Tin bronze is a family of copper-based alloys in which tin is the principal alloying element. Compared with pure copper, adding tin increases strength, hardness, wear resistance and corrosion resistance, making these alloys particularly useful for bearings, bushings, gears, valve components, pump parts and marine hardware.
However, “tin bronze” should not be treated as one material with one set of properties. C90500, C90700 and other copper-tin alloys contain different amounts of tin, zinc and other elements, and their machining behavior can differ significantly. High-leaded bearing bronzes such as C93200 are also frequently discussed alongside tin bronze because they serve similar bearing and bushing applications, although their composition and behavior are different.
For engineers selecting a bronze for a CNC-machined component, the important questions are therefore not simply “Is tin bronze strong?” or “Is bronze easy to machine?” The correct alloy depends on load, sliding speed, lubrication, corrosion environment, casting quality, required tolerances and manufacturing cost.
What Is Tin Bronze?
Tin bronze is a copper alloy containing tin as its major strengthening alloying element. Traditional industrial tin bronzes typically contain roughly 8–12% tin, although the exact range depends on the specific alloy standard.
The copper forms the base of the alloy, while tin increases hardness, mechanical strength and resistance to wear. Some grades also contain zinc to improve casting characteristics or small controlled quantities of other elements.
Tin bronze has been used for thousands of years, but modern engineering tin bronzes are much more precisely controlled than historical bronze. Commercial grades are produced to defined chemical compositions and mechanical-property requirements so they can be reliably used in components such as bearings, gears and pressure-containing parts.
One terminology issue is worth clarifying. Tin bronze and phosphor bronze overlap, but they are not automatically identical. Phosphor bronze normally refers to copper-tin alloys containing a deliberate small addition of phosphorus. Many wrought phosphor bronzes are used for springs, electrical contacts and fatigue-resistant components, whereas cast tin bronzes such as C90500 and C90700 are widely used for bearings, gears and fluid-handling components.
What Is Tin Bronze Made Of?
The two essential elements are copper and tin, but small composition changes can have a large effect on manufacturing and service performance.
| 合金 | 典型的な組成 | Main Characteristics | 代表的な用途 |
|---|---|---|---|
| C90300 | Approximately 88% Cu, 8% Sn, 4% Zn | Good strength, corrosion resistance and bearing performance | Bearings, pump components, valve bodies and fittings |
| C90500 | 86–89% Cu, 9–11% Sn, 1–3% Zn | Good load capacity, wear resistance and corrosion resistance | Bearings, bushings, gears, pump parts and valve components |
| C90700 | 88–90% Cu, 10–12% Sn | Higher tin content, high hardness and good resistance to heavy wear | Heavy-duty bearings, gears, bushings and high-load components |
| C93200* | 81–85% Cu, 6–8% Pb, 6.3–7.5% Sn, 1–4% Zn | Excellent machinability and widely used bearing properties | Bushings, washers, bearings and sliding components |
*C93200 is technically classified as a high-leaded tin bronze rather than a conventional copper-tin bronze. It is included because engineers frequently compare it with C90500 and C90700 when selecting bearing materials.
According to the Copper Development Association’s C90500 data, C90500 contains 9–11% tin, while C90700 contains approximately 10–12% tin. This apparently small difference is enough to influence strength, hardness, ductility and machinability.
How Does Tin Content Affect Bronze Properties?
Increasing tin content generally strengthens copper and increases hardness and wear resistance. This is one reason higher-tin bronzes are useful for gears and heavily loaded sliding components.
However, more tin is not automatically better.
As tin concentration increases, the alloy generally becomes more difficult to deform and machine. At sufficiently high tin contents, brittle phases can develop and ductility can decrease significantly. The ideal composition is therefore a balance between wear resistance, load capacity and manufacturability.
This helps explain why C90700 may be selected for heavily loaded bearing applications even though C90500 can be easier and more economical to manufacture.
What Are the Main Properties of Tin Bronze?
Good Wear Resistance
Tin increases the hardness of the copper matrix, giving tin bronze good resistance to adhesive and sliding wear. This is particularly valuable in bushings, bearings, worm gears and thrust components where two surfaces repeatedly move against one another.
High Load-Carrying Capacity
Cast tin bronzes combine hardness with useful mechanical strength. They can therefore support relatively high bearing pressures when the shaft, lubrication and bearing geometry are properly designed.
