Polishing Metal & Non-Wood Applications

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These recommendations should be used as general guidelines for beginning your processes. Variables such as machine type, type of metal, coolants, belt speeds, etc. can require different recommendations. It is up to each operator to determine how best to approach each specific application using these guidelines as a starting point.

Ferrous Metals

In general, steel of all types tends to generate much frictional heat during the grinding and polishing process. The ones particularly heat sensitive are tool steels, stainless steels and high nickel or cobalt alloy steels. Thus, these materials require slower belt speeds and the possible use of a belt lubricant or coolant.

Steels of all types are generally ground with Ceramic, Alumina Zirconia, or Aluminum Oxide although occasionally Silicon Carbide is used on stainless steel to impart a fine satin finish. Cast iron is mostly ground in coarser grits using CR, AZ, or SC, but most polishing operations will use AO.

The normal grit range for ferrous metal polishing is 50-80 for general polishing and 180 - 320 for fine polishing leading to buffing operations. Only for very fine satin finishes or applications requiring a precision tolerance will grits of 500 or finer be used.

Non-ferrous Metals

Non-ferrous metals are those which do not contain iron as a principal element. The most common types used in metalworking are aluminum, zinc, and brass. Other non-ferrous metals that necessitate grinding and polishing are bronze, copper, magnesium, and certain "exotic" metals such as titanium and zirconium.

Compared with steel, non-ferrous metals are usually softer and can become "gummy". This difference causes abrasive belts to "load" (clog) more quickly unless a lubricant or a slower belt speed is used. Most non-ferrous polishing operations will use Aluminum Oxide belts, although Silicon Carbide is preferred for satin finishes or where fast cutting is a priority over durability. Silicon Carbide is highly preferred for polishing on titanium and zirconium because these metals have a degree of high heat sensitivity and may react chemically with Aluminum Oxide. Normally AZ is used for the removal grits 24 - 120.

Common grit ranges for non-ferrous polishing are 80 to 180 for most operations, and 150 to 240 for fine work. Aluminum that is to be anodized may need to be polished to 320 grit to remove imperfections in the metal.

Non-metal Materials

Glass

Silicon Carbide is used almost exclusively. Water is used as a coolant to control heat build-up and harmful dust. Grit range is 180 to 320. Special cork-based polishing belts are used for final polishing to full clarity.

Plastic

Thermo-plastics (nylons, acrylics and urethanes) are relatively soft and are quite heat-sensitive. These plastics should be polished wet to prevent melting or surface damage. Thermosetting plastics (epoxies, polyesters, polycarbonates and phenolics) do not soften with heat and can be polished wet or dry. Silicon Carbide is the preferred abrasive mineral for all plastics, although AO can be used on thermosetting plastics. Grit range is 180 to 320 for polishing.

Stone

These include concrete, brick, ceramics, granite, marble etc. Silicon Carbide is almost exclusively used in grits 120 to 220. Stone is often ground and polished with water as a coolant and to control dust.

Rubber

Silicon Carbide is used almost exclusively in grit range 80 to 240. It is normally ground and polished dry. Most rubber polishing is seen in roll resurfacing operations.

Grit Sequencing

A proper progression of grits is necessary to ensure that unwanted coarse grit lines are not visible on the finished product. Especially in the case of fine plating or thin layers of plating, it is important to avoid having scratch lines that are coarse enough to show through the metal plating. The proper selection of grit is mostly a trial-and-error scenario because the vast number of variables in workpiece type, machinery; contact wheels, operators, etc. can affect the sanding results.

Generally, it is best not to skip more than one grit when moving to finer grits. For example, a possible sequence might be 80 - 120 - 180 - 240 - 320. When sanding soft metals, a larger jump may be practical because the metal may "flow" better to cover coarser grit lines. Conversely, hard steels require a greater number of grit steps to remove coarser grit lines most efficiently.

For the best possible "mirror finish" on hard steels, it is not a good idea to skip grits when moving through a grit series before buffing. The CAMI and FEPA (P) grade grit size standards have been established so each finer grit in the series will remove the next coarser grit scratch. The time required for buffing is directly related to the size of the last belt polishing grit size. Therefore, use the finest grit possible as the last step before buffing.

Belt Tension/Machine Requirements

Belts are very strong and can perform under rigorous operating conditions. However, excessive belt tension can cause the failure of belts in addition to problems with the finish and wear of machine bearings. Almost all belt machines are tensioned by spring mechanisms or by an air cylinder. Air-operated tensioning devices are superior because they allow for belt stretch during use and can be easily adjusted and monitored. Belt tension should be maintained in the range of 4 to 5 pounds per inch of belt width to a high of 30 to 40 pounds.

Polishing operations should use lower belt tension, especially when using soft contact wheels. A rule of thumb is to use the lowest tension possible before the belt "walks" off center.

Fixturing

Most workpieces that are polished on abrasive belt machinery are done "off-hand", i.e. when the workpiece is held against the belt by the operator's hands. Certain shapes lend themselves to simple fixturing, such as a cylindrical part placed into a rotating handle which allows the part to spin freely against the moving belt. More complex fixturing such as rotary automatics and even robotic set-ups are possible for a variety of shapes.

