General Questions
Q: Why can’t I just use parts straight from the CNC machine?
A: While possible for some non-critical internal components, as-machined parts have sharp edges (burrs) that are safety hazards, visible tool marks that may be aesthetically unacceptable, and surfaces more prone to corrosion and wear. Finishing improves safety, performance, longevity, and appearance.
Q: How do I specify the surface finish I want on a technical drawing?
A> Use standard surface roughness symbols (the checkmark symbol) to call out Ra (average roughness) or Rz (maximum height) values for machined surfaces. For specific processes like anodizing or powder coating, call out the standard (e.g., MIL-A-8625 for anodizing) or a proprietary name and color code (e.g., RAL 9003). Always include notes for critical non-visual requirements like “Passivate per ASTM A967.”
Q: Does surface finishing affect part dimensions?
A> Yes, most finishing processes add or remove a small amount of material. Coatings like powder coating and anodizing add thickness. Processes like electropolishing and brushing remove material. It is crucial to discuss this with your supplier during design. Critical dimensions should be specified as “after finish” or include a tolerance that accounts for the finishing layer’s thickness.
Process-Specific Questions
Q: What’s the difference between Type II and Type III anodizing?
A> Type II (standard) anodizing creates a coating typically 0.0001″ to 0.001″ thick. It offers good corrosion resistance and is fully dyeable for color. Type III (hardcoat) anodizing is much thicker (0.002″ and above), significantly harder (comparable to case-hardened steel), and offers superior wear and corrosion resistance. It can be dyed, but colors are often limited to darker shades like black, dark green, or dark gray due to the thickness.
Q: When should I choose powder coating over paint?
A> Powder coating is generally more durable, environmentally friendly (no solvents), and efficient for medium to high volumes. It creates a thicker, more consistent coating that is highly resistant to chipping, scratching, and chemicals. Liquid paint may be preferable for very low volumes, complex assemblies where “wrapping” coverage isn’t needed, or when a specific custom color effect is required that is difficult to match in powder.
Q: Is passivation the same as electropolishing for stainless steel?
A> No. They are complementary but different. Passivation is a purely chemical process that removes contaminants and enhances the existing chromium oxide layer; it does not significantly alter the surface texture or remove material. Electropolishing is an electrochemical process that removes a thin layer of surface material, smoothing, deburring, and polishing the part while also improving corrosion resistance. Often, parts are electropolished first to clean and smooth, then passivated to maximize the corrosion-resistant layer.
Q: What is bead blasting actually doing to my part?
A> Bead blasting propels small, spherical media (often glass beads) at the part surface at high pressure. This impacts the surface, creating a uniform, matte texture by peening over microscopic peaks. It effectively removes light burrs, masks tool marks, cleans the surface, and provides a consistent base for subsequent coatings. It does not provide significant corrosion protection on its own.
Material & Application Questions
Q: Can aluminum be powder coated?
A> Yes, aluminum is an excellent substrate for powder coating. Proper surface preparation, which often includes a chromate or non-chromate conversion coating, is critical to ensure adhesion and prevent corrosion under the film (filiform corrosion).
Q: What is the best corrosion-resistant finish for steel?
A> There is no single “best” answer as it depends on the environment and part function. For high performance, electroless nickel plating offers excellent, uniform protection. For cost-effective protection, zinc plating with a chromate seal is very common. Powder coating provides a thick, barrier-layer protection. For components that must retain precise dimensions, black oxide or passivation (for stainless) offer mild protection with no measurable buildup.
Q: How do I achieve a mirror finish on a CNC part?
A> A true mirror polish is achieved through a multi-step mechanical polishing process, often starting with progressively finer abrasive sanding (e.g., using Scotch-Brite wheels or sandpaper) and culminating with buffing compounds on cloth wheels. For stainless steel, electropolishing can also produce a bright, smooth, near-mirror finish. These processes are labor-intensive and add significant cost.
Cost & Logistics Questions
Q: Which finishes add the most to part cost and lead time?
A> Specialized, multi-step, or labor-intensive processes add the most. Mirror polishing, hardcoat anodizing (Type III), and electropolishing are typically higher cost. Standard deburring or bead blasting add minimal cost. Processes requiring external vendors (like some plating) can add shipping and queue time to the lead time. Always request a quote that includes the finishing step.
Q: Can I get multiple finishes on a single part?
A> Yes, but it requires careful planning and masking. For example, you might specify an electrical contact area to remain uncoated (masked off) while the rest of the part is anodized. Or, a part could be bead blasted overall, with a specific logo area polished. Masking adds to the complexity and cost, so discuss this early in the design phase.
Q: How do I ensure color consistency across production batches?
A> For colored finishes like anodizing or powder coating, always specify using an industry-standard color system such as Pantone (PMS), RAL, or a physical color chip from the supplier. Understand that anodized colors can vary slightly with alloy composition and batch conditions. Powder coating is generally more consistent. The best practice is to approve a physical sample from your first production run to use as a master for future orders.