Grooving and Parting-Off Tools: Insert Width, Grade, and Chipbreaker Selection
Grooving and parting-off are among the most demanding turning operations. The tool is fully enclosed in the cut, chip evacuation is restricted, and cutting forces are concentrated on a narrow cutting edge. Selecting the right insert width, chipbreaker geometry, and carbide grade is essential for tool life, surface finish, and cycle time.
1. Insert Width: The Primary Decision
Grooving insert width (also called blade width) directly determines the groove width for external grooving and the kerf width for parting-off. Standard widths range from 1.0 mm to 10.0 mm, with the most common sizes being 2.0, 3.0, 4.0, 5.0, and 6.0 mm.
| Operation | Typical Insert Width (mm) | Cutting Force Level | Application Notes |
|---|---|---|---|
| Parting-off (bar Ø ≤25 mm) | 1.5–2.0 | Low | Minimizes material waste, Swiss-type lathes |
| Parting-off (bar Ø 25–65 mm) | 2.5–3.5 | Medium | CNC lathes, standard production |
| Parting-off (bar Ø 65–150 mm) | 4.0–6.0 | High | Oil pipe, large shaft cutoff |
| Seal grooves (O-ring) | 1.5–3.0 | Low–Medium | Precise width tolerance ±0.02 mm |
| Retaining ring grooves | 1.0–2.5 | Low | DIN 471/472 groove dimensions |
| Wide profile grooving | 4.0–10.0 | Very High | Stepped grooves, keyway forms |
Key rule: For parting-off, use the narrowest insert that provides adequate tool life and rigidity. Every extra 0.5 mm of width increases cutting force by roughly 15–20% and wastes material. For bar work on CNC lathes, 3.0 mm is the standard compromise for diameters up to 50 mm.
2. Chipbreaker Geometry: Controlling the Chip
In grooving, the chip must curl and evacuate from a narrow slot. Without proper chipbreaking, long stringy chips wrap around the blade, scratch the groove surface, and can cause insert fracture. Insert manufacturers offer multiple chipbreaker profiles optimized for different feed rates and materials.
| Chipbreaker Type | Feed Range (mm/rev) | Material Suitability | Chip Form |
|---|---|---|---|
| Sharp (finishing) | 0.03–0.10 | Stainless, aluminum, thin-wall | Short spirals |
| Medium (general purpose) | 0.08–0.20 | Carbon steel, alloy steel | 6-shaped curls |
| Heavy (roughing) | 0.15–0.35 | High-temp alloys, hard steel | Tight helical coils |
| Flat (no chipbreaker) | 0.01–0.05 | Very light finishing, honing passes | Long ribbon (managed by feed) |
Korloy’s grooving inserts (MGMN and MGGN series) come with multiple chipbreaker options. The MGMN300-M with the “M” chipbreaker handles 0.08–0.18 mm/rev in carbon and alloy steels — a reliable default for production CNC turning. For stainless steel (304/316), switch to the sharper “F” chipbreaker and reduce feed by 20%.
3. Carbide Grade Selection by Material
Grooving demands a carbide grade with high edge toughness because the cutting edge is fully engaged and subject to impact at entry. Avoid hard, brittle finishing grades — a medium-toughness grade with PVD coating is ideal.
| Workpiece Material | Recommended Grade | Coating | Hardness (HRA) | Speed Range (m/min) |
|---|---|---|---|---|
| Carbon steel (C45, 1045) | Korloy NC3220 | TiCN + Al₂O₃ CVD | 90.5 | 150–250 |
| Alloy steel (4140, 4340) | Korloy NC3225 | Multi-layer CVD | 89.8 | 120–200 |
| Stainless steel (304/316) | Korloy NC3115 | TiAlN PVD | 91.2 | 80–150 |
| Cast iron (GG25) | Korloy NC9125 | Thick Al₂O₃ CVD | 90.0 | 150–300 |
| Aluminum (6061-T6) | Korloy NC500H | DLC or uncoated | 92.0 | 300–600 |
| Inconel / Ti alloys | Korloy NC6015 | AlTiN PVD | 91.5 | 30–70 |
4. Parting-Off: Specific Considerations
Parting-off introduces unique challenges as the tool approaches the center of the bar:
- Surface speed decreases: As the tool approaches center, the effective cutting speed drops proportionally to the remaining diameter. On a 50 mm bar at 1,200 RPM, surface speed drops from 188 m/min at the OD to 19 m/min at 5 mm remaining diameter. Program constant surface speed (CSS/G96) to compensate.
- Feed reduction near center: Reduce feed rate by 30–50% in the last 5–8 mm of the cut to prevent the remaining “pip” from tearing and damaging the insert edge.
- Coolant pressure: Minimum 20 bar through-coolant for effective chip evacuation. High-pressure coolant (70+ bar) dramatically improves tool life in stainless and Inconel.
- Sub-spindle catch: When parting off on a twin-spindle lathe, engage the sub-spindle before the cut completes to eliminate the pip and support the part.
5. Toolholder Systems: Block vs. Cassette
Grooving toolholders come in two main types:
Solid block holders: The insert clamps directly into a steel shank (e.g., MGEHR/MGIVR series). Simple, rigid, and economical. Best for high-volume production where one groove width dominates.
Cassette/blade systems: A thin carbide or steel blade holds the insert, and the blade clamps into a base holder. This allows quick width changes without replacing the entire holder. Preferred in job shops with varied groove widths. The Korloy modular grooving system allows blade swaps in under 30 seconds.
6. Decision Framework
- Determine groove width → select the nearest standard insert width.
- Identify material → select grade from the table above.
- Set feed rate → start at 60% of the chipbreaker’s mid-range, increase if chips are long.
- Choose holder type → solid block for production, cassette for job shop flexibility.
- Coolant strategy → through-coolant mandatory for parting-off; flood acceptable for shallow external grooves.
Summary
Grooving and parting-off tool selection comes down to three choices: insert width (narrowest practical), chipbreaker (matched to feed rate and material), and grade (tough enough for full-engagement cutting). Proper coolant delivery and feed reduction near center are equally critical for parting-off success.
Find Korloy MGMN/MGGN grooving inserts, blade holders, and parting tools at hooguu.com — competitive pricing, fast delivery, and expert technical support for CNC shops worldwide.
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