Abrasive Belt Grinding of Metals HUGH N. DYER Behr-Manning Division, Norton Co., Troy, N . Y .
Failure of coated abrasive belts used for grinding metals may occur either by fragmentation of the abrasive grains or by an attritious type of wear which dulls the grain points without appreciable fragmentation. The relative rates of the two types of wear depend on certain controllable grinding conditions, such as belt speed, work pressure, and type of grinding fluid. The balance between fragmentation and attritious wear also changes with the metal being ground. Some tests on both laboratory and commercial grinding equipment are reported which show how metal removal and belt wear vary with different grinding conditions and with different metals.
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HE recent development of machines and methods which can take advantage of the inherent characteristics of coated abrasives as machining tools has resulted in a tremendous increase in the industrial use of coated abrasive belts. Figures 1 and 2 show views of an abrasive belt centerless grinder. Several of these grinders are used in steel milk to grind the outside diameter of seamless stainless steel tubing. A typical production record is the removal of 50 pounds of Type 304 stainless steel a t an average rate of 1pound of metal removed per minute. The machine uses an abrasive cloth belt 9 inches wide by 14 feet long for grinding, and a shorter abrasive regulating belt (Figure 2). Figure 3 shows a flat sheet grinder that uses cloth and paper belts of various grit sizes for grinding and finishing sheet metal. This machine was developed specifically for mechanical pickling or scale removal by grinding of hot rolled titanium alloy sheet. It uses an abrasive belt 50 inches wide by about 20 feet long. With the increasing number of applications of the coated abrasive machining method, there is an increasing demand for data corresponding with the extensive published machinability data already available to those concerned with the older metal cutting methods-turning, planing, milling, and drilling. The volume of such data directly applicable to coated abrasives is still very small, but some extremely helpful preliminary information is available.
Figure 1. Abrasive belt centerless grinder
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GRINDING MECHANICS AND TYPES OF BELT FAILURE
Abrasive articles such as grinding wheels, stones, and coated abrasive belts are referred to as multipoint cutting tools. Examination of the swarf from an abrasive belt grinding operation, especially one where a grinding fluid is used, shows clearly that the abrasive grains remove small metal chips, much like the tools in milling machines and lathes; but the geometry of abrasive grains is quite complicated and random and is not subject t o accurate control as in the case of tool bits. Although certain features of grinding and of orthogonal metal cutting are basically similar (I--@, the mechanics of chip removal is mainly of theo-
Figure 2.
Abrasive belt centerless grinder
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 12
ABRASIVES AND REFRACTORIES retical interest in connection with coated abrasives and has not been worked out in detail as in the case of tools of simple geometry. The grinding swarf usually contains fragments of abrasive grains and bond mixed with the chips, as a result of the fracturing and dislodging of abrasive grains from the coating by impact against the work. I n addition t o this fragmentation wear, cutting points in contact with the work also become dulled or flattened, either through fragmentation on a microscopic scale or through actual solution in or chemical reaction with the work. These flattened cutting points remain in the bond matrix without being dislodged or fractured further. This type wear has been called attritious wear in order to distinguish it from the fragmentation-type wear ( 6 ) .
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50-Inch abrasive belt surface grinder
Ideally, an abrasive belt should fail by alternate attritious and fragmentation wear until all the abrasive grain has been used. Each grain would then remain in the coating until it becomes too dulled, when it would be fractured or dislodged from the coating, exposing fresh sharp cutting edges. In practice, there is usually either too much fracture wear or too little. In the first case, the belt fails by shedding, or premature loss of abrasive; in the second, the belt fails by dulling or glazing, and although the belt remains coated with abrasive, the flattened exposed grains are not capable of cutting. In conjunction with the dulling-type failure, there may also be more or less loading of the abrasive surface with metal chips which adhere to the cutting points and wedge themselves among the grains. This loading condition creates additional frictional heat and reduces the cutting efficiency of the exposed grains.
