Turning Insert Nose Radius Guide: R0.2 to R1.6
The last two digits of an insert code set the corner nose radius, and that single number changes edge strength, surface finish, chatter tendency and how tightly you can profile. This guide explains how nose radius interacts with feed and depth of cut and gives a practical way to pick R0.4, R0.8 or R1.2 instead of copying the last job.
The last two digits of a designation such as CNMG120408 are the nose radius in tenths of a millimetre — so 08 means rε = 0.8 mm. Shape, clearance and inscribed circle decide what fits the machine; the nose radius decides how the edge behaves in the cut. It affects how strong the corner is, what surface finish you can expect, how hard the tool pushes the workpiece sideways, and whether the tool can reach into a tight internal corner. Get it wrong and you get either a vibrating, deflecting cut or an edge that chips for no obvious reason.
This guide treats nose radius as an independent choice from grade and shape. The radius lives in the same body across many grades, so you can often change radius without changing anything else. If any position of the code is unfamiliar, the ISO 1832 walkthrough decodes it, and the interactive decoder confirms the radius for any code.
The radius ladder you actually see
| Code | Nose radius | Catalogued examples | Typical character |
|---|---|---|---|
| 02 | 0.2 mm | CCMT09T302 | Fine profiling, tight corners, very low force |
| 04 | 0.4 mm | CNMG120404, DCMT11T304 | Finishing and light cuts, slender parts |
| 08 | 0.8 mm | CNMG120408, WNMG080408 | The general-purpose default |
| 12 | 1.2 mm | CNMG120412, WNMG080412 | Stronger edge, medium to heavy cuts |
| 16 | 1.6 mm | CNMG190616, SNMG120416 | Heavy roughing on rigid machines |
What a larger radius gives you
As the corner radius grows, two things improve. The edge has more material behind the cutting point, so it withstands interrupted cuts, scale and high feeds far better than a sharp small corner. And the theoretical surface roughness falls: the residual scallop height between feed marks scales roughly with f2 / (8·rε), so at a fixed feed a larger radius leaves a smoother surface. That combination — strength plus finish — is why heavy roughing and high-feed operations reach for R1.6 and R1.2 corners on large, rigid setups.
The cost is force, and it points the wrong way for delicate work. A large radius spreads the cut over a longer edge and generates substantially more radial (passive) force that pushes the workpiece away from the tool. On a slender shaft, a thin wall, a long boring bar or a lightly built machine, that force shows up as taper, chatter and poor dimension — exactly the symptoms people wrongly blame on the grade. A big radius on a small insert is also weak in a different way: when rε becomes a large fraction of the inscribed circle there is little supporting material, which is why you rarely see an R1.6 on a small IC body.
What a smaller radius gives you
A small radius such as R0.2 or R0.4 cuts with far less radial force, so it is the first thing to reach for when a part deflects or chatters and you cannot reduce the feed. It is also mandatory for geometry access: when profiling an internal corner, the tool radius has to be equal to or smaller than the smallest radius on the drawing, or the corner will be gouged. Fine finishing of small features, copy turning and operations on low-horsepower machines are the natural home of the R0.4 and R0.2 corners.
You trade away edge strength and, at the same feed, surface finish. A sharp R0.2 corner is easy to chip in an interrupted or scaly cut, and to hold a fine roughness you must drop the feed considerably, which costs cycle time. Small radii are a deliberate choice for access and stability, not a starting point for heavy work.
How radius, feed and depth of cut fit together
Three practical rules keep the geometry consistent. First, match feed to radius: in finishing, a feed per revolution around 0.3–0.5 times rε is a common starting band that uses the radius for finish without overloading it; roughing feeds can run up to roughly half to two-thirds of rε, which is one reason heavy roughing wants a bigger corner. Second, keep the nose radius at or below the depth of cut for general turning; when the radius is much larger than the cut depth, engagement becomes unstable and the chip thins unpredictably. Third, if chatter appears, reducing radius is usually more effective than chasing it with speed alone, because it removes the radial force that drives the vibration.
A reasonable default flow: start from R0.8 for general external turning, move down to R0.4 or R0.2 when profiling tight corners, cutting slender parts or fighting chatter, and move up to R1.2 or R1.6 only when the machine and workpiece are rigid enough to absorb the force and the operation is genuinely heavy. Confirm that the radius exists on the exact geometry you run — the same radius ladder repeats across WNMG, DCMT and the other series, but on the body size that suits the cut.
The short version
Larger radius = stronger edge and better finish but more sideways force and chatter risk; smaller radius = low force, fine profiling and corner access but a weaker edge and lower allowable feed. Start at R0.8, step down for stability or tight features, step up for rigid heavy cuts, and keep feed and depth of cut in proportion to the radius. Once the radius is set, pair it with the right grade and coating for the workpiece material.
Browse radius variants in the model index, compare catalogued brands on the cross-reference finder, and use the inquiry box on any model page for a specific radius and grade.