Parting & Grooving Buyer's Guide: Insert, Holder, Speed Selection (2026)

What Is Parting vs Grooving? Two Related but Different Operations

Parting (also called cut-off or part-off) and grooving are both axial-cutting operations on a lathe, but they serve very different purposes and use different tool geometries.

Parting (Cut-Off)

Parting separates a finished workpiece from the bar stock by cutting radially inward from the outer diameter until the part is free. The cut goes all the way through the workpiece — once the insert reaches the center, the part drops off (or is caught by a parts catcher). The cutting edge geometry is straight (no curvature) and the cut width is the part-off insert width.

Grooving

Grooving cuts a recess into the workpiece without going all the way through. The cut may be on the outside diameter (external grooving), on the inside diameter (internal grooving), or on the face (face grooving). Grooving creates features like O-ring grooves, retaining ring grooves, relief grooves, and decorative grooves.

The same insert geometry can often be used for either operation, but the parameters and tool holder differ significantly. Parting requires maximum rigidity because the insert is cutting full width into a free part; grooving allows more flexibility in holder choice and depth-of-cut strategy.

Part-Off Insert Selection: Width, Geometry, and Grade

Insert Width Selection

The first and most important parameter is insert width. The rule of thumb:

Insert width = 60 – 75% of the smallest workpiece diameter

Practical values:

• Workpiece Ø 10 – 25 mm → insert width 1.0 mm
• Workpiece Ø 25 – 40 mm → insert width 1.5 mm
• Workpiece Ø 40 – 60 mm → insert width 2.0 mm
• Workpiece Ø 60 – 100 mm → insert width 3.0 mm
• Workpiece Ø 100+ mm → insert width 4.0 mm or larger

Why not use the widest insert you can? Wider inserts (3+ mm) are more rigid, but they also waste more material (the kerf is wider, so the cut-off slug is bigger and more material is lost as chips). They also require more spindle power. For typical production turning, 1.5 – 2.0 mm is the universal default.

Insert Geometry: Neutral vs Lead-Angle

Parting inserts come in two lead angles:

• Neutral (0° lead angle) — straight radial cut, simplest geometry, good for general purpose. Standard choice for 80% of part-off work.
• Lead-angle (3 – 15°) — angled cutting edge that pulls chips away from the workpiece. Reduces chip jamming on stringy materials (stainless, aluminum, low-carbon steel). Essential for parting stainless 304/316 — without lead angle, the chip welds to the insert and breaks it.

Insert Grade by Material

For the full 8-brand grade cross-reference by material, see our carbide inserts by ISO code guide. Quick rules:

• Steel (P): CVD-coated with Al₂O₃ + TiN multi-layer (IC8150, GC1015, AC5015S, T9125)
• Stainless (M): PVD-coated with positive chipbreaker and lead angle (IC808, VP15TF, AC6020M)
• Aluminum (N): Uncoated polished or DLC-coated, sharp edge, high helix (IC9000 series, AK10)
• Cast iron (K): CVD-coated ceramic or cermet (CC650, CC6050)

Grooving 3 Types: External, Internal, and Face

Grooving inserts come in three configurations depending on where the groove is located.

External Grooving (SCLC / QLC Holder)

The most common grooving application. The insert cuts radially into the outer diameter of the workpiece. Holder style: SCLC (square shoulder, neutral cutting), QLC (positive rake, sharper edge), or CLC-style with through-coolant. External grooving inserts have a straight cutting edge perpendicular to the holder axis. Common groove widths: 1.0 – 6.0 mm; common depths: 0.5 – 8 mm.

Internal Grooving (SCLP / Boring Bar Holder)

Cutting a groove on the inside diameter of a bored hole. Holder style: SCLP (positive rake) or straight boring bar holder. Internal grooving is the most challenging because of the rigidity limit — the holder is inside the bore, which restricts diameter and length. For internal grooving deeper than 1× bore diameter, use a vibration-damping boring bar holder. Common groove widths: 1.0 – 4.0 mm.

Face Grooving (T-Style Holder)

Cutting a groove on the face of the workpiece, typically for retaining rings or face seals. Holder style: T-style cutter with the insert perpendicular to the workpiece face. Face grooving is unique because the cutting force is axial (pulling the insert away from the workpiece), not radial. Use a positive rake insert with sharp edge to reduce axial force. Common groove widths: 1.0 – 4.0 mm; common diameters: 20 – 200 mm.

For 8-brand grade recommendations for grooving by material and groove type, see our carbide grade cross-reference hub.

Blade Holder Selection: SCLC, QLC, and Modular Options

The blade holder is the second-most-important factor after the insert itself. A bad holder wastes even the best insert.