Comparative stock removal-accelerated shedding test
a severe shedding test, and is designed to cause an accelerated failure of the belt from fracture wear. Comparisons between stock removal rates as well as between belt shedding rates are possible, and Figure 6 shows the appearance of the used belts for which the stock removal rates are shown in Figure 4. When the belts are run dry the rate of removal of carbon steel is 2 to 3 times the rate of removal of stainless steel under the same conditions. The belt used on the carbon steel shed nearly to the backing along the wear path. The belt used on stainless steel, even after being run nearly twice as long, showed relatively little shedding but rather failed by a combination of dulling plus a smearing of the metal over the flattened grain points. Figure 6 does not show this comparison clearly between the two belts 3 and 6 because of a lack of contrast between the worn and unworn parts of belt 3. The loading of belt 6 is visible, however, as a lighter streak near the right-hand edge. I n this test belt failure on carbon steel was primarily from a fragmentation-type wear; whereas, the test belt failure on the stainless steel was from attritious wear and loading. When grinding fluids are used the stock-removal and belt-wear patterns are modified. The cutting fluid emulsion-a selfemulsifying mixture of mineral oil and soap, diluted with 80 parts water-has a minor effect on initial rate of stock removal. The greatly accelerated fragmentation wear of the abrasive belt
LABORATORY GRINDING TESTS AT HIGH UNIT PRESSURE
It is more difficult to grind certain stainless steels than carbon steels with coated abrasive belts. Figure 4 shows the results of a series of tests using grit 50 aluminum oxide, resin bonded, cloth backed, belts to grind both low carbon steel (1020 hot rolled) and stainless steel (Type 304) under the same conditions of high unit work pressure. The belt speed was 5000 feet per minute, and the metal test piece was oscillated across the belt at a speed of 7 feet per minute under a constant pressure of 11 pounds. As shown in Figure 5 , the rubber contact wheel over which the belt runs and the work piece are so positioned that only a narrow path of the belt approximately 8 / 8 inch wide is in contact with the work piece. This is a standard test in this laboratory, called December 1955
Figure 5.
Testing machine for laboratory grinding tests
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PRODUCT AND PROCESS DEVELOPMENT 1
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2.
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Used belts accelerated shedding test
1020 Steel, oil 1020 Steel, emulsion 1020 Steel, dry
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Type 304 stainless steel, oil Type 304 stainless steel, emulsion
Type 304 stainless steel, dry
reduces effective belt life, thereby reducing total stock removal, particularly on carbon steel. The commercial cutting oil, which contains chemically active additives-sulfur, chlorine, and lard oil-gives results comparable with the emulsion on carbon steel but further accelerates both initial cutting rate and belt wear on stainless steel. CENTERLESS GRINDER TESTS
The abrasive belt centerless grinder of Figure 7 was used to obtain additional data on the grinding of low carbon and stainless steels with various grinding fluids and a t several belt speeds. The machine is equipped with a steel contact wheel, with powered work supporting fixtures on both sides of the contact wheel, and with variable speed drives so that a wide range of belt speeds and of work speeds is available. There is also a hydraulically
Figure 7.
