Guides / Process

Finishing a Machined Brass Part: Tumbling to Chrome

September 21, 2026

On a visible or plated brass part, the finishing routinely costs more than the machining. Buyers who only look at the cycle time are looking at the smaller half of the quote.

The options, cheapest first

As machined. The surface straight off the tool. On free-cutting brass this is already bright and clean, which is one of the reasons brass is popular. Cost: nothing. Still specify the edge condition, because “as machined” does not mean “burr free”.

Deburred. Edges broken, burrs removed. Can be a machine operation if the edges were designed with chamfers, or hand work if they were not. Designing the chamfers in is far cheaper than filing them off later.

Vibratory tumbling. Parts go into a bowl or tub with ceramic or plastic media and run for hours. Removes light burrs, rounds sharp edges, produces an even satin surface. Cheap per part because it is a batch process. The standard answer for volume fittings.

Polishing and buffing. Progressively finer abrasives and then a buffing wheel to a mirror. Largely manual, which makes it slow and expensive per part, and difficult to keep consistent across a batch. Specify it only where the part is seen.

Lacquer or clear coat. A thin organic film that stops the brass tarnishing. Cheap, and it wears through on handled surfaces.

Electroplating. Nickel, chrome, tin, silver or gold over a nickel underlayer. Adds appearance, wear resistance, solderability or contact properties depending on the metal.

Passivation and cleaning. Not a finish in itself, but a required step before plating and often before packing on a part that must be free of machining oil.

What plating actually does to your drawing

This is the part that surprises people.

Plating does not hide anything. It is a thin conformal layer that follows the surface underneath. A tool mark under bright nickel is a tool mark with a shine on it. On a bright finish the defects are arguably more visible than they were before.

So a part going to bright plate needs a better machined surface than the same part going out as machined. That is a real cost, and it is created by a note in the finish block rather than by anything in the dimensional drawing.

Plating has thickness. Typically a few to tens of micrometres, and it builds up on external corners and thins in internal ones. On a close fitting part or a thread, that thickness has to be allowed for before plating. Say on the drawing whether dimensions apply before or after plating. Threads in particular need this, because a thread plated after gauging may no longer gauge.

Plating needs a plan for the wetted or masked areas. If part of the surface must stay unplated, masking is a manual operation and it is priced accordingly.

Nickel and chrome, the two you will meet most

Nickel. Hard, corrosion resistant, mildly yellow-white. Used both as a finish and as the underlayer for chrome. Electroless nickel gives very even thickness including inside bores, which matters on a fitting with a long internal passage.

Chrome. Almost always over nickel. Very hard, bright blue-white, the finish people picture on a tap. The chrome layer is thin, so nearly all the corrosion protection comes from the nickel underneath it. That is why “chrome plated” without a nickel thickness spec is an incomplete instruction.

A typical decorative sanitary spec looks like a nickel thickness in the region of 10 to 20 micrometres with a thin decorative chrome over it, but the exact numbers belong to your product standard, not to a general article.

Writing the finish block

Vague finish notes are the most common reason a brass quote comes back with a disclaimer.

Weak:

Finish: polished and chrome plated

Better:

1. Deburr all edges, break sharp corners 0.2 to 0.4 mm.
2. Bores to be free of burrs at all cross hole intersections.
3. Surface Ra 0.8 max on faces A and B, Ra 3.2 elsewhere.
4. Nickel plate to [your spec], then decorative chrome.
5. Dimensions apply after plating.
6. Mask thread at feature C, no plating permitted.

Six lines, and every one of them removes an assumption a shop would otherwise have to make and price defensively.

Note item 3. Specifying a fine finish on two faces instead of the whole part is one of the cheapest savings available on any brass drawing. See tolerances and surface finish on brass parts.

The lead time question

Ask this early, because it changes your schedule more than your budget.

Most machine shops do not plate in house. Your parts get packed, shipped to a plating house, queued, processed in a batch and returned. Two weeks of calendar time is normal even where the plating itself takes an afternoon.

Shops that tumble and polish in house but outsource plating are the common case. A shop that plates in house is worth knowing about if your lead times are tight.

If you need plating, say so in the RFQ rather than adding it after the quote. It affects which shops can sensibly take the job.

One material caution

Polishing and buffing are cold working operations on the surface, and bright dipping and some pre-plating chemistries are aggressive. On a yellow brass part that will live near ammonia or amines, heavy surface cold work adds to the residual stress picture. It is a small effect next to forming or crimping, but if the part is already marginal it is worth raising. See season cracking in brass parts.

Have a brass part with a finish spec? Send the drawing and put the plating requirement in the finish field. It narrows the shop list usefully rather than being an afterthought.

Frequently asked questions

Keep reading

• Brass CNC Machining: Why It Is Cheap, and Where the Cost Hides
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