Insert Tolerance Classes: A, C, E, G, J, K, L, M, N, U — Complete Reference
The third character in any ISO 1832 insert designation is the tolerance class — a single letter that tells you how precisely the insert was ground. Tolerance directly affects dimensional accuracy, surface finish, and whether your finished part hits its print tolerances. This comprehensive reference covers every ISO tolerance class with exact dimensional limits and practical selection guidance.
What Does Tolerance Control?
The tolerance class defines the permissible deviation for three critical insert dimensions:
- IC (Inscribed Circle diameter): Controls the cutting point position, directly affecting part diameter
- s (Thickness): Affects insert seating depth and cutting height
- m (Distance from cutting edge to inscribed circle): Affects tool nose position and length compensation
The tighter the tolerance, the more precisely the insert will produce to-size parts — but also the higher the manufacturing cost.
Complete Tolerance Class Table
| Class | IC Tolerance (mm) | Thickness Tolerance (mm) | m Dimension (mm) | Grinding Required? | Typical Application |
|---|---|---|---|---|---|
| A | ±0.005 | ±0.005 | ±0.005 | Yes (peripheral + top/bottom) | Swiss-type, precision boring, medical implants |
| C | ±0.008 | ±0.010 | ±0.008 | Yes (fully ground) | High-precision finishing, aerospace components |
| E | ±0.010 | ±0.010 | ±0.010 | Yes (fully ground) | Precision finishing, boring bars |
| G | ±0.013 | ±0.015 | ±0.013 | Yes (peripheral ground) | Finishing, semi-finishing, general precision |
| J | ±0.020 | ±0.020 | ±0.020 | Yes (peripheral ground) | Semi-finishing, general turning |
| K | ±0.025 | ±0.025 | ±0.025 | Optional | General purpose turning, milling |
| L | ±0.038 | ±0.038 | ±0.038 | No (sintered) | Roughing, heavy stock removal |
| M | ±0.050 | ±0.050 | ±0.050 | No (sintered or lightly ground) | General roughing, most common economy class |
| N | ±0.063 | ±0.063 | ±0.063 | No (sintered) | Heavy roughing, where precision is not critical |
| U | ±0.075 | ±0.100 | ±0.075 | No (as-sintered) | Economy roughing, heavy interrupted cuts |
Tolerance Selection by Operation
| Operation | Surface Finish (Ra µm) | Part Tolerance (mm) | Recommended Insert Class | Common Korloy Inserts |
|---|---|---|---|---|
| Swiss-type / Micro machining | 0.1–0.4 | ±0.003–0.010 | A or C | CCGT 060202, DCGT 070202 |
| Precision boring / finishing | 0.2–0.8 | ±0.005–0.015 | C or E | CCMT 09T304, DCMT 11T304 |
| Finish turning | 0.4–1.6 | ±0.010–0.030 | E or G | CNMG 120404, DNMG 150404 |
| Semi-finishing | 1.6–3.2 | ±0.020–0.050 | G or J | CNMG 120408, WNMG 080408 |
| General turning | 3.2–6.3 | ±0.050–0.100 | J, K, or M | CNMG 120408, WNMG 080408 |
| Roughing | 6.3–12.5 | ±0.100–0.500 | M, N, or U | CNMG 120408, SNMG 120408 |
| Heavy interrupted roughing | >12.5 | ±0.200+ | L, M, or U | SNMG 150612, RNMG 120408 |
How Tolerance Affects Part Accuracy
The insert tolerance directly impacts your finished part dimensions. Here is the practical relationship:
| Insert Tolerance Class | Max IC Variation | Impact on Part Diameter | Impact on Length (Z-axis) |
|---|---|---|---|
| A (±0.005) | 0.010 mm total | ±0.010 mm on diameter | ±0.005 mm |
| C (±0.008) | 0.016 mm total | ±0.016 mm on diameter | ±0.008 mm |
| E (±0.010) | 0.020 mm total | ±0.020 mm on diameter | ±0.010 mm |
| G (±0.013) | 0.026 mm total | ±0.026 mm on diameter | ±0.013 mm |
| M (±0.050) | 0.100 mm total | ±0.100 mm on diameter | ±0.050 mm |
Important: On a CNC lathe, the diameter tolerance is twice the IC tolerance because the IC variation shifts the cutting point radially — affecting both sides of the workpiece. A ±0.050 mm IC variation (class M) can cause up to ±0.100 mm diameter variation on the part.
Ground vs. Sintered Inserts: What Is the Difference?
| Factor | Ground Inserts (A–K) | Sintered Inserts (L–U) |
|---|---|---|
| Manufacturing Process | Sintered then ground to final dimensions | Sintered to near-net shape, minimal grinding |
| Dimensional Accuracy | High (±0.005 to ±0.025 mm) | Low (±0.038 to ±0.100 mm) |
| Edge Quality | Precise, sharp edges | Slightly rounded, as-sintered |
| Cost | Higher (grinding adds cost) | Lower (fewer processing steps) |
| Best For | Finishing, precision work | Roughing, where economy matters |
| Korloy Example | CCGT 060202-HF (class A/E) | CNMG 120408-HM (class M) |
Tolerance Class Cost Comparison
Tighter tolerances require additional grinding operations, which increase insert cost. As a general rule:
| Tolerance Class | Relative Cost Index | When Justified |
|---|---|---|
| A / C | 3.0–5.0× | When part tolerance is <0.015 mm |
| E / G | 1.5–2.5× | When part tolerance is 0.015–0.050 mm |
| J / K | 1.0–1.3× | General purpose, standard parts |
| M | 1.0× (baseline) | Roughing, economy applications |
| N / U | 0.8–0.9× | Heavy roughing, maximum cost savings |
Cost optimization tip: Use class M (or L) inserts for roughing passes where dimensional accuracy does not matter, then switch to class E or G for finishing. This two-grade approach saves 30–50% on tooling costs versus using precision inserts for all operations.
When to Upgrade Tolerance Class
Consider stepping up to a tighter tolerance class when you experience:
- Inconsistent part sizes after insert indexing — variation may be the insert, not the machine
- Failed SPC/Cpk checks on critical dimensions — tighter insert tolerance reduces process variation
- Excessive trial cuts after insert changes — precision inserts reduce setup time
- Chatter or vibration during finishing — ground inserts seat more consistently
- Customer complaints on dimensional accuracy — start with the insert tolerance
Quick Selection Decision Tree
Part tolerance < ±0.010 mm? → Use class A or C
Part tolerance ±0.010 to ±0.030 mm? → Use class E or G
Part tolerance ±0.030 to ±0.080 mm? → Use class J or K
Part tolerance > ±0.080 mm or roughing? → Use class M, N, or U
Need precision inserts for your finishing operations? hooguu.com carries Korloy inserts in all tolerance classes from A to U. Filter our catalog by tolerance class to find the right insert for your part requirements. Contact us for technical support on tolerance selection.
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