Good Corrosion Resistance
Tin bronze performs well in water, seawater and many brine environments. This has led to extensive use in pumps, marine hardware, valves and other fluid-handling equipment.
It should not, however, be described as corrosion-proof. Ammonia, certain aggressive chemicals and unfavorable chloride conditions can still attack copper alloys.
Good Fatigue and Impact Performance
The combination of copper and tin provides useful resistance to repeated mechanical loading. This makes properly selected tin bronze suitable for gears, bearings and mechanical components exposed to cyclic loads.
Low Friction Against Suitable Mating Materials
Bronze is widely selected as the softer sacrificial member in steel shaft-and-bushing systems. Instead of allowing two hardened steel surfaces to gall or seize, the bronze component can provide a more favorable sliding interface and can be replaced when worn.
Tin Bronze vs Phosphor Bronze: Are They the Same?
Not exactly.
Phosphor bronze is also fundamentally a copper-tin alloy, but it contains a controlled phosphorus addition. Phosphorus acts as a deoxidizer during processing and can improve stiffness, wear resistance and fatigue behavior.
Wrought phosphor bronze grades are particularly common in:
- Springs
- 電気接点
- Clips
- Diaphragms
- Fasteners
- Thin fatigue-resistant components
Cast tin bronzes such as C90500 and C90700 are more strongly associated with:
- Bushings
- Bearings
- ギア
- Valve bodies
- Pump components
- Wear parts
Consequently, a drawing that simply specifies “phosphor bronze” or “tin bronze” may be insufficient for a precision CNC project. The actual UNS or equivalent material grade should normally be specified.
C90500 vs C90700: Which Tin Bronze Should You Choose?
C90500 and C90700 are two of the most commonly discussed cast tin bronzes, but they are intended for somewhat different priorities.
Choose C90500 for a More Balanced General-Purpose Tin Bronze
C90500 contains approximately 9–11% tin together with a small amount of zinc. It offers a useful combination of strength, wear resistance, corrosion resistance and castability.
It is suitable for many industrial bearings, gears, bushings, pump components and valves where good mechanical performance is needed without going to the higher tin content of C90700.
Choose C90700 When Load and Wear Resistance Are More Important
C90700 normally contains 10–12% tin with very little zinc. Its higher tin concentration provides greater hardness and strong resistance to wear and pounding.
This makes it attractive for heavily loaded gears and bearing applications, particularly at relatively low speeds.
The trade-off is manufacturing difficulty. Copper Development Association data gives C90700 a machinability rating of approximately 20 compared with about 30 for C90500.
What About C93200 Bearing Bronze?
C93200, commonly associated with SAE 660 bearing bronze, frequently appears in engineering forums because it is one of the most widely used materials for machined bushings.
Unlike C90500 and C90700, C93200 contains significant lead. The lead improves machinability and helps the material tolerate sliding contact conditions.
The machinability difference is substantial. Copper Development Association data gives C93200 a machinability rating of approximately 70, compared with 30 for C90500 and 20 for C90700.
This is an important content gap in many general articles about bronze. Saying that “bronze has good machinability” can be misleading because the actual machining behavior depends strongly on the alloy.
C93200 may therefore be a better choice when easy machining and general bearing performance are important, while higher-strength tin bronzes may be preferred when load capacity and resistance to mechanical pounding are more important.
Is Tin Bronze Easy to CNC Machine?
Tin bronze can be CNC machined accurately, but it should not automatically be treated like free-cutting brass.
The machining difficulty increases as the alloy becomes harder and contains more tin. A machinist who finds C93200 easy to turn may have a very different experience with C90700 or a high-strength aluminum bronze.
CNC Turning Tin Bronze
Many tin bronze components are rotational parts such as bushings, sleeves, thrust washers and bearing rings, making CNC turning one of the most common manufacturing methods.
A stable setup and sharp cutting edge help produce a clean finish. Positive cutting geometry can reduce cutting forces, particularly on thin-wall bushings where excessive force may distort the part.
One practical issue repeatedly mentioned by machinists is chip control. Bronze chips can be small and can spread throughout the machine, mixing with coolant and accumulating in difficult-to-clean areas.