SCLC (Square Shoulder, Negative Rake)

The workhorse for steel and stainless. Strong insert seating, handles 80% of part-off and external grooving work. Available in shank sizes 16×16 / 20×20 / 25×25 mm to fit all common lathes.

QLC (Positive Rake, Sharp Edge)

Better for aluminum, copper, and other sticky non-ferrous materials. The positive rake reduces cutting force and improves chip flow. Use for parting aluminum bar stock where the chip would otherwise weld to a negative-rake insert.

Through-Coolant Holders

Modern best practice for part-off and grooving. Coolant is delivered through internal channels in the holder directly to the cutting edge. Extends tool life 30 – 60% over flood coolant by preventing the chip from welding to the insert, especially in stainless and low-carbon steel. Downside: requires a CNC lathe with through-coolant spindle and tooling setup.

Modular Holders (with Cartridge)

For shops running many different insert widths on the same machine, a modular holder with interchangeable cartridges reduces inventory cost. Cartridge swap time: 30 – 60 seconds. For matching the right holder to your lathe and your operation, see our CNC tool holder guide.

Cutting Parameter Table by Operation and Material

Start with these values for your first cut, then adjust based on chip color, surface finish, and tool wear.

For the full speed-and-feed calculation per material and operation, see our CNC speeds and feeds hub.

Chip Control Strategy: Solving Stringy Chip Problems

Chip jamming is the #1 failure mode for part-off and grooving. The chips don't have a clear evacuation path (unlike turning, where chips fall away), so they accumulate around the insert and break it.

Strategy 1: Use Through-Coolant

The single most effective fix. Coolant jet at 70+ bar aimed at the cutting edge breaks chips into short pieces and washes them out of the groove. For stainless and low-carbon steel, through-coolant is essentially mandatory.

Strategy 2: Use Lead-Angle Inserts (3 – 15°)

Lead angle pulls chips away from the workpiece and curls them into shorter shapes. For stainless 304/316 part-off, a 15° lead angle is the standard. Without lead angle, the chip welds to the insert and breaks it within minutes.

Strategy 3: Reduce Feed at the End of the Cut

For part-off, when the insert approaches the center of the workpiece, the chip becomes thin and weak. Reduce feed by 30 – 50% in the last 0.5 mm of cut to prevent the chip from "pinching" against the insert and breaking it.

Strategy 4: Use a Part-Off Insert with a Chip Breaker

Modern part-off inserts have integral chip-breaker geometries that curl the chip into a tight C-shape, breaking it into short pieces automatically. Look for inserts labeled "with chipformer" or "with chipbreaker". For matching chipbreaker geometry to your specific material, see our carbide inserts by material guide.

Common Parting & Grooving Mistakes and How to Fix Them

Mistake 1: Insert Width Too Wide

Symptom: Excessive material waste, parts dropping into the chip conveyor instead of the parts catcher, higher cutting force causing spindle overload on small lathes. Fix: Use the narrowest insert that provides adequate rigidity. For Ø 30 mm part, use 1.5 mm not 2.0 mm. For Ø 50 mm part, use 2.0 mm not 3.0 mm.

Mistake 2: No Through-Coolant for Stainless

Symptom: Insert fails within 30 – 60 seconds on 304/316 stainless part-off. Fix: Through-coolant is essentially mandatory for stainless. If your machine doesn't have through-coolant, switch to a high-pressure flood setup (8+ bar at the cutting edge) or use a different operation.

Mistake 3: Same Feed for All Insert Widths

Symptom: Insert chipping or breakage. Fix: Feed rate scales with insert width. A 1.0 mm insert can typically take 50% of the feed of a 2.0 mm insert because it has half the cross-section. Reduce feed proportionally to insert width.

Mistake 4: Ignoring Workpiece Runout

Symptom: Insert chipping on the first pass, or poor cut quality. Fix: Check workpiece concentricity with a dial indicator. Runout over 0.05 mm causes the insert to engage unevenly and chip. For workpieces with intentional runout (forged parts), reduce feed to 0.05 mm/rev maximum.

Mistake 5: Using a Single-Side Insert for Part-Off

Symptom: Wasted insert — one corner wears, the other three are unused. Fix: Use a double-sided (two-corner) insert for part-off. The 2-corner insert has the same width as a single-side but uses both cutting edges, doubling tool life per insert.

Mistake 6: Not Spinning Down the Spindle Before Retracting

Symptom: Part-off insert catches on the un-cut center stub, breaking the insert or the workpiece. Fix: Always stop the spindle (RPM = 0) before retracting the part-off insert. Retracting while spinning risks the insert jamming into the un-cut center.

Frequently Asked Questions

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