Problem Description
A bore drilled off position is a costly problem in deep hole drilling. I’ve had parts where the bore exit was 0.5 mm off location on a 20 mm diameter hole through 300 mm of 4140 steel. The part was otherwise perfectly machined, but the positional error made it scrap unless I could rework it. In deep hole drilling, bore position errors are especially painful because so much machining time has already been invested when the error is discovered.
The error can show up at entry, exit, or both. Entry error usually means the drill started in the wrong spot. Exit error that is worse than entry error means the drill wandered during the cut. Measuring both ends with a CMM tells me which case I’m dealing with. I always measure both ends — measuring only the entry gives a false sense of security if the drill wandered mid-cut.
Root Causes
Position error at entry comes from worn guide bushings, incorrect bushing-to-workpiece gap, or a misaligned spindle. I check the guide bushing ID first — if it’s worn more than 0.03 mm oversize on a 10 mm drill, the drill can shift before engaging the workpiece.
| Cause | Position Error Pattern | Typical Magnitude |
|---|---|---|
| Worn guide bushing | Random at entry and exit | 0.1 - 0.3 mm |
| Wrong bushing gap | Consistent at entry, less at exit | 0.2 - 0.5 mm |
| Spindle misalignment | Tapered bore error | 0.3 - 1.0 mm |
| Drill wandering | Exit error larger than entry | 0.5 - 2.0 mm |
| Chuck or collet runout | Consistent radial error | 0.05 - 0.15 mm |
| Workpiece movement during cut | Sudden offset change along bore | 0.2 - 1.0 mm |
| Thermal growth of spindle | Gradual drift over long cuts | 0.05 - 0.2 mm |
Drill wandering makes things worse. When the drill walks off center, the exit position drifts. I’ve measured wandering as high as 1.5 mm on a 600 mm deep hole in 316 stainless when the feed rate was 20% too high. The material work-hardened ahead of the drill and deflected it off course.
Assessment Before Rework
Before I touch the part, I check the remaining wall thickness. If the bore offset leaves less than 2 mm of wall on a hydraulic cylinder application, the part is scrap — no rework can fix that. I also check the print tolerance. If the original callout is +/- 0.1 mm and the bore is off by 0.15 mm, I might only need to open the hole to the next standard size.
I use a coordinate measuring machine to map the bore centerline at three depths: entry, mid-span, and exit. This tells me if the error is a straight shift or a curve. A straight shift can be reworked. A curved bore usually cannot be salvaged. I have only successfully salvaged a curved bore once, and that was because the curvature was under 0.1 mm over 200 mm depth.
Salvage Methods Comparison
I use five different salvage methods depending on the error magnitude, the material, and the component’s end use. Here is the full comparison I reference when deciding which approach to take.
Rework Methods Comparison
| Rework Method | Max Offset Corrected | Cost Factor (vs original) | Success Rate | Wall Thickness Needed | Applicable Materials |
|---|---|---|---|---|---|
| Weld and recut | Up to 0.5 mm | 3x | 90% | 3 mm minimum | Carbon steel, stainless |
| Oversize bore/hone | Up to 0.3 mm per side | 1.5x | 95% | Design-dependent | All materials |
| Plug and redrill | Up to 1.0 mm | 5x | 75% | 4 mm minimum | Low-stress applications |
| EDM rework | Up to 0.2 mm | 4x | 85% | 2 mm minimum | Any conductive material |
| Sleeve insertion | Up to 0.8 mm | 6x | 70% | 5 mm minimum | Any, requires redesign |
Decision Matrix for Rework Method Selection
| Condition | Best Method | Backup Method | Avoid |
|---|---|---|---|
| Offset < 0.3mm, any material | Oversize bore/hone | Weld and recut | Plug |
| Offset 0.3-0.5mm, steel | Weld and recut | Oversize | Plug for thin walls |
| Offset 0.5-1.0mm, low pressure | Plug and redrill | Sleeve | Weld (distortion risk) |
| Offset < 0.2mm, hardened material | EDM rework | Oversize | Weld (heat damage) |
| Offset > 1.0mm | Part scrap | Sleeve with redesign | Any direct rework |
| Hydraulic cylinder bore | Weld and recut only | Oversize if design allows | Plug (pressure risk) |
Rework Methods in Detail
Weld and Recut
For a straight shift under 0.3 mm, I machine a weld buildup on the off-side entry face, then recut the spot face and drill from the corrected location. I use a low-heat TIG process with a matching filler rod — 309L for stainless, ER70S-6 for carbon steel. I keep interpass temperature under 150 degrees C to avoid distortion. I measure the temperature with an infrared gun between passes. If the part gets above 150 degrees C, I stop and let it cool to room temperature before continuing.
