CNMG PM vs MM vs MF vs MRR vs R: Complete Chipbreaker Family Guide
5 chipbreaker families decoded · 8-brand naming map · geometry matrix · workpiece selection decision tree · migration cheat sheet
1. Where the Chipbreaker Letter Lives in the ISO Code
The chipbreaker is encoded as the letter after the nose radius in the ISO 1832 standard. Here's how CNMG 120408-PM decodes:
The 8th position letter (P, M, F, R, etc.) is what differentiates chipbreaker geometries within the same insert shape and size. It's also the most commonly misunderstood part of the ISO code because each brand uses their own letter conventions. For the full ISO 1832 letter-by-letter decoder see our carbide inserts by ISO code pillar.
2. The 5 Universal Chipbreaker Families
Despite brand-specific naming differences, every turning insert chipbreaker falls into 5 universal families based on chip geometry, edge prep, and intended application. Here's the framework:
| Family | Edge Prep | Rake Face | Typical ap | Typical f | Best For |
|---|---|---|---|---|---|
| PM (positive medium) | Sharp honed (T-land 0.05 mm) | Positive 8-12° | 1-6 mm | 0.10-0.40 mm/rev | Universal 80% of turning — steel, stainless, finishing to medium roughing |
| MM (medium-medium) | Light hone (T-land 0.10 mm) | Neutral 0-5° | 2-8 mm | 0.20-0.50 mm/rev | Medium-roughing, cast iron, interrupted cuts, tougher workpiece |
| MF (medium finishing) | Very sharp polished | Positive 12-18° | 0.3-3 mm | 0.05-0.20 mm/rev | Finishing pass only, Ra <0.8 μm, low cutting force thin-wall parts |
| MR (medium roughing) | Heavy hone (T-land 0.20 mm) | Negative -5 to 0° | 4-10 mm | 0.30-0.55 mm/rev | Roughing passes only, deep ap, high feed |
| MRR / R (heavy roughing) | Strongest hone (T-land 0.30+ mm) | Strong negative -8° | 5-12 mm | 0.40-0.70 mm/rev | Heavy roughing, cast iron shell, scaled-up workpieces (ap >8 mm) |
Pattern: chipbreaker letter roughly maps to ap + f intensity. Sharp positive (PM, MF) for light cuts; heavy negative (MR, MRR) for heavy cuts. The trade-off is always edge strength vs cutting force. PM gives lowest cutting force but fastest edge wear; MRR gives longest edge life but highest cutting force.
3. 8-Brand Chipbreaker Naming Map
Each brand uses different letter codes that map to the 5 universal families. Here's the cross-reference matrix for CNMG 120408 across 8 brands:
| Brand | Universal Family | Equivalent Code | Edge Prep Detail | Typical Grade Pairing |
|---|---|---|---|---|
| Mitsubishi | PM (positive medium) | PM / PF / PS | Sharp T-land 0.05, positive rake 10° | VP15TF, US7020, MC6015 |
| Sandvik | PM (positive medium) | PM / MF / MM | Sharp honed, neutral rake | GC2025, GC4325 |
| Kennametal | PM (positive medium) | FP / MP / RP | Polished edge, light hone | KC7315, KC9325 |
| Iscar | PM (positive medium) | PF / PM / MW | Sharp T-land, positive rake | IC8150, IC8250 |
| Walter | PM (positive medium) | PM5 / MM5 / RM5 | 5-micron edge prep, polished | WPP05, WSM35 |
| Korloy | PM (positive medium) | PM / MF / MG / VH | Sharp hone, +8° positive rake | NC3220, NC3120 |
| Sumitomo | PM (positive medium) | SE / SA / SH / GU | Sharp T-land, polished (different naming) | AC6020M, AC8025P |
| Tungaloy | PM (positive medium) | NS / TM / TH | Sharp honed, positive rake 12° | T9115, T9125 |
How to read this table: To find an equivalent of Mitsubishi CNMG 120408-PM in another brand's catalog, look up the row for that brand and find the equivalent code in the same family column. For example, Sandvik CNMG 120408-PM is the direct equivalent, Korloy CNMG 120408-PM is the same, but Sumitomo uses CNMG 120408-SE for the same geometry. For complete grade equivalency across all chipbreaker families see our grade cross-reference hub.
