Parting and Grooving, Decoded: Insert Width, Corner Form and Why Cut-Off Breaks Tools

Every other operation on a lathe has somewhere to run. Parting does not. The tool goes straight down into the work, buried on three sides, with chips that have nowhere to go and a cut that gets less stable the closer it gets to finishing. It is the operation where the most inserts die, usually on a part that was already good. Here is how the tooling is organized, what the numbers in the codes actually mean, and the handful of decisions that separate a clean cut-off from a snapped insert.

Why is parting the least forgiving cut on the machine?

In turning, the insert is engaged on one side, and the chip curls away into open air. In parting, the insert is down in a slot barely wider than itself. Three things follow from that, and every rule further down is a consequence of one of them.

The chip has to come back out the way the tool went in. There is no other exit. A chip that jams in the slot is not a nuisance; it is the load that snaps the insert.

The tool gets less supported as it goes deeper. The overhang grows with every revolution of feed while the section carrying the load stays the same. The cut is at its least rigid exactly when it is most nearly finished.

The cutting speed falls to zero at the center. As the diameter shrinks, surface speed drops with it. The last fraction of an inch is cut at a speed no insert is designed for, which is why parts tear off instead of parting cleanly and why the pip in the middle is a permanent fact of the operation.

The insert width is the groove width.

This is the difference that catches people coming from turning. On a turning tool you choose a nose radius, and the toolpath makes the shape. On a parting or grooving tool, the width of the insert is the width of the feature you get. You are not buying a cutting edge; you are buying a dimension.

That is why the range is specified in widths rather than in geometries, and why it is so much deeper than it looks. Across the families we stock, groove and cut-off widths run from 0.031" to 0.375"—the Top Notch NG series alone covers 30 distinct widths in that span.

Two consequences worth holding on to. First, a narrower insert is not automatically the right answer: narrow saves material but is weaker and deflects more, and on a deep cut-off, that deflection is what breaks it. Take the narrowest insert that will survive the depth, not the narrowest insert on the shelf. Second, if the groove is a feature on the print—a seal groove, a retaining-ring groove—the insert width is a tolerance decision, not a preference.

Three systems, three different problems

Parting and grooving tooling splits into three families, and they are not competing products—they solve different problems. Picking the wrong family is the first mistake, before any code is read.

Double-ended or single-ended: two edges or one

Look at the insert families, and the split is immediate:

• The WDM series—WDMG for grooving, WDMP and WDMR for parting, and WDMT for turning on the same seat—are double-ended. Two cutting edges per insert, five widths each from 0.078" to 0.236", grade TL30.
• The MRCN series is single-ended, with six widths from 0.063" to 0.236", and grade TL40. One edge, but a seat that can be built more rigidly because it only has to support one end.
• The PTNT series runs from 0.083" to 0.358" and is the one family we stock across five different grades—KM15, PM25, PM40, TIN21, and TN30—which is what you want when the same groove has to be cut in five different materials.

Double-ended is the obvious economy: two edges for one price. The catch is that the second edge is buried in the pocket while the first one works, so the seat has to grip on the flanks rather than support the whole body. For general grooving, that is fine. For heavy cut-off, a single-ended insert in a seat built around it is the more stable machine.

Top Notch: one holder, two operations

Top Notch deserves its own section because it is the only system here that does a second job. The holders are, by name, grooving and threading tool holders: the same NER or NEL body at 93° takes three different insert families.

If a part needs a relief groove and a thread, that is normally two tools and two turret stations. With Top Notch it is one holder and an insert change. On a small lathe with four stations, that is not a convenience—it is the difference between running the job and not.

The same logic extends inside the bore: the Top Notch boring bars cover 5/8" through 2" diameters, right and left hand, and take the same insert families. For how bar diameter and overhang drive everything inside a bore, see boring bar nomenclature, decoded.

And if you are cutting the thread rather than the groove, the companion article on threading insert designations covers how those codes work.

Square corner or radius? The print decides, and so does fatigue.

NG and NR differ by one thing: NG leaves a square corner at the bottom of the groove; NR leaves a radius. It looks cosmetic. It is not.

A square internal corner is a stress raiser. On a shaft that sees bending or torsion, a sharp-cornered relief groove is where a fatigue crack starts. That is why so many drawings specify a root radius on a groove that is otherwise unremarkable—and why grinding a square-corner insert to fake a radius is a bad trade. If the print calls for a radius, the radius is structural.

The reverse case matters too: a retaining-ring groove or a seal groove usually wants a square corner, because the ring or the seal has to sit flat against the wall. Use NR there and the part gauges wrong.

Center height: the setup error that eats inserts

Every parting tool has to be on center. Not close—on it.

