A Comprehensive Guide to Bronze CNC Machining

Few materials carry as much historical significance as bronze. First developed over 5,000 years ago during the Bronze Age, this copper-based alloy revolutionized tools, weapons, and art, ushering in a new era of human craftsmanship. Although bronze is often associated with casting and hand forging, in modern manufacturing, it is widely used in bearings, bushings, gears, and valve components, where cast parts are finished by precision machining to meet tight tolerances.

This guide will discuss what bronze CNC machining is, the types of bronze available, the machining processes, common challenges, and how to overcome them.

What Is Bronze CNC Machining?

Bronze CNC machining is a process that uses CNC machines to produce parts from bronze—an alloy of copper with typically 5–12% tin. Small amounts of other elements，such as aluminum, phosphorus, manganese, or lead,are often added to achieve specific performance.

Bronze, like brass and other copper alloys, has a range of important electrical, thermal, and corrosion resistance properties. But its mechanical strength is generally lower than those of many other machinable metals (though higher than that of copper and brass). While it doesn’t match the exceptional machinability of free-cutting brass (rated at 100%), many bronze grades still provide good machinability. Typical leaded tin bronzes have a machinability rating between 60–75%, so they are best used on low-stress CNC-machined components. It also has low friction and excellent wear resistance, making it well-suited for sliding-fit parts.

Types of Bronze Available for CNC Machining

Bronze refers to a family of copper-tin alloys whose performance varies based on their specific alloying elements. Below, we’ll explore the bronze grades most commonly used in CNC machining.

Leaded Tin Bronze (Bearing Bronze)

Leaded tin bronze typically contains 83–92 % copper, 7–12 % tin, and 4–8 % lead. It’s one of the easiest bronzes to machine—the lead acts as a built-in lubricant and greatly promotes chip breakage. The tin provides solid strength and corrosion resistance.

However, its tensile strength and hardness are below those of high-tin or aluminum bronzes, and its lead phase melts above ~300 °C, making it unsuitable for hot or heavily loaded parts. It also can’t match the fatigue resistance of phosphor bronzes or the extreme wear resistance of high-tin grades, and its corrosion resistance is limited in aggressive media such as seawater or acidic or alkaline environments. Additionally, leaded tin bronze is unsuitable for food-grade or potable-water applications due to lead toxicity.

Common Grades: C93200 (SAE 660), C93600, C93700
Typical Usages: Bearings, bushings, thrust surfaces, wear plates, general machine parts

Phosphor Bronze

Phosphor bronze is an alloy of copper with typically 4–6% tin and a small addition of phosphorus (around 0.01–0.35%). The phosphorus improves wear resistance, stiffness, and acts as a deoxidizer during alloying, resulting in clean grain structures.
This alloy offers high fatigue strength and excellent corrosion resistance. Historically, phosphor bronze was used in marine hardware—for example, some ship propellers were made from it for its seawater durability. Today, it’s most often found in springs, electrical connectors, bushings, bearings, and bolts where a combination of toughness and wear resistance is required. However, it is notably harder to machine: a common grade like C51000 (5% tin, 0.2% phosphorus) has a machinability rating of only about 20% relative to free-cutting brass.

Common Grades: C51000, C52100
Typical Usages: Springs, electrical connectors, bolts, small bushings

Aluminum Bronze

Aluminum bronze generally contains around 5–12 % Al, with the balance copper (≈85–92 %), plus 3–5 % Fe and up to 1.5 % Ni (with minor Mn, Si) for added strength and corrosion resistance. It is the strongest of the common bronzes, with tensile strength reaching 500–620 MPa in heat-treated tempers, comparable to medium-grade steel. It also has excellent corrosion resistance, particularly in marine and chemical environments, because aluminum in the alloy forms a protective oxide on the surface.

In terms of machinability, aluminum bronze is moderately machinable. C95400, for instance, has a machinability rating around 60%. Because of its high hardness and tendency to work-harden, CNC machining aluminum bronze requires rigid setups, sharp carbide tooling, and precise feed control to maintain accuracy and tool life.

Common Grades: C95400, C95500, C95900
Typical Usages: Marine hardware, valve/pump components, high-load bushings, gears

Silicon Bronze

Silicon bronze is generally made up of approximately 96% copper and 2–4% silicon, with small additions of zinc or manganese. This alloy offers a good balance of moderate strength, excellent corrosion resistance, and good weldability. It also has a warm, golden appearance, often preferred for architectural or artistic applications.

In CNC machining, silicon bronze is considered to have fair machinability, around 30% relative to free-cutting brass. It produces clean chips and good surface finishes when machined under moderate cutting speeds and feeds. Although slightly gummy, it remains manageable with sharp tools and effective chip control.

