Boring bar: A single-point cutting tool used to enlarge an existing hole (typically drilled or rough-bored) to a precise diameter with tolerance IT6-IT7 (±0.005-0.01mm) and surface finish Ra 0.4-1.6 μm. Six types cover 95% of work: standard boring bars ($89-300), U-drills ($67-220 shank), indexable drills ($120-450), head finishers ($250-650), fine boring heads ($450-1200), and solid carbide boring bars ($80-260).
Six common boring tools cover 95% of hole-enlarging work: standard boring bars, U-drills (indexable), indexable drills, head finishers, fine boring heads, and solid carbide boring bars. Each excels in a specific diameter and tolerance range — choosing the right one saves hours of rework and tooling cost.
This guide covers the 6 tool types side-by-side (with diameter range, tolerance capability, cost, and best application), recommended speeds and feeds by material, and decision rules for picking the right tool on the first try.
Boring vs Drilling: 4 Key Differences
Drilling and boring are often confused but serve different purposes:
- Purpose: Drilling creates a new hole. Boring enlarges an existing hole (typically from a drill or rough bore) to a precise diameter.
- Starting point: Drilling starts from solid material. Boring requires a pre-existing hole (0.5-2mm undersize is typical).
- Tolerance: Drilling typically achieves IT10-IT12. Boring achieves IT6-IT7 (down to ±0.005mm).
- Surface finish: Drilling produces Ra 1.6-6.3 μm. Boring produces Ra 0.4-1.6 μm, often eliminating the need for a separate finishing operation.
Use drilling for rough hole creation (production, large diameter, deep holes). Use boring for precision diameter control, tight tolerance, and fine surface finish.
6 Boring Tools Compared: Specs, Tolerance, Cost
Here's the full comparison of the 6 boring tool types:
| Tool | Ø Range | Tolerance | Surface Finish | Cost | Best For |
|---|---|---|---|---|---|
| Standard Boring Bar (single-point) | 10-500mm+ | IT7-IT8 (±0.01mm) | Ra 0.8-1.6 μm | $89-300 | General precision boring, mid-volume |
| U-Drill (indexable) | 15-60mm | IT9-IT10 | Ra 1.6-3.2 μm | $67-220 (shank only) | Drilling + boring in one pass, 2-3×D depth |
| Indexable Drill | 12-80mm | IT10 | Ra 3.2-6.3 μm | $120-450 | Deep hole drilling up to 5×D |
| Head Finisher (combo) | 20-100mm | IT7-IT8 | Ra 0.8-1.6 μm | $250-650 | Drill + bore + chamfer in one pass |
| Fine Boring Head | 20-300mm | IT6-IT7 (±0.005mm) | Ra 0.4-0.8 μm | $450-1200 | Precision finishing, mold work, tight tolerance |
| Solid Carbide Boring Bar | 3-25mm | IT6 | Ra 0.4-0.8 μm | $80-260 | Small-diameter precision, finishing |
Key fact: Six boring tool types cover the full diameter range from 3mm to 500mm+: solid carbide boring bar (3-25mm), standard boring bar (10-500mm+), U-drill (15-60mm), indexable drill (12-80mm), head finisher (20-100mm), and fine boring head (20-300mm). For most CNC work, only U-drill + standard boring bar combination is needed (covers 90% of jobs).
Quick selection: For Ø10-25mm precision holes → solid carbide boring bar. For Ø15-60mm production → U-drill. For Ø20-100mm tolerance ±0.01mm → head finisher. For mold-making precision (<±0.005mm) → fine boring head.
Speeds & Feeds for Boring (By Material)
Recommended starting parameters for boring with carbide inserts. Always adjust based on rigidity, coolant pressure, and surface finish requirements:
| Material | Vc (m/min) | Feed f (mm/rev) | Depth of Cut ap (mm) | Coolant Pressure |
|---|---|---|---|---|
| Mild steel (1018, A36) | 150-220 | 0.10-0.25 | 0.5-2.0 | 30-50 bar flood |
| Alloy steel (4140, 4340) | 120-180 | 0.08-0.20 | 0.4-1.5 | 50-70 bar HP |
| Stainless (304, 316) | 100-160 | 0.05-0.15 | 0.3-1.0 | 70+ bar HP mandatory |
| Cast iron (gray, ductile) | 120-180 | 0.10-0.30 | 0.5-2.0 | Dry or MQL |
| Aluminum (6061, 7075) | 200-400 | 0.10-0.30 | 0.5-2.5 | Flood (15+ bar) |
| Hardened steel (HRC 50+) | 60-100 | 0.03-0.10 | 0.1-0.5 | 70+ bar HP |
| Titanium (Ti-6Al-4V) | 40-80 | 0.05-0.15 | 0.3-1.0 | 70+ bar HP mandatory |
Key fact: Recommended cutting speeds for boring by material: mild steel 150-220 m/min, alloy steel 120-180, stainless 100-160 (with 70+ bar HP coolant), cast iron 120-180 (dry OK), aluminum 200-400, hardened steel (HRC 50+) 60-100, titanium 40-80. Going 20% faster than these baseline values increases tool wear 3-4×.
