Parting, Grooving, and Threading Inserts: Which for What

Turning inserts are the familiar ones. Parting, grooving, and threading inserts are a different world — narrow, specialised, and often mis-ordered by shops new to them. This post maps the common codes (MGMN, MRMN, TDC, TDT, 16ER, 16IR) to their operations.

Parting off (cut-off)

Removing the finished part from the bar stock. The insert makes one axial groove that meets the bar axis, severing the part.

Common codes:

• MGMN — general-purpose parting, single-ended insert, 2-edge design. MGMN200 = 2 mm width, MGMN300 = 3 mm width, MGMN400 = 4 mm width.
• MRMN — round-corner variant of MGMN. Used when the parting slot needs a round inside corner (rare in parting, common in some grooving jobs).
• TDC — double-ended parting/grooving insert. Two cutting edges per insert. Widths from 1-4 mm.
• GFN / GFR — deep parting inserts, narrower width for economical bar utilisation.

Width choice: narrower is better — less material wasted in the kerf. But narrower inserts break easier and can't handle as much feed. For parting steel, 2-3 mm is typical; for aluminium, 2 mm works fine; for hardened steel, 3 mm or wider for rigidity.

External grooving (turning grooves)

Cutting slots, O-ring grooves, circlip seats, or decorative grooves on the outside of a part.

Common codes:

• MGMN / MRMN — the same parting inserts work for standard external grooves.
• TDT — dedicated external-groove insert, 2-edge design, optimised for groove shape.
• TDJ — triangular grooving insert, 3-edge economy. Good for production shops.
• TDXU — tapered-flank grooving for specific profile shapes.

Internal grooving

Cutting grooves on the inside of a bored hole — O-ring glands, retaining-ring seats, internal reliefs.

Common codes:

• MGMN in an internal grooving holder (ICMG / SCGR series).
• TGIR / TGIL — dedicated internal grooving inserts.
• GIM / GIL series — for deep internal grooving in production jobs.

Face grooving

Cutting grooves on the flat end face of a part — rare operation but needed for some specific seals and assemblies.

Requires a dedicated face-grooving holder and narrow insert. The insert flanks have a curvature matching the diameter — you must specify the diameter range when ordering.

External threading

Cutting external threads (bolts, studs, custom screws) directly on the CNC lathe rather than using a die.

Common codes:

• 16ER / 16IR — the ubiquitous external / internal threading inserts. "16" = 16 mm IC. "ER" = external right-hand, "IR" = internal right-hand. EL / IL = left-hand.
• 11ER / 11IR — smaller IC for small-OD work (below 10 mm diameter).
• 22ER / 22IR — larger IC for heavy threads (above 30 mm OD).

Thread profile: threading inserts come in profile-ground forms for specific thread standards:

• ISO metric (60° flank) — most common. "IR" or "ER" followed by thread pitch (1.0, 1.25, 1.5, 1.75, 2.0, 2.5, 3.0 mm). Example: 16ER-1.5ISO.
• UNC / UNF (60° flank, inch pitches) — marked with TPI (threads per inch). Example: 16ER-8UN.
• BSW (55° flank) — British Standard Whitworth, legacy equipment.
• NPT (tapered pipe thread) — American pipe threads. Tapered profile on the flank.
• Trapezoidal / ACME — power transmission threads. 29° (ACME) or 30° (Tr) flanks.

Threading technique — partial vs full profile

• Partial profile — insert has a 60° V-flank but no crest radius. You must make multiple passes to generate the full thread (modified feed, step infeed). Cheaper insert, more flexible across pitches, longer cycle time.
• Full profile — insert has the full thread shape with crest radius and pitch-specific flanks. One pass = one correctly profiled thread. Faster cycle, but insert is pitch-specific (one insert per pitch).

Partial profile for job shops running multiple pitches on one setup; full profile for production shops running the same pitch in volume.

Parting-off mistakes to avoid

• Pushing feed too hard — the most common reason parting inserts snap. Start at 0.05 mm/rev and increase only if chip colour stays straw-coloured (not blue).
• Not using coolant — parting generates heat in a tight space. Coolant matters more here than in most operations.
• Parting too close to chuck — increases chatter. Stick the part out enough that the cut has room to breathe.
• Parting at edge of capacity — if your part is 40 mm OD and your parting holder is rated for 40 mm depth, you're at the edge. Add a small bore-out or step to reduce the parting depth.

Browse our grooving inserts and threading inserts, or ask on WhatsApp for a grade recommendation tied to your specific operation.