Milling Insert Designations Decoded: APKT, SEKT, RDMT and How to Match One to Your Cutter

You learned to read CNMG 432 and felt like the catalog opened up. Then you pulled a milling insert out of the drawer, and it said APKT 100308 PDTR, and four of those characters are not in the turning system at all. Milling inserts use the same ISO code with a tail on the end, and once you can read the tail, you can tell whether an insert will fit a cutter, cut the hand you need, and sit at the same height as its neighbors—before you order it.

The first four letters are the system you already know.

ISO 1832 describes the insert outline in four letters, and they mean the same thing on a milling insert as on a turning insert.

Position 2 is worth a sentence on its own. Turning inserts are often N—zero clearance, double-sided, with cutting edges on both faces, and the strongest edge you can buy. Most of the milling inserts in this article are P (11°) or E (20°). A positive clearance angle cuts with less force, which is what you want when a tooth is entering and leaving the cut thousands of times a minute instead of staying buried.

Position 3 is the difference between APKT and APMT.

These two look like two names for the same insert, and shops swap them constantly. They are the same shape (A), the same clearance angle (P, 11°), and the same hole and breaker type (T). The only thing that changes is the letter in the middle, and that letter is the tolerance class.

Position 4 decides whether the insert can even be clamped.

The fourth letter covers the hole and the chipbreaker together, and it is the one that makes an otherwise identical insert useless in your cutter.

• T—hole with a 40–60° double countersink, chipbreaker on one side. The common screw-down milling insert.
• H—hole with a 70–90° single countersink, chipbreaker on one side.
• M—cylindrical or double-countersunk hole, chipbreaker on one side.
• G—cylindrical hole, chipbreaker on both sides, so the insert can be flipped.
• N—no hole and no chipbreaker. These are clamped, not screwed.

An insert with N in position 4 physically cannot go into a pocket that expects a screw. That is a dimension, not a preference, and no grade or price makes up for it.

The numbers: edge length, thickness, and corner radius.

After the four letters come the numbers, in that order. On a milling insert, they are read in millimeters.

Take APKT 100308, the size that moves more than any other across our counter:

• 10—cutting edge length, 10 mm.
• 03—thickness code, 3.18 mm (1/8").
• 08—corner radius code, 0.8 mm (1/32").

The same arithmetic runs through the rest of the family. APKT 160416 is a 16 mm edge, 04 thickness (4.76 mm, 3/16"), and a 16 radius (1.6 mm, 1/16"). SEKT 12T3 is a 12 mm square with the T3 thickness code, 3.97 mm (5/32"). And on a round insert such as RDMT 1204 M0, the M0 in the radius position means zero—a round insert has no corner to put a radius on.

The tail: four letters that only exist on milling inserts.

This is the part the turning code never prepared you for, and it is the most useful part of the string. Four more positions, in order: wiper lead angle, wiper clearance angle, cutting edge preparation, and cutting direction.

Position 7 is the one that changes how the insert behaves in your material. A sharp edge (F) cuts with the least force and is what you want in aluminum and other soft, gummy material. A T-land (T) puts a small negative chamfer behind the edge, trading some cutting force for an edge that survives an interrupted cut in steel. A honed edge (E) is the middle ground. Two inserts with identical shape and size and a different letter here are not interchangeable in practice, even though they will both bolt in.

Position 8 is the one that stops the job cold. An R insert cuts right hand only. Put it in a left-hand cutter and the clearance faces the wrong way: it will mount, it will look right, and it will rub instead of cut.

Three real codes, read straight through.

APKT 100308 PDTR—85° parallelogram, 11° clearance, K tolerance, countersunk hole with a single-sided breaker, 10 mm edge, 3.18 mm thick, 0.8 mm radius, 0° wiper lead, 15° wiper clearance, T-land edge, right hand only. See it in grade FM125.

SEKT 1204 AFTN—square, 20° clearance, K tolerance, countersunk hole with a single-sided breaker, 12 mm edge, 4.76 mm thick, 45° wiper lead, 25–26° wiper clearance, T-land edge, either hand. See it in grade FM125.

APMT 160404 PDER—same outline as the APKT above but class M, 16 mm edge, 4.76 mm thick, 0.4 mm radius, 0° lead, 15° wiper clearance, honed edge rather than a T-land, right hand only. See it in grade FM125.

The grade is not part of the code, and it is half the decision.

