The surface of the part may require a specific metal surface treatment for a variety of reasons, whether it is appearance, product function, or customer preference. Certain metal finishes can also help the material resist damage or rust.
Some applications require surface treatment for precise measurement of metal parts, while others only need to look attractive. In many jobs, the consistency of surface appearance is the most important factor in judging the finish of a metal. Each finish can also have a variety of coatings and treatments, such as clear coating, bronze and steel blackening. Which abrasive product you use, the grain size and size of the sand, and the skill of the operator will affect the surface finish and produce different results.
Measurement and visual effects
Grind and polish metal to achieve subjective visual effects or objective measurement effects. Metal surfaces may look smooth to the naked eye, but they actually contain peaks and valleys that can be accurately measured, usually measured using Ra (average roughness) or RMS (root mean square). If the operation requires a measurable finish, measuring these is essential.
Quality personnel use a profiler to measure the surface roughness. The profiler determines the Ra value by moving the diamond pen needle a specified distance on the surface and using a specified contact force. Then, they specified the Ra value as the average of the surface deviations. A smaller number means that the surface is smoother or less deviated.
Although not as common as the appearance finish, a measured finish may be required in applications where two parts are in contact with each other and require a precise, smooth finish to avoid friction. In food-grade and aerospace applications that require rigorous quality testing, a measurable surface finish is almost always specified.
In applications that require powder spraying or treatment of materials, surface treatment may not be so critical, and visual treatment or unmeasured surface treatment is usually acceptable. However, if the parts are to be painted, they should be relatively free of deep scratches.
Ordinary metal finish
The finished metal parts have a variety of finishes. The four common ones are grinding, directional, non-directional and mirror polishing.
A polished surface (also known as 2B finishing) is an unground material from the production plant. How the material (whether it is carbon steel, cold rolled steel or hot rolled steel) appears directly on the production machine. The surface finish of milling is difficult to match with hand-held tools or abrasive products. Therefore, it is important to ensure that the surface is not significantly scratched during welding or finishing. Any type of abrasive applied to the surface of the mill will cause scratching or rotation.
Directional finishing is common on stainless steel products (such as refrigerator doors). There are multiple varieties of these finishes, including hairline and #4 finishes. The hairline finish has light-colored “hairline” direction markings, while the #4 finish is rougher and has more pronounced stripes.
On the other hand, non-directional finishes have no clear directional patterns. Manufacturers create consistent, non-directional patterns by upgrading to high-grainity hook-and-loop or pressure-sensitive adhesive (PSA) sand discs on orbital sanders. Fabs use most non-directional finishes as the final stage before performing certain surface treatments (such as colored varnish, patina, steel blackening, bluing, painting or waxing).
Some jobs require mirror polishing. It is not easy to imitate machine-made mirror polishing by hand. If done manually, mirror polishing is one of the most labor-intensive visual finishes and one of the most difficult to maintain consistent finishes. In order to obtain a perfect mirror polishing effect, the operator must follow specific grinding and polishing steps. Because it is difficult to achieve surface treatment, many companies have purchased mirror-polished metal, cut it to a certain size, welded as needed, and then blended it by hand sanding and polishing the area around the weld.
Ordinary grinding wheel
The flap disc and the mini-flap cousin that can extend into narrow areas are ideal for clean cutting, quickly removing material and forming a consistent profile. In order to obtain a smoother finish, the operator can reduce the particle size, for example from 36 to 80 or 120 particle size. In other words, discs that rely solely on flaps usually cannot provide visual effects. Applications ending in flap discs usually involve products for which no surface treatment is specified or workpieces that will be surface treated.
There are many types of petals, including ceramic and zirconia alumina. Ceramic flaps are usually topcoated and are ideal for heat-sensitive applications. Heating will discolor the base material (especially stainless steel) and cause a rough surface. Both ceramic particles and topcoat can reduce heat.
Resin fiber discs that can mix base materials around the weld can also be used instead of flap discs. Skilled operators can use these discs to mix and create surface patterns. In order to obtain an extremely smooth finish, the operator can use an orbital sander with high-granularity resin fiber abrasive in an appropriate sequence. In order to improve the efficiency of the sander, the operator can add compounds or perform wet polishing.
The surface treatment of the resin fiber disc can reflect the selected crystal grain size, and like the folded plate, the general rules for particles and particle size are applicable: the finer the particle size, the smoother the surface. The finishes ending with the resin fiber discs usually have no designated finishes or will undergo surface treatment.
