CNMG 120408-PM Grade Recommendation: Complete Buyer's Guide
8-brand cross-reference · ISO code decoding · PM chipbreaker geometry · Vc / f / ap parameters · migration cheat sheet
1. What is CNMG 120408-PM?
CNMG 120408-PM is a negative rake (N), 80° diamond shape, with a 12.7 mm inscribed circle (IC), 4.76 mm thickness, 0.8 mm nose radius, and the PM chipbreaker geometry. The "CNMG" prefix decodes as:
- C — 80° diamond shape (negative rake, hole through center)
- N — 0° relief angle (negative rake, strongest edge)
- M — tolerance class (M = ±0.005 mm IC, ±0.025 mm thickness, ±0.013 mm nose radius)
- G — hole + chipbreaker on both faces (double-sided)
The 120408 trailing code specifies:
The -PM suffix identifies the chipbreaker geometry: P = positive cutting geometry, M = medium chipbreaker (mid-range feed / depth). Combined, this insert is the most common medium-roughing geometry in CNMG lineup worldwide, used by every major brand with the same physical dimensions.
2. CNMG 120408 vs CNMG 120404 vs CNMG 120412: PM Variant Comparison
The PM chipbreaker is offered in three nose radius variants within the CNMG 1204 family:
| Variant | Nose Radius | Best For | Surface Finish (Ra) | Feed Range |
|---|---|---|---|---|
| CNMG 120404-PM | 0.4 mm | Finishing, thin-wall | 0.4-0.8 µm | 0.10-0.25 mm/rev |
| CNMG 120408-PM | 0.8 mm | Medium-roughing (most common) | 0.8-1.6 µm | 0.20-0.40 mm/rev |
| CNMG 120412-PM | 1.2 mm | Heavy roughing, strong edge | 1.6-3.2 µm | 0.30-0.55 mm/rev |
The 120408 variant is the sweet spot: it balances edge strength (1.2 mm radius is too blunt for finishing) and surface finish (0.4 mm is too fragile for roughing). For most steel roughing applications, 120408-PM is the default choice.
3. PM Chipbreaker Geometry: How It Cuts
The PM chipbreaker is a "positive-on-negative" design — meaning the chipbreaker geometry itself has a positive rake face, but it sits on a negative-rake insert body. The geometry looks like this from the side:
- Land width (T-land): 0.05-0.10 mm at the cutting edge (sharp, not honed)
- Rake face angle: +6° to +12° (positive, low cutting force)
- Chipformer pocket: wide and shallow (suitable for medium chip thickness)
- Corner strengthening: small T-land at the nose radius for edge stability
The PM chipbreaker's main advantage over heavier breakers (like MM or MR) is reduced cutting force — typically 15-25% lower cutting forces than MR at the same feed. This translates to:
- Lower spindle load (use on lower-power machines)
- Better surface finish (lower vibration)
- Longer tool life on less rigid setups
The trade-off is reduced chip control on heavy roughing — for Vc > 280 m/min or ap > 5 mm, MM or MR breakers handle the heavier chip load better.
4. The 8-Brand Grade Cross-Reference Matrix
Every major brand sells a CNMG 120408-PM with the same physical dimensions. The grade (coating + substrate) varies by brand and application. Here is the cross-reference matrix:
| Brand | Grade | ISO | Coating | Best For | $/corner |
|---|---|---|---|---|---|
| Mitsubishi | US7020 | P15-P25 | CVD TiCN-Al2O3 + TiN | Steel roughing, continuous cut | ~$5.20 |
| Sandvik | GC4325 | P15-P35 | CVD TiCN-Al2O3 | Steel universal, roughing to medium | ~$5.40 |
| Kennametal | KC9325 | P15-P30 | CVD TiCN-Al2O3 + TiN | Steel roughing, high-Vc | ~$5.50 |
| Iscar | IC8250 | P15-P30 | CVD TiCN-Al2O3 | Steel universal, roughing | ~$4.95 |
| Walter | WSM01 | P05-P15 | CVD TiCN-Al2O3 | Steel finishing, sharp edge | ~$6.10 |
| Korloy | NC3030 | P20-P35 | CVD TiCN-Al2O3 | Steel roughing, value | ~$4.30 |
| Sumitomo | AC630M | P15-P35 | CVD TiCN-Al2O3 | Steel + stainless roughing | ~$5.20 |
| Tungaloy | T9125 | P15-P30 | CVD TiCN-Al2O3 + TiN | Steel roughing, high feed | ~$5.55 |
How to read this: All 8 grades are CVD-coated for steel (P-class). Mitsubishi US7020 and Sandvik GC4325 are the most established cross-compatible pair. Korloy NC3030 is the most cost-effective at ~17% below US7020. Walter WSM01 has a tighter ISO range (finishing only) and is not a true 1:1 substitute for roughing.