For high-volume production, chip evacuation and coolant filtration should therefore be considered as part of the process rather than only selecting cutting speed and feed.
CNC Milling Tin Bronze
CNC milling is used for flat surfaces, bolt patterns, keyways, slots, gear-related features and non-axisymmetric bronze components.
Rigid workholding is important because bronze parts such as bearing housings and wear plates may include relatively thin sections. Excessive clamping force can deform the workpiece and lead to dimensional error after unclamping.
Sharp carbide tooling is generally suitable for production machining, although the final tool geometry should be selected according to the exact bronze grade and operation.
Drilling and Boring
Precision bores are especially important in bronze bushings and bearings. The designer should distinguish between the tolerance required during machining and the final bore dimension after installation.
A press-fit bushing can contract internally after being installed into its housing. If the operating clearance is critical, the engineering drawing should account for this dimensional change rather than assuming that a free-state bore will remain unchanged after assembly.
Threading Tin Bronze
Bronze is also used for ACME nuts, worm-drive components and threaded wear parts because it can run against a steel screw without the same galling risk as identical steel-on-steel contact.
Internal threads require good chip evacuation and sufficient tool clearance. For high-load screw-and-nut applications, thread strength alone should not be used to choose the material; wear, lubrication and operating duty must also be considered.
Why Do Porosity and Spots Sometimes Appear After Machining Tin Bronze?
This is one of the most useful questions raised in machining and metallurgy forums.
A cast bronze blank may look completely acceptable before machining, but pores, inclusions or irregular patches can appear when the cutting tool removes the external surface.
That does not necessarily mean the CNC machining process created the defect.
Possible causes originate in the casting process and include:
- Gas porosity
- Shrinkage porosity
- Insufficient feeding during solidification
- Improper melt treatment
- Oxide or inclusion entrapment
- Composition segregation
- Non-uniform cooling
A 2024 metallurgy forum discussion described severe porosity in centrifugal-cast Cu-12Sn bronze that only became clearly visible after machining. Similar machining discussions show that visible patches in leaded bronze can originate from the cast microstructure or segregation rather than coolant damage.
This distinction matters when troubleshooting production. Changing inserts, feeds or coolant cannot eliminate an internal casting defect.
Does Raw Material Form Matter for CNC Machining?
Yes. An engineer should not consider only the alloy designation.
Tin bronze can be supplied as sand castings, centrifugal castings, continuous-cast bars or other product forms. The manufacturing route influences structure, dimensional allowance and the probability and location of casting defects.
For CNC-machined bushings, rings and sleeves, continuously or centrifugally cast stock is frequently attractive because near-net cylindrical forms reduce material waste.
For a critical component, the supplier should also consider whether the drawing includes deep machining that could expose internal casting imperfections.
Material certification and an appropriate inspection plan can be more important than simply specifying “bronze.”
Why Is Tin Bronze Commonly Used for Bearings and Bushings?
Tin bronze is not selected for bearings simply because it is “low friction.” A bearing system depends on the interaction of the bronze, shaft, lubricant, load, velocity, temperature and alignment.
その Copper Development Association bearing guide notes that C90300, C90500 and C90700 combine high load capacity, wear resistance and resistance to pounding.
However, their hardness creates an important trade-off.
Hard tin bronze does not conform as easily to a misaligned shaft and is less capable of embedding dirt particles than softer bearing materials. It therefore benefits from:
- Good shaft alignment
- Clean lubrication
- Correct operating clearance
- Suitable shaft hardness
- Controlled surface finish
The CDA recommends relatively hard mating shafts for these tin bronze bearing alloys, commonly in the 300–400 BHN range.
Why Is PV Important When Selecting a Bronze Bushing?
A recurring problem in engineering forums is trying to select a bushing material using only static load.
That is incomplete.
Bearing performance is strongly influenced by PV, where P represents bearing pressure and V represents sliding velocity. A material that works reliably under a high load at very low speed may fail at the same load when sliding speed rises significantly.
Lubrication condition also changes the acceptable operating range.
This is why a statement such as “C90700 supports heavy loads” does not mean it is automatically suitable for every high-load bearing. The engineer still needs to consider speed, lubrication, shaft condition, temperature and duty cycle.
Does Tin Bronze Need Lubrication?
In most conventional bearing applications, yes.
