Cast Bronze Alloy Bearing Bushings: Hardness and Machinability

Understanding Machinability Ratings

• Baseline: Machinability ratings are typically expressed as a percentage relative to Free-Cutting Brass (Alloy C36000 / CZ121 / CW614N), which is set at 100%. This brass is considered very easy to machine, producing small, broken chips.

• Higher % = Easier to Machine: Materials with ratings closer to 100% (or even above, though rare for bronzes) are easier to machine.

• Lower % = Harder to Machine: Materials with lower ratings require lower cutting speeds, generate more heat, cause faster tool wear, and may produce long, stringy, or tough chips.

• Approximate Nature: These ratings are guidelines. Actual machinability depends heavily on the specific machining operation (turning, milling, drilling), tooling (material, geometry, coating), coolant, machine rigidity, and desired surface finish.

• Key Factor: Lead (Pb): Lead is the single most significant element for improving machinability in copper alloys. It acts as an internal lubricant and promotes chip breaking. Alloys with higher lead content generally have much better machinability.

• Other Factors: Aluminum (Al), Nickel (Ni), Silicon (Si), and high Tin (Sn) content generally make bronzes tougher, stronger, and consequently, less machinable.

Machinability Ratings for Common Bearing Alloys (ASTM B505 & EN 1982)

Here’s a comparison of common bearing alloys, grouped by type, showing approximate machinability ratings. EN equivalents are provided where common parallels exist.

Notes:

1. C93700 vs CuSn12 (CC483K): C93700 is a high-leaded tin bronze (typically ~7% Pb). CuSn12-C (CC483K) is primarily a high-tin bronze (~12% Sn) with low or no intentional lead. They are not direct equivalents in terms of machinability despite both being used for bearings. C90700 is a closer ASTM match to CuSn12-C in composition (high tin, low lead). The table reflects C93700’s high lead content for machinability.

2. C83600: While listed under B505, C83600 (Leaded Red Brass / Ounce Metal) is more common for plumbing/fittings but sometimes used for low-load bushings. Its high rating is due to the lead/zinc content. It’s included for comparison but C93200 is the more typical leaded bearing bronze.

3. C90300/C90500 vs CuSn10 (CC480K): These are reasonable compositional equivalents (approx. 10% Sn, 2% Zn). Machinability is similar – fair to poor due to the lack of lead.

4. C95400 vs CuAl10Fe2-C (CC331G): These are roughly equivalent aluminum bronzes (around 10% Al, some Fe).

5. C95400 Rating Variability: Some sources rate C95400 as low as 20%, others up to 60%. 30-50% represents a common middle ground, acknowledging it’s significantly harder than leaded bronzes but perhaps slightly easier than the nickel-bearing grades depending on exact temper and machining conditions.

Summary Trends:

• Leaded Bronzes (C932, C937): Best machinability among the common bearing bronzes.

• Tin Bronzes (C903, C905, C907): Machinability decreases as tin content increases. Generally tougher and gummier than leaded alloys.

• Aluminum Bronzes (C954, C955, C958): Strong, tough, wear-resistant, but significantly harder to machine. Nickel additions (C955, C958) further reduce machinability.

• Manganese Bronzes (C863, C865): High strength, machinability is fair, often better than high-tin or Ni-Al bronzes but worse than leaded tin bronzes.

Always consult the material supplier’s specific data sheet and use recommended speeds, feeds, and tooling for the best results when machining these alloys.