Here is an article written by our engineering team that will guide you on how to make the most out of your design experience. We hope to give you a succinct yet complete depiction of the different kinds of surface finishes that you can design your parts to have. This would help you optimize your design and assemblies to ensure that they can perform the intended role best!

Importance of surface finishes

Surface finishes are generally depicted on the drawings and are essential in describing key features of the parts that the designer envisions. These can be coated surfaces that need to be extremely hard and smooth, to provide a contact surface for motion or can be painted surfaces that are visually appealing.

Mechanical finishes

Surface finishes of metals can be compared by looking at their respective surface roughness values. The roughness of a surface refers to its perception to human touch and can be quantified using several different parameters. The most commonly used parameter is the Ra (arithmetic average of roughness over a range), which is measured in units of micrometers (μm). Under this parameter, the arithmetical mean deviation of the surface is measured. In general, a smaller Ra value indicates a smoother surface and vice-versa. However, contrary to the popular opinion, a smoother surface is not always advantageous, and the best surface roughness value is actually dependent on the final intended application(s) and cost optimization.

The surface finish of any part can be altered by a variety of mechanical finishing processes, each suitable for achieving different values of surface roughness. Some of these most commonly used processes are listed below:

• Sanding — The method of rubbing abrasive particles on the surface of a workpiece creating random, non-linear surface texture is known as sanding. Based on the use-case, different abrasive materials are used, either glued onto a backing paper or plate. These abrasive media are characterized by the size of the cutting particles, known as grit. In essence, the higher the grit number of the sanding tool, the smaller and finer the particles are, resulting in a finer and smoother surface finish. Occasionally, water or some other form of liquid lubricant is used to wash off the removed surface material and expose a fresh cutting surface. Sanding is most useful when applied to curved or contoured surfaces. It is, however, not very efficient when it comes to tight corners or pockets.

Using electric sander on metal surface [Source: The Tool Geeks]

• Ultrasonic Polishing — Ultrasonic polishing is used mostly for internal finishing of tight corners, holes and pockets that are otherwise quite difficult to access. In this method, a soft, fine tipped tool mounted onto an ultrasonic spindle vibrates at 30 kHz which, in combination with an abrasive slurry, induces a pressure wave that safely works away at the surface to create a fine polish. This method is effective as the tip of the tool does not even touch the work surface and this technique works even on hardened steel, with very little chance of causing any damage to the workpiece.
• Rumbling and Tumbling — Rumbling and tumbling, similar to sanding, also use abrasive particles to alter the surface roughness of the workpiece(s). The major difference is that, unlike sanding where the abrasive particles are attached to a backing paper, these particles are loose and not attached to anything during rumbling and tumbling. Depending on the workpiece and the desired surface finish, various different types of abrasive materials are used including walnut shells, stones, garnets, or coarse sand. In tumbling, the parts are put into a tub or a box along with the abrasive particles and the tub is then rotated to mix together everything in a random fashion. On the other hand, in rumbling the parts and the abrasive medium are placed in a trough and vibrated to increase the cutting action of the particles. Both these methods are also used to remove any sharp edges left on the part’s edges after machining, otherwise known as de-burring.
• Magnetic Polishing — In order to reduce the frictional resistance experienced by air or liquid passing through them, parts with deep channels, holes or large internal chambers need to be highly polished to have a smooth surface. As these areas are quite difficult or impossible to reach by hand, metallic parts can instead be put in a box like chamber, filled with magnetized particles. Using a focused magnetic field, these particles can then be directed to abrade away against the interior surfaces to achieve the desired fine polish.
• Sandblasting — Sandblasting refers to treating a part’s surface by blasting it with abrasive particles under high pressure, which when used in combination with air and water, can cover a large surface area fairly quickly. Different levels of surface finish can be achieved by using varying types of abrasive media. In some cases, sandblasting can also improve the mechanical properties of metal by increasing its fatigue strength and improving corrosion resistance through shot-peening.

Sandblasting a metal part [Source: My Dreamality]

• Lapping — A very high degree of surface finish and flatness can be achieved using the precision technique known as lapping. This method involves slowly moving a soft iron tool in combination with a mild abrasive slurry in random and non-linear motions by hand, filling microscopic voids and flattening higher bumps.
• Filing — Filing is a versatile and easy to use method to shape metal quickly using handheld tools called files. A file is a very hard steel piece with parallel grooved cut into it with a combination of sharp cutting teeth. The surface finish from files can be quite rough and might require additional processes such as sanding or polishing to achieve the desired final surface finish.

Coatings for metals

For prototyping purposes, our customers commonly choose Aluminium or Steel as the material. While some processes are common with the two, there are certain differences between coating processes that can be used for each of these. Stainless steel is usually never coated but surface treated, this is because it is inherently protected from corrosion. Mild steel, which is cheaper and more commonly used needs to be coated as it is prone to corrosion. The kind of material (sheet metal/CNC milled part/turned part) does also affect the coatings and how they can be applied.

Aluminium:

• Anodization — Anodization essentially means developing an inert oxide coating around the part. This coating is extremely durable, porous and makes the surface resistant to corrosion. A variety of colors can be chosen to coat the part with, but the surface finish is always matte. There are 2 commonly used types of anodization:
  • Type II: This adds corrosion protection about 0.001” inch (0.02 mm — 0.03 mm) thick, thus the part dimensions don’t change by much. The user can choose from a large basket of colors for Type II anodization.
  • Type III: Hard anodization provides a thicker layer 0.006” inch (0.15 mm), but colors are limited to clear or black.

Anodized Aluminium (Type II) [Source: 3ERP]