Surface conditioning abrasives

mechanical surface preparation

Surface Conditioning Abrasives BY JAN REYERS 3M CO., ST. PAUL,MINN.; WWW.MMM.COM Surface conditioning, or three-dimensional abrasive media, provides a means of producing uniform finishes and is used in the intermediate operations between heavy grinding or dimensioning and final bu~mg, plating, or application of coatings. The media are made from three basic materials: fiber, resin, and abrasive mineral (Fig. 1). These components are oriented in a manner that allows the mineral to follow along a surface in a springlike action. One of the major advantages of this type of construction is controlled cut, the ability of an abrasive media to follow over contours, removing only surface contaminants and small amounts of the suhstrate material. Low-density abrasives can &burr, clean, and finish surfaces without changing the geometry of the part. Production requirements, such as rate of cut and surface roughness, can be regulated by varying the rotational speeds and pressures.

~

~dve

Fig. 1. Standard abrasive web.

(Fig. 4): 1- to 18-in. diameter X

1/4-

FORMS AND GRADES

The four forms of low-density abrasive products presently available are the following: Cleaning brushes. Abrasive web cut into disk shape, compressed together, and held inplace mechanically (Fig. 2) to 18-in. diameter X 8- to 120-in. width. Flap brushes. Single sheets of abrasive web cut to length and adhered at a 900 angle to a steel or fiber core (Fig. 3): 6- to 18-in. diameter X 1/2- to 64-in. width. Unitized wheels. Disks of abrasive web compressed together and abrasively bonded to form a solid wheel to 1-in. width.

Convolute wheels. Abrasive web convolutely wrapped around a fiber core and adhesively bonded between layers to form a hard cutting wheel (Fig. 5)- 4 to 24-in. diameter X 2/2 to 18-in. COMPRESSED width. 3 Dimen~nalAbrasiveDiscs Each of these forms is available in various grades and hardnesses that affect the amount of cut and subsequent surface roughness (rms) on a substrate (Fig. 6). Because of their three-dimensional Fig. 2. Cleaning brush construction.

construction, these products are not graded by grit or mesh numbers as are conventional single-layer abrasive media such as oil wheels, setup NONDIRECTIONAL wheels, greaseless compounds, and coated abrasives. Low-density media are available in six grades: coarse, medium, fine, very fine, superfine, and ultrafine. This method of gradFig.3. Flapbrushconstruction. ing in sequence for a given brush or wheel form takes the guesswork out of determining what grade should be used to prepare a surface for buffing or plating. For example, a very fine grade would be used to reduce the scratches left by a fine grade, which was used to reduce scratches remaining from a medium grade, and so forth. The surface roughness left after use of each grade can be predetermined, as illustrated in Fig. 6. As indicated in the area of nonferrous materials, the coarser the product, the wider the microinch range, which is dependent on the type of material being worked on and its hardness. Abrasive minerals used in low-density abrasives include aluminum oxide, silicon carbide, flint, and garnet. Varying in shape and hardness, these minerals are utilized in different products designed for specific applications. The type of material to be worked on will dictate which mineral should be used. Aluminum oxide and silicon carbide, for example, are most commonly used on metal, glass, plastic, and rubber, whereas garnet is used primarily on wood. The amount of abrasive web used to make up a given size and converted form determines the product's hardness. Hardness is related directly to product life, rate of cut, and finish. Densities are rated by a numbering scale, from 1 to 10. The higher the number, the harder the product. Generally, the softer products, 5 density and lower, are used for decorative finishes. These softer products conform more readily to surface contours and generate a uniform scratch finish. Typical applications are finishing nameplates, automotive and appliance trim, satin finishing of stainless steel, and a wide variety of jewelry items. They are particularly well suited for the finishing of aluminum. The lower density, nonwoven, open-type construction creates less heat at the working surface than conventional media, thereby reducing galling problems associated with finishing aluminum. The harder converted forms, 6 density and higher, are recommended for heavier stock removal, cleaning of surface contaminants, and deburring operations. DISKSBONDED TOGETHER

WHEEL SPEEDAND PRESSURE Rotational speed directly affects the rate of cut and the type of finish. The higher the surface feet per minute (sfpm), the higher the rate of cut. Of course, the maximum safe operating speed of the wheel or brush should not be exceeded. Higher speeds are used only when high rates of cut are required for polishing operations.

