Milling operations

Consider machining the workpiece shown in Fig. 11.17, using in the first instance a horizontal milling machine and second producing the same workpiece using a vertical milling machine. 

The two essential requirements are opposite faces parallel to each other and square with their adjacent faces, and as many operations as possible done at a single setting. The most convenient method of holding such a workpiece is to grip it in a machine vice. 

The machine vice is set relative to the machine movements using a parallel gripped in the jaws and is checked by means of a dial indicator attached to a fixed part of the machine. The machine vice, having been satisfactorily set up, is securely clamped to the machine table. 

Set the block in the vice on parallels and ensure that at least 32 mm is protruding above the vice jaws. Tighten the vice, ensuring that the workpiece is seated on the parallels. May require hitting with a soft faced mallet. 

Machine face C to clean up, using a cylindrical cutter. Fig. 11.18 stage 1. 

Release the workpiece and reset with face B against the fixed jaw and face C seated on the parallels. 

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Economic Aspects. The hardwood and decorative plywood industry has decreased in size and production significantly in the past few years. In 1994, there were an estimated 100 mills operating in the United States having a production volume of 1.135 x 10 m (2). The doUar value of this production is extremely difficult to estimate because of the very wide range of prices for the products. 

Increases in U.S. demand for caustic soda have been unpredictably high in the last few years. Between 1987 and 1989, the annual increase in demand was about 3% (6). However, the caustic soda market is mature and new areas of significant growth have not surfaced in recent years. The unexpected recent demand is generally related to two factors the pick-up in the U.S. economy after the slump of 1986 and pulp mills operating at fiiU capacity, leading to less efficient caustic use. 

Typically, dry potassium nitrate is pulverized in a ball mill. Sulfur is milled into cellular charcoal to form a uniform mix in a separate ball mill. The nitrate and the sulfur—charcoal mix are screened and then loosely mixed by hand or in a tumbling machine. Magnetic separators may be used to ensure the absence of ferrous metals. The preliminary mix is transferred to an edge-mimer wheel mill with large, heavy cast iron wheels. A clearance between the pan and the wheels is required for safety purposes. The size of this gap also contributes to the density of the black powder granules obtained. Water is added to minimize dusting and improve incorporation of the nitrate into the charcoal. The milling operation requires ca 3 to 6 h. 

Based on previous spHts ia milling operations, about a 60% yield or 146 x 10 t of acid-grade spar could be expected. At the production rates of the early 1990s, this would be a 24-yr supply. Additional suppHes are expected to be brought iato production, however, and no decline ia available reserves is expected through the year 2000. 

Locust Bean Gum. Locust bean gum [9000-40-2] is produced by milling the seeds from the leguminous evergreen plant, Ceratonia siliqua or carob tree, which is widely grown in the Mediterranean area. Pods produced by the carob tree consist of a husk, embryo, and endosperm. The latter, the source of the gum, is separated from the tough outer husk and the yeUow embryo tissue by a variety of rolling and milling operations, and subsequently is milled into a fine powder (60). 

Other uses of oxyacetylene flames in mill operations are in building up or hardfacing metal, lancing (piercing a hole in a metal mass), and a variety of metal cleaning procedures. A minor but interesting fuel use of acetylene is in flame spectrophotometry where oxygen and nitrous oxide are used as oxidants in procedures for a wide variety of the elements. 

Sodium sihcate (41°Bh, 1 3.22 ratio Na20 Si02) is added in the milling operation to disperse the slime, mosdy kaolin. Dispersion also aids the grinding process. The rod mill serves to grind the ore to 0.833 mm (—20 mesh) or to the point where mica, quart2, feldspar, and iron minerals are Hberated. Cyclones, or rake, hydrauhc, or other types of classifiers, are used after grinding to produce coarse and fine mica fractions that are treated separately. 

Annealing is also employed as a final mill operation to produce a material having high formabiUty for subsequent customer shaping or forming... 

Less mechanization is used on smaller tracts of individually owned land, where the terrain is mountainous or otherwise difficult to reach, or where thinning as opposed to clear-cutting is the preferred silvicultural practice. The harvesting operation maybe done by the landowner or small contractor using chainsaws and tractors. In some operations where minimal damage to the forest is critical, horses may be used. Sale of pulpwood to the mill operation is usually through dealers. 

Roller Mills Pendulum, Table, and Bowl Type. This is a group of machines commonly applied for grinding of mineral powders down to approximately 97% below 75 p.m, or even finer in some instances. The mills operate at medium speed, up to approximately 30 m/s, and can handle materials with up to Mohs hardness 5 before wear rates become prohibitive. Many different designs are available the two most commonly encountered variants are pendulum mills and the table roUer mill. 