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operated device for automatically reversing the work fwd angle a t the end of each pass through the machine so that the direction of through feed is reversed. The work pieces were bars about 2 inches in diameter and 10 feet long. Bars of both low carbon and stainless steel were trued up a t the beginning to provide straight, round, and scale-free test pieces. The automatic reversing fixtures and feed belt were set a t 7" feed angle and 120 feet per minute surface speed to give a through-feed rate of about 14.5 feet per minute in each direction. The infeed was manually controlled to maintain a nearly constant load of about 7 horsepower on the main drive motor, and the test bars were removed and weighed periodically. The same bar was used throughout each individual test. The test belts were grit 50, aluminum oxide grain, resin bonded, cloth backed, 6 inches wide by 14 feet long. A series of preliminary tests, in which two belts each of two different runs were used under the same conditions, established that the product variability is low and the test reproducibility is good. Nevertheless, all the belts used in this series of tests were from the same run of material, and duplicate tests were run occasionally. Figure 8 shows the results of grinding stainless and carbon steels at a belt speed of 4000 feet per minute with the same two grinding fluids used in the test of Figure 4. Evidently, the tests of Figure 8 correspond to a much lower unit work pressure than the tests of Figure 4, as shown by the much longer belt life in the centerless grinding tests. When the water-base grinding fluid is used, there is very little fracture wear with either type of steel. Failure is primarily from attritious wear. In the case of the stainless steel, serious loading or welding of metal to the flattened grain points developed, and the test had to be discontinued after a short time because of excessive chatter. On both alloys, cutting performance is greatly improved when grinding with the active cutting oil. The belts show much more extensive fracture wear than did the belts used with the emulsion, and no loading is evident. Under these conditions, the active cutting oil is a good grinding fluid for both carbon and stainless steels. The emulsion is a fair grinding fluid for carbon steel but a very poor grinding fluid for stainless steel. Unfortunately, photographs of the used belts are not available.
Machine for centerless grinding tests
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 12
ABRASIVES AND REFRACTORIES GRINDING FLUID COMPOSITION EFFECT ON STOCK REMOVAL
A COMMERCIAL CUTTING OIL
It was known that the commercial cutting oil which worked
M. 0. + SULFWZEO CONC.
so effectively on the stainless steel was a mineral oil base containing 3.3% sulfur, 1.0% chlorine, and 11.4% animal fat, with a viscosity of 174 Saybold seconds a t 100' F. A series of tests was run on the centerless grinder to compare the performance of a commercial 100 second mineral oil with the performance of
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blends of the mineral oil with commercially available additives.
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mater base emulsion is a poor cutting fluid for Type 304 stainless steel. Because of the relative difficulty of machining materials of the nature of Type 304 stainless steel, free machining alloys are available which have better machining properties. Thus Types 303 and 304 have similar analyses except that the former has a fairly high percentage of sulfur, phosphorous, or selenium incorporated, providing a built-in extreme pressure lubricant. Type 303 stainless grinds much more readily than Type 304, even with straight mineral oil, and the various additives increase the effectiveness of the mineral oil only slightly (Figure 11).
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BELT SPEED EFFECT ON STOCK REMOVAL
These additives were Sulfurized black still bottom oil containing 6% active sulfur Chlorinated hydrocarbon containing 50% active chlorine 3. No. 1 lard oil 1.
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The additives were used in the following proportions, by volume: 1, Mineral oil-70%, 2 . Mineral oil-98%,
3, Mineral oil-90%,
sulfurized concentrate-30% chlorinated concentrate-2% lard oil-lOR
Figure 9 shows that on carbon steel a t 4000 feet per minute the sulfur additive alone is equally as effective as the combined additives of the commercial cutting oil. Straight mineral oil and the blends of mineral oil and additive are all more effective than the emulsified mineral oil as far as stock removal rate and total metal removal are concerned. All the belts used with the various mineral oil blends failed primarily by shedding. Thus, it is the mineral oil itself which is responsible for promoting the fracture-type wear, rather than the additives. The variation in effectiveness of the single additives is probably related to their effectiveness in preventing rewelding of metal chips t o the work and to the cutting points of the abrasive belt. Figure 10 shows the cutting curves obtained a t 4000 feet per minute for Type 304 stainless steel. Here again the sulfur is the most effective of the single additives, and is equally as effective as the combined additives of the commercial cutting oil On the stainless steel, there is a greater spread between the most effective and the least effective additives, and it is noteworthy that the relative effectiveness of lard oil is much less on stainless steel than on carbon steel. As has already been pointed out the December 1955
Figure 12 shows the curves obtained with l o a carbon steel a t 8000 feet per minute belt speed. Each curve has a lower slope than the corresponding curve of Figure 9, but the relative effectiveness of the individual additives is unchanged. The higher speed does appear to have a greater proportionate effect on the poorer additives, and a t the higher speed the sulfur additive alone does not give performance equal to that obtained with the commercial cutting oil. At 8000 feet per minute on the Type 304 stainless steel, the
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MINERAL OIL
BELT SPEED -4000 FT./MIN
0 TIME (MINUTES) Figure 10. Effect of cutting fluid on centerless grinding
INDUSTRIAL AND ENGINEERING CHEMISTRY
2503
PRODUCT AND PROCESS DEVELOPMENT commercial cutting oil and the individual additives are less effective than a t 4000 feet per minute. The sulfur additive used alone is no longer equal in effectiveness to the commercial oil. If the three additives are combined in an attempt to synthesize the commercial oil, the cutting curve falls between that for the commercial oil and that for the mineral oil with sulfur additive.