The weld buildup adds material on the side opposite the error direction. After welding, I machine the face flat again and recut the spot face at the corrected center. This method works best when the error is purely at entry and the bore is straight from entry to exit.
Oversize Bore/Hone
For errors that allow a bore size increase, I bore or hone to the next standard oversize. Going from 20.00 mm to 20.25 mm gains me 0.125 mm of center adjustment per side. This only works if the wall thickness and the design allow a larger diameter. I check with the design engineer before using this method because the wall thickness reduction might affect the pressure rating or fatigue life.
Plug and Redrill
The most aggressive method is plugging. I bore the existing hole to 1.5x the offset diameter, press in a matching plug with Loctite 638 retaining compound, and redrill. I’ve saved several thousand-dollar parts this way, but it only works on holes where the plug won’t see extreme pressure. The plug must be the same material as the parent material to avoid galvanic corrosion and differential thermal expansion.
EDM Rework
Electrical discharge machining can rework the bore position by eroding material from the off-side wall. I use this method for hardened materials where conventional machining is difficult. The EDM process adds 0.05-0.1 mm of recast layer that needs to be polished off afterward.
Cost Comparison of Rework vs Scrap
| Rework Method | Setup Time | Execution Time | Total Cost | Cost as % of New Part Value |
|---|---|---|---|---|
| Oversize bore/hone | 30 min | 20 min | $85 | 15-25% |
| Weld and recut | 45 min | 60 min | $175 | 30-45% |
| EDM rework | 60 min | 90 min | $250 | 40-60% |
| Plug and redrill | 90 min | 120 min | $350 | 55-75% |
| Scrap and remake | 0 min | Full cycle | $500-800 | 100% |
I use this cost comparison to decide whether rework makes economic sense. If the rework cost exceeds 70% of the new part value, I scrap the part. Below 50%, rework is usually worth doing. Between 50% and 70%, I discuss with the customer before proceeding.
Prevention
I prevent off-position bores by verifying the guide bushing condition before every setup. I also check spindle runout with a dial indicator — anything over 0.005 mm gets corrected before I start cutting.
On the programming side, I add a dwell of 0.5 seconds before the drill engages the workpiece. This lets the spindle stabilize after the rapid move. I’ve found this single change reduces entry position scatter by about 40%.
I also use a pilot drill before the finishing deep hole drill on critical position applications. The pilot drill creates a starter hole that guides the deep hole drill. I use a short, rigid carbide spot drill that is 0.05 mm smaller than the deep hole drill diameter.
For more on preventing alignment problems, see the coolant leak detection and repair guide and the ejector drilling troubleshooting guide.
Key Takeaways
- Measure bore position at both ends to distinguish entry error from wandering — measuring only the entry gives a false sense of security.
- Remaining wall thickness is the deciding factor for whether rework is possible — less than 2mm on hydraulic cylinders means scrap.
- The salvage methods comparison table covers five methods: weld and recut, oversize bore/hone, plug and redrill, EDM rework, and sleeve insertion.
- Weld-and-recut works for offsets up to 0.5 mm on steel parts; plugging handles up to 1.0 mm only for low-pressure applications.
- Use the decision matrix to select the right method based on offset amount, material, and application pressure requirements.
- The cost comparison shows oversize bore/hone at 15-25% of new part value is the cheapest option, while plugging at 55-75% is borderline.
- Check guide bushing wear and spindle runout before every setup to prevent the problem in the first place.
- A 0.5-second dwell at entry and a pilot drill reduce position scatter significantly — these are low-cost preventive measures.