Should a turned part be C360 brass or 303 stainless steel?

Which machines faster, C360 brass or 303 stainless?

C360 brass machines faster, and by a wide margin. It is the reference alloy of the copper industry’s machinability scale, while 303 is the easiest-cutting common austenitic stainless but still behaves like stainless on the lathe.

The Copper Development Association rates C36000 at 100 and scores every other copper alloy against it (CDA, C36000). Lead that does not dissolve in the brass sits in the structure as fine particles, so chips break off short and tool edges stay cool. In practice that means high spindle speeds, long tool life, clean threads and burrs that are easy to remove.

Type 303 gets its free-cutting behavior differently. Sulfur combines with manganese to form sulfide inclusions that act as chip breakers. The British Stainless Steel Association places 303 at an index of 0.85 against free-machining low-alloy steel at 1.0, with standard 304 at 0.52 (BSSA, machinability by steel family). Note that this is a different scale from the copper one, so the two numbers cannot be compared directly. What matters is the direction: 303 cuts far better than 304, but it still work hardens, so it needs sharp tools, positive feeds and a cut that never rubs.

For a buyer, the machining difference shows up as cycle time and tool cost on the quote. The same geometry typically takes longer to turn in 303 than in C360, and features such as small deep holes, fine threads and thin walls are typically easier to hold in brass. Material price is a separate driver. Copper and nickel are both traded metals, so bar prices for either alloy move with commodity markets; ask for a quote in both materials when the choice is still open.

Which one resists corrosion better in service?

It depends on the environment rather than on a single ranking. Brass handles many fresh-water, air and fuel applications well, while 303 gives the look and general corrosion resistance of stainless but is the weakest of the common austenitic grades in chloride service.

The sulfide inclusions that help 303 cut are also its weak point. Where they break the surface, they act as starting points for pitting, which is why the BSSA advises that 303 should not be considered for seawater service (BSSA, 316, 304 and 303 in seawater). Compared with 304, the BSSA also lists lower resistance to stress corrosion cracking and corrosion fatigue, lower ductility and toughness, and a nickel-release result that matters for parts in skin contact (BSSA, free-machining stainless grades). If a part sees salt spray, pool water or cleaning chemicals with chlorides, 304 or 316 is often the better stainless even though it cuts more slowly.

Brass fails in a different way. In some waters, zinc leaches out of the alloy and leaves weak, porous copper behind, a process called dezincification. Where that is a risk, specify a dezincification-resistant or low-lead alloy instead of C36000, such as C69300, which the CDA lists with a machinability rating of 85 (CDA, C69300). Brass also tarnishes, which is harmless to function but matters on visible parts; plating or clear coating takes care of it.

Stainless parts should be passivated after machining to remove free iron picked up from tools and to restore the passive surface. Per ASTM A967/A967M, passivation treatments are chosen by grade, and the standard includes tests that confirm the treatment worked (ASTM A967/A967M). Write the treatment and the verification test on the drawing rather than “passivate” alone.

What finishing does each alloy need after turning?

Brass is often plated for appearance or tarnish resistance, commonly with nickel, and it plates readily. Stainless is usually left bare and passivated, because plating would hide the property you chose it for.

Brass also solders and brazes well, per its CDA fabrication ratings, so it suits parts that are joined to copper tube or wire. Fusion welding is not recommended for C36000. Stainless 303 is weldable only with care, and the BSSA describes its weldability as more limited than that of 304, so if a turned part is later welded into an assembly, 304 or 304L bar may be the safer call.

Surface finish itself is rarely the deciding factor. Both alloys turn to a clean surface, and brass typically leaves fewer burrs at thread starts and cross holes. For the finish options on turned parts, see surface finishing and the guide comparing anodizing, powder coating and zinc plating, which explains how to call out a finish on a drawing.

What should the drawing and RFQ say about the material?

Name the alloy by UNS number, the bar specification with its edition, and the temper or condition, and ask for a mill test report that shows the heat number. A note like “brass” or “SS” lets a supplier substitute a different alloy that may behave very differently in service.

A complete material note reads like “UNS C36000 per ASTM B16/B16M, half-hard” or “UNS S30300 per ASTM A582/A582M, condition A”, followed by the finish. If the part is regulated, add the regulation, such as the lead-free requirement, so that the supplier checks the bar chemistry against it rather than against the base specification alone. List critical diameters and threads separately; turned tolerances for either alloy are reviewed feature by feature on CNC turning before the quote.

Sourcing from Vietnam or elsewhere adds one more reason to be specific. Bar stock for an overseas order may come from a different mill or country than your domestic stock, so the specification on the drawing is what keeps the chemistry identical from lot to lot. Ask the supplier to state the bar specification on the quote, keep the mill reports with each shipment, and see the RFQ package guide and the tolerance guide for the rest of the drawing. When it is ready, send the drawing for a quote, in both alloys if the choice is still open.