Metal finishing is the final step in the manufacturing process used to provide aesthetics and environmental protection. It is also used to reduce surface roughness beyond the capabilities of machining operations for parts that must mate or seal. It also includes methods of metal cleaning, descaling, deburring, etc. In short, numerous types of metal finishing processes are used for a variety of purposes. This guide reviews some major finishing methods, as well as applications and considerations for choosing a metal finishing process. Some of the general advantages of metal finishing treatments include:
Increased durability
Improved decorative appeal
Enhanced electrical conductivity
Higher electrical resistance
Higher chemical resistance
Higher tarnish resistance
Types of Metal Finishing
Below are a few of the types of finishing processes available:
Plating
Metal plating uses chemical baths to coat or alter the surface of substrates with thin layers of metal such as zinc, nickel, cadmium, or chromium. The electroplating method generates an electric current to coat the substrate, while electroless plating employs an autocatalytic process in which the substrate catalyzes the reaction. As plating is a chemically intensive, large scale process, it is almost invariably performed by plating service providers who specialize in the field.
Metal plating can improve a product’s durability, corrosion resistance, surface friction, and exterior appearance. In the case of galvanized metal, the zinc plating serves as a sacrificial anode, itself corroding while protecting the integrity of the underlying substrate. However, plating machines are generally not suited for smoothing out surface defects.
Sherardizing is a process used to coat small steel parts such as bolts and washers with a corrosion-resistant zinc-iron alloy. Parts are tumbled in a heated drum containing zinc dust, producing a highly corrosion-resistant finish. In high-volume production runs, a barrel-finishing machine is a fast and efficient coating solution.
More information on plating may be found in this article on corrosion-resistant coatings.
Brushing/Cleaning
Unlike plating, metal finishing with brushes is an effective method for removing surface imperfections. These finishing machines create a uniform, parallel grain surface texture to smooth out a product’s exterior. An abrasive belt or wire brush is usually employed to achieve this effect. In addition, the singular direction of the belt or brush can create slightly rounded edges perpendicular to the grain. This type of finishing is used for aluminum panels that are used for metal photo prints and signage.
In addition, wire brushing is used to remove the slag produced by some welding operations and to remove scale and grit from metal surfaces prior to cleaning.
Chemical baths and acid baths are used to clean oil residue left over from the machining, forming, and other processes prior to plating or coating.
Polishing/Buffing/Honing/Lapping
These abrasive methods are used to reduce surface roughness in metals after machining. Roughness is measured in micro-inches with about the most exacting machining processes (reaming, broaching) producing surface finishes of 16-125 µ-in. Precision or fine grinding (see below) can reduce surface roughness to 8-16 µ-in. using abrasive wheels or mounted points. Beyond that, abrasive powders are used in combination with leather or felt wheels to polish (1-32 µ-in.) and buff (0.5-16 µ-in.) as required. Honing and lapping use abrasive compounds to improve the surface roughness of mating parts by rubbing them against each other (such as lapped engine valves) or running a mating shape through a hollow (such as a honed cylinder).
Metal polishing can be done either manually or via automatic robotic metal polishing. The process that reduces surface roughness also increases a metal’s luster as reflected light is scattered less from smoother surfaces. Thus, these same methods are often employed simply for improving surface aesthetics.
Grinding
Grinding machines use abrasive wheels to smooth out surfaces. There are several types of grinding machines designed to deliver different levels of finite smoothness. As noted above, grinding is used to reduce surface roughness left over from machining and as a final step in the machining process to close in on a tolerance. A surface grinder is the most common form of grinding machine but there are numerous specialty grinders such as centerless grinders and blanchard grinders in everyday use as well, with precision blanchard grinding being a widely-used metalworking technique ideal for oversized parts.
Numerous varieties of hand grinders are used for metal removal, slag cleaning, etc. Die grinders are employed for detailed work in passages of intake manifolds, dies, etc. These are commonly fitted with special shaped abrasive wheels known as mounted points.
Vibratory Finishing
Vibratory finishing machines are used to deburr products and remove sharp edges, sprue, etc. They position parts inside a drum filled with abrasive pellets and apply tumbling vibration to create a uniform random texture. The machine’s cycle speed and magnitude of vibration are usually variable, allowing effective treatment for a range of small- to large-sized parts.
Blasting
Blast machinery, such as sand-blasting machines, is typically employed in projects requiring a uniform matte texture. The sandblasting process (also known as bead blasting) forces sand, steel shots, metal pellets, or other abrasives onto a substrate at high speed. This results in a smooth, clean product texture, particularly in soft metals.
Shot peening is a blasting method used to impart compressive stress in the surface of metals as a way of improving fatigue resistance, stress corrosion cracking resistance, fretting resistance, etc. The cold working process imparts compressive stress to surfaces as a way of countering tensile stress induced during manufacturing. Burnishing is another cold-work surface modification method used to impart compressive stress for fatigue resistance.
Heat Treating
An important step in many metal manufacturing operations is heat treatment, which is used to procure desirable characteristics in the material after its finished shape is attained. For example, after gears are cut they are usually hardened through an inductive or flame heating process to increase wear resistance at the teeth surfaces while improving the strength of the underlying teeth. Hardening usually applies a quenching operation which is a critical step in transforming the grain structure of the metal. Case hardening is used on the shackles of padlocks, for example, to produce a shell that resists mechanical attack. Heat treating is applied too to remove any residual stresses left by manufacturing. Nitriding is another surface hardening process which can be done below the transformation temperature of steel, eliminating the need for quenching and any part distortion that could result.
Coating
Water- and oil-based paints are used to provide inexpensive protection to metals against corrosion. Special paints are available for harsh environments, such as anti-fouling paints for marine applications.
Powder coating applies a decorative finish that is similar to paint but with greater durability. The process involves UV or heat curing of dry thermoplastic or thermosetting polymer powder onto the metal to produce a textured, matte, or glossy coating. It is an electrostatic process wherein a charge placed on the substrate attracts the powder particles to it. A textured powder-coat finish is highly effective in hiding surface defects.
Aluminum is often anodized, a chemical surface treatment, to provide corrosion resistance, aesthetic effects, or, in the case of hard-coat anodizing, greater resistance to wear.
Hot Blackening
Hot blackening machines spread a thin layer of black oxide onto a product’s surface to create a matte black finish with high abrasion resistance. It is a high-temperature hot finishing process in which the product is inserted into a series of tanks containing cleaners, caustics, and coolants. Hot blackening is most commonly used in the production of automotive parts, tools, and firearms. Bluing is a subset of black oxide coating that is used to protect steel from corrosion.
Choosing a Metal Finishing Process
There are a few considerations in narrowing down choices when it comes to selecting a metal finishing technique. Some helpful things to keep in mind are:
Production speed: How quickly does the technique apply finish to the product?
Cost-effectiveness: Certain finishing machines (such as vibratory tumblers) can be expensive, but may compensate for their price by delivering faster cycle rates
Metal hardness: harder metals usually require more intense finishing techniques, like grinding, or may need tougher abrasives than those used on softer materials
Metal Finish Types—Summary
This article presented a brief discussion of the different types of metal finishing. For more information on other products, consult our other guides or visit the XTJ Platform to locate potential sources of supply or view details on specific products.
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