Experience has revealed in most cases it is more convenient and adaptable to use the fluid type lubricant applied freely
through a recycling system directly to the blade and work stock.
3-210. GRINDING. The grinding characteristics of the various aluminum alloys vary in many instances. The harder
free-cutting aluminum alloys may be ground satisfactorily with free cutting commercial silicon carbide grinding wheels,
such as crystalon, carborundum and natalon. Rough grinding operational are usually performed by use of resin bonded
wheels of medium hardeners and grit sizes of 24 to 30. Also the aluminum abrasives from No. 14 to No. 36 have been
found to be satisfactory for rough grindings.
3-211. Common alloys, particularly in their softer tempers have a tendency to clog the wheels and do not finish to as
bright and smooth a surface as the harder materials.
3-212. Caution should be taken in selecting the proper grade of each commercial make of wheel. Once the grinding
wheel has been selected there are three variables that affect the quality of a finish; these are the wheel speed, work
speed and grinding compound. Experienced operators have proven that their own good judgment is a determining factor
as to the correct wheel and work speeds, however, wheel speeds of about 6,000 feet per minute have given good results.
3-213. For finish work, a soft silicon carbide wheel of 30 to 40 grit in a vitrified bond have proven to be very satisfactory.
A grinding compound of soluble cutting oil and water works well. However, the fine grindings of aluminum must be
strained from the compound before reusing in 'order to prevent deep scratches on the finished surface.
3-214. Special care should be exercised when grinding castings and wrought alloy products that have been heat treated,
since their greater resistance to cutting or grinding generates a considerable amount of heat which may cause warping
and damage to the material.
3-215. Lubricants and Coolants. Generous applications of stick grease are recommended to prevent clogging of the
grinding wheels during rough grinding, while copious quantities of a low viscosity coolant type grinding compound are
essential and recommended for finish grinding. Soluble oil emulsions of the proportions of 30 or 40 to 1 are most
3-216. POLISHING. Polishing or finishing aluminum and most of its alloys, by the application of proper machining
procedures, gives it a smooth lustrous finish. Aluminum and its alloys are polished in the same manner as other metals,
but a lower wheel-to-metal pressure is used for aluminum.
3-217. Polishing is the act of removing marks, scratches or abrasion on the metal resulting from previous handling and
operations; it must be understood that a more gentle cutting action or finer abrasives are used for polishing aluminum
than used for steel. The various operations covered under the polishing category include roughing greasing or oiling,
buffing and coloring. These operations are briefly described in the following paragraph.
3-218. ROUGHING. This is a term used to describe the preliminary finishing operation or process, used to prepare
aluminum surfaces having deep scratches gouges or unusually rough surfaces, for subsequent polishing procedures.
Roughing is not required on smooth undented or unscratched surfaces. The preliminary finishing or roughing process
usually employs a flexible aluminum oxide paper disc, a semi flexible bonded muslin or canvass wheel, faced with
suitable abrasives. Usually 50 100 grit abrasives are for this process and are set in an adhesive in accordance with
standard practice. The peripheral speed of these discs runs around 6,000 feet per minute; faster wheel speeds would
cause heating or ridging of the soft metal surface. Heating is also reduced by small applications of tallow or a tallow oil
3-219. GREASING OR OILING. This is a refined or gentle roughing procedure for finishing aluminum surfaces.
Application is visually employed by a soft wheel faced with 100 to 200 grit aluminum oxide emery, plus a light coat of
tallow or beeswax lubricant to prevent excessive heating. Here again, peripheral speeds of about 6,000 FPM are used.
3-220. Greasing or oiling is a necessary operation in finishing coatings and other fabricated work which has been marred
by previous operations. Excess aluminum pick-up on the wheels as results from overheating will cause deep scratches
in the metal.
3-221. BUFFING. This is a term used to describe a finishing procedure employed to obtain a smooth high luster on an
aluminum surface. This high luster finish is obtained by use of a fine abrasive, such as tripole powder mired with a
grease binder, which is applied to the face of the wheel. These wheels usually consist of muslin discs sewed together,
turned at a peripheral speed of 7,000 FPM.
3-222. Many factors, such as, the thread count of the buff, the pressure applied to the buff against the work, the buffing
compound used, the speed of the buff or wheel and the skill and experience of the operator must be considered in
obtaining a satisfactory and quality type finish.