For specific Vc/f calculations and chipbreaker selection, see our Speeds and Feeds Calculator.
Boring Bars vs U-Drills: Which to Choose?
The most common production question — boring bar or U-drill — comes down to tolerance and production volume:
- Choose U-drill if: Production volume, you can live with IT9-IT10 tolerance, diameter 15-60mm, depth 2-3×D or less. Faster cycle time, lower per-hole cost.
-
Choose boring bar if: Tight tolerance (IT7 or better), larger diameter (>60mm), surface finish
Most production shops use both: U-drill for rough boring (1st pass), then boring bar for finish (2nd pass). This gives cycle time savings + precision.
How Does a Boring Bar Work?
A boring bar is essentially a single-point cutting tool held in a boring head. The cutting diameter is adjusted by:
- Manually adjustable boring heads: Dial indicator on the head moves the insert radially in 0.01mm or 0.0025mm increments. Set before mounting.
- Digital boring heads: Electronic digital readout to 0.001mm. More accurate and faster to set, but 3-4× the cost.
- Cartridge-style boring bars: Cartridges with different diameters screw into a common holder. Quick change for production runs of multiple sizes.
The boring bar must be rigidly supported — overhang beyond 4×D causes chatter and poor surface finish. For deep boring, use a damped boring bar (internal vibration absorber).
Choosing the Right Boring Tool for the Job
Decision framework based on your situation:
- One-off prototype or repair: Standard boring bar with HSS or brazed carbide insert. Most flexible, lowest cost.
- Production run of 100+ parts: U-drill for roughing, boring bar for finishing. Inserts replaceable per machine.
- Mold or die work (tight tolerance): Fine boring head with digital readout. ±0.005mm achievable.
- Deep hole (>5×D): Indexable drill with internal coolant, or damped boring bar. Avoid standard boring bars — they chatter.
- Small diameter (<10mm): Solid carbide boring bar. Indexable tools don't have room for inserts below 10mm.
Key fact: Boring bar overhang rule — never exceed 4×D overhang (D = bore diameter). At 5×D, chatter becomes likely. At 6×D, you need a damped boring bar with internal vibration absorber. Going beyond 6×D without damping causes unacceptable surface finish and rapid insert failure.
Common Boring Tool Failure Modes
If your boring tool is failing prematurely, check these causes first:
- Chatter (poor surface finish): Too much overhang, insufficient rigidity, or wrong insert geometry. Reduce overhang to <4×D, use sharper insert.
- Insert chipping: Interrupted cuts, hard inclusions in material, or excessive feed. Reduce feed by 20%, use tougher grade.
- Rapid wear: Wrong Vc for material, or insufficient coolant. Check speeds & feeds chart, verify 70+ bar coolant for stainless.
- Built-up edge (BUE) on aluminum: Insufficient Vc or wrong insert. Increase Vc to 300+ m/min, use polished or DLC-coated insert.
Where to Buy Boring Bars: BEYOND / Sandvik / Mitsubishi
For production boring without the OEM price tag, our BEYOND house-brand boring bars and U-drills offer Sandvik/Mitsubishi-equivalent performance at 30-50% lower cost. In-stock diameters from 8mm to 60mm.
Shop Boring Bars & U-Drills
Standard boring bars, U-drills, fine boring heads, damped boring bars. From $89. 50+ countries, DHL/UPS/FedEx 3-7 day delivery.
Browse Boring Bar Collection Browse VN-Insert Boring Bars Browse 3D U-DrillsPeople Also Ask
What is the difference between a drill and a boring bar?
A drill creates a new hole from solid material using a multi-point cutting edge, achieving IT10-IT12 tolerance and Ra 1.6-6.3 μm surface finish. A boring bar enlarges an existing hole using a single-point insert, achieving IT6-IT7 tolerance and Ra 0.4-1.6 μm finish. Drilling is faster; boring is more precise.