Bronze is sometimes casually described as “self-lubricating,” but this can create the wrong expectation. Some porous sintered bronze bearings can retain oil internally, and leaded bronze may have favorable emergency sliding behavior, but a solid cast tin bronze bushing should not automatically be operated dry.
Lubrication reduces friction, removes heat and separates the bronze from the shaft.
Forum machinists using bronze steady-rest pads make the same point in practical terms: keeping the bronze contact surface lubricated helps control heat and wear.
Tin Bronze vs Brass
Brass uses zinc as its principal alloying element, while tin bronze uses tin.
For CNC machining, free-cutting brass such as C36000 is generally easier to machine than conventional high-tin bronze. Brass can therefore offer lower manufacturing cost for fittings and general-purpose components that do not require the wear performance of bronze.
Tin bronze is generally preferred when the component requires:
- Better sliding wear resistance
- Higher bearing load capacity
- Strong resistance to seawater and brine
- Better resistance to repeated mechanical contact
Cost and performance therefore need to be considered together rather than replacing bronze with brass solely because both are copper alloys.
Tin Bronze vs Aluminum Bronze
Aluminum bronze uses aluminum rather than tin as the main alloying element and can achieve considerably greater strength.
It is widely used in severe marine, heavy-load and high-strength applications.
However, aluminum bronze can be substantially more demanding to machine. Machinists frequently describe some high-strength aluminum bronze grades as tough, abrasive and difficult to deburr.
Tin bronze may therefore provide a better balance when extreme strength is unnecessary but wear resistance, corrosion resistance and reliable bearing behavior remain important.
Can Tin Bronze Be Used in Marine Environments?
Yes. Resistance to seawater and brines is one of the major advantages of tin bronze.
Typical marine applications include:
- Pump components
- Valve components
- Bushings
- Bearings
- Marine fittings
- Wear components
However, corrosion performance still depends on the exact alloy, water chemistry, flow conditions and contact with other metals.
Galvanic interaction should be reviewed when bronze is assembled with stainless steel, aluminum or other metals in an electrolyte.
What Surface Finishes Can Be Applied to Tin Bronze?
Many functional bronze components are used in the machined condition because the base alloy already provides useful corrosion and wear resistance.
Possible finishing options include:
Machined Finish
The most common choice for functional bearing surfaces. Surface roughness should be controlled according to the lubrication and mating-shaft requirements.
研磨
Polishing may be used for decorative components or exposed bronze hardware. It improves visual appearance but does not fundamentally change the mechanical properties of the alloy.
Protective Clear Coatings
Decorative bronze naturally oxidizes and develops a patina. A suitable clear protective finish may be used when maintaining a polished appearance is important.
めっき
Nickel or other plating systems may be specified for specialized appearance, corrosion or engineering requirements. The coating should not be applied automatically to a bearing surface because it can alter clearances and sliding behavior.
What CNC Parts Are Commonly Made From Tin Bronze?
Tin bronze is particularly valuable where wear, sliding contact or corrosion resistance is required.
- ブッシュ： Support rotating or oscillating shafts and provide a replaceable wear surface.
- Sleeve bearings: Provide sliding support in pumps, machinery and industrial equipment.
- Worm gears: Bronze gear teeth can run against hardened steel worms with favorable wear behavior.
- Thrust washers: Carry axial loads while allowing relative movement.
- バルブ部品： Take advantage of corrosion and wear resistance in fluid systems.
- Pump parts: Used where water or corrosive fluid exposure occurs.
- Wear plates: Provide replaceable wear surfaces between moving assemblies.
- ACME nuts: Bronze can provide a compatible sliding partner for steel lead screws.
- Seal rings: Used in rotating and fluid-handling assemblies where dimensional stability and wear resistance are important.
What Factors Affect the Cost of CNC Machined Tin Bronze Parts?
Tin bronze is usually more expensive than carbon steel and many common aluminum alloys because copper and tin are relatively costly raw materials.
However, raw material price is only one part of CNC component cost.
Bronze Grade
C93200 is substantially easier to machine than C90700. If the application does not require the mechanical advantages of a high-tin grade, specifying an unnecessarily difficult alloy can increase machining cost.
材料形状
Using hollow centrifugal-cast stock for a large bushing can significantly reduce material removal compared with machining the same component from a solid bar.
