The deep hole drilled hole is rarely the finished hole. Secondary operations correct geometry, improve surface finish, and achieve final tolerances that primary drilling cannot deliver. Process planning that integrates primary and secondary operations as a complete system produces better holes at lower cost than treating each operation independently.
Secondary Operation Overview
Capability Comparison
| Operation | Typical Tolerance (IT grade) | Typical Surface Finish (Ra) | Material Removal | Tooling Cost per Hole |
|---|
| Gun drilling (primary) | IT7–IT10 | 0.4–1.6 µm | Full material | Low |
| BTA drilling (primary) | IT8–IT11 | 0.8–3.2 µm | Full material | Medium |
| Reaming | IT6–IT8 | 0.2–0.8 µm | 0.1–0.5 mm stock | Low to medium |
| Boring (single-point) | IT5–IT7 | 0.1–0.4 µm | 0.2–1.0 mm stock | Medium (adjustable) |
| Roller burnishing | IT6–IT8 (improves from prior) | 0.05–0.2 µm | 0 (displaces material) | Low per hole |
| Honing | IT5–IT6 | 0.05–0.2 µm | 0.02–0.10 mm stock | High (stone cost) |
| Skiving (Skive and burnish) | IT6–IT8 | 0.05–0.15 µm | 0.1–0.3 mm stock | Medium to high |
| Ballizing | IT7–IT9 (depends on prior) | 0.1–0.4 µm | 0 (displaces) | Low per hole |
Process Selection by Requirement
| Requirement | Primary Choice | Secondary Operations | Total Process Steps |
|---|
| General tolerance, any finish | Gun drill alone | None | 1 |
| IT7 tolerance, good finish | Gun drill | Ream | 2 |
| IT6 tolerance, good finish | Gun drill | Bore | 2 |
| IT6 tolerance, excellent finish | Gun drill | Bore + burnish | 3 |
| IT5 tolerance, mirror finish | Gun drill | Bore + hone | 3 |
| IT5 tolerance, production rate | BTA drill | Skive and burnish | 2 |
| Surface hardened requirement | Gun drill | Bore (with CBN) | 2 |
Reaming
Reaming Stock Allowance
| Hole Diameter (mm) | Stock per Side (mm) | Expected Tolerance Improvement | Expected Ra Improvement |
|---|
| 5–15 | 0.10–0.20 | IT7→IT6 | 1.6→0.4 µm |
| 15–30 | 0.15–0.25 | IT8→IT7 | 1.6→0.4 µm |
| 30–60 | 0.20–0.35 | IT9→IT7 | 3.2→0.8 µm |
| 60–100 | 0.25–0.50 | IT9→IT8 | 3.2→0.8 µm |
Deep Hole Reaming Considerations
| Factor | Recommendation | Reason |
|---|
| Reamer length | Minimum required + 20% | Long reamers deflect, cause taper |
| Flute design | Spiral flute for deep holes | Better chip evacuation |
| Coolant | Through-coolant reamer preferred | Flush chips from cutting zone |
| Speed | 50–70% of drilling speed | Lower speed reduces wear |
| Feed | 2–3× drilling feed per revolution | Each cutting edge takes light cut |
| Back taper | Minimum 0.01 mm per 100 mm | Prevents binding in deep holes |
Common Reaming Problems
| Problem | Cause | Solution |
|---|
| Tapered hole | Reamer deflection, worn leading edge | Reduce stock, check reamer condition |
| Oversize hole | Reamer runout, feed too high | Check holder concentricity, reduce feed |
| Poor surface finish | Insufficient coolant, dull reamer | Increase coolant flow, regrind or replace |
| Chatter marks | Speed too high, insufficient rigidity | Reduce speed, increase feed |
| Bell mouth at entry | Misalignment, reamer not guided | Use bushing or guide, check alignment |
Boring (Single-Point)
Boring Applications
| Application | Tool Type | Achievable Tolerance | Surface Finish |
|---|
| Correct hole straightness | Single-point boring head | IT5–IT7 | 0.1–0.4 µm |
| Correct diameter after primary | Adjustable boring head | IT5–IT7 | 0.1–0.4 µm |
| Remove drill breakage remnant | Indexable insert boring bar | IT9–IT11 | 0.8–3.2 µm |
| Stepped or counter-bored holes | Custom boring tool | IT7–IT9 | 0.4–1.6 µm |
Boring Bar Considerations for Deep Holes
| L/D Ratio | Boring Bar Type | Max Depth | Limitation |
|---|
| < 4:1 | Solid carbide | Unlimited in diameter range | Cost at large diameters |
| 4:1–8:1 | Steel with carbide insert | Moderate | Chatter at overhang |
| 8:1–12:1 | Damped boring bar | Moderate | High cost, limited availability |
| > 12:1 | Guided boring system | Machine length | Requires bushing support |
Roller Burnishing
Process Parameters
| Parameter | Typical Range | Effect |
|---|
| Interference | 0.02–0.08 mm per side | Higher interference = smoother finish (to a point) |
| Feed rate | 0.05–0.20 mm/rev | Lower feed = better finish |
| Speed | 50–150 m/min | Minimal effect on finish |
