Metal Finishing Guide Book

2013

Issue link: https://metalfinishing.epubxp.com/i/218436

Contents of this Issue

Navigation

Page 27 of 843

als as fatty acids, tallows, and glycerides. Waxes can be from vegetable, insect, or petroleum-based products. Petroleum-based or vegetable-based oils also may be used. The animal and vegetable materials are more saponifiable and will produce a water-soluble soap when combined with alkali. Petroleum, mineral oils and waxes are unsaponifiable and, therefore, might create subsequent cleaning problems. Each ingredient is added to the binder to transmit a specific effect to the bar compound such as lubricity, degree of hardness, or improved adherence to a buffing wheel. A binder also controls the amount of frictional heat that can be developed on a surface. This is called slip. There is a wide range of abrasives used in buffing compounds, a few of which will be described. BUFFING ABRASIVES Aluminum Oxide and Other Powders Aluminum oxide powders, fused and unfused, are the abrasives most commonly used in the buffing of hard metals. Chromium oxide is used to achieve the highest reflectivity (color) on stainless steel, chromium, and nickel plate. To achieve a high reflectivity (color) on brass, gold, copper, and silver, iron oxide is generally used. Aluminum oxide is chemically represented as Al2O3. The unfused aluminum oxide is white in color. This is manufactured from bauxite or hydrated aluminum oxide by heating it at elevated temperatures. This heating process, called calcination, gives the abrasive the common name calcinated alumina. The higher the calcination temperature, the more water of hydration is driven off and the harder the crystalline material becomes. When the calcinated temperature is about 950oC, the product produced is a soft alumina having a porous structure. This type of abrasive is used for luster or color buffing. When the calcined temperature is about 1,250oC, a harder alumina is produced. This type of abrasive is used for cutting. Soft aluminas are used to produce luster or a higher reflectivity on all metals, both ferrous and nonferrous. The harder aluminas will cut and remove more metal from the surface of castings or extrusions of aluminum, brass, and other metals. When alumina is heated to 1,850oC, fused aluminum oxide (Al2O3) is produced. This material is made in an electric furnace at approximately 2,000oC. Bauxite, when mixed with alumina and other oxide materials, produces a specific crystalline structure whose hardness can be varied to meet specified physical properties. This fused mass is then cooled and crushed. In the crushing process, the material is ground, screened to the appropriate size, treated magnetically, and acid washed. It is then rescreened to its final classification (grit sizing). The difference between fused aluminum oxide and calcined alumina is that the fused oxide is of a crystalline structure that is much harder than that of the calcined alumina. Fused aluminum oxide is used mainly on abrasive belts or setup wheels for polishing. As for buffing, fused aluminum oxide is used for cutting down ferrous metals. The abrasive sizing is generally from 60 grit to -325 grit for buffing compounds. Tripoli Tripoli is considered to be microcrystalline silica, which is made naturally. It is highly suitable for buffing of aluminum, brass, copper, and zinc die cast or other white metals. Tripoli and silica can be used as a cutting abrasive or a so-called cutand-color abrasive for nonferrous metals. Tripoli should not be classified as an amorphous silica, but it is microcrystalline in nature. Crystalline silica may cause delayed lung injury for people when exposed to it over a long period. Users of products containing these abrasives should be aware of this possibility and should wear a mask and work in a ventilated area. 24

Articles in this issue

view archives of Metal Finishing Guide Book - 2013