Deep hole drilling is a high-risk process. When something goes wrong – a drill breaks in the bore, the bore wanders off position, or the surface finish is out of spec – the part is not necessarily scrap. I have salvaged thousands of dollars worth of components by knowing how to repair and rework bores.
The key is knowing when a part can be saved and when it cannot. I have a simple rule: if the repair costs less than 60% of the replacement cost, it is worth doing. Anything over that and the customer is better off starting fresh.
Broken Tool Extraction
Every driller breaks tools. When a gun drill or BTA drill snaps in the bore, the first step is to assess how it broke and where it is stuck.
Extraction Methods by Situation
| Situation | Method | Success Rate | Time Required |
|---|---|---|---|
| Small drill (<6mm), near entry | Carbide burr on flexible shaft | 80% | 1-2 hours |
| Medium drill (6-20mm), stuck tight | Sinker EDM | 95% | 2-4 hours |
| Large drill (>20mm), near entry | Mechanical extractor | 70% | 1-3 hours |
| Any size, deep in bore | EDM with long electrode | 90% | 4-8 hours |
| Broken carbide tip only | Acid etch (HF-based) | 60% | 2-6 hours |
I prefer EDM for extraction. A sinker EDM burns out the broken tool without damaging the bore wall. The electrode matches the shape of the broken tool, and the spark gap is small enough (0.05-0.15mm) that the parent material is unaffected.
The limitations of EDM are the depth and the access. If the broken tool is more than 1 meter deep in the bore, the electrode has to be that long and the EDM machine has to have enough throat depth. Not all shops have that capability.
Preventing Breakage
The best extraction is the one you do not need. I have a pre-start checklist that prevents most breakage:
- Check coolant pressure at the tool tip (not at the pump)
- Verify guide bushing wear is under 0.05mm clearance
- Inspect the drill for edge wear or chipping
- Confirm the workholding is rigid – no vibration at the entry
- Check the material for hard spots or scale at the entry
Weld Repair and Re-Drilling
When a bore is out of position or oversized, welding and re-drilling can save the part. The process closes the old bore and creates a new one in the correct position.
Weld Repair Process
- Inspect – determine the defect (off-position, oversized, tapered)
- Machine preparation – open the bore to clean metal, create a V-groove if needed
- Weld – fill the bore with compatible filler material
- Stress relieve – heat treat to relieve welding stresses (600-1200 F depending on material)
- Machine entry face – create a flat, square start surface
- Re-drill – deep hole drill at the correct position and size
Weld Repair Limitations
| Factor | Acceptable | Not Acceptable |
|---|---|---|
| Wall thickness after repair | 5mm minimum | Under 5mm |
| Bore position error | Up to 5mm | Over 5mm |
| Number of repair attempts | 1 | 2 or more |
| Material type | Weldable steels | High-carbon >0.5%C, tool steels |
| Part value | Over $500 replacement cost | Under $500 |
I have successfully repaired hydraulic cylinder barrels, pump housings, and valve bodies this way. On a recent job, a 2-meter-long cylinder barrel was drilled 1.5mm off center. The replacement cost was $4,000. The weld repair and re-drilling cost $850. The part passed hydrostatic testing.
Reworking Undersized Bores
Sometimes the bore comes out within position tolerance but undersized. This is usually caused by a worn drill that is cutting under size, or thermal contraction after drilling that closes the bore slightly.
For undersized bores, I have three options:
| Rework Method | Stock Removal | Surface Finish | Best For |
|---|---|---|---|
| Reaming | 0.1-0.5mm | 0.4-1.6 Ra | Tight tolerances |
| Skive and roller burnish | 0.05-0.15mm | 0.1-0.4 Ra | Hydraulic cylinders |
| Honing | 0.05-0.3mm | 0.1-0.8 Ra | Any bore |
| Ball burnishing | 0.01-0.05mm | 0.05-0.2 Ra | Surface finish only |
I check the bore size after the part has stabilized at room temperature. A bore that was drilled hot can shrink by 0.02-0.05mm when it cools. If I measure immediately after drilling, the reading is misleading.
Salvaging Oversized Bores
An oversized bore is harder to fix than an undersized one. The options are:
- Accept it – if the bore is within the upper tolerance limit, use it as-is
- Sleeve it – press in a thin-walled sleeve and re-drill to size
- Weld and re-drill – as described above
- Coat the part – thermal spray or chrome plate to restore the bore to size
Sleeving is the most common fix for oversized bores. I use a 0.5-2.0mm wall sleeve made from the same material as the parent part. The sleeve is pressed in with an interference fit of 0.02-0.05mm, then welded or bonded at the ends.
Key Takeaways
- EDM is the most reliable method for broken tool extraction
- Weld repair and re-drilling is cost-effective for high-value parts (under 60% of replacement cost)
- Minimum 5mm wall thickness required after weld repair
- Measure bore after thermal stabilization – hot bores read small
- Sleeving is the simplest fix for oversized bores
- Prevention is cheaper than any repair – check coolant, bushings, and tool condition before every job
For related topics, see my articles on thin wall tubing and casting and forging rework.