| Lubrication | Oil or high-lubricity coolant | Required for tool life |
| Number of passes | 1–2 | Second pass improves finish 10–20% |
Achievable Results
| Starting Ra (µm) | Final Ra (µm) | Tolerance Improvement |
|---|
| 1.6 | 0.10–0.20 | IT8→IT7 (if prior consistent) |
| 0.8 | 0.05–0.10 | IT7→IT6 (if prior consistent) |
| 0.4 | 0.05–0.08 | IT7→IT6 |
Honing
Stock Removal and Results
| Material | Stock per Side (mm) | Achievable Tolerance | Achievable Ra (µm) |
|---|
| Steel (hardened) | 0.02–0.08 | IT5–IT6 | 0.05–0.20 |
| Steel (soft) | 0.05–0.15 | IT5–IT6 | 0.10–0.40 |
| Cast iron | 0.05–0.15 | IT5–IT6 | 0.05–0.20 |
| Stainless steel | 0.03–0.10 | IT5–IT6 | 0.10–0.30 |
Honing vs Other Finishing Methods
| Factor | Honing | Roller Burnishing | Skive and Burnish |
|---|
| Tolerance capability | IT5–IT6 | IT6–IT8 | IT6–IT8 |
| Surface finish | 0.05–0.20 µm | 0.05–0.20 µm | 0.05–0.15 µm |
| Cycle time | Moderate to slow | Fast | Fast |
| Tool cost per hole | Medium to high | Low | Medium |
| Corrects straightness? | Yes (slightly) | No | Yes |
| Requires prior operation | Bore or ream | Bore or ream | BTA or gun drill |
Process Sequence Planning
Recommended Sequences by Requirement
| Target Quality | Sequence | Total Operations |
|---|
| Hydraulic cylinder, IT7, 0.4 µm | Gun drill → Roller burnish | 2 |
| Hydraulic cylinder, IT6, 0.1 µm | Gun drill → Skive and burnish | 2 |
| Precision bushing bore, IT5, 0.1 µm | Gun drill → Bore → Hone | 3 |
| Gun barrel, IT6, 0.2 µm | Gun drill → Ream (×2 passes) | 3 |
| Structural, IT9, 1.6 µm | Gun drill only | 1 |
Stock Allowance Summary
| Operation After Drilling | Minimum Stock per Side | Maximum Stock per Side |
|---|
| Reaming | 0.10 mm | 0.50 mm |
| Boring | 0.15 mm | 1.00 mm |
| Roller burnishing | 0.01 mm | 0.08 mm |
| Honing | 0.02 mm | 0.15 mm |
| Skiving | 0.05 mm | 0.30 mm |
| Ballizing | 0.005 mm | 0.03 mm |
FAQ
What is the most common secondary operation for deep hole drilling?
Reaming is the most common secondary operation. It improves tolerance by approximately one IT grade and reduces surface finish from 1.6–3.2 µm Ra (as-drilled) to 0.2–0.8 µm Ra. Reaming is preferred when the primary drilling produces reasonable geometry but the final hole requires tighter tolerance or better finish than drilling alone can achieve.
Can I ream a deep hole on the same machine?
Yes — many deep hole drilling machines are designed for combined drilling and reaming operations. The reamer mounts in the same spindle, and the bushing guides the reamer at the hole entry. Use through-coolant spiral flute reamers for deep holes. Reduce speed to 50–70% of drilling speed and increase feed to 2–3× the drilling feed per revolution.
What is skive and burnishing for deep holes?
Skive and burnish (also called skiving and roller burnishing) is a combined finishing operation that uses a cutting edge to remove a thin layer (0.1–0.3 mm) followed immediately by roller burnishing elements that smooth the surface. It produces IT6–IT8 tolerance and 0.05–0.15 µm Ra surface finish in a single pass. It is commonly used for hydraulic cylinder tubes.
How much stock should I leave for honing a deep hole?
Leave 0.02–0.15 mm stock per side for honing, depending on material and prior operation. Hardened steels need less (0.02–0.08 mm), soft steels need more (0.05–0.15 mm). The stock must be uniform — honing cannot correct significant geometry errors from prior operations. The prior operation (boring or reaming) should produce a hole within 0.05 mm of final size for honing to be effective.
When should I use roller burnishing instead of honing?
Use roller burnishing when: the primary drilling produces consistent geometry within 0.05 mm of final size, surface finish below 0.2 µm Ra is needed, cycle time matters (burnishing is 2–5× faster than honing), and through-wall fatigue strength improvement is desired (burnishing work-hardens the surface). Use honing when: the hole needs straightness correction, IT5 or tighter tolerance is required, or the material is too hard for roller burnishing.
Secondary operations complete what primary drilling starts. Choosing the right finishing method and leaving the correct stock allowance determines whether the final hole meets specification on the first pass. This article reflects industry practice as of 2026.
