (random) | Material hardness variation | Check hardness | Normalize material |
| Oversize (increasing) | Progressive guide pad wear | Compare early vs late parts | More frequent pad replacement |
| Undersize (consistent) | Guide pads too large for application | Verify pad size | Use smaller pads |
| Undersize (random) | Built-up edge on pads | Inspect pad surface | Adjust coolant concentration |
| Undersize (increasing) | Pad material transfer | Check coolant lubricity | Increase EP additives |
Exit Defects
| Defect | Cause | Fix |
|---|
| Exit burr | High feed at breakthrough | Reduce feed to 50% for last 5 mm |
| Exit breakout | Material fracture at exit | Support exit face with backup material |
| Exit oversize | Loss of guide pad support at exit | Reduce feed at breakthrough |
| Exit chip jamming | Chips accumulate at exit | Increase coolant flow during breakthrough |
| Exit wander | Uneven breakthrough forces | Ensure exit face is perpendicular |
Defect Identification Flowchart
Step-by-Step Diagnosis
| Step | Question | Yes → | No → |
|---|
| 1 | Is the defect at the hole entry only? | Check bushing and entry feed (Bell mouth) | Go to step 2 |
| 2 | Does the defect run the full length? | Check for consistent pattern (Barber pole, spiral) | Go to step 3 |
| 3 | Does the diameter change along the length? | Check for taper causes | Go to step 4 |
| 4 | Is the defect intermittent or random? | Check material, chips, coolant | Go to step 5 |
| 5 | Is the surface finish affected? | Check for chatter or scoring | Check for vibration |
Measurement and Inspection
Defect Detection Methods
| Defect | Detection Method | Measurement Tool |
|---|
| Bell mouth | Diameter measurement at entry and 10 mm deep | Bore gauge, inside micrometer |
| Taper | Diameter at multiple depths | Air plug gauge, bore gauge |
| Spiral marks | Visual, profilometer trace | Borescope, surface profilometer |
| Barber pole | Visual, light reflection check | Borescope |
| Lobing | Roundness measurement | Roundness gauge |
| Oversize/undersize | Diameter measurement | Bore gauge, air plug gauge |
| Surface finish | Ra measurement | Profilometer, comparator |
FAQ
What causes bell mouth in deep hole drilling?
Bell mouth is caused by the drill oscillating at entry before establishing full diameter contact. The primary causes are a worn starting bushing (bushing bore oversize allows drill movement), excessive entry feed rate (drill tip deflects on initial contact), and bushing misalignment to the spindle axis.
How do I fix barber pole defects?
Barber pole defects (diagonal banding) are caused by uneven guide pad burnishing. Check guide pad wear patterns first — if one pad is more worn than others, replace all pads. Verify drill alignment to the bore centerline. Stabilize coolant pressure, as fluctuating pressure causes uneven pad lubrication.
What causes spiral marks on a gun drilled hole?
Spiral marks are caused by the interaction between the drill's feed rate and its rotational frequency. When the feed per revolution does not match the tool geometry, the cutting edge leaves a helical pattern on the bore surface. Reduce feed rate or adjust tool geometry to eliminate spiral marks.
What is the difference between bell mouth and taper?
Bell mouth is a localized enlargement at the hole entry only — typically the first 2–5 mm. Taper is a gradual diameter change along the full length of the hole. Bell mouth is caused by entry conditions (bushing, feed). Taper is caused by thermal effects (tool heating or cooling during the cut) or coolant pressure variation.
Can I fix a bell mouth defect without replacing the bushing?
If the bushing is within specification (less than 0.02 mm oversize), you can reduce bell mouth by lowering entry feed rate to 30% of production feed and ensuring the workpiece face is square to the drill axis. However, if the bushing is worn beyond 0.02 mm oversize, replacement is required — the bell mouth will not improve until the bushing is replaced.
Hole defects are the diagnostic output of the drilling process. Identify the pattern correctly, and the root cause is usually obvious. This article reflects industry practice as of 2026.