What is the best boring bar for precision work?
For precision work (IT6-IT7 tolerance, Ra 0.4-0.8 μm), a fine boring head with digital readout is the best choice. Brands like BIG KAISER, Sandvik Coromant, and Mitsubishi offer heads with 0.001mm adjustment increments. Cost: $450-1200 per head, but achievable tolerance is ±0.005mm — 10× better than standard boring bars.
What is a U-drill?
A U-drill (also called indexable drill) is a drilling tool that combines drilling and rough boring in one pass. It has a steel shank with indexable carbide inserts (typically 2-3 inserts) that can be replaced when worn, rather than reground. U-drills cover Ø15-60mm diameters at 2-3×D depth, with IT9-IT10 tolerance. Cost: $67-220 per shank.
How accurate is a boring bar?
Standard boring bars with digital readout achieve ±0.005mm (IT6-IT7) tolerance. Manual boring heads with dial indicators achieve ±0.01mm (IT7-IT8). Fine boring heads with vernier scales reach ±0.0025mm. Achievable accuracy depends on machine rigidity, tool wear, and operator skill — even the best boring head won't compensate for a worn machine spindle.
What is the difference between rough and finish boring?
Rough boring removes most material in a single pass (depth of cut 1-3mm, feed 0.15-0.30 mm/rev) using a strong insert geometry like CNMG or TNMG. Finish boring takes a light pass (depth 0.2-0.5mm, feed 0.05-0.15 mm/rev) using a sharp insert with wiper geometry (DCMT, TCMT). Most precision holes need both passes for optimal cycle time and surface finish.
What RPM should I use for boring?
For boring, RPM = (1000 × Vc) / (π × D), where Vc is cutting speed in m/min and D is bore diameter in mm. Example: boring Ø50mm in mild steel at Vc 180 m/min = 1146 RPM. For stainless, reduce Vc by 20-30%. For hardened steel (HRC 50+), use Vc 60-100 m/min with PVD-coated carbide.
Frequently Asked Questions
What is the difference between a boring bar and a drill?
A drill creates a new hole from solid material using a multi-point cutting edge. A boring bar enlarges an existing hole using a single-point insert. Boring achieves tighter tolerance (IT6-IT7 vs IT10-IT12) and better surface finish (Ra 0.4-1.6 μm vs 1.6-6.3 μm) but is slower per cubic inch of material removed.
How accurate is a boring bar?
Standard boring bars with digital readout achieve ±0.005mm (IT6-IT7). Manual boring heads with dial indicators achieve ±0.01mm (IT7-IT8). Fine boring heads with vernier scales can reach ±0.0025mm. Achievable tolerance depends on machine rigidity, tool wear, and operator skill.
Can a boring bar cut on a drill press?
Not recommended for precision work. Drill presses lack the rigidity for boring — you can get within 0.05mm but won't achieve IT7 tolerance. For occasional boring, a boring bar in a VMC or boring mill is the proper tool. Drill presses are limited to light finishing with HSS bits.
What RPM should I use for boring?
For boring, RPM = (1000 × Vc) / (π × D), where Vc is the cutting speed in m/min and D is the bore diameter in mm. For example, boring Ø50mm in mild steel at Vc 180 m/min: RPM = (1000 × 180) / (3.14 × 50) = 1146 RPM. Reduce by 20% if using HSS, increase by 20% for aluminum.
What is a damped boring bar?
A damped boring bar has an internal vibration absorber (typically a heavy metal mass in an elastomer sleeve) that reduces chatter at deep overhangs (>4×D). It allows boring to 6-8×D depth with acceptable surface finish. Standard boring bars are limited to 4-5×D. Damped bars cost 2-3× more but enable otherwise impossible operations.
How do I choose between rough and finish boring?
Rough boring removes most of the material (ap 1-3mm, f 0.15-0.30 mm/rev) using a strong insert geometry (e.g., CNMG / TNMG). Finish boring takes a light pass (ap 0.2-0.5mm, f 0.05-0.15 mm/rev) using a sharp insert with wiper geometry (e.g., DCMT / TCMT). Most precision holes need both passes.
Conclusion
Choosing the right boring tool comes down to diameter range, tolerance required, and production volume. For most CNC work, the U-drill + boring bar combination covers 90% of needs. For tight-tolerance mold work, invest in a fine boring head.
Need help choosing the right boring setup for your job? Contact our engineering team at /pages/contact or WhatsApp +86 159-9066-3752 for application engineering support.