4. Geometry Family Coverage Matrix
The same chipbreaker geometry (PM, MM, etc.) is available across multiple insert shape families. Here's how each chipbreaker family is typically deployed across the 7 main turning shapes:
| Chipbreaker | CNMG | DNMG | SNMG | WNMG | VNMG | TNMG | APMT (mill) |
|---|---|---|---|---|---|---|---|
| PM | — Universal | — Universal | — Universal | — Universal | — Universal | — Universal | — (use APKT) |
| MM | — Common | — Common | — Common | — Available | — Available | — Available | — |
| MF | — Finishing | — Finishing | — Finishing | — Finishing | — Finishing | — Finishing | — |
| MR / R | — Roughing | — Roughing | — Roughing | — Roughing | Limited | Limited | — |
| MRR | — Heavy roughing | — Heavy roughing | — Heavy roughing | Limited | — | — | — |
Pattern: PM is universally available in all 6 turning shape families (CNMG / DNMG / SNMG / WNMG / VNMG / TNMG). MRR is only available in CNMG / DNMG / SNMG because those shapes handle the heaviest roughing loads. APMT (milling) has its own chipbreaker family (APKT for Mitsubishi / Korloy) — totally different geometry. For milling-specific chipbreaker selection, see our APMT 1135 stainless steel milling guide.
5. When to Use Each Chipbreaker Family — Decision Tree
Use this 5-step sequence to pick the right chipbreaker for your job:
- Step 1 — workpiece material: Steel → PM or MR. Stainless 304/316 → PM (low Vc). Cast iron → MM or MRR. Aluminum → MF (sharp polished). For material-specific insert selection see carbide inserts by material.
- Step 2 — depth of cut (ap): ap <3 mm → MF or PM. ap 3-6 mm → PM. ap 6-10 mm → MR. ap >10 mm → MRR. If ap >6 mm and you're using CNMG 120408, consider sizing up to CNMG 120412 or CNMG 190616 — see our CNMG size progression guide.
- Step 3 — feed rate (f): f <0.15 mm/rev (finishing) → MF. f 0.15-0.35 (general) → PM. f 0.35-0.55 (heavy) → MR. f >0.55 (aggressive) → MRR.
- Step 4 — surface finish target (Ra): Ra <0.4 μm → MF (sharpest edge). Ra 0.4-1.6 μm → PM (universal). Ra >1.6 μm → MR or MRR (don't waste PM on rough finish).
- Step 5 — rigidity of setup: Thin-wall / low rigidity → PM (lowest cutting force). Heavy chuck / rigid setup → MR or MRR (heavier cuts safe).
Practical heuristic: For 80% of steel shaft turning jobs, CNMG 120408-PM with a universal grade (Mitsubishi US7020 or Sandvik GC4325) is the right choice. Only switch to MR / MRR when ap >6 mm AND f >0.5 mm/rev AND you have rigid setup. Switch to MF only for the final finishing pass on Ra-critical parts.
6. Tool Holder Compatibility (Same as Shape Family)
The chipbreaker geometry doesn't change tool holder requirements — you use the same DCLNR 2525 M12 for any CNMG 120408-XX (PM, MM, MF, MR, or MRR). This is one of the benefits of the universal insert naming system: same holder, swap inserts based on application. Holder family for CNMG is the DCLNR / DCLNL / DCLCR series at 95° lead angle. For complete holder family selection including shank sizes, see our tool holder guide.
7. Cutting Parameters by Chipbreaker Family
| Chipbreaker | Vc on Steel (CVD) | Vc on Stainless (PVD) | Vc on Cast Iron (CVD) | f (mm/rev) | ap (mm) | Surface Finish Ra |
|---|---|---|---|---|---|---|
| MF | 220-280 m/min | 160-200 m/min | 200-260 m/min | 0.05-0.20 | 0.3-3.0 | 0.3-0.8 μm |
| PM | 200-260 m/min | 140-180 m/min | 180-240 m/min | 0.10-0.40 | 1.0-6.0 | 0.6-1.6 μm |
| MM | 180-240 m/min | 120-160 m/min | 160-220 m/min | 0.20-0.50 | 2.0-8.0 | 1.0-3.2 μm |
| MR / R | 160-220 m/min | 100-150 m/min | 140-200 m/min | 0.30-0.55 | 4.0-10.0 | 2.0-4.0 μm |
| MRR | 140-200 m/min | 80-130 m/min | 120-180 m/min | 0.40-0.70 | 5.0-12.0 | 3.0-6.0 μm |
Trend: as chipbreaker gets heavier (PM → MRR), Vc drops 15-30%, f rises 25-30%, ap potential grows 50%. For Vc/f/ap formulas and material-specific calculator, see our speeds & feeds hub.