Wall Thickness
Thin-wall bronze bushings require controlled clamping and machining strategies because they can deform during turning or inspection.
Tight Bore Tolerances
Bearing fits often require close control of diameter, roundness and cylindricity. These requirements increase finishing and inspection time.
表面仕上げ
Very low roughness requirements may require additional finishing passes or secondary operations.
数量
For production quantities, dedicated soft jaws, mandrels or other fixtures can reduce handling time and improve repeatability.
How Should You Specify Tin Bronze on a Drawing?
A purchasing drawing should avoid simply stating “bronze.”
Where relevant, specify:
- Exact UNS or equivalent alloy grade
- Material standard
- Casting or stock form if important
- Required material certificate
- Critical dimensions
- Bore and shaft fit
- 表面粗さ
- Concentricity or runout requirements
- 検査要件
- Restrictions on porosity or casting defects
- Required surface treatment
For a bushing, the drawing should also distinguish between the free-state dimensions and the required installed condition if press fitting will alter the bore.
Common Questions About Tin Bronze
Is Tin Bronze Stronger Than Brass?
For many comparable engineering applications, tin bronze offers greater hardness and wear resistance than common brass alloys. Exact strength still depends on grade and product condition.
Does Tin Bronze Rust?
No. Rust specifically refers to iron oxide, and tin bronze does not rely on iron as its base metal. It can still oxidize, tarnish and corrode under unsuitable environmental conditions.
Why Does Tin Bronze Turn Green?
Copper-containing alloys can form green corrosion products or patina when exposed to moisture and environmental contaminants. A stable patina is not automatically the same as destructive chloride-related corrosion.
Is C93200 Better Than C90700?
Neither is universally better. C93200 provides much easier machining and excellent general bearing performance. C90700 provides higher tin content and is suited to demanding load and wear applications where proper alignment and lubrication are available.
Can Tin Bronze Run Against Steel?
Yes. This is one of its common applications. However, shaft hardness, surface finish, lubrication, alignment and PV must all be considered.
Can Tin Bronze Be Used Without Lubrication?
Do not assume that a solid tin bronze bearing is maintenance-free or self-lubricating. Most conventional tin bronze bearing systems perform better with appropriate lubrication unless the bearing design has specifically been engineered for dry running.
Why Does My Bronze Look Porous After CNC Machining?
If pores appear below the original surface, the cause may originate from casting rather than CNC machining. Gas, shrinkage, feeding and melt-control problems can produce internal defects that become visible only after material is removed.
CNC Machining Tin Bronze at Tuofa CNC Germany
Choosing a bronze alloy should start with the actual operating requirements of the component rather than selecting the highest-strength material by default.
For a CNC-machined bushing, bearing, gear or wear component, Tuofa CNC Germany can review the requested material together with load conditions, mating geometry, machining tolerances, wall thickness, surface finish and assembly requirements.
This DFM process is particularly useful for tin bronze because alloy selection affects both service performance and machining cost. A C90700 component may provide excellent wear performance but require more machining effort, while C93200 may offer a more economical option where its bearing properties are sufficient.
Tuofa CNC Germany supports CNC turning, CNC milling, drilling, boring, threading and secondary finishing for custom metal components. For bronze projects, customers can provide a 2D drawing and 3D CAD model together with the required bronze grade, quantity, tolerance and operating requirements.
If the alloy has not yet been finalized, identifying the application—such as a bushing, gear, ACME nut, valve component or marine wear part—allows the material choice to be reviewed before production begins.
結論
Tin bronze is not a single universal bronze grade. It is a family of copper-tin alloys whose properties change with tin content and additional alloying elements.
C90500 provides a balanced combination of strength, wear resistance and corrosion resistance. C90700 contains more tin and is suited to demanding load and wear conditions but is more difficult to machine. C93200, although technically a high-leaded tin bronze, remains an important comparison because its high machinability and bearing performance make it one of the most common choices for CNC-machined bushings.
For engineering applications, material selection should therefore consider not only hardness and tensile strength, but also PV, lubrication, shaft alignment, machining requirements, casting quality, corrosion environment and total manufacturing cost.
When these factors are evaluated together, tin bronze can provide long service life in gears, bearings, bushings, pumps, valves and other demanding CNC-machined components.