The process of surface finishing requires a variety of tools, materials, and most importantly, highly specialized skills. It is an art that takes years of experience to perfect, but some basic principles and techniques can provide a good start. This article will examine the three major processes necessary to produce a highly polished, lustrous metal surface: grinding, hand stoning and diamond polishing.
1. Grinding
While machining is technically the starting point of the surface finishing process, grinding is where the real work begins.
After metal is machined, its surface condition is usually rough with ridges and cutter marks. Finishers must use a hand grinder, the basic tool for light metal removal, to get the surface smooth enough to eventually finish with hand stones and, ultimately, diamond polish.
Grinders have a tendency to follow the wavy contours of the rough surface, which makes it difficult to develop the smooth or flat surfaces required for subsequent stoning. Whenever possible, direct the grinding strokes at a 45-degree to 90-degree angle relative to the grooves developed in the machining process.
Give careful consideration to the direction of rotation. When taking a stroke in a direction opposite to a cutting tool rotation, the grinder has a tendency to run away from the cut. On the other hand, stroking into the rotation of the wheel can cause the stone to cut deeper than is desired.
Don’t try to cover too large an area at any one time. It’s difficult to maintain control and uniform pressure on the wheel over a large area. An experienced craftsman will work on a number of small areas and then blend them together.
Another good practice is to crisscross the grinding strokes to ensure a uniform surface. When smoothing a surface, grind in one direction to completely cover the area being worked on. Then cross the grinding strokes until all of the previous grinder marks have been removed.
2. Hand Stoning
Once you’ve arrived at the point where all the metal has been ground to satisfaction, the surface should be examined to determine if it’s ready for hand stoning. Check milled surfaces to see if the cutter marks are fine enough to make further hand grinding unnecessary.
Choosing the initial grit of stone depends upon the degree of finish left by the machining, grinding or filing operation. Machining usually results in a coarser finish than grinding; therefore, a coarser grit stone should usually be used. For a ground finish, stoning can begin with a finer grit stone.
Preliminary stoning may be done with a 240 grit stone to remove final dips, depressions, waves or other imperfections and achieve a flat or properly contoured surface. If defects are not too great, a 320 grit stone will be sufficient. The stone should be moved back and forth, with medium pressure applied, over the surface in a direction 45 degrees or 90 degrees from the direction made by the last tool marks.
Polishing, from a purely physical point of view, is the process of producing a series of overlapping “scratches” that get finer and finer. To accomplish this, it’s important that for each finer grade of stone used, the angle (direction) be changed relative to the marks made by the preceding coarser stone. In this way, the marks of one grit size are erased by the subsequent finer-grit stone. It is critical that each finer grit stone completely removes the marks of the last grit. Neglecting to do so will result in a shiny, but nonetheless, scratched surface.
3. Diamond Polishing
Diamond polishing is the last step of the polishing process—depending upon how much of a smooth luster must be achieved. A variety of diamond compounds—diamond particles suspended in some proprietary vehicle, such as an oil—are available in the marketplace. Begin by applying a small amount of coarser diamond compound to the surface being worked.
Then, by means of a bristle, brass or steel brush, swirl the compound over the surface using a rotary tool at a slow speed to avoid throwing the compound off the surface. A speed of 500 rpm for roughing, or 5,000-10,000 rpm maximum for final polishing, is a good rule. Brush the surface until all that is visible are fine swirly marks left by the brush’s rotary action. There should be no stoning marks visible at all.
The next step—removing the swirly marks left by the bristle brush—is accomplished with a felt product, usually a felt bob. Felt bobs are available in various degrees of hardness, pre-assembled to a shank or retained in a shanked nylon holder. Mount the bob in a rotary tool and, using light to moderate pressure, polish the surface with diamond compound until all that is visible are felt swirls.
Before progressing to a finer grade of diamond compound, thoroughly clean the mold surface to remove all residual particles of the previous grade. This is usually done with clean tissue paper or cotton and a very refined oil or alcohol. Don’t use the brushes and felts used with one grade compound on the next grade of compound.
These steps—brushing, felting and cleaning—should be followed with each grade of diamond compound to arrive at the final step. Polish the surface with fine tissue paper, felt sticks or cotton swabs with an ultra-fine grade of compound to arrive at the final high gloss luster.
Similar to sanding, both rumbling and tumbling use an abrasive media but in this case the sanding particles are “loose” and not fixed onto any backing paper. Depending on the workpiece and the type of finish desired, many different types of media can be used, including garnet, walnut shells, stones, or coarse sand.
In tumbling, components are placed into a box or tub along with the abrasive particles, which is then rotated to mix everything together randomly. This is often used for “deburring”, or to remove the sharp metal points left on a part’s edge after it’s been machined.
Rumbling is much the same but the parts and abrasive are in a trough that is then vibrated to increase the cutting action.
Deep holes, pockets, or large interior chambers sometimes need to be highly polished in order to make a smooth surface with a low frictional resistance to air or liquid. But these areas can be difficult or impossible to reach by hand, so a different method is needed.
Like with tumbling, metal parts can be put into a box or chamber that’s filled with magnetized particles. Using a focused magnetic field, these particles can then be directed to abrade away against the interior surface to achieve a fine polish.
Sandblasting is the process of treating the surface of a part by blasting it with an abrasive media under high pressure. The type of media used again will determine the quality of the resulting finish. Sandblasting, or blasting with a combination of air and water, can cover a large surface area quickly. It can also improve some of the mechanical properties of metal, by increasing fatigue strength and improving corrosion resistance through shot-peening.
Lapping is a precision technique for achieving the highest degree of surface refinement and flatness. It involves a skilled craftsperson who uses a soft iron tool together with a mild abrasive slurry to slowly work the surface with random, light and non-linear motions by hand. This fills microscopic surface voids and flattens high spots.
Machines can also be used for this process but it requires a very slow speed to avoid heating the workpiece.
A file is a piece of very hard steel that has a series of parallel grooves cut into it, which leave behind rows of sharp cutting teeth. Files are typically used by hand and, depending on how coarse the teeth are, can aggressively remove material and shape metal quickly. The surface finish is somewhat rough and imprecise and often needs additional sanding or polishing for good results. But files are still versatile, easy to use and can produce fine results in skilled hands.
Sanding is simply a method for rubbing abrasive particles against the surface of a workpiece to create a random, non-linear surface texture. Different abrasive media are used, glued onto a backing paper or plate. The size of the cutting grains is referred to as “grit”: the higher the grit number, the smaller and finer the particles are and hence the finer the surface finish they are able to achieve.
Very coarse grits can remove a lot of material quickly, while finer grits are able to achieve a mirror polish. Water or some other lubricant is often used to flush material away and expose a fresh cutting surface. Sanding is especially useful for contoured or curved surfaces, but it’s not ideal for getting into very tight corners or pockets.
When making a tool for plastic injection molding or pressure die casting, it’s often necessary to give the inner cavity a fine polish to make a beautiful finished part. But tool steels are heat-treated and very hard, so polishing them is difficult. It’s also not easy to reach into holes, pockets, and other complex shapes.
Ultrasonic polishing is used in these cases. A soft, fine-tipped tool is mounted onto an ultrasonic spindle that vibrates at 30KHz. In combination with an abrasive slurry medium, the tool tip does not actually touch the work surface but it induces a pressure wave that safely works away at the surface to create a fine polish. This technique works even on hardened steels and there’s little chance of damaging the workpiece.
The surface roughness of the mould is very important to the quality of injection molding，Polishing is an essential process，your blog about this issue is very helpful for us, thanks for sharing