Free-Cutting Brass for Micro Turned Parts: Grades, Lead Rules and Plating Allowances

By the Micropartz editorial team · Published November 2, 2026

If a micro turned part can be brass, make it brass. No other common metal cuts as easily at small scale, holds a tolerance as reliably or leaves as little burr. That is why connector pins, watch screws, instrument fittings and fluidic nozzles are so often brass, and why the guide to materials for micro machining uses brass as the machinability reference. This article covers the grades, the lead question, plating allowances and the points that cause trouble.

Why free-cutting brass cuts so well

Free-cutting brass is an alloy of copper and zinc with about 2.5 to 3.7 percent lead. The lead sits in tiny globules between the grains. When the tool pushes the metal, chips break at those globules into short pieces, and the lead acts as a lubricant at the cutting edge. The result is low cutting force, little heat and long tool life.

For micro turning on a Swiss-type lathe, this matters in three ways. Tiny chips do not wrap around a 0.3 mm tool. Burrs are small and often removed by light tumbling. The part can run at high speed with a fine, consistent surface. See the guide to Swiss-type turning for micro parts for how the machine handles the bar.

Grade equivalents

Common name	UNS	EN designation	Lead content, approx	Notes
Free-cutting brass	C36000	CuZn36Pb3, CW603N	2.5 to 3.7 percent	Reference for machinability, ASTM B16
Free-cutting brass, higher zinc	C38500	CuZn39Pb3, CW614N	2.5 to 3.5 percent	Common in Europe, good for hot forging
Lead-free silicon brass	C69300 and similar	CuZn21Si3P, CW724R	Under 0.1 percent	Machines well, less than C36000
Naval and other special brasses	Various	Various	Low	Used for corrosion, not for machinability

Typical grade families for orientation. Check the exact standard and certificate.

Equivalence is approximate. Standards differ in composition limits, so state the grade and the standard on the drawing, not “brass”.

The lead question

Lead in brass is the main reason brass parts get rejected from a project. Three situations matter.

1. RoHS and electronics. RoHS has had an exemption allowing lead in copper alloys up to 4 percent by weight, but exemptions get reviewed and customers may demand lead-free regardless.
2. Drinking water and food contact. Rules in several regions limit lead in wetted parts, which means lead-free or low-lead brass for fittings and nozzles in those markets.
3. Customer policy. Some medical and consumer brands specify lead-free materials for all parts.

Lead-free brasses cost more to machine because chips do not break as freely, tools wear faster and cycle time rises. Silicon brasses give acceptable results but need tuned speeds and chip breakers. Ask the shop about the actual cost difference before changing the alloy on the drawing.

Lead-free brass in practice

Moving from C36000 to a lead-free brass is more than a material swap. Silicon brasses and bismuth brasses break chips by different means, and the chips tend to be longer and harder than leaded ones. Shops often find that speeds and feeds drop, tools wear faster and cycle time rises, sometimes by 20 to 40 percent, though results vary by alloy and part shape. Some lead-free grades are also harder, which changes burr behavior and tapping.

Practical advice: run a trial lot before committing a program, check thread and bore quality, and ask for both prices. Mind the supply side as well, because lead-free bar in ground form and small diameters is less widely stocked than C36000, and lead times can be longer.

Typical tolerances and finishes

Feature	Typical tolerance	Notes
Turned diameter	±0.005 mm	±0.002 mm on ground bar with care
Length	±0.01 to ±0.02 mm	Depends on cut-off method
Hole diameter	±0.005 to ±0.01 mm	Drilled and reamed
Concentricity	0.01 mm	Features turned in one clamping
Surface finish	Ra 0.4 to 0.8 μm	Finer with finishing passes

Typical values for orientation. Confirm with the shop.

Brass holds tolerance well because it does not work harden or distort much and the temperature effect is modest. Brass expands about 19 μm per meter per kelvin, a little more than steel, so measure at 20 °C as with any micro part.

Plating allowances

Many brass micro parts are plated. Nickel is a common underlayer for corrosion and wear. Gold, tin or silver go on contact surfaces. Every layer adds thickness on all exposed surfaces, so a diameter grows by twice the layer thickness.

Layer	Typical thickness	Effect on diameter
Nickel underplate	2 to 5 μm per side	+4 to 10 μm
Gold flash	0.05 to 0.2 μm per side	+0.1 to 0.4 μm
Hard gold	0.5 to 1.5 μm per side	+1 to 3 μm
Tin	2 to 8 μm per side	+4 to 16 μm

Typical values for orientation. The plating specification sets the real ones.

Plating also fills small holes, rounds sharp edges and builds up at corners. Parts with threads or tight holes need allowance both in the machined size and in the plating specification. Say whether the tolerance applies before or after plating.

Season cracking and dezincification

Two long-term issues deserve a mention.

Season cracking. Cold-worked brass under residual tension can crack when exposed to ammonia, amines or some cleaners. Stress relief annealing after machining lowers the risk. Brass drawn bar and machined parts carry stress, so shops add a low-temperature anneal for critical parts.

Dezincification. In some waters, zinc leaches from brass and leaves a weak copper structure. Dezincification-resistant brasses, labeled DZR or CR, exist for water service. If your part sees hot or aggressive water, say so.

Hypothetical example

A brass contact pin of ⌀0.8 mm and 6 mm length is to be nickel plated at 3 to 5 μm and gold plated at 0.5 μm. The drawing shows ⌀0.800 ±0.005 mm after plating.

Plating adds about 7 to 11 μm to the diameter, so the machined size must target about ⌀0.791 mm with a tight band, or the plated part will exceed the limit. A shop might propose turning at ⌀0.791 ±0.002 mm from ground C36000 bar, then plating, then checking a sample. That works, but only if the drawing states that the tolerance is after plating. If it does not, the part is rejected either at the shop or at the customer, and both blame the drawing.

What to put in the RFQ

1. Alloy and standard. For example C36000 to ASTM B16, or CW614N, or a named lead-free grade.
2. Lead and compliance. RoHS, drinking water or customer rules that apply.
3. Bar form and temper. Ground bar for Swiss-type, with diameter tolerance.
4. Plating. Layers, thickness and whether tolerances are before or after.
5. Stress relief. Needed or not, especially if exposed to ammonia.
6. Surface and burr limits. Values and inspection method.

The tolerance and materials reference lists typical figures. When the drawing is clear, send an RFQ. Micropartz matches it with up to 3 shops that turn brass at micro scale and replies within 24 business hours.

Frequently asked questions

Reviewed against

• Wikipedia: Brass
• ASTM B16, Free-Cutting Brass Rod, Bar and Shapes for Use in Screw Machines

Numbers in this guide are typical values for orientation. The drawing, the material certificate and the matched shop set the real ones.

Keep reading

• Materials for Micro Machining: Stainless, Titanium, Brass, PEEK and More
• Swiss-Type Turning for Micro Parts: When a Sliding-Head Lathe Is the Right Call
• What Micro Machined Parts Cost and Why Inspection Is Half of It