Microboring: A guide to boring at very small diameters
Micro boring in the ~2–6 mm range straddles the line between conventional and precision lathe work. While commercial boring bars exist for this domain, the constraints imposed by physical tool size and rigidity mean that even these small-scale operations behave more like fine watchmaking or precision instrumentation work than typical lathe practice.
This article outlines methods, tooling, and techniques for boring at the lower limit of standard commercial tool holders—focusing on internal diameters where typical boring bars bottom out: ~2 mm.
1. Tooling and Boring Bar Selection
A. Tool Holders
At this scale, most tool holders are proprietary or modular systems designed to accept very small insert- or solid-tip boring bars. Look for:
High-precision modular holders (e.g., Sandvik Coromant’s CoroTurn XS, Seco MDT Micro, or Big Kaiser Micro Boring).
Monoblock shank boring bars down to 2 mm diameter, often in solid carbide.
Through-tool coolant is often standard in these systems—essential for chip clearing.
B. Bar Materials
Solid Carbide: Standard below 6 mm, essential below 3 mm.
Steel shanks with brazed carbide heads are rare in this size due to manufacturing constraints.
Minimum bore sizes:
Ø6 mm boring bar → ~6.5–7 mm min bore
Ø2 mm bar → ~2.2–2.5 mm min bore, depending on relief
2. Tool Geometry and Inserts
Tool geometry for micro boring must maximize clearance and minimize cutting pressure:
Rake Angle: Positive (10–20°), particularly in non-ferrous materials.
Relief: Radial and axial clearance is crucial; even small interference causes rubbing and heat.
Insert Choice: Small, proprietary inserts or solid-carbide brazed tips are common.
Edge Prep: Honed or lapped edges improve finish but may increase cutting forces—use with caution.
Some systems use single-piece ground solid-carbide tools, effectively combining insert and bar into one unit.
3. Machining Considerations
A. Rigidity and Setup
Overhang: Minimize—ideally <3× bar diameter.
Tool alignment: Critical—use indicators or test cuts to ensure the bar is exactly on center.
Workholding: Collet chucks or precision 3-jaws are mandatory to control runout.
B. Speeds and Feeds
Cutting Speed: Use manufacturer recommendations—typically 100–250 m/min for carbide.
Feed per rev: 0.01–0.03 mm/rev typical; lower feeds for finishing.
Depth of Cut: Start with 0.05–0.1 mm; reduce for finishing or near final dimension.
C. Chip Evacuation
Boring small bores traps chips, causing surface defects or tool breakage.
Coolant-through tooling is highly recommended.
If not available, peck boring manually or with canned cycles is necessary.
For blind bores, add a clearance groove at the bottom if possible.
4. Surface Finish and Tolerance Control
At these diameters, small changes in tool pressure, deflection, or temperature have outsized effects:
Spring cuts are often needed to remove tool deflection.
Finishing Passes: Keep DOC under 0.02 mm.
Use air gaging or small bore dial gauges for inspection. Avoid calipers or micrometers for ID measurement at this size.
5. Materials and Behavior
Aluminum, Brass: Easy to cut; use sharp, high-rake tools.
Tool Steel, Stainless: Tool deflection and chip welding more likely; use coolant, lower DOC.
Titanium: Requires sharp tools and high surface speed; don’t dwell.
6. Tool Wear and Breakage
Breakage Risk: High with poor chip evacuation or deflection.
Detection: Monitor sound and spindle load for signs of chatter or tool rub.
Edge Wear: Can quickly degrade surface finish in small bores—have spare bars or inserts ready.
7. Example Applications
| Part Type | Bore Ø | Notes |
|---|---|---|
| Valve seat bores | 2.5–3.5 mm | Requires concentricity and surface finish |
| Instrument bearings | ~2.2 mm | Requires tight geometry tolerance |
| Microfluidic fittings | 3–5 mm | Thin walls—tool pressure must be minimized |
Conclusion
Boring at the lower limit of commercially available tooling (~2–3 mm) requires treating the operation as a hybrid of macro- and micro-machining. While you can leverage the precision of standard lathe equipment, you'll need the care, patience, and tool selection normally reserved for ultraprecision work. With proper tooling, short overhangs, sharp geometries, and pecking strategies, you can achieve exceptional results in this constrained space.