Guides / Materials
Brass, Bronze or Copper: Which One Your Part Actually Needs
September 21, 2026
Copper alloys come in three families that look similar on a drawing and behave nothing alike on a machine. Picking the wrong one is expensive in a way that does not show up until the quote comes back.
The short version
Brass is copper and zinc. Cheapest of the three to machine, by a wide margin. The default for fittings, valves, connectors and hardware.
Bronze is copper and tin, or copper and aluminium. Chosen for bearing behaviour, wear under load and seawater. Harder and slower to cut.
Copper is copper. Chosen when electrical or thermal conductivity is a number on the drawing. Difficult to machine despite being soft.
If you take nothing else: the question is not which is best, it is which property is on your drawing.
Machinability, the number that decides the price
Everything is rated against free-cutting brass C360 at 100.
| Material | Machinability | Why |
|---|---|---|
| C360 free-cutting brass | 100 | Lead particles break the chip and lubricate the edge |
| C385 architectural brass | 90 | Same mechanism, more zinc |
| C69300 silicon brass, lead-free | 80 | Silicon does the job instead of lead |
| C464 naval brass | 30 | Tin addition, long chips |
| C260 cartridge brass | 30 | Made for forming, not cutting |
| Pure copper, C101 and C110 | 20 | Does not break its chip at all, conducts heat into the tool |
Published typical values for the alloy. They describe the material, not a promise from any shop.
Bronzes span a wide range depending on the family. Leaded tin bronzes machine reasonably. Aluminium bronzes are tough, abrasive and slow, closer to machining a stainless than a brass.
The practical read: there is a five times spread in cutting cost inside the copper alloys. That is larger than the spread in bar price for most of them, so the alloy choice moves the quote through machine time rather than through metal.
Brass: the default and why
Copper plus zinc, usually with lead added for machinability. C360 is 61.5 percent copper, 35.5 percent zinc, 3 percent lead.
Pick it for: anything threaded, anything made in volume, fittings, valve bodies, connectors, hardware, instrument parts.
Watch out for: dezincification in hot water and seawater, lead limits in drinking water and electrical equipment, and stress corrosion cracking if the part is cold worked and lives near ammonia.
The lead question has become the defining one. See leaded and lead-free brass compared and which brass alloy to specify.
Bronze: for load and seawater
Copper plus tin, or copper plus aluminium, sometimes with lead or nickel.
Pick it for: plain bearings and bushings, worm gears, valve trim in seawater, marine propellers and pump components, anything sliding under load.
The reason is behaviour rather than raw numbers. Tin bronzes are forgiving as a bearing surface. Aluminium bronzes hold up in seawater where brass dezincifies.
Watch out for: cost, machining time and the naming. Architectural bronze C385 is a brass. Commercial bronze C220 is a brass. Read the UNS number.
Copper: only when conductivity is the requirement
Pick it for: busbars, heat sinks, RF and waveguide parts, welding and EDM electrodes, anything where a conductivity or thermal number appears on the print.
Watch out for: the machining. Pure copper is soft and still difficult. It smears instead of cutting, burrs on every exit edge, and pulls heat out of the cutting edge instead of letting it leave with the chip. Machinability near 20.
There is a free-machining copper family, C145 tellurium copper and C147 sulphur copper, that reaches machinability near 85 while staying above 90 percent IACS conductivity. If your part needs conductivity and has threads, that family is usually the answer rather than pure copper.
The question that decides it
Work down this list and stop at the first yes.
- Is there an electrical or thermal number on the drawing? Copper, or a free-machining copper if there are threads.
- Is it a bearing, a wear surface or a gear tooth? Bronze.
- Does it live in seawater under load? Bronze, or naval brass for lighter duty.
- Does it touch drinking water? Lead-free brass. See NSF/ANSI 61 and 372.
- Is it hot water or a dezincification risk? Silicon brass or a DZR grade. See dezincification resistant brass.
- Otherwise: brass, and the question becomes which brass.
Most parts land on line 6. That is why brass dominates machined copper alloy work.
The substitution that is worth checking
A surprising number of drawings specify bronze because a previous drawing did, not because anyone tested the requirement.
If the bronze on your print is not doing bearing work, not in seawater and not carrying a specific corrosion requirement, ask whether a brass would do. The machining time alone often moves the price enough to matter, and on a threaded part the difference is large.
The reverse substitution is rarer and more dangerous. Putting brass where bronze was doing real bearing or seawater work fails in service rather than at inspection.
Not sure which family your part belongs in? Send the drawing and describe what the part has to survive. The environment picks the alloy more often than the geometry does.
Frequently asked questions
Is bronze stronger than brass?
Usually yes, and that is not the reason to pick it. Bronzes are chosen for bearing behaviour, seawater corrosion resistance and wear under load. Brass is chosen for machinability and cost. If you only need strength, a tin bronze is an expensive and slow way to get it compared with picking a stronger brass such as C69300 silicon brass.
Why is copper so much harder to machine than brass?
Pure copper is ductile and does not break its chip, so it comes off as a long ribbon that wraps the tool. It also conducts heat away from the cut into the part instead of letting it leave with the chip. Machinability sits near 20 against free-cutting brass at 100. The lead in C360 acts as a chip breaker, which is the whole difference.
Can I substitute brass for bronze to save money?
Sometimes, and check the failure mode first. If the bronze was specified for a bearing surface or for seawater, brass will not do the same job. If it was specified out of habit for a dry structural part, a brass often works and machines several times faster. Ask why the bronze is there before you change it.
What about naval brass, is that a bronze?
No. C464 naval brass is a brass with about 1 percent tin added for dezincification and seawater resistance. Architectural bronze C385 is also a brass despite the name. Copper alloy naming is historical rather than logical, so read the UNS number rather than the name.
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