Parting-Off Large Diameters: Insert Width and Coolant Strategy

Parting-off (cutoff) operations on large diameter workpieces present unique challenges that differ significantly from cutoff of small bar stock. When the workpiece diameter exceeds 80 mm, the cutoff tool must travel a long radial distance, the chip evacuation path becomes extended, cutting forces increase substantially, and the severed piece can weigh enough to cause safety hazards or damage the machine. This guide addresses the specific requirements for parting large diameters on CNC lathes, covering insert selection, cutting parameters, coolant strategies, and safety considerations.

Insert Width Selection for Large Diameters

The cutoff insert width must balance blade rigidity against material waste and cutting forces. Wider inserts provide a stiffer blade that resists deflection but remove more material, generate higher cutting forces, and waste more stock. For large diameter cutoff, the following insert width guidelines apply:

• Diameter 80 to 120 mm: insert width 3.0 to 4.0 mm
• Diameter 120 to 180 mm: insert width 4.0 to 5.0 mm
• Diameter 180 to 250 mm: insert width 5.0 to 7.0 mm
• Diameter 250 to 400 mm: insert width 6.0 to 8.0 mm (or use a blade-type cutoff tool with 8 to 12 mm blade width)

The blade height must be sufficient to reach the workpiece centerline plus clearance below center. For a 150 mm diameter workpiece, the blade must extend at least 80 mm from the holder (75 mm to center plus 5 mm clearance). This long overhang makes blade rigidity the limiting factor. Carbide-reinforced steel blades or solid carbide blades provide the necessary stiffness.

Insert Geometry for Parting

Parting inserts for large diameters should have specific geometry features:

A positive rake angle (15 to 20 degrees) reduces cutting forces by 20 to 30 percent compared to neutral rake, which is critical when blade rigidity is limited. The insert should have a narrow land (0.05 to 0.10 mm) at the cutting edge followed by a relief angle of 7 to 10 degrees to minimize flank contact with the groove walls. A slight front angle (1 to 2 degrees) on the insert creates a self-centering effect that helps keep the blade tracking straight through the cut.

Cutting Parameters for Large Diameter Parting

• Carbon steel (AISI 1045), 120 mm diameter: Cutting speed 100 to 140 m/min at the outer diameter, decreasing as the tool approaches center. For constant surface speed (CSS/G96), the spindle speed increases automatically. Feed rate: 0.08 to 0.15 mm/rev for the outer portion (first 30 mm of depth), reducing to 0.05 to 0.10 mm/rev for the center portion (last 20 mm of radius). The reduced feed near center prevents insert overload as the surface speed drops.
• Stainless steel (AISI 316), 100 mm diameter: Cutting speed 60 to 90 m/min. Feed rate: 0.05 to 0.10 mm/rev outer, 0.03 to 0.07 mm/rev center. Maintain constant feed without dwelling. Flood coolant mandatory.
• Aluminum (6061-T6), 150 mm diameter: Cutting speed 200 to 350 m/min. Feed rate: 0.12 to 0.25 mm/rev. Sharp, polished uncoated carbide inserts. Minimal burr formation expected with correct parameters.
• Ductile iron (GGG50), 130 mm diameter: Cutting speed 80 to 120 m/min. Feed rate: 0.06 to 0.12 mm/rev. CBN or ceramic inserts for production volumes above 500 parts.

Coolant Strategy

Coolant delivery is the most critical factor in large diameter parting. The deep, narrow groove traps chips and heat, leading to rapid insert wear and potential tool failure. The following coolant strategies are recommended:

High-pressure coolant (minimum 50 bar): Directed through the toolholder at the cutting edge, high-pressure coolant flushes chips from the groove and provides effective cooling at the insert tip. Pressures of 70 to 100 bar are recommended for diameters above 150 mm and for tough materials like stainless steel and Inconel.

Dual coolant delivery: Some advanced cutoff systems deliver coolant from both above and below the insert. The upper stream cools the rake face and flushes chips forward, while the lower stream cools the flank face and prevents chip welding on the blade body. Dual delivery is highly effective for deep cuts in stainless steel.

Peck cutoff cycle: For diameters above 200 mm or when high-pressure coolant is not available, program a peck cutoff cycle where the tool retracts 1 to 2 mm every 10 to 15 mm of depth to allow chip clearing and coolant penetration. The retracts add cycle time but prevent catastrophic tool failure in deep cuts.

Managing the Cutoff Piece

When parting large diameters, the severed piece can be heavy enough to damage the machine, the part, or the tooling if it falls uncontrolled. A 150 mm diameter solid steel bar cutoff piece, 50 mm long, weighs approximately 7 kg. Solutions include:

• Subspindle (counter-spindle) pickup: the subspindle catches the cutoff piece before separation, providing full support. This is the preferred method for production CNC lathes.
• Steady rest support on the cutoff side: a programmable steady rest supports the cutoff piece during the final portion of the cut.
• Parts catcher: a mechanical or pneumatic catcher deployed below the cut zone to receive the piece.
• Programmed feed reduction: reduce the feed rate to 30 to 50 percent of normal for the last 2 to 3 mm of radius to create a thin connecting web, then stop the spindle and break the piece manually or with a push-off mechanism.

Troubleshooting Common Issues

Blade deflection and taper: If the groove walls show taper (wider at the OD, narrower at the center), the blade is deflecting under radial force. Increase insert width, reduce feed rate, or use a stiffer blade material.

Insert chipping at center: As the tool approaches center, the surface speed drops toward zero while the feed rate remains constant. This causes the insert to rub rather than cut, leading to chipping. Program a linear feed reduction over the last 20 percent of the radius.

Summary

Parting large diameters requires wider inserts, high-pressure coolant delivery, and careful feed rate management as the tool approaches the center. Blade rigidity is the primary constraint, and tool selection must account for the required overhang to reach the workpiece centerline. Subspindle pickup or parts catchers are essential for safe and damage-free production. With correct parameters, large diameter cutoff can be performed reliably with acceptable tool life and part quality.