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metal, and the interval between contacts, is of the order of 10-4 to 10-6 seconds. It is not surprising that the additives are less effective a t 8000 feet per minute than at 4000 feet per minute, because of the short time available for the fluid to reach the metal-grain interface and then to enter into reaction before the next chip is removed. The combined additives (sulfur, chlorine, and lard oil) of the commercial cutting oil are more effective than sulfur alone a t 8000 feet per minute but not at 4000 feet per minute. This synergistic effect indicates a catalytic action of the other additives which results in increased effectiveness of the sulfur a t high cutting speeds when the combined additives are present. I n addition to the chemical effect of the additives, the mineral oil itself has a tendency to increase fragmentation wear and to promote shedding failure of the abrasive belt. This is probably a physical effect as a result of a tendency for the oil to penetrate betveen the grains and the adhesive film, and to saturate the fibers of the cloth backing, thus reducing the effective bond strength. This effect is independent of the additive. Besides affecting the time available for the cutting oil additives to react chemically, the belt speed also affects the relative amounts of attritious and fragmentation wear. High belt speeds result in higher temperatures at the abrasive grain-metal contact. These higher temperatures encourage attritious wear by accelerating any chemical or physical reaction which takes place between abrasive grain and metal. At low belt speeds fragmentation wear of the abrasive belt is promoted, according to the
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TIME (MINUTES) Figure 11. Effect of cutting fluid on centerless grinding
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2 203 Figures 13 and 14 show, on a larger scale, the lower portions of the cutting curves obtained with the commercial cutting oil and the emulsion at belt speeds of 2000, 4000, and 8000 feet per minute. In each case belt life was poor at 2000 feet per minute because of rapid shedding failure (fracture wear) of the belts. For carbon steel highest stock removal rates were obtained with both fluids at 4000 feet per minute. At 8000 feet per minute the cutting rates were lower and the belts showed less tendency to shed than a t 4000 feet per minute, Similar results were obtained on Type 304 stainless, except for the very high initial stock removal rates obtained a t 2000 feet per minute with both oil and emulsion. DISCUSSION OF RESULTS
The chemically active cutting oils usually contain compounds of phosphorous, chlorine, or sulfur. Chemical reaction between additive and the freshly cut metal surfaces forms contaminating films of low shear strength which help prevent galling, seizing, and rewelding of the chips to the woik or to the cutting tool. These antiweld agents are particularly effective when grinding the austenitic stainless steels, such as Type 304, which are known for their tendency to gall and reweld in machining operations. Sulfur and chlorine are effective chemical grinding fluid additives for both carbon steels and stainless steels. Lard oil is believed to have some boundary lubricating properties as a result of thermal breakdown under severe conditions to form fatty acids which in turn react with the metal surface to form metallic soaps in situ. This additive is effective on carbon steel but there is no obvious explanation fgr the fact that poorer results were obtained on stainless steel with the lard oil additive than with the straight mineral oil (Figure 10). The Chemically active additives are effective a t belt speeds as high as 8000 feet per minute. Calculations show that a t this speed the actual time of contact between individual grains and 2504
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Effect of cutting fluid on centerless grinding
