US 3204371 A
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Description (OCR text may contain errors)
Sept. 7, 1965 A. c. BOOTH 3,204,371
MOUNTING INSERT FOR RESINOID CUP GRINDING WHEELS Filed Feb. 4, 1963 2 Sheets-Sheet 1 FIG. I
/ ,frzy k1 98 FIG.4 22 @Mwg Xhzawey Sept. 7, 1965 A. c. BOOTH 3,204,371
MOUNTING INSERT FOR RESINOID CUP GRINDING WHEELS Filed Feb. 4, 1963 2 Sheets-Sheet 2 FIG. 5
United States Patent 3,204,371 MOUNTING INSERT FOR RESINOID CUP GRINDING WHEELS Arthur C. Booth, Fayville, Mass., assignor to Bay State Abrasive Products Company, Westboro, Mass, a corporation of Massachusetts Filed Feb. 4, 1963, Ser. No. 255,825 5 Claims. (Cl. 51168) This invention relates to a mounting insert for use in the manufacture of resinoid cup grinding wheels. More particularly this invention relates to a mounting insert of the threaded prong-anchor bushing type adapted to be molded integrally in said wheel and provided with an internally threaded shank or spindle receiving portion having a substantially spherical exterior configuration and with an integral safety ring.
Resinoid cup grinding wheels are normally used in hand operated portable grinders for rough heavy duty grinding operations, such as snagging steel or other metal castings, forgings, weldments and the like. Because of the nature of the use the bottom fiat edge of the cup, which comprises the grinding Wheel and a portion of the side thereof, must of necessity remain unshielded and unguarded. Thus even though the grinding wheel is carefully manufactured so that it will remain intact under centrifugal forces substantially in excess of any that will be encountered in actual use, it is impossible to protect the wheel from damage due to impact such as that which might occur if the portable grinder is dropped, or if the exposed portion of the grinding wheel is hit against some object. Such an impact usually results in a fracture of the wheel and this fracture frequently occurs along a diameter. Since the wheel is rotated at a high speedgenerally in the order of about 6000 r.p.m.any such fracture is likely to hurl large segments of the wheel at great force for long distances causing injury to the operator or to by-standers, or damage to the work.
As the result of this potential for causing damage or injury various expedients have been proposed for the purpose either of strengthening the wheel or of keeping the segments of the wheel intact even if the wheel is fractured. A typical such protective device is the revolving cup guard (Morrison guard) specified for use with cup Wheels when used on portable grinders in Section 4.4 of the American Standard Safety Code for the Use, Care, and Protection of Abrasive Wheels, American Standards Association, New York, Publication B7.11956. The revolving cup guard, a cup formed of steel or other material with adequate strength, is mounted on the spindle behind the wheel in a manner as to turn with the spindle and the wheel and provides a peripheral member for the wheel extending outward from the base at least onethird of the wheel thickness with a clearance from the sides of the wheel of inch or less. In some instances a peripheral steel guard meeting the requirement of Code Regulations 4.4 is molded right onto the wheel as part of the mounting insert. This type of guard is not particularly popular with grinder operators since it adds appreciably to the weight of the portable grinder, and if, as usually is the case, it is not premolded as part of the grinding wheel, it adds appreciably to the time required for mounting the Wheel.
Another expedient frequently used to retain the fractured pieces of a cup wheel and to prevent these from flying off is to provide the wheel with an internal safety ring molded right into the wheel itself such as is disclosed in Gregory US. Patent 2,250,580, July 29, 1941, or in the article by Gormly and Kibbey entitled Resinoid Cup Grinding Wheels appearing in the magazine Foundry for June 1959. Such rings while quite effective for their purpose, complicate the manufacturing process because 3,204,371 Patented Sept. 7, 1965 they require that a pre-measured portion of the grinding wheel mix be placed in the mold, the ring placed in position on top of this first portion and then the remainder of the mix be inserted in the mold during the molding process. Not only is the manual placing of the ring costly, but there is always the danger that the ring will be incorrectly placed and will therefore be off centered or at an angle to the back in the completed wheel.
In considering these and other safety and mounting devices (including those of the present invention) for use with resinoid cup grinding wheels certain basic facts should be borne in mind. For example, a grinding wheel is of necessity of limited strength since to operate properly it is essential that fresh abrasive grains be constantly exposed during the use of the wheel. Fresh grains can become exposed only if the wheel wears down during the grinding operations. The rate of wear of the wheel and the strength of the wheel are interrelated functions and for a given size and style of wheel are both dependent upon the nature and size of the abrasive grain and the nature and relative quantity of bonding material used.
Grinding wheels are formed by mixing abrasive grains With a binding material and forming and curing the mixture in a mold. The most satisfactory binding agents for cup type wheels are the various thermosetting resins such as the phenol-formaldehyde resins, the urea formaldehyde resins or the like. Other binding materials may be used such as inorganic cementitious or vitreous materials or Various natural and synthetic organic polymeric and film forming agents. Thus while the present invention is described in terms of resinoid cup grinding wheels it will be readily apparent that the advantages flowing from the mounting insert of the present invention are in no way dependent upon the particular chemical or physical characteristics of the particular abrasive mix, but rather the mounting insert of the present invention would show proportional advantages with any type of bonded abrasive material.