8. Migration Cheat Sheet — Moving Between Families
When the existing chipbreaker isn't working, here's how to migrate between families. Read each row as "if you're running X and seeing Y problem, switch to Z":
| If You Run | Problem Symptom | Switch To | Parameter Adjustment |
|---|---|---|---|
| PM, edge chipping at high feed | Insert fractures at corner after 10 min | MM (stronger edge) | Vc -10%, f same, ap same |
| PM, surface finish too rough | Ra >1.6 μm despite slow feed | MF (sharper edge) | Vc same, f -50%, ap -50% |
| MM, slow material removal | Cycle time too long, low MRR | MR (heavier edge) | Vc -5%, f +30%, ap +30% |
| MR, chatter on thin-wall part | Vibration marks on Ra 1.6 μm | PM (lower force) | Vc same, f -25%, ap -25% |
| MRR, premature corner wear | Plastic deformation on the nose radius | MR (less aggressive) | Vc -10%, f -10%, ap -10% |
| MF, tool life too short | Edge wears in 5 minutes | PM (slightly stronger) | Vc -10%, f +20%, ap +30% |
| PM, work hardening on stainless | 304/316 hardens ahead of insert | MF (sharper, less rubbing) | Vc +10% (counter-intuitive), f -20%, ap -20% |
For deeper migration paths (CNMG vs DNMG geometry, or single-size to size scaling), see our DNMG vs CNMG geometry guide and CNMG 120408-PM 8-brand grade recommendation.
9. Frequently Asked Questions
What does the letter after the nose radius in ISO 1832 mean?
The 8th character in a turning insert ISO code (e.g., CNMG 120408-**P**) is the chipbreaker family letter. It encodes the rake face angle, edge prep, and intended application range. The letter is brand-specific but follows the 5 universal families: PM (positive medium, universal), MM (medium-medium, cast iron / interrupted), MF (medium finishing, Ra-critical), MR (medium roughing, heavy ap), MRR (heavy roughing, cast iron shell). See our ISO code decoder for the full letter-by-letter breakdown.
Can I use the same holder for PM, MM, MF, MR, and MRR inserts?
Yes. The chipbreaker letter does not change the holder requirements. All CNMG 120408-XX inserts (any chipbreaker letter) fit the same DCLNR 2525 M12 holder at 95° lead angle. This is one of the key benefits of the universal ISO naming system — same physical dimensions across chipbreaker families. The only thing that changes is the chip control and edge prep, both of which are contained within the insert body itself.
What is the difference between Mitsubishi PM and PF?
Mitsubishi uses PM for positive medium (universal 80% application, sharp edge with T-land 0.05 mm) and PF for positive finishing (very sharp, smaller T-land or no T-land, optimized for finishing Ra-critical work). PF gives slightly better surface finish but reduced edge strength. For most steel roughing operations, PM is the right choice. Reserve PF for stainless finishing where built-up edge is the main failure mode.
Should I use PM or MR for general steel turning?
For 80% of general steel turning, CNMG 120408-PM is the right choice. It gives the lowest cutting force, longest tool life at moderate ap (1-6 mm), and best surface finish range (Ra 0.6-1.6 μm). Only switch to MR when you consistently run ap >6 mm AND f >0.5 mm/rev AND have rigid setup. For mixed production, run PM for both roughing and finishing passes (with reduced ap for finishing), or do PM roughing + MF finishing for optimal cycle time vs surface finish balance.
Why does my insert wear too fast when I use MRR on stainless?
MRR (heavy roughing) inserts have a very strong negative rake and heavy edge prep, which generates 30-50% higher cutting forces than PM. On stainless 304/316, this high force combined with work hardening accelerates plastic deformation at the nose radius. For stainless roughing, use PM with PVD grade (VP15TF, GC2025, KC7315) at slightly reduced Vc (100-140 m/min). Reserve MRR for cast iron roughing where the geometry works exceptionally well due to the chip form matching cast iron's continuous chip behavior.
Is there a universal chipbreaker that works for all turning applications?
PM is the closest to universal — it's the default choice for 80% of turning applications across steel, stainless, finishing, and medium roughing. However, no single chipbreaker handles all scenarios optimally: for finishing at Ra <0.8 μm, switch to MF; for cast iron roughing, switch to MM or MRR; for high-feed turning above 0.5 mm/rev, switch to MR. The recommended workflow is to keep PM as your default and keep MM and MF as backup inserts for specific conditions.
10. Related Pillar Guides
- Carbide inserts by material — steel (P), stainless (M), cast iron (K), aluminum (N) selection.
- Carbide inserts by operation — turning, milling, drilling, threading geometry families.
- Carbide inserts by ISO code — letter-by-letter ISO 1832 decoder.
- Grade cross-reference hub — 8-brand grade matrix for steel + stainless + cast iron.
- Speeds & feeds calculator — Vc / f / ap formulas by material.
- Tool holder guide — DCLNR/DDJNR family selection.