Non-woven abrasives are an excellent choice for processing a variety of metals and can produce consistent cohesive smoothness on the metal surface. They can achieve cross patterns well and can eliminate discoloration on stainless steel. They can be used for hand pads, integrated wheels and surface treatment discs.
There are many grades of surface treatment discs or non-woven abrasives, from very fine to medium and rough. At the beginning of the process, the rough disk will remove surface imperfections and prepare the surface for refinement. Medium-sized discs can enhance the surface left by rough grades and leave a bright satin finish. The extremely high grade will continue to improve the finish and leave a smooth, silky finish.
When metal removal is not allowed or desired, the wire brush works well. Although wire brushes are generally not recommended for painted surfaces, brushes with smaller wire diameters will produce slight scratch patterns to blend and improve the appearance. The thinner the wire, the smaller the scratches and the better the finish.
Remember, wire brushes cannot remove the scratches left by the flap disc; they can only improve the existing finish. Because they do not remove the substrate, wire brushes are usually good at eliminating thermal discoloration on stainless steel. Nylon abrasive products or nylon products can also eliminate thermal discoloration, eliminate burrs, round edges and provide scratched surface treatment, which can improve the surface finish to a certain extent.
For some commercial and retail equipment workpieces, applications that need to specify very fine and can measure surface finish may require mechanical polishing and polishing. Manufacturers sometimes integrate mechanical grinding systems to perform the time-consuming work of repetitive products. However, most industrial applications can achieve basic surface treatment by installing suitable abrasive products on bench grinding wheels, right angle grinders and other hand tools.
The most suitable abrasive for work
Regardless of the tool used in the finishing operation, the use of different grinding wheels and polishing wheels will produce very different results, and each grinding wheel has a specific type and size of grains.
If the base material must remove a large number of mill scales, the 120-grain flap will take longer to remove, but it will provide a good surface finish. In contrast, the use of 36-grain or 40-grain flaps can remove oxide scale faster, but leaves deeper scratches and requires more time to obtain a smooth surface.
To achieve a specific effect, it is usually necessary to use a series of products. The operator can start with coarse sandpaper flaps, which will quickly remove material and lower the weld to almost surface level, and then move to higher, finer sand grains such as 60, 80 or 120 (or even higher) to provide Smoother finish.
Depending on the desired finish, the next step may be to incorporate non-woven abrasives into the directional scratches. The high-grain flaps and non-woven abrasive allow the operator to measure the paintable topcoat.
Pressure, granularity and technology
In grinding, pressure is very important. If operators (or related machines) use the same abrasive product to apply different pressures, they will get significantly different results.
Laboratory tests have proven this, especially when higher pressure is applied to the low particle size (36 and 40) discs. Applying more pressure to smaller abrasives (such as 60, 80, and 120 grit) will produce less change in results. In other words, applying a greater pressure to a 36-grain disk will produce a greater difference in finish compared to applying a greater pressure to a 120-grain disk. This is important because pressure is one of the few variables that the operator can adjust.
Operators have different grinding methods, no matter which abrasive product they use, their technology will affect the results. For example, whether it is from north to south or east to west, you can pass through the initial grinding mode, thereby improving the finish and reducing the finishing steps required for a job.
Note that the scratch pattern will also vary. Unless they need to achieve directional trimming, the operator can rotate each step 90 degrees from the previous step. This will remove and mix the previous step more effectively.
Maintain quality and consistency
Appropriate technical training of employees can help prevent rework and make the finishing department of the shop more consistent. In other words, the best-looking finishes in the world cannot overcome the waste caused by poor communication. This is correct when producing visual effects. Customers who want an appearance trim want to get a certain appearance, so it is important to understand the appearance accurately.
To this end, the finishing process can retain samples and finishing codes to illustrate the exact steps taken to achieve certain finishing. They are usually displayed on codebooks or boards, providing customers and employees with visual templates containing high-quality photos or actual samples. The customer can then choose the desired finish from the beginning of the project, thus avoiding rework and confusion.
Think of well-supported finishing operations as a three-legged stool. The right training will strengthen one foot; having the right tools and abrasive products will strengthen the second leg; good communication with customers and with employees is the third item. All three legs of the stool are firm and stable, and the inefficiency of finishing is a thing of the past.
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Link to this article：What are the ways to achieve a specific metal finish?
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