5. Workpiece-Specific Grade Recommendations
Match the grade to your workpiece for best results:
| Workpiece | Recommended Grade | Vc (m/min) | f (mm/rev) | ap (mm) | Coolant |
|---|---|---|---|---|---|
| Mild steel (1018, 1020) | GC4325 / US7020 / IC8250 | 200-280 | 0.25-0.40 | 2.0-5.0 | Dry or flood |
| Alloy steel (4140, 4340) | US7020 / KC9325 / GC4325 | 160-220 | 0.20-0.35 | 2.0-4.0 | Dry or flood |
| Tool steel (D2, H13, A2) | KC9325 / T9125 / US7020 | 120-180 | 0.15-0.30 | 1.5-3.0 | Dry or MQL |
| Stainless steel (304, 316) | Switch to PVD grade (VP15TF, KC7315, IC8150) — see our VP15TF equivalent guide | 120-180 | 0.20-0.30 | 1.5-3.0 | Flood |
| Ductile cast iron (60-40-18) | GC4325 / US7020 / NC3030 | 180-260 | 0.25-0.40 | 2.0-5.0 | Dry (preferred) |
| Gray cast iron (class 30+) | KC9325 / GC4325 / T9125 | 150-220 | 0.20-0.35 | 2.0-4.0 | Dry |
Important: CNMG 120408-PM with CVD grades (US7020, GC4325) is for steel. For stainless steel, the PM chipbreaker geometry is correct, but the grade must switch to PVD (VP15TF or equivalent). CVD on stainless causes built-up edge and crater wear.
6. Mitsubishi US7020 vs Sandvik GC4325: Direct Comparison
The two most commonly cross-referenced CNMG 120408-PM grades (also see our carbide inserts by material for steel-grade selection):
| Dimension | US7020 | GC4325 | Winner |
|---|---|---|---|
| ISO range | P15-P25 (narrower) | P15-P35 (wider) | GC4325 (versatility) |
| Coating thickness | ~12 µm CVD | ~10 µm CVD | US7020 (thicker) |
| Substrate grain size | Medium-grain (~1.0 µm) | Medium-grain (~0.8 µm) | GC4325 (finer) |
| Vc on mild steel (1018) | 220-280 m/min | 200-260 m/min | US7020 (faster) |
| Tool life at Vc 250 m/min on 1018 | ~25 min | ~28 min | GC4325 |
| Notch wear resistance | High | Higher | GC4325 |
| Edge strength on interrupted cuts | Strong (T-land 0.05-0.08) | Strong (T-land 0.05-0.10) | Tie |
| Price per corner | ~$5.20 | ~$5.40 | US7020 (~4%) |
Verdict: Pick US7020 for high-Vc continuous roughing on mild steel; pick GC4325 for broader ISO coverage (alloy steel, cast iron) and longer tool life. Both are first-tier choices.
7. Kennametal KC9325 vs Iscar IC8250: Budget Tier Comparison
KC9325 and IC8250 target the same application space (steel roughing P15-P30) but at different price points:
| Dimension | KC9325 | IC8250 | Winner |
|---|---|---|---|
| ISO range | P15-P30 | P15-P30 | Tie |
| Coating | CVD TiCN-Al2O3 + TiN top | CVD TiCN-Al2O3 | KC9325 (post-coat treatment) |
| Vc on alloy steel 4140 | 160-220 m/min | 160-220 m/min | Tie |
| Tool life at Vc 200 m/min on 4140 | ~22 min | ~20 min | KC9325 |
| Wear pattern on steel | Uniform crater | Slight notch tendency | KC9325 |
| Price per corner | ~$5.50 | ~$4.95 | IC8250 (~10%) |
Verdict: KC9325 has marginally better tool life (+10%) but costs 10% more per corner. Net cost per part is roughly equivalent. Pick KC9325 for premium tool life or shop standardization on Kennametal tooling; pick IC8250 for value-oriented bulk production.
