Grooving & Parting Tools

Grooving and cutoff are interrupted, two-edge cuts on a thin protruding blade. Chip control and rigidity are everything: the blade cannot deflect, and the chip must break instead of wrapping the part.

Concept

A grooving / parting tool is a narrow blade with one or two cutting edges, fed radially into the rotating work. Grooving makes a recess of controlled width and depth; parting (cutoff) feeds through to separate the part from the bar. The narrow blade cuts on its side edges, so chip flow and rigidity are the two constraints.

Why It Matters

A grooving blade is a cantilever like a boring bar — but thinner, so it deflects and chatters easily. The chip comes out of a confined space and must break; a long chip wraps the part and jams. Grooving at too high a feed overloads the two edges; too low a feed rubs and work-hardens. Getting feed and chip control right prevents broken blades and scrapped parts.

How It Works

The blade feeds radially at a low feed per revolution. The insert has a chipformer that curls and breaks the chip. For parting, the blade reaches near center, often with a small end geometry to cleanly separate the part. Keep the blade supported and the overhang minimal; use the recommended feed so the chip forms instead of packing.

Worked Example

Cut off a Ø30 bar to length with a 3 mm wide cutoff blade. Set the blade exactly on center height. Feed at a conservative per-rev rate so the chip breaks into short pieces rather than a coil, and keep overhang to the depth needed. A wrapped chip around the bar means the feed or chipformer is wrong — not a reason to push harder.

Why groove width matters

The blade width sets both the groove dimension and the strength of the edge: a narrow blade is more prone to vibration. For a retaining groove you often feed radially to depth, then traverse along the part to widen or square the groove. For parting, the blade reaches near center; below-center cutting on a lathe causes drag and a poor finish, which is why exact center-height setting is emphasized. Always part with the spindle at the right speed for the material, and never let a long unbroken chip wind around the workpiece — it can pull the tool, damage the part, or trap the operator.

Why feed sits in a narrow window

Grooving is unforgiving because the blade cuts on two edges at once in a confined space, and the feed has to thread a small window. Feed too low — say a finishing-style 0.05 mm/rev — and the edge rubs rather than shears: it work-hardens the surface, builds up heat, and gives a torn groove. Feed too high and the two edges overload at once, overloading the thin blade and chipping it. The maker's grooving feed is deliberately a band, not a point, and the chipformer inside that band curls the chip into short C-shaped pieces. When the chip comes out as a long coil instead, the first lever is feed within the recommended band and the second is the chipformer window — not a jump to a much higher feed. On parting specifically, the last fraction of a millimetre near centre is the hardest cut: speed drops to zero at the axis, the edge rubs, and a slightly lower feed on the final touch reduces burr and edge chip. Because the blade is a thin cantilever, the same cube-law of deflection as a boring bar applies (page 16): keep the overhang to only what reaches the cut, and support the blade block firmly, or chatter sets in long before the edge wears.

Parting and grooving put the edge in a narrow, confined cut where heat and chip packing concentrate. A ground insert that looks sharp can still fail if the chip cannot escape the groove, so the chipformer shape and coolant feed matter as much as the edge material. Plan the parting path for a clean break and an unobstructed chip, not only for the width of the groove.

Common Mistakes

• Running too low a feed. Rubbing and work-hardening instead of cutting. Fix: use the recommended grooving feed.
• Ignoring a wrapped chip. It jams. Fix: adjust chipformer/feed to break the chip.
• Blade not on center height. Poor cut and edge chipping. Fix: set the blade tip on the spindle centerline.

Grooving and parting push the tool into a highly interrupted, constrained cut: keep overhang short, feed controlled and coolant aimed at the edge. A jammed chip in the groove scores the wall, so the chipbreaker window and feed matter more here than in facing.

Practice