Introduction
Selecting the right bar stock material is critical for automatic lathe and CNC Swiss-type machining operations. The ideal material must balance machinability, corrosion resistance, mechanical strength, and cost — and the optimal choice depends heavily on the end application, surface treatment requirements, and production volume.
This guide summarizes the most commonly used free-machining bar stock materials for automatic lathes, organized by material family, with practical selection guidance for each grade.
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Free-Machining Stainless Steel Bar Stock (JIS G4318)
The following stainless steel grades are specifically designed or optimized for automatic lathe machining. All belong to the JIS G4318 standard for cold-finished stainless steel bars.
| Grade (CNS/JIS) | Key Characteristics | Best For |
|---|---|---|
| SUS303F | Balanced machinability and corrosion resistance; the standard free-machining austenitic grade | General precision parts; see full guide: SUS303 |
| SUS303Cu | 303 series with copper (Cu) addition for enhanced machinability; particularly suited for drilling operations | High-speed drilling, complex turned parts |
| SUS316F | Higher nickel (Ni) content than 303 series; better corrosion resistance; free-machining variant of SUS316 | Marine, chemical, and corrosive environment parts |
| SUS316LHS | Sulfur (S) and copper (Cu) added; excellent machinability; superior to SUS316F in both electroplating adhesion and mirror surface finish | Parts requiring plating or high-gloss surface finish |
| SUS416F | Martensitic free-machining grade; excellent machinability; heat-treatable to HRC 38+; see full guide: SUS416 | High-hardness parts after heat treatment; wear-resistant components |
| SUS420F | Slightly lower machinability than SUS416F; heat-treatable to HRC 50+ | High-hardness applications requiring HRC 50+ |
| SUS420J2 | Slightly lower machinability than SUS420F; heat-treatable to HRC 50+ | Cutlery, blades, and high-hardness precision parts |
| SUS430F | Ferritic free-machining grade; better corrosion resistance than 416; higher Cr than 416; commonly used for electronic components | Electronic parts, decorative components, non-magnetic applications |
Free-Machining Carbon Steel / Easy-Iron Bar Stock
| Grade (CNS/JIS) | AISI/SAE Equivalent | Key Characteristics | Best For |
|---|---|---|---|
| SCM24L | C12L14 | Equivalent to AISI 12L14; leaded free-machining carbon steel; excellent machinability; see full guide: 12L14 vs 1215 | General electroplated shafts (SHAFT) and precision parts; not RoHS-compliant |
| 45C | S45CF | Medium-carbon free-machining steel; can achieve HRC 50+ after heat treatment | Parts requiring moderate strength with heat treatment capability |
Free-Machining Brass Bar Stock (JIS H3250)
| Grade (CNS/JIS) | Key Characteristics | Best For |
|---|---|---|
| C3604-BD-F | Standard free-cutting brass; no special mechanical property requirements from customer; lead-free copper variant available; see full guide: C3604 | General precision parts; connectors; valve cores; standard hardware |
Free-Machining Aluminum Bar Stock
| Grade | Key Characteristics | Best For |
|---|---|---|
| A6061-BD-F | 6061 aluminum alloy bar; customer has specific mechanical property requirements; mechanical properties reference JIS B1057; see full guide: 6061 Aluminum | Precision aluminum parts with defined mechanical property requirements; lightweight structural components |
Material Selection Guide
Use the following decision framework to select the right bar stock for your automatic lathe application:
| Priority | Recommended Grade(s) |
|---|---|
| Maximum machinability, general parts, no RoHS required | SUS303F, C12L14 (SCM24L), C3604-BD-F |
| Maximum machinability + RoHS compliant | SUS303F, 1215, C3604-BD-F (lead-free variant) |
| High corrosion resistance (marine / chemical) | SUS316F, SUS316LHS |
| High hardness after heat treatment (HRC 38+) | SUS416F |
| Very high hardness after heat treatment (HRC 50+) | SUS420F, SUS420J2, S45CF |
| Electronic components, non-magnetic | SUS430F |
| Plating / mirror finish quality | SUS316LHS |
| Drilling-intensive operations | SUS303Cu |
| Lightweight aluminum parts with mechanical specs | A6061-BD-F |
| General brass hardware, connectors | C3604-BD-F |
Key Considerations for Automatic Lathe Material Selection
- Machinability vs corrosion resistance trade-off: Free-machining additives (S, Pb, Cu) improve chip-breaking but may slightly reduce corrosion resistance. Choose the grade that meets the minimum corrosion requirement for the application.
- Heat treatment requirement: Only martensitic grades (SUS416F, SUS420F, SUS420J2) and carbon steels (S45CF) can be hardened by heat treatment. Austenitic grades (SUS303, SUS316) cannot be through-hardened.
- RoHS compliance: Leaded grades (C12L14/SCM24L) are not RoHS-compliant. Specify 1215 or lead-free C3604 variants for RoHS-required applications.
- Surface treatment compatibility: SUS316LHS is specifically optimized for electroplating adhesion and mirror polishing. For general plating on carbon steel, 12L14 or 1215 are standard choices.
- Magnetic requirements: Austenitic grades (303, 316 series) are non-magnetic in annealed condition. Martensitic (416, 420) and ferritic (430) grades are magnetic.
Standards reference: JIS G4318 | JIS H3250 | JIS B1057 | AISI/SAE | GB/T 8731
Content authorized for publication by FULLERKREG.