Turning Chipbreaker Codes Explained: PF/PM/PR, MF/MM/MR and How to Choose
The two letters glued between the size and the grade — the -PM in CNMG120408-PM4325 — are a chipbreaker code. The first letter tracks the workpiece family, the second tracks finishing, medium or roughing. This guide decodes both from the catalogue's own depth-of-cut and feed ranges, then shows how to pick.
A full ordering code is geometry, chipbreaker and grade glued together. In CNMG120408-PM4325, the geometry is the ISO 1832 part CNMG120408, the grade is 4325, and the short word in the middle — PM — is the chipbreaker. Buyers often search on exactly that fragment, because the chipbreaker is what matches the insert to an operation. Get the geometry and grade right but the chipbreaker wrong and the cut either smears the finish, strains the machine or throws long unbroken chips.
These codes look arbitrary but they are systematic. The first letter tracks the workpiece family the groove is optimised for; the second tracks the cutting duty — finishing, medium or roughing. The ranges below are taken from the manufacturers' own catalogue depth-of-cut (ap) and feed (fn) fields, not from memory. Chipbreaker sits on top of two independent choices — shape and grade — and changes for the same body as the operation changes.
Read the two letters separately
The first letter is the workpiece focus. Catalogue ISO-group coverage makes this concrete rather than a guess: the P designs (PF, PM, PR) are centred on steel and also cover cast iron and stainless; the M designs (MF, MM, MR) centre on stainless and heat-resistant materials; the K designs (KF, KM, KR) are listed for cast iron only; and the S designs (SF, SM) target stainless and superalloys. The W designs are wiper geometries and the U designs are the broadest, catalogued across P, M, K, N and S.
The second letter is the duty, and the catalogue's own cutting ranges separate the three cleanly. F designs are labelled finishing and run shallow cuts at low feed; M designs are "pre-machining with demand on surface" — the medium, general-duty band; R designs are roughing and take heavy depths at high feed. The numbers in the table below are the catalogue ranges for the P and M steel/stainless designs.
| Code | Duty | Catalogue ap range | Catalogue fn range |
|---|---|---|---|
| PF (steel) | Finishing | 0.06–2.4 mm | 0.03–0.53 mm/r |
| PM (steel) | Medium | 0.3–8.6 mm | 0.06–0.65 mm/r |
| PR (steel) | Roughing | 0.7–10 mm | 0.12–0.80 mm/r |
| MF (stainless) | Finishing | 0.06–4.0 mm | 0.03–0.61 mm/r |
| MM (stainless) | Medium | 0.15–8.5 mm | 0.06–0.65 mm/r |
| MR (stainless) | Roughing | 0.25–12 mm | 0.12–0.90 mm/r |
Each row is the catalogue envelope across the steel (P), stainless (M) and cast-iron (K) subgroups the groove is listed for; the working range narrows for a specific subgroup and insert size. Treat the table as the duty band, not a fixed setting.
F, M and R in practice
F — finishing. Shallow, low-feed cuts where surface quality and tight tolerance matter more than metal removal. The groove is open and light so it shears cleanly at small depths without work-hardening the surface. This is where you run fine feeds on stainless and high-temperature alloys to leave a controlled finish. Example chipbreakers: PF, MF, KF, SF, UF.
M — medium / "pre-machining with demand on surface". The widest, most forgiving band and the usual starting point for general production. It removes real stock while still protecting surface quality, spanning from light semi-finishing to fairly heavy cuts. If one groove has to cover a mixed steel and stainless shop floor, the M family — PM, MM, QM, UM — is where you look first. The CNMG120408 offers a spread of these, which is why that geometry is the default for so many machines.
R — roughing. Heavy depth and high feed where the goal is maximum metal removal and chip control on long, stringy stock; finish is secondary. The groove is stronger and more enclosed to curl and break thick chips. Run these on rigid machines with enough power and a secure setup — the high feeds generate large forces, so they pair naturally with the larger nose radii. Example chipbreakers: PR, MR, KR.
The first letter: match the material
Once the duty is fixed, choose the material family. For steel, start in the P column (PF/PM/PR); for stainless and heat-resistant alloys, the M column (MF/MM/MR) uses a more positive, freer-cutting groove that reduces work-hardening; for cast iron, the K designs (KF/KM/KR) handle the abrasive, short-chipping material; for superalloys and titanium, the S designs (SF/SM) run shallow and controlled. The U family is the catalogue's broadest — listed across P/M/K/N/S — and suits low-volume shops that would rather stock one groove across many materials than optimise each one.
Wipers: the W designs behave differently
The W chipbreakers — WF, WM, WL, WMX — are not a fourth duty band so much as a different edge. A wiper uses a special nose profile that flattens the feed scallops, letting you hold a fine surface finish at feeds two to three times higher than a conventional edge. The catalogue labels WF and WL as finishing and WM as medium, and their ranges overlap the normal F and M bands; the point of choosing a wiper is productivity on finish and semi-finish passes, not heavier roughing. Use them where cycle time on finishing is the bottleneck and the machine is rigid enough to run the higher feed without chatter.
How to choose, in order
Pick the duty first from the stock and finish requirement: F for shallow finishing, M for general production, R for heavy stock. Then pick the first letter from the workpiece — P for steel, M for stainless and high-temperature, K for cast iron, S for superalloys, U for an honest generalist. Consider W only when faster finishing is the specific goal. Confirm the chipbreaker exists on the exact geometry and pair it with the right coating by following the grade-selection guide; then verify the complete code in the ISO 1832 decoder or find catalogued offers on the cross-reference finder.
The short version: read -PM as "steel, medium duty" — first letter the material, second letter the cut. F finishes shallow, M is the general-duty core, R takes heavy stock, and W buys faster finishing through a wiper edge. Browse specific geometries in the model index and use the inquiry box on any model page for a particular chipbreaker and grade.