Common Grades: C65500, C65100
Typical Usages: Marine fasteners, valve stems, electrical terminals, architectural fittings

High-Tin Bronze (Gunmetal)

High-tin bronze, often historically called gunmetal, is a bronze alloy with elevated tin content and minimal or no lead. A typical composition is 88–90 % copper and 10–12 % tin, with trace zinc or nickel. This formulation produces a hard, strong alloy that was famously used to cast cannon barrels—hence the name “gunmetal.” High-tin bronze has excellent wear resistance and carries heavy loads without deforming, making it ideal for heavy-duty components. It serves many of the same roles as aluminum bronze, albeit with slightly lower corrosion resistance but good casting qualities.

Its machinability is around 30 % (similar to silicon bronze). The absence of lead means the alloy is less free-cutting, so machining requires sharper tools and possibly more patience than working with a leaded alloy.

Common Grades: C90300, C90500, C90700
Typical Usages: Worm gears, heavy-load bushings, pump impellers and bodies, valves, steam fittings

A Step-by-Step Guide on Bronze CNC Machining Process

Bronze CNC machining transforms raw bronze stock into precision parts through a series of controlled steps. From design to final inspection, each stage must be optimized for bronze’s material behavior, tool wear, and surface finish. Here's a concise overview of the process:

1. Material Selection and Preparation

The process begins with selecting the right bronze alloy based on mechanical strength, wear resistance, and corrosion requirements. For example, leaded tin bronze is preferred for bushings because of its excellent machinability and self-lubricating properties, while aluminum bronze is chosen for high-strength, marine-grade components.

Once the alloy is selected, the raw material—typically rods, bars, or plates—is cut to size, deburred, and inspected for surface quality. It is then securely clamped to the CNC machine’s worktable using precision fixturing to ensure stability during machining.

2. CAD/CAM Programming

The digital workflow starts by creating a detailed CAD model of the part. That model is imported into CAM software, where toolpaths are generated and optimized for the chosen bronze alloy and the part’s geometry. The resulting CNC program then specifies tool movements, spindle speeds, feed rates, and cut sequences—each tuned to the material’s machinability and thermal properties.

3. Machining Operations

With the CAM-generated toolpaths loaded, the CNC machine is set up—tools such as end mills, drills, and inserts are selected, installed, and calibrated for the specific bronze alloy and required cuts. Once the machine is ready, the actual machining begins. Depending on the part design and bronze type, common operations include:

• Milling: Milling bronze uses rotating cutters to remove material and create slots, pockets, and complex contours. Because bronze can work-harden if the cutter dwells, it’s best to take light radial cuts (≤ 0.5× cutter diameter) with moderate axial engagement.

• Turning: Turning bronze involves rotating the workpiece against a stationary cutting tool to produce cylindrical features such as shafts, sleeves, pins, or bushings. Bronze’s low friction and good thermal conductivity allow higher spindle speeds (up to 400 m/min) than steel, but its tendency to work-harden means you should use sharp carbide or PVD-coated inserts with a slight positive rake (7–10°) and take light finishing passes (≤ 0.5 mm depth of cut).

• Drilling and Tapping: Drilling creates round holes in bronze. Bronze’s softness allows faster drilling speeds than harder metals, but care must be taken to prevent overheating and ensure clean hole walls. Once the hole is drilled, switch to a tap to cut matching threads. For blind holes, use a bottoming tap to get threads right to the bottom without burrs.

4. Post-Processing

After CNC machining, bronze parts undergo deburring and chamfering to remove burrs and sharp edges. If required by part geometry or tolerance demands, a stress-relief anneal stabilizes dimensions and relieves machining stresses. Each part then goes through inspection and testing (CMM, gauges, surface-roughness checks) to verify dimensional and surface quality. Depending on the end use, one or more surface finishes may be applied to bronze machined parts.

• As-Machined Finish

This is the natural bronze surface straight from the CNC machine. It typically shows visible tool-path lines and has a roughness of Ra 1.6–3.2 μm. This finish is cost-effective and well suited for internal bushings, hidden structural parts, or any component where appearance isn’t critical. Note that minor tool marks or burrs may remain, which can impair performance in very tight fits or high-friction applications.

• Mechanical Polishing

Polishing uses successive abrasives or buffing wheels to smooth the surface and create a bright, reflective finish. It can reduce surface roughness from around Ra 1.6–3.2 µm down to below Ra 0.2 µm and improve sliding performance in moving assemblies. Polished bronze is commonly used for decorative hardware, visible fittings, and dynamic components that require low surface drag. However, polishing can be labor-intensive and may increase production time and cost, particularly when uniform gloss is required on complex geometries.