grain depth of cut concept of Alden (1). For constant work speed, work diameter, contact wheel diameter, and feed, the grain depth of cut varies inversely as belt speed. At low belt speeds the grain depth of cut, and thus the force on the individual grain, is greater than a t high speeds. The centerless grinding tests were not conducted under constant feed conditions, but rather under conditions more nearly approximating constant pressure. The infeed rate was actually higher at 4000 feet per minute belt speed than a t 8000 feet per minute. Since grain depth of cut varies directly as some function of infeed, the grain depth of cut and thus the force on each grain was still further increased a t the lower speed. The very rapid shedding failure observed a t 2000 feet per minute is accounted for on the basis of a very high grain depth of cut and correspondingly high forces tending to fracture the abrasive grains and to break them out of the coating.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 12
ABRASIVES AND REFRACTORIES
coxcLUSION s A COMML. CUTTING OIL
The results of the centerless grinding tests show that chemically active cutting oil additives such as sulfur and chlorine are effective with coated abrasive belts a t belt speeds as high as 8000 feet per minute. Abrasive belts are unique among cutting tools in their ability to make effective use of chemically active cutting fluids a t such high speeds. The stock removal ability of abrasive belts can be markedly effected by the choice of grinding fluid. I n general, belts used with water-base grinding fluids tend to shed faster than when used dry, while oil-base fluids further increase the shedding tendency. The oil-base grinding fluids with chemically active additives are particularly effective on Type 304 stainless steel.
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Effect of belt speed on centerless grinding
speeds, in qualitative agreement with the Alden grain depth of cut concept (1). As a practical matter, this means that high belt speeds give better finishes, slower stock removal, and slower belt shedding. Lower belt speeds give poorer finishes, more rapid stock removal, and more rapid belt shedding. Once the kind of belt failure associated with a given grinding operation is known, recommendations can be made concerning the best belt speed, grinding fluid, and contact wheel for the job, depending on whether stock removal or finish is the primary consideration. With the accumulation of Rome experience, recommendations which give near optimum results can often be made in advance of any trials.
Effect of belt speed on centerless grinding ACKNOWLEDGMENT
The effective use of abrasive belts depends on obtaining a balance between attritious wear and fracture wear. With carbon steels a suitable balance is achieved under ordinary grinding conditions, and satisfactory belt performance is obtained. With certain stainless steels there is a predominance of attritious wear under ordinary grinding conditions, with the result that abrasive belts fail by dulling and loading before an economically useful belt life is realized. Fracture wear of abrasive belts can be controlled to some extent by controlling unit work pressure and the type of contact wheel or belt support used. High unit work pressures and hard contact wheels promote fracture wear and give more aggressive cut. Softer contact wheels and lower unit work pressures give better finishes. The results of a series of centerless grinding tests with abrasive belts show that the choice of belt speed also affects the way in whrich the belt fails, and consequently the rate of stock removal and the finish obtained. The effect of belt speed is to promote attritious wear a t high speeds and fragmentation wear a t low
December 1955
The photographs for Figures 1, 2, and 3 were kindly supplied by Production Machine Co., Greenfield, Mass., and by Mattison Machine Works, Rockford, 111. The experimental measurements were made by D. R. Lowther, R. W. Degener, and the 1at)eJohn F. Scanlon. LITERATURE CITED
(1) Alden, G . I., Trans. Am. SOC.Mech. Engrs., 36, 451-60 (1914). (2) Ernst, Hans, “Metal Cutting; Art t o Science” in “Machining-
Theory and Practice,” Am. SOC.Metals, Cleveland, Ohio, 1950. (3) Guest, J. J., Proc. Inst. Mech. Engrs. (London), 79, 543-66 (1915). (4) Letner, H. R., Steel Processing, 40, 774-9, 798 (1954).
(5) Merchant, M. E., “Metal Cutting Research-Theory and Application” in “Machining-Theory and Practice,” Am. Soc. Metals, Cleveland, Ohio, 1950. (6) Wagner, H. W., Grits and Grinds, 41, No. 3, 9-13 (1950). RECEIVED for review April 6, 1955.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ACCEPTED $ugust 10, 1955.
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