Finally a grinding wheel is designed to operate and in fact is tested under centrifugal forces much greater than any that may be exerted under normal operating conditions. For example, standard resinoid cup wheels are speed tested at a peripheral rotational velocity of 14,250 surface feet per minute (s.f.p.m.) even though the operating velocity is limited to a maximum of 9,500 s.f.p.m. Actually the destructive velocity of such resinoid cup wheels is substantially higher than the control velocity of 14,250 s.f.p.m. This destructive velocity is substantially independent of the mounting and/or safety devices employed, but rather is almost solely dependent upon the physical and chemical nature of the bonded abrasive itself, and upon the size, shape and uniformity of the wheel. In other words, the destructive velocity for wheels of a given size and style made from identical mixes under identical conditions is substantially independent of the mounting or of the particular safety device employed.
It the ordinary resinoid cup grinding wheel is in itself strong enough to withstand centrifugal forces much greater than any to which it will be sul ected during use, what then is the cause of breakage at these lesser velocities, especially the characteristic cracking along a diameter? Certainly such damage to the wheel is not related to internal strain set up'during the molding and curing operation for any such strain would show up when the wheel is subjected to excess rotational velocities in routine testing. Neither would it appear to be likely that such breakage could be caused by ordinary wear, since as the wheel is worn it loses mass and diameter both of which would tend to reduce the resultant centrifugal force imposed upon the wheel at a given rotational velocity. In-
deed there is evidence to show that a worn but not abused wheel can withstand a greater velocity at destruction than can a new unused wheel.
I It would appear that the breakage of a pre-tested wheel under normal use is due entirely to abuse and rough handling of that wheel. Normally such abuse can occur under two conditions: when the wheel is unmounted, for example during shipment, storage or the like; or when the wheel is mounted, for example, as by dropping the grinder or knocking it against some solid object. The wheel itself is very strong and it can be expected that any abuse incurred when the wheel is unmounted will show visibly. Thus if you drop an unmounted wheel onto a hard surface, or bang away at it with a hammer, the damage is likely to show up as chips knocked off of the edge of the wheel. To produce an incipient crack in an unmounted Wheel of this shape is definitely more diflicult than it would be in the case of a mounted wheel.
The mounted wheel is another matter entirely. When the wheel is mounted it is maintained axially about a stud or spindle which in turn is part of a portable grinder, which weighs perhaps 18 pounds on the average. Any force that would tend to tilt the wheel is resisted by the mounting stud. Since the mounting stud is normally cylindrical and extends axially into the cup for the full depth of the base of the cup any tilting motion of the cup relative to the mounting stud would tend to create a force locally at the center of the cup acting to push the upper or inner edge of the base of the cup in one direction, and the lower or outer edge of the base of the cup in the opposite direction. This prying action exerted axially of the cup could very readily cause the characteristic cracking along the diameter typical of the normal failure of a cu grinding wheel.
To test this theory, resinoid cup grinding wheels with several different types of conventional mounting devices were attached to a threaded stud mounted at the end of an 18 pound steel cylinder (to duplicate a normal portable grinder in size and weight) and the assembly was dropped from a height of four feet onto a steel plate placed on a concrete floor. In tests with five separate samples of each of three different types of conventional mountings every wheel cracked apart along a diameter, the crack being visible either initially orafter the dropped wheel was spun to a velocity of 16,000 s.f.p.m. in a speed tester.
It is the object of this invention to provide a resinoid cup grinding wheel with an integrally molded mounting nut which will not tend to pry the wheel into two halves should there be any force applied to the wheel tending to tilt the wheel relative to the attached portable grinder. It is a further object of this invention to provide a mounting bracket of the threaded prong-anchor bushing insert type with integral safety ring whereby a wheel havinga molded safety ring may be made in a single mold filling operation.
It is a further object of this invention to provide a grinding wheel with mounting means whereby a visible and audible warning is given when any attempt is made to use the wheel after it has been subjected to a potentially damaging force.
These and other objects of the invention will be apparent from the drawing and from the description which follows:
In the drawings:
FIG. 1 is a plan view of a resinoid cup grinding wheel.
FIG. 2 is a plan view of the preferred mounting bracket.
FIG. 3 is a bottom view of the bracket of FIG. 2.
FIG. 4 is a side view of the bracket of FIG. 2.
FIG. 5 is a section along lines 5-5 of FIG. 2 showing in phantom a partial cross-section of a typical grinding wheel to which such a bracket might be applied.
FIG. 6 is a plan view of an alternate form of the mounting bracket.
FIG. 7 is a side view of the bracket of FIG. 6.
FIG. 8 is a bottom view of the bracket of FIG. 6.
The typical resinoid cup grinding wheel 11 as shown in FIG. 1 is in the shape of a truncated right cone with a threaded nut 12 integrally mounted in base 13 with the axis of the nut corresponding with the axis of the cone. Base 13 has a smaller radius than face 14 whereby the outer edge 15 of wheel 11 tapers outwardly from the base to the face. A tapered recess 16 is provided in face 14 of the wheel to provide a constant wall thickness for wheel 11. The thickness of base 13 is normally equal to or slightly less than the wall thickness. In a typical wheel for example the diameter of the face 14 is approximately 6", the diameter of base 13 is approximately 5", the length of edge 15 is approximate 2%" (i.e., the distance between the face and the base along a line normal to both is approximately 2 /8"), the wall thickness is approximately 1%" and the base thickness is approximately A5". The wheel comprises grains for abrasive grit bonded together by a thermosetting resinous material. The mounting insert may include a backing plate in which case the backing plate remains exposed in the back of the base of the wheel. The size, shape and structure of the grinding wheel itself is no part of the present invention which relates solely to an improved mounting bracket for such wheels.
In its simplest form, as shown in FIGS. 6, 7 and 8, the present invention contemplates modifying in two significant respects a conventional mounting bracket 17 of the threaded prong-anchor bushing insert type. Such a mounting bracket normally comprises a mounting nut 12, a circular backing plate 18 mounted at one end of the nut, and a plurality of integral prongs 19 extending from the edge of'backing plate 18 and bent upwards at an oblique angle thereto.
The first modification comprises welding or otherwise fastening -a safety ring 21 to the tips of prongs 19. In a typical prong-anchor bushing for a 6" wheel, safety ring 21 has a diameter of about 3". The safety ring in this position has proven to be very effective in preventing the propagation of cracks originating at the mounting nut or bushing to points beyond the ring, and thereby preventing the creation of a typical diametric fracture in such grinding wheels. Thus the safety ring 21 not only ties the parts of the wheel together as was the case in the previous isolated safety ring, but also prevents major cracks and fractures from forming. In addition since ring 21 is positively fastened to prongs 19 the grinding wheel can be molded in a single operation with positive assurance that the ring will always be properly located.
The second modification comprises replacing the conventional cylindrical mounting nut with a mounting nut 12 having an outside contour substantially that of a spherical segment. More particularly mounting nut 12 of the present invention is formed so that every point on its side wall 22 is at a substantially equal distance from a point on the axis of nut 12 midway between top edge 23 and bottom edge 24. The spherical exterior surface of nut 12 permits nut 12 to be offset under stress without applying any fracture initiating strains to the molded abrasive.
Nut 12 is provided with a mounting extension 25 adapted to extend within and cooperate with the edge of mounting hole 26 provided in backing plate 18. A plurality of cut back stake points 27 are provided around the periphery of mounting hole 26 and the metal comprising mounting extension 25 is swaged into the stake points to fasten nut 12 to backing plate 18. Alternatively mounting nut 12 may be fastened to backing plate 18 by other means.
The preferred modification of the invention is shown in FIGS. 2, 3, 4, and 5. In this modification rather than fastening a safety ring 21 to the upturned ends of preformed prongs 19, mounting bracket 17' is formed from a one piece stamping provided with a flat back 28, an upturned edge 29, and a plurality of apertures 31 provided in upturned edge 29 adjacent to backing 28. Mounting nut 12' is identical with nut 12 described in connection with FIGS. 6, 7, and 8 and is fastened to back 27 by swaging portions of mounting extensions 25' into stake points 27' provided around the edge of mounting hole 26.
Preferably bracket 17 is formed by providing a circular plate with apertures 31 and thereafter forming upturned edge 29 as by stamping.
In a typical mounting bracket 17' for a 6" resinoid cup grinding wheel the bracket is formed from a circular piece of 12 gauge steel plate 3%" in diameter. Apertures 31 are high and 7 inches wide with rounded edges. Edge 29 is upturned to an angle of about 60 to the horizontal with an inside diameter of back 28 of 2 /8" and an outside diameter of the bracket of 3%". Stamping out the upturned edge 29 after the apertures 31 are formed results in a characteristic scalloped edge 32. Edge 32 acts as the full equivalent of ring 21 and the portions of edge 29 between apertures 31 as full equivalents of prongs 19. In this typical mounting nut 12 has an overall height of about 0.735 inch, an over-all width of 1%," with side walls 22' formed on a radius of 0.950". Mounting extension 25' is about thick.
Mounting nut 12 (12) may if desired be provided with one or more peripheral grooves 33. The function of grooves 33 is to provide a better bonding between nut 12 (12') and the molded abrasive composition comprising the grinding wheel. As shown a single groove A wide and deep is provided at the maximum diameter of nut 12 (12').
The sphere shaped mounting nut apparently acts as a ball in a socket. If the grinding wheel is stressed excessively the nut tips from the vertical thereby causing a state of out of truth or dynamic unbalance. If such a wheel remains on a grinder, and especially if it is applied to work, it chatters, vibrates or bumps in a very obvious and alarming manner and, for this reason alone, must be replaced. Thus a wheel