Above center, the clearance angle under the cutting edge is effectively reduced, the tool rubs instead of cutting, heat builds, and the cut finishes on a piece that will not shear.

Below center, the rake changes, and the tool is drawn into the work. As the diameter shrinks toward the middle, the error grows in relative terms, and the last moments of the cut are where the insert is dragged in and broken.

On a turning tool a small center-height error changes the effective angles slightly, and the part still comes out. On a parting tool, it is the single most common reason inserts fail early. Set it against a known reference, and recheck it whenever the blade moves in the block.

Seven ways a cut-off goes wrong

1. Tool not on center. Covered above, and it is still the most common. Check it first, before blaming the insert.
2. Too much overhang. The blade is sticking further out of the block than the cut needs. Set the reach for the largest diameter you will actually part, and no more.
3. Feed reduced near the center. The instinct is to ease off as the cut closes. Easing off makes the tool rub at exactly the moment speed is already collapsing. Hold the feed.
4. Chip jammed in the slot. The chip has one way out. If it packs, the insert takes the load. Coolant is aimed into the slot, and a chip that curls narrow enough to clear is not optional on a deep cut-off.
5. Insert wider than the job needs, or narrower than it can survive. Too wide wastes material and multiplies cutting force; too narrow deflects. Choose for the depth, not the material saving.
6. Wrong corner form. NR where the print wanted a square shoulder, or NG where it wanted a radius. The part is scrap either way.
7. One grade for every material. The PTNT family exists in five grades for a reason. A grade that works in steel is not the grade for stainless or for an interrupted cut.

Ordering without guessing

Four questions, in this order:

1. Parting or grooving? 'Cut-off' means 'depth,' which points to a blade and block. A feature on the diameter means a shank holder or top notch.
2. What width? For a groove, the print gives it. For a cut-off, the narrowest insert that will survive the depth.
3. Square corner or radius? NG or NR. Structural, not cosmetic.
4. Will the same part need a thread? If yes, Top Notch does both in one holder.

All of it is on the shelf: parting & grooving inserts, parting & grooving tool holders, parting blade tool blocks, the full Top Notch system, and the complete parting & grooving range. Send us the groove dimension and the material, and we will match the insert and the holder.

FAQ

Why do parting inserts break so often?

Because parting is the least rigid cut on the lathe. The tool is buried on three sides, the chip has only one way out, the overhang grows as the cut deepens, and the surface speed falls to zero at the center. Most failures trace back to one of four things: the tool is not on center, the overhang is greater than the cut needs, the chip is packing in the slot, or the feed was reduced near the middle instead of held.

How do I choose the width of a parting insert?

For a groove that is a feature on the print, the insert width is the groove width, so it is a tolerance decision. For a cut-off, take the narrowest insert that will survive the depth. Narrower saves material but deflects more, and on a deep cut-off, that deflection is what breaks the insert.

What is the difference between NG and NR grooving inserts?

NG leaves a square corner at the bottom of the groove; NR leaves a corner radius. It is structural, not cosmetic. A square internal corner is a stress raiser and is where a fatigue crack starts on a shaft that sees bending or torsion, which is why many prints specify a root radius. A retaining ring or seal groove, on the other hand, usually needs the square corner so the ring or seal seats flat.

Is a double-ended parting insert better than a single-ended one?

It depends on the cut. Double-ended gives two cutting edges per insert, which is the cheaper edge. But the second edge sits in the pocket while the first works, so the seat has to grip on the flanks instead of supporting the whole body. For general grooving that is fine; for heavy cut-off, a single-ended insert in a seat built around it is more stable.

Why does tool center height matter so much when parting?

Above the center, clearance under the edge is effectively reduced, and the tool rubs instead of cutting, so heat builds and the cut finishes on a piece that will not shear. Below center, the rake changes and the tool is drawn into the work, and the error grows in relative terms as the diameter shrinks. On a turning tool a small error is tolerable; on a parting tool it is the most common cause of early insert failure.

Can one holder do both grooving and threading?

Yes, with the Top Notch system. The 93-degree NER and NEL holders take NG grooving inserts, NR radius grooving inserts, and NT threading inserts in the same body, so a part needing a relief groove and a thread is one holder and an insert change instead of two turret stations.

Do I need a blade and tool block, or will a shank holder do?

Depth decides it. A shank-type holder handles grooving and shallow parting where reach is modest. When you are cutting off a larger diameter, the blade carries depth a shank tool cannot reach, and the blade sits in a tool block sized by its seat, not by your turret.

See the full parting & grooving range →

Insert families, widths, grades, shank sizes, and blade seat sizes in this article that are taken from the FM Carbide catalog as stocked.