8. Korloy NC3030: The Budget Champion
Korloy NC3030 is the most cost-effective CNMG 120408-PM in this cross-reference (see our carbide inserts by ISO code for the full CNMG geometry family):
| Dimension | US7020 | NC3030 | Saving |
|---|---|---|---|
| ISO range | P15-P25 | P20-P35 | Wider in NC3030 |
| Coating | CVD TiCN-Al2O3 + TiN | CVD TiCN-Al2O3 | US7020 (more layers) |
| Vc on mild steel | 220-280 m/min | 200-260 m/min | US7020 faster |
| Tool life at Vc 240 m/min on 1018 | ~25 min | ~23 min | US7020 +9% |
| Price per corner | ~$5.20 | ~$4.30 | NC3030 -17% |
| Cost per part (240 m/min, 25-min life, 50 parts/shift) | ~$10.40 | ~$8.60 | NC3030 saves $1.80/part |
Verdict: For high-volume steel roughing (5,000+ parts/week), NC3030 saves ~17% per corner with only 9% lower tool life — net cost per part drops ~$1.80. For low-volume or premium applications, US7020 remains preferred.
9. Migration Cheat Sheet: From US7020/GC4325 to All 8 Brands
Use this when switching grade suppliers or qualifying a new equivalent:
| If You Run US7020 At | Switch To | Adjust Vc | Adjust f | Coolant |
|---|---|---|---|---|
| Vc 250 m/min, f 0.3 mm/rev, 1018 mild steel | GC4325 / IC8250 | -5% | 0% | Dry or flood |
| Vc 220 m/min, f 0.25 mm/rev, 4140 alloy steel | KC9325 / T9125 | 0% | 0% | Dry or flood |
| Vc 200 m/min, f 0.35 mm/rev, 1018 production run | NC3030 (budget) | -5% | +5% | Dry |
| Vc 180 m/min, f 0.2 mm/rev, H13 tool steel | KC9325 / WSM01 | 0% | 0% | Dry or MQL |
| Vc 240 m/min, f 0.3 mm/rev, ductile cast iron | GC4325 / NC3030 | -5% | 0% | Dry (preferred) |
| CNMG 120408-PM on 304 stainless | ⚠️ SWITCH to PVD grade: VP15TF equivalent guide | |||
For milling on 304/316 stainless with a similar geometry, see our APMT 1135 stainless steel milling guide — the milling equivalent uses PVD grades in the APMT family.
10. Frequently Asked Questions
What is the best grade for CNMG 120408-PM on mild steel?
For mild steel (1018, 1020) at Vc 220-280 m/min, Mitsubishi US7020 and Sandvik GC4325 are the two strongest choices. US7020 has slightly better high-Vc performance; GC4325 has slightly longer tool life. Both are CVD TiCN-Al2O3 with similar ISO ranges (P15-P25 / P15-P35). For budget production, Korloy NC3030 delivers ~17% lower cost with ~9% lower tool life.
Can I use CNMG 120408-PM on stainless steel?
The PM chipbreaker geometry works on stainless, but the CVD coating (US7020, GC4325) is not optimal — stainless's gummy chips weld to the rounded CVD edge and cause rapid crater wear. Switch to a PVD grade for stainless: Mitsubishi VP15TF, Sandvik GC2025, Kennametal KC7315, Iscar IC8150. See our VP15TF equivalent guide for cross-reference and parameters.
What is the difference between CNMG 120408-PM and CNMG 120408-MM?
PM = positive-on-negative chipbreaker with positive rake face for medium-roughing (low cutting force, sharp edge). MM = medium-roughing with stronger edge prep and more aggressive chipformer for higher feeds. Use PM for Vc 180-280 m/min and f 0.20-0.40 mm/rev. Use MM for Vc 150-220 m/min and f 0.30-0.55 mm/rev with heavier interrupted cuts. PM delivers ~15-25% lower cutting force than MM at the same feed.
Is Mitsubishi US7020 cross-compatible with Sandvik GC4325?
Yes — both are CNMG 120408-PM compatible in the same holder. The grades have similar ISO ranges (P15-P25 / P15-P35) and both are CVD-coated for steel. US7020 has slightly higher Vc capability; GC4325 has slightly longer tool life. Switching between them requires no holder change — only Vc adjustment of ±5%.
What tool holder do I need for CNMG 120408-PM?
CNMG 120408-PM fits any standard CNMG 1204 holder with negative rake geometry. Common holders: MCLNR/L (85° lead angle), MCLNR/R (95° lead angle), DCLNR/L (75° for turning toward chuck). For the full holder family selection and shank sizes, see our tool holder guide. To pick the right Vc / f / ap for your specific machine setup, see our speeds & feeds hub. For operation-specific recommendations, see our carbide inserts by operation.
Where can I find a complete grade cross-reference for all CNMG 120408 grades?
For a complete 8-brand grade cross-reference covering CNMG, DNMG, SNMG, TNMG, VNMG and all common turning insert geometries — across steel (P), stainless (M), cast iron (K), and HRSA (S) — see our grade cross-reference hub. It includes price-per-corner, ISO range, and best-fit workpiece recommendations.