RADIALLY ~

~

Abr~veSheetst L

~AOHESIVE

~

FIBER orSTEEL CORE Fig.4. Unitizedwheelconstruction. 45

CONVOLUTELY WOUND~

~ DIRECTION Slower speeds will lengthen the scratch and create a more uniform, ~ ~\~RROW

~//~/"FIBER CORE AOHESIVE Fig. 5. Wheel construction.

decorative finish. Some typical speeds for different applications are listed in Table I. Work pressures for a given application depend on the selected product form and the application. Unitized and convolute wheels require higher work pressure, whereas brush forms require lower pressure and horsepower. Recommended work pressures and horsepower (hp) requirements are listed by applica-

tion in Table II. It is important that the correct pressure be used, both to generate the desired cut and finish and to obtain the most economical life from the low-density abrasive. The work pressures suggested here indicate relative amounts of work required over and above no-load or free idle of the motor. Pressure for offhand operations will be lower, usually in the 3- to 5-1b range. In practice, the optimum is obtained when the minimum pressure is used to achieve desired results. Excessive pressure can cause premature wear and does not necessarily increase the rate of cut due to the controlled cut characteristic of three-dimensional abrasives.

LUBRICANTS Lubricants, such as soaps, waxes, tallow, water-soluble coolants, and grinding aids, can be used. Water-based coolants and grease- or oil-based lubricants can beneficially affect the life, rate of cut, and the finish produced while eliminating damage to heat-sensitive materials, such as thermoplastics and glass, by preventing heat buildup. With the use o f a coolant, it is possible to increase the rate of cut of a low-density abrasive while producing a duller finish. In the case of lubricants, Ultra Fine

Super Fine Very Fine

Fine Medium

Coarse ExtraCoarse

20

40

60

80

100

120

140

160

Average Surface Roughnmm(micro inches) Fig. 6. Average surface roughness. 46

180

200

TabLe I. WheeL Speeds for SeLected AppLications, s f p m

Cleaning Deburring Decorative finish Polishing (steel)

2,000-5,000 3,000-6,500 500-2,500 6,000-9,000

Table II. Recommended Work Pressures for Various Applications Applications Decorative finishing Cleaning Deburring Polishing

Work Pressures (lb/working inch) 5-10 10-20 10-20 20-40

Horsepower (hp/working inch) 0.1-0.15 0.1-0.2 0.15-0.2 0.5-2

surface roughness can be reduced. The higher velocity lubricants produce lower surface roughness; that is, grease produces a lower rms than oil. Consequently, it is desirable to use either a coolant or a lubricant whenever possible.

SHAPING An important feature of the low-density abrasive is that it can be readily shaped or formed to follow complex part contours. The shape will be retained throughout its usable life. One of the most effective and easiest methods used to shape wheels or brushes is to adhere a piece of 36-grit, coated abrasive to the part and hold it against the wheel until the shape is formed. Low-density abrasive wheels will retain intricate shapes without undercutting or creating flats. SUHNER ABRASIVE EXPERT AG

SUHNER introduces the first model in an entirely new generation of flexible-shaft machinery--the Rotomax. This new engineering design retains the specific properties that boosted the Rotovar to success through the years. Separating the drive unit and the tool gives the user a powerful drive without having to hold a heavy tool. The lightweight tool holders make for delicate and fatigue-free work wherever high power levels are neede6--even in locations with restricted accessibility. Phone: 41 (0)56 464 28 28 • Fax: 41 (0)56 464 28 29 E-mail: [email protected]• www.suhner,com www.metaffinishing.com/advertisers

www.metalfinishing.com/advertisers 47