Refining to a High Purity Product. The normal yeUowcake product of uranium milling operations is not generaUy pure enough for use ia most nuclear appHcations. Many additional methods have been used to refine the yeUowcake iato a product of sufficient purity for use ia the nuclear iadustry. The two most common methods for refining uranium to a high purity product are tributyl phosphate (TBP) extraction from HNO solutions, or distiUation of UF, siace this is the feedstock for uranium enrichment plants. 

In recent years, several wet milling operations have been initiated with obvious advantages in dust control and potential advantages in the separation of mineral contaminants from the fiber product. On the other hand, large-scale industrial wet classification methods are not in use at present. 

Extraction of Bertrandite. Bertrandite-containing tuff from the Spor Mountain deposits is wet milled to provide a thixotropic, pumpable slurry of below 840 p.m (—20 mesh) particles. This slurry is leached with sulfuric acid at temperatures near the boiling point. The resulting beryUium sulfate [13510-49-1] solution is separated from unreacted soflds by countercurrent decantation thickener operations. The solution contains 0.4—0.7 g/L Be, 4.7 g/L Al, 3—5 g/L Mg, and 1.5 g/L Fe, plus minor impurities including uranium [7440-61-1/, rare earths, zirconium [7440-67-7] titanium [7440-32-6] and zinc [7440-66-6]. Water conservation practices are essential in semiarid Utah, so the wash water introduced in the countercurrent decantation separation of beryUium solutions from soflds is utilized in the wet milling operation. 

Pulp bleaching with chlorine dioxide is most often performed at an acidic pH, so that the final pH of the bleach Hquor is in the range of 2—5. Under these conditions, the residual concentration of chlorite and chlorate ions in the bleach Hquor are minimized and chloride ion is the predominant chlorine species in the spent bleach (77). In addition to direct addition to pulp in bleaching, chlorine dioxide also finds use in wastewater treatment from pulp mill operations as a means to remove effluent color (85). 

After the molten glass has become a homogeneous Hquid, it is poured in a thin stream into water or between closely spaced, water-cooled rotating metal rollers. This quenched glass, termed frit, is a friable material easily reduced to small-sized particles by a ball-milling operation. Ball-milling the glass frit into small-sized particles can be carried out whether the frit is to be used wet or dry (see Size reduction). 

Tables usually are surfaced either with heavy battleship linoleum or with rubber. The riffles may be a clear grade of sugar pine or may be rubber strips. Such riffles usually taper from the feed end of the table to the discharge end. Almost all mill operators employ different styles of riffling table, which they believe best for their particular separations. The usual method of riffling is shown in Fig. 19-28.

In the Premier Mill the rotor is shaped hke the frustrum of a cone, similar to that in Fig. 20-53. Surfaces are smooth, and adjustment of the clearance can be made from 25 [Lm (0.001 in) upward. A small impeller helps to feed material into the rotor gap. The mill is jacketed for temperature control. Direct-connected hquid-type mills are available with 15- to 38-cm (6- to 15-in) rotors. These mills operate at 3600 r/min at capacities up to 2 mVh (500 gal/h). They are powered with up to 28 kW (40 hp). Working parts are made of Invar alloy, which does not expand enough to change the grinding gap if heating occurs. The rotor is faced with Stellite or silicon carbide tor wear resistance. For pilot-plant operations, the Premier Mill is available with 7.5- and 10-cm (3- and 4-in) rotors. These mills are belt-driven and operate at 7200 to 17,000 r/min with capacities of 0,02 to 2 mVh (5 to 50 gal/h). 

Talc milling is largely a grinding operation accompanied by air separation. Most of the industrial talcs are diy-ground. Diyers are commonly employed to prediy ahead of the milling operation because the wet material reduces mill capacity by as much as 30 percent. 

A wet-process plant maldug cement from shale and hmestoue has been described by Bergstrom [Roc/c Prod., 64—71 (June 1967)]. There are separate facilities for grinding each type of stone. The ball mill operates in closed circuit with a battery of Dutch State Mines screens. Material passing the screens is 85 percent minus 200 mesh. The entire process is extensively instrumented and controlled by computer. Automatic devices sample crushed rock, slurries, and finished product for chemical analysis by X-rav fluorescence. Mill circuit feed rates and water additions are governed by conventional controllers. 

Pellet Mills Pellet mills operate on the principle shown in Fig. 20-92. Moist, plastic feed is pushed through holes in dies of various shapes. The friction of the material in the die holes supplies the resistance necessary for compaction. Adjustable knives shear the rodlike extrudates into pellets of the desired length. Although several designs are in use, the most commonly used pellet mills operate by applying power to the die and rotating it around a freely turning roller with Fixed horizontal or vertical axis. 

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