• Bead Blasting

Bead blasting bronze uses a pressurized stream of fine media—commonly glass beads—to gently abrade the surface and produce a uniform matte finish. It removes visible tool marks and smooths the ridges left by milling passes, improving adhesion for coatings or patinas. Since the rounded beads gently peen rather than cut the surface, the result is a consistent satin texture ideal for architectural hardware and decorative fittings. However, the process can leave tiny dimples that may trap debris or interfere with sealing surfaces, so tighter-tolerance parts often require a light polish or lapping afterward.

• Patination (Chemical Coating)

Patination uses controlled oxidation or chemical agents to alter the color and tone of the bronze surface. Common patina hues—brown, green, and black—give an antique or artistic finish. While this process is primarily aesthetic, it can provide a degree of surface protection. Patinas are popular on signage, decorative panels, and period-style fixtures. However, patination requires skilled handling to achieve consistent results, and it may not be suitable for functional surfaces or high-contact mechanical parts.

• Anodizing (Bronze Conversion Coating)

Although anodizing is most often used on aluminum, certain bronze alloys can undergo a similar electrolytic conversion process. In this treatment, the bronze acts as the anode in a specialized bath, forming a thin, porous oxide layer that can be sealed, or even dyed, to produce decorative colors. This oxide film improves corrosion resistance, enhances adhesion for paints or coatings, and adds a modest increase in surface hardness. Because it requires custom electrolytes and tight process control, bronze anodizing isn’t a standard service and tends to be more costly and less widely available than aluminum anodizing. When applied, it delivers unique visual effects and light-duty protection for specialty decorative or corrosion-prone applications.

• Electroplating

Electroplating deposits a thin layer of another metal, such as nickel, silver, gold, or chrome, onto the bronze surface. This improves corrosion resistance, electrical conductivity, and visual appeal. Electroplated finishes are common in high-spec or decorative parts but require tight process control and add to cost and lead time.

Common CNC Machining Challenges and Solutions for Bronze

Bronze has many desirable properties, but its machinability can vary greatly between alloys. Below are five challenges you may encounter when CNC machining bronze, along with practical solutions.

Work Hardening

Certain bronze types (e.g., aluminum bronze, phosphor bronze) are prone to work hardening under improper cutting conditions. If the tool dwells or re-cuts a hardened surface, tool stress and failure risk increase. To prevent this:

• Keep radial depths of cut light, typically no more than 0.5× the tool diameter, to reduce tool load.
• Avoid dwell by using continuous, uninterrupted toolpaths.
• Apply climb milling wherever possible, as it directs chips away from the cutting zone and minimizes rubbing.
• Use sharp carbide tools or PVD-coated inserts (e.g., TiAlN) to minimize heat buildup and resist wear in hardening-prone alloys.

Tool Wear

Harder bronze grades, such as aluminum bronze and silicon bronze, contain abrasive oxides or hardening elements that can dull carbide cutting edges. To mitigate tool wear:

• Use premium, wear-resistant carbide inserts with TiAlN or similar coatings for better heat resistance and extended tool life.
• Monitor insert condition regularly, and replace tools at the first sign of wear .
• Ensure sufficient coolant flow to dissipate heat and flush away abrasive particles.
• For highly abrasive grades, slightly reduce cutting speeds to control thermal load and tool degradation.

Chip Control & Built-Up Edge

Many bronze alloys produce long, stringy chips or form a built-up edge (BUE) that welds to the tool, spoiling finishes and causing tool breakage. To improve chip control and prevent BUE:

• Choose tools with a positive rake angle and polished flutes to encourage clean chip curls.
• Adjust feed rates so chips shear off rather than stretch.
• Run flood coolant or air blast to flush chips away immediately.

Thermal Expansion

Bronze conducts heat well, but excessive cutting temperatures can cause thermal expansion in the workpiece and wear down cutting edges. To maintain dimensional stability:

• Use flood coolant or mist lubrication to carry heat away from the cutting zone.
• Reduce cutting speeds slightly on long runs or tight-tolerance jobs to limit thermal buildup.
• Leave a small finishing allowance and perform a final light pass—this corrects any heat-induced distortion and enhances surface precision.

Workholding & Vibration

Due to bronze’s relatively low stiffness and softness, improper clamping can cause part deflection, chatter, or surface deformation. To maintain stability and accuracy:

• Use rigid fixturing with soft jaws or custom contoured clamps to distribute pressure evenly and prevent marring.
• Minimize tool overhang to reduce deflection and vibration.
• Where applicable, use dual clamping or vacuum chuck systems to enhance rigidity, especially for thin-walled or high-precision parts.

Conclusion

Bronze CNC machining delivers parts with durability, corrosion resistance, and electrical conductivity to meet a wide range of applications. With over a decade of manufacturing expertise, Chiggo is your trusted provider of Bronze CNC Machining Service for precision, efficiency, and consistency. Contact us today for a custom quote and to learn more about our CNC Machining Service.