Cast Iron Speeds & Feeds Calculator 2026
Optimized parameters for gray iron, ductile iron, malleable iron, and CGI. Includes dust control and tool wear considerations.
Calculate Cast Iron Parameters
Cast Iron Machining Guide
Cast iron is widely used in machine tool beds, engine blocks, brake rotors, and countless other applications. Its excellent damping properties, castability, and machinability make it a staple of manufacturing. However, graphite particles make it abrasive and dusty.
Cast Iron Types
⬜ Gray Iron
Graphite: Flake structure
Most common type. Graphite flakes interrupt the matrix, making it brittle but easy to machine. Excellent vibration damping. Produces fine graphite dust.
- • Class 20-30: Soft, easy machining
- • Class 40-60: Harder, slower speeds
- • Best machined dry or with air blast
🔵 Ductile Iron
Graphite: Spheroidal nodules
Also called nodular or SG iron. Round graphite nodules give ductility and higher strength than gray iron. Machines more like steel than gray iron.
- • 60-40-18: Ferritic, easy machining
- • 80-55-06: Pearlitic, moderate
- • 100-70-03 and up: Similar to steel
🔴 CGI (Compacted Graphite Iron)
Graphite: Vermicular (worm-like)
75% stronger than gray iron with 2× fatigue strength. Used in diesel engines. Extremely abrasive! Tool wear is 5-10× faster than gray iron.
- • Requires CBN or SiAlON ceramic
- • Reduce speeds vs gray iron
- • Production challenge
🟣 Malleable Iron
Graphite: Temper carbon nodules
Created by heat treating white iron. Good ductility and shock resistance. Machines well, similar to gray iron. Used for pipe fittings and hardware.
Cutting Speed Reference
| Grade | Machinability | Milling (m/min) | Turning (m/min) | Notes |
|---|---|---|---|---|
| Gray 20-30 | 100-110% | 150-350 | 190-420 | Easy machining |
| Gray 40-60 | 65-85% | 80-190 | 100-240 | Harder, slower |
| Ductile 60-40 | 80% | 130-280 | 160-360 | Similar to mild steel |
| Ductile 100-70 | 55% | 75-165 | 95-215 | High strength |
| CGI | 50% | 70-160 | 90-200 | Very abrasive! |
* Speeds for coated carbide (Al₂O₃). Ceramic can run 2-3× faster.
Tool Selection Guide
Carbide Inserts
Al₂O₃ (Alumina) coating is best for cast iron - excellent abrasion resistance. TiCN is also good. Use K-grade (uncoated) or Al₂O₃ coated grades.
Ceramic (High Speed)
Si₃N₄ (silicon nitride) and SiAlON excel at high-speed roughing of gray iron. Can run 500-1000+ m/min. Avoid flood coolant - thermal shock risk.
Frequently Asked Questions
Gray iron has graphite in flake form, making it brittle but excellent for damping (machine beds, engine blocks). Ductile iron has spheroidal graphite nodules, giving it ductility and higher strength, more similar to steel. Gray iron is easier to machine but produces more dust.