Insert Shapes & ISO Designation

Insert shapes & ISO designation are the two halves of choosing an indexable insert — the physical shape that decides what the insert can do, and the code that names it. The indexable insert, the carbide cutting edge that this wiki’s cutting-tool entries treat, is a small, precise body of carbide with several cutting edges — corners — round its outline. When one corner wears, the operator indexes the insert to the next corner; when all are worn, the insert is thrown away. Because the insert is made in standard shapes, and because the shape decides how many corners it offers and how strong each is, the shape is the first thing chosen for a cut. And because inserts are made and sold worldwide, each is named by the ISO designation — the standard code, set out in the ISO 1832 standard, that names in a few characters the insert’s shape, clearance, tolerances, size, thickness and corner. This entry sets out the common insert shapes and how to read the code that names them.

The shapes and what they mean

An insert’s shape is its outline, and the outline decides the arithmetic of the cut. Each straight side of an insert is a cutting edge, and where two edges meet is a corner — a cutting point. The more corners an insert has, the more edges it offers and the cheaper each is to use; but the more acute its corners, the weaker each point is. The common shapes balance the two. The triangle, the T-shape, has three corners at sixty degrees, offering three strong-ish edges for general turning; the square, the S-shape, has four corners at ninety degrees, the strongest point of the common shapes and four edges to use, ideal for the square shoulder and the strong roughing cut. The diamonds and the trigons trade corners for point angle: the 80-degree diamond and the 80-degree trigon (the C and W shapes) give a point sharp enough for profiling with good strength; the 55-degree diamond (D) reaches sharper into corners; and the 35-degree diamond (V) is the sharp, pointed insert for the finest profiling, threading and the cuts that need a keen point. The round insert (R) has no point at all — a full circle of edge that presents its strongest, largest-radius form to the work, ideal for heavy roughing and for the cuts where a radius is wanted. The choice is a bargain: corners against strength, edge economy against point sharpness.

What each corner does

The shape is chosen for what the cut demands, and the connection is through the included angle and the nose of the insert. The included angle — the angle between the two edges that form a corner — decides the insert’s strength: the ninety degrees of the square and the eighty of the C and W shapes give a robust point that takes a heavy cut and a high feed without breaking, while the acute fifty-five and thirty-five degrees of the sharp diamonds give a keen point that reaches into a corner or a profile but must cut lighter. The corner’s nose radius — the rounding ground on the point, named in the code — governs the finish and the strength of the point: a larger radius gives a stronger point and a smoother surface finish, at the cost of the ability to cut a sharp internal corner; a smaller radius cuts keenly into corners but is weaker and finishes more coarsely. The machinist chooses shape and nose radius together with the depth of cut and the feed — the strong, blunt corner for the deep, fast rough; the sharp, keen corner for the fine finish and the tight profile.

Reading the ISO code

The insert’s name is the ISO designation, and it is a code of ten or eleven characters set by the ISO 1832 standard, each position with its own meaning. Take a common turning insert, CNMG 120408. The first letter, C, names the shape — the code letters follow the shapes above: T for triangle, S for square, C for the 80-degree diamond, D for the 55-degree diamond, V for the 35-degree diamond, W for the trigon, R for round, and more. The second letter, N, names the clearance angle — the relief ground under the edge — N meaning no clearance, a negative insert that is turned over and used on both faces. The third letter, M, names the tolerance class — the precision to which the insert’s dimensions are held. The fourth letter, G, names the insert’s type and chipbreaker — whether it has a hole for screw mounting and what chip-control form it carries. Then the digits: 12 is the insert’s size, the diameter of the circle that the shape inscribes, in millimetres; 04 is its thickness; and 08 is the corner radius, in tenths of a millimetre — 0.8 millimetre. The code thus names the whole insert: CNMG 120408 is an 80-degree diamond, negative clearance, 12-millimetre size, 4-millimetre thick, with a 0.8-millimetre corner radius.

The variations the code hides

Beneath the standard characters, the code’s last positions and its manufacturers’ extensions carry the rest of the insert’s identity. The tenth character — after the corner radius — names the cutting edge condition and the chipbreaker: whether the edge is sharp or honed, and which chipbreaker design the face carries, the form-ground grooves that this wiki touches under chip control that curl and break the chip. Different makers use their own chipbreaker codes, and the full insert name often appends the maker’s chipbreaker and grade — so CNMG 120408-MP names the shape, size and edge, and then the MP chipbreaker, while the grade, the specific carbide and coating, is named separately. Two inserts that share the standard code but carry different chipbreakers or grades cut differently — the code names only the geometry, while the grade and chipbreaker name the material and the chip control.

Choosing an insert by its name

The ISO designation and the shapes behind it turn the vast catalogue of inserts into a language the machinist reads at a glance, and choosing an insert is reading that language against the job. For a heavy roughing cut the machinist reaches for the strong shape and the large nose radius — the square or the 80-degree shape, the C or S first letter, the generous corner — to take the deep cut and the high feed; for a fine finish the keen shape and the small radius, the D or the V, to leave the smooth surface and reach the corners; for the general run of turning, the triangle or the 80-degree diamond that balances corners and strength. The second letter tells whether the insert is negative, for the two-sided turning and the rigid machines that negative geometry needs, or positive, for the lighter machines and the freer cut; and the digits give the size that fits the holder and the edge that fits the cut. The insert a shop stocks is the insert it trusts, but it is chosen, ordered and understood by the code this entry describes — the characters that name the shape, the size and the edge.