Everything after the ISO string—FM125, TIN21, TIN28, TL20, TL40, KM15—is the manufacturer's own grade designation. There is no standard behind it.

Keep the two apart in your head, because they answer different questions. The ISO code tells you whether the insert will fit. The grade tells you whether it will work. An insert that fits your pocket perfectly in a grade meant for cast iron will not last in stainless, and nothing in the eight-character code will warn you. When you substitute on code alone, this is the thing you have substituted away.

Matching an insert to a cutter you already own

If the cutter is in your hand and you need the right insert for it, go in this order. The first four are pass or fail. Only the last one is a judgment call.

1. Shape, edge length, and thickness have to match the pocket. No exceptions.
2. Position 4 has to match how the pocket holds. A screw pocket needs a hole; a clamped pocket does not.
3. Position 8 has to match the hand of the cutter.
4. The corner radius has to be no larger than the corner the part drawing allows.
5. Grade and edge preparation are where you choose, once the four above are fixed. Tell us the material and we will narrow it.

What FM Carbide stocks

Our own line is ground to these codes and sold in our own grade, FM125:

• APKT—100304, 100308, 100316, 160404, and 160416.
• APMT—160404 PDER and 160408 PDTR.
• SEKT and SEKN—SEKT 1204 AFTN, SEKT 12T3 AGTN, and SEKN 1203 AFTN.
• RDMT round—1204 M0 and 10T3 M0.
• ADKT—150508 PDTR.

If you run our own cutter bodies, the indexable end mill cutters and shell mill cutters take XNEX inserts, which carry a manufacturer family name rather than a full ISO string—there the cutter's own specification is the only thing to go by, and we will match it for you.

Questions we get

Are APKT and APMT inserts interchangeable?

Dimensionally they share the same shape, clearance angle, and hole type, so they often drop into the same pocket. What changes is the tolerance class. K holds the corner point to ±0.013 mm and the thickness to ±0.025 mm; M allows 0.05–0.13 mm on the corner and ±0.13 mm on the thickness. In a single-pocket cutter the difference rarely shows. In a multi-tooth face mill, it becomes insert-to-insert height variation, which means one tooth takes more than its share of the chip load.

What do the numbers in APKT 100308 mean?

Ten millimeters of cutting edge length, a thickness code of 03, which is 3.18 mm or 1/8", and a corner radius code of 08, which is 0.8 mm or 1/32". The same reading works across the family: 160416 is a 16 mm edge, 4.76 mm thick, with a 1.6 mm radius.

What does the R at the end of APKT 100308 PDTR mean?

Cutting direction. R is right hand only, L is left hand only, and N cuts either hand. An R insert in a left-hand cutter will mount and look correct, but the clearance faces the wrong way, and the insert rubs instead of cutting.

What is the difference between a T-land and a sharp edge on a milling insert?

That is position 7 of the code. F is a sharp edge, which cuts with the least force and suits aluminum and soft, gummy material. T is a T-land, a small negative chamfer behind the edge that trades cutting force for an edge that survives interrupted cuts in steel. E is honed, between the two. Two inserts that are identical except for this letter are not interchangeable in practice.

Does the ISO code tell me which grade to buy?

No. Everything after the code—FM125, TIN21, KM15, and the rest—is the manufacturer's own grade designation with no standard behind it. The ISO code tells you whether an insert fits. The grade tells you whether it works in your material. Substituting on code alone substitutes away the grade.

Why do milling inserts have a positive clearance angle when turning inserts often do not?

A turning insert stays buried in the cut, so a zero clearance angle (N) buys the strongest possible edge and lets the insert be double-sided. A milling tooth enters and leaves the cut thousands of times a minute, and a positive clearance angle—P at 11°, E at 20°—cuts with less force, which is worth more than the extra edge strength on most milling work.

If you want the turning half of the same system, Turning Insert Designations Explained covers CNMG, TNMG, and the rest. For choosing between two common milling insert families by application rather than by code, see AOMT vs. XNEX, and if the job is a square shoulder, the 90-degree shoulder mill covers the cutter body itself.

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Code positions and value tables in this article follow the ISO 1832 designation system as published in standard industry reference charts. Grade designations are manufacturer-specific and carry no standard meaning. Dimensions are nominal; the cutter manufacturer's pocket specification governs fit. Send us the cutter and the material, and we will match the insert with you.