Coolants and Lubricants

Liquid coolants and lubricants involve more sophisticated machinery to contain the fluids and possibly to filter and re-circulate these fluids as well.

Liquids

Water is the most effective coolant and is used for glass and plastic. The amount of water used may range from a mist to a heavy flood.

Soluble oils are mixtures of water and oil, which are common in metal grinding and other machining operations. These mixtures serve to cool and lubricate.

Straight oils such as mineral oil or special cutting oils are used for grinding heat-sensitive metals or where optimum finish is desired.

Solid Lubricants (Grease Sticks)

Grease sticks are very common in the polishing trades for improving finish and reducing loading. The grease stick reduces scratch depth, especially on fresh belts, by filling the space between the grains. Additionally, grease sticks reduce loading by inhibiting soft metals such as aluminum from sticking directly to the belt. Once the belt becomes loaded, it can be cleaned with solvent. Finally, grease sticks can reduce heat, which improves belt life and finish.

The composition of a grease stick is usually fat or wax packed into a cardboard tube and applied directly to the moving belt. A light coating of grease on the belt is required because too heavy a concentration will simply smear and cause clean-up problems.

The abrasive manufacturers may also add a dry lubricant to the coated abrasive material. The added lubricant can be added as a top-coat onto the finished product or mixed into the resin bonding. These lubricants are useful for stainless steel and high-performance alloys. They are also used on aluminum and brass polishing.

Non-metal Application Guide

Machine Type Typical Sizes Heavy Stock Grits Medium Stock Grits Light Stock Grits Finishing Grits Polishing Grits KAI Products
Glass 4x106, 3x24, 7x7/8 - 80 100 - 120 180 - 220 320+ CS320, 321, 333
Rubber Roll grinding is typical 24 - 36 50 - 60 80 - 100 120 - 220 150 - 320 CS320, 333, 411, or 565
Stone, brick ceramics, or concrete Various types; Belt and disc machines 24 36 60 120 220 CS320, 321, 333, or 565
Thermoplastics Belts and discs 36 60 120 180 - 220 320 CS320, 321, 14
Leather Wide belts and drum sanders - - 80 100 - 180 220 - 400 PS20, 22, 29, 24
Notes:
  • Thermoplastics are those that soften when heated, such as acrylics, nylons and urethanes. They should be wet sanded whenever practical.
  • Thermo-setting plastics do not soften when heated, such as phenolics, epoxies, polyesters, polycarbonates and melamines. This includes solid surface plastics such as DuPont Corian. These can be sanded wet or dry.
  • Rubber is normally ground dry, or using a dry lubricant such as powdered soapstone or zinc stearate powder. Re-finishing rubber rolls, such as those used in printing or other processes, may done wet or dry.
  • Glass is ground wet or dry, but wet is preferred to reduce heat and dust.
  • Stone products are ground wet or dry, but wet is preferred to reduce dust.
  • Leather sanding operations are good abrasive users when located. Leather is always sanded dry, and typical operations are called "buffing" and "sueding". Watch for moisture from the hides!

*These recommendations are general guidelines only. Many variables influence abrasive applications.

Metal Application Abrasive Guide

Machine Type Typical Sizes Heavy Stock Grits Medium Stock Grits Light Stock Grits Finishing Grits Polishing Grits KAI Products
Backstand or Conveyorized 2x132, 3x132, 2x118, 3x118 36 - 40 50 - 80 100 - 150 180 - 240 280 - 400 J, X and Y materials
Grinder 2x148, but common length is 132" 36 - 40 50 - 80 100 - 120 180 - 240 320 - 400 -
Pedestal (Burr King) 1-1/2x60, 2x60, 2x72 36 - 40 50 - - - -
Benchstand 1x30, 1x42, 2x48, 6x48 36 60 120 220 320 CS308, CS311, CS411
Wide belt (deburring & finishing) Various widths and lengths 60 80 120 220 320 CS412, CS341, CS310, CS311, CS411
Stroke Sander 6x300 or similar sizes 60 100 180 240 320 PS20, PS22, CS310
File Belt Sander Less than 1" width & 30" length 50 - 60 60 - 80 120 180 220 CS310XF, 411Y, 710XF, 409Y
Portable Belts 3x21, 3x24, 4x21, 4x24 36 60 120 220 320 LS309XH, CS311Y, CS308Y
Flapwheel - 50 - 60 120 150 220 320 SM611, KM613
Centerless and Roll Grinding 4x132, 6x168, 12x132 24 - 40 60 120 220 320 CS811, CS411Y, CS341
Orbital Sanders 5-8" diameter - 60 - 80 120 150 - 240 320 - 400 AO in paper, film, and latex backings
Stationary Disc 10+ diameter PSA 24 40 80 120 220 CS310, 311, 411
Fiber Discs 4, 4-1/2, 5, 7 & 9" 24 - 36 50 - 60 80 100 120 561, 565, 570, 661
Spiral Bands - 50 80 120 180 240 AO and ZR
Cartridge Rolls - 50 60 80 120 - AO
Hand Sanding Sheets and Rolls 50 - 60 80 - 100 120 220 320 KL361 and most paper-backed products


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