Surface Treatment for CNC Prototype Sheet Metal Parts

• 1
  Why a Finish Changes CNC Prototype Sheet Metal Parts
• 2
  As-Machined and Ra: The Numbers Behind the Surface
• 3
  Choosing a Finish for CNC Prototype Sheet Metal Parts
• 4
  Functional Coatings: Powder, Black Oxide, Nickel, Chromate
• 5
  Anodizing Aluminum Prototype Parts: Type II vs Type III
• 6
  Finish Options for CNC Prototype Sheet Metal Parts

Sheet metal and machined prototypes leave the shop with a surface that reflects how they were made. Sheared edges carry a roll-over burr. Laser-cut faces show a fine oxide line. Milled faces show tool marks in the direction of travel.

A finish either preserves that surface, removes it, or adds a new layer on top. Each route moves the part in a different direction: some add 5 to 25 μm of thickness, some etch away 2 to 10 μm, some only change the topography.

That matters for CNC prototype sheet metal parts because prototypes are usually fit-checked against other components. A 0.02 mm growth per face is harmless on a cover plate. On a mating flange with a 0.05 mm clearance, it can turn an easy assembly into a press fit.

The right question is not which finish looks best. It is which finish keeps the dimension, the corrosion budget, and the appearance target inside the limits the design already set.

• 1
  Additive finishesAnodizing, plating, powder coating. They grow the part.
• 2
  Subtractive finishesEtching, electropolishing, heavy blasting. They remove material.
• 3
  Topography-only finishesBrushing, light blasting, tumbling. Thickness stays close to nominal.

Ra is the arithmetic mean of the surface profile deviation, measured in micrometers. Lower Ra means a smoother surface. It says nothing about waviness, scratches, or color, so treat it as one axis of the specification, not the whole story.

As-machined finishes on our parts typically land at Ra 1.6–3.2 μm. That is the baseline for a prototype that will be painted, powder coated, or hidden inside an assembly. Tool marks are visible under raking light.

Smooth machining pushes the same surface to Ra 0.8–1.6 μm by reducing feed per tooth and using a finishing cutter. Fine machining reaches Ra 0.2–0.8 μm, which needs a sharp tool, a rigid setup, and a lighter step-over. It costs more time and is harder to hold on thin sheet.

A practical trap: specifying Ra 0.2 μm on a 1.5 mm aluminum panel. The panel will deflect under the finishing pass, and the measured Ra will vary across the face. If the function only needs a smooth feel, Ra 0.8–1.6 μm is usually enough.

Start with the material. Aluminum takes anodizing well and can be colored in the same bath. Carbon steel takes black oxide, zinc, or powder coating. Stainless steel usually needs only passivation or a brushed texture, because its own oxide layer already resists corrosion.

Then ask what the part touches. A housing that sits indoors needs appearance only. A bracket bolted to a vehicle chassis needs salt spray resistance. A sliding cover needs wear resistance and a low-friction surface.

Thickness tolerance is the third filter. Anodizing Type II adds roughly 5 to 15 μm per surface, Type III hardcoat adds 25 to 50 μm. Powder coating can add 60 to 120 μm. On a tight-tolerance prototype, mask the mating faces or choose a topography-only finish.

Finally, check conductivity. Anodizing is an insulator. Chromate conversion keeps conductivity, which is why it is common on EMI housings and grounding plates.

• 1
  Appearance firstAnodizing, powder coating, brushing, polishing.
• 2
  Corrosion firstAnodizing Type III, electroless nickel, zinc plating.
• 3
  Tolerance firstBead blasting, brushing, tumbling, or mask the critical faces.

Powder coating sprays a dry polymer onto a grounded part, then cures it in an oven at roughly 180 to 200 °C. The result is a thick, tough layer that hides minor surface defects and comes in many colors and gloss levels. It is a poor choice for tight tolerances and for parts that cannot take oven heat.

Black oxide converts the steel surface into a thin magnetite layer, usually under 2 μm. It does not add meaningful thickness and it holds oil well. On its own it gives limited corrosion protection, so it is often paired with a light oil or wax.

Electroless nickel deposits without electric current, so the coating follows the part uniformly, including recesses and blind holes. That uniformity is why it appears on prototypes with complex geometry. It adds hardness and corrosion resistance but changes the color to a warm gray.

Chromate conversion, often called Alodine on aluminum, leaves a conductive film that still accepts paint. Use it when the part must ground through its own surface, or when a low-resistance path matters more than color depth.

Anodizing is not a coating. An electric current in an acid bath grows aluminum oxide out of the base metal. The oxide is part of the part, so it cannot flake off like paint. It is also an electrical insulator, which is a benefit on some housings and a problem on others.

Type II builds a clear or dyed oxide layer of roughly 5 to 15 μm. It is the standard choice for enclosures, panels, and brackets that need color and moderate wear resistance. Clear anodizing keeps the metal look. Dyed anodizing gives a repeatable brand color, though the shade shifts with alloy.

Type III, or hardcoat, builds 25 to 50 μm and reaches a much higher surface hardness. It suits sliding surfaces, wear plates, and parts that see abrasion. The trade-off is cost and a darker, less uniform appearance on some alloys.

On 6061 and 7075 the color response is predictable. On 5052 or cast alloys, the same dye bath can produce a noticeably different shade. If color matching matters, keep the alloy consistent across the whole prototype build.