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Burnishing is the process of removing the surface coating on metal, while polishing the exposed material, and. is another method of engraving on brass and aluminum. The significant differences between burnishing and diamond drag engraving are the type of tool used and the use of the spindle motor. Keep in mind that a burnishing tool does not really "cut" the metal, but is more of a marking technique. Burnishing can also be used to engrave on glass.
I don't recommend burnishing on aluminum, because the softer material can produce too ragged a cut if burnished, but there are exceptions. However, if the aluminum is anodized, such as for trophy plates, it may be burnished with a diamond burnishing tool. A diamond tool will tend to be sharper and will break through the anodized surface much better than a carbide tool.
The main advantage to burnishing on metals is that the tool produces a wider stroke width than drag engraving without the need for deeper cuts. The process removes only the top coating from the metal. The application is generally used on coated metals such as lacquered brass where burnishing will only remove the top coating, exposing the bright brass underneath. To the engraver operator, this means that the size of the engraved character can be significantly larger than with diamond drag. On large plaques this is the preferred method.
If you have a computerized engraving system, it would be helpful to invest in it spring-loaded burnishing adapter to use as a substitute for the normal cutter knob at the top of the tool. When properly set, this adapter allows enough pressure to float the burnisher over the surface of the engraving material and still assures even contact.
There are two commonly available types of burnishing tools: rotating diamond and carbide. Diamond burnishers have a very hard tip and generally last longer than carbide, but usually cost two to three times more. They come in tip sizes from .005" to .030". Larger sizes are available by special order, but can be very expensive. For jobs that require a stroke width larger than .030", most engravers choose to use a carbide cutter instead of a diamond. If you are engraving on glass, you must use a diamond burnisher.
Be sure to notify your supplier when you order a burnishing tool if you are using a burnishing adapter. They will provide your tools with a longer shank, since the adapter is longer than a regular cutter knob. If the tool is a bit short for your spindle, simply remove the nosecone and micrometer ring. These are not used at all when burnishing.
As a burnisher becomes dull from use, it will produce a rougher cut. This can result in the surface coating being ground into the engraved area. In this instance, it will be difficult to effectively oxidize the engraving. As with any tool, diamond burnishers can be re-lapped when worn, and carbide burnishers can be re-sharpened.
NOTE: Non-rotating (drag) diamonds and rotating diamond burnishers are two distinctly different tools and cannot be used interchangeably. A non-rotating diamond has a pointed, cone-shaped tip, whereas a rotating diamond is wedge-shaped. At best, running the spindle motor with a non-rotating diamond or trying to drag a rotating diamond through metal will produce a poor quality result. At worst, it will likely knock the diamond out of its shank. Since resetting a diamond is as expensive as buying a new one, at this point your expensive tool will be scrap.
The key to good quality burnishing is the amount of downward pressure exerted on the tool. The goal is to achieve enough pressure to evenly remove the surface coating without digging into the metal and leaving a rough cut. The burnisher should "float" across the surface of the material with a light, even pressure. Before the introduction of the burnishing adapter, engravers would stay away from applications where the material was soft such as aluminum or pewter. Uneven surfaces also caused considerable problems. But now, this has been eliminated in almost every application.
The machine set-up is basically the same for manual and computerized engraving systems. The down pressure should be set at its lowest setting on computerized machines. For the manual operator, down pressure is determined by touch, often by disabling the spindle return spring and allowing the weight of the spindle itself to produce the correct pressure. With this arrangement, be sure to lift the spindle between characters.
To set-up your machine for burnishing, first insert the burnisher into the spindle, and adjust it so that it extends below the spindle's nosecone. If the burnisher is too short in length, then you should remove the nosecone and retaining ring from the spindle.
Burnishing is usually done without the use of the nosecone to prevent scratching the material. The next step is to lower the spindle to its full down position. Using a piece of scrap material, adjust the burnisher by loosening the spline screw so that it makes firm contact with the engraving surface. If you don't want to remove the nosecone, remember at least not to allow the nosecone to touch the engraving surface. Since there will be variations in the thickness of the material you are engraving and in the engraving table itself that may make it not perfectly level, you will need to set the burnisher to touch the lowest point on the engraving material. This will assure even contact over the entire surface of the material. If you are using a burnishing adapter, you want to position the burnisher so that the spline screw in the adapter is about in the middle of the slot that it travels in.
Adjust the spindle motor so that it is relatively fast (about 8-12,000rpm), and the engraving or travel speed about mid-range between fast and slow. By reducing the engraving speed, the resulting stroke will have a more polished, smoother finish. For systems with a dwell setting, reduce the dwell to eliminate the cutter stalling in one area of the cut.
If you are engraving on glass, be sure to mist the surface of the glass with a 50/50 mixture of glass cleaner or dish detergent and water to reduce the chances of chipping the glass. Rinse the finished piece thoroughly before wiping to avoid scratching. If the glass is tempered and especially hard, using a burnishing adapter may result in "skipping" of the burnishing tool across the surface. You may achieve better results by changing to a standard cutter knob.
NOTE: The use of the burnishing adapter is extremely helpful when tackling odd-shaped items. Surfaces that are concave or convex where the change in surface height is significant can oftentimes be cut with the use of the adapter. The adapter will give you the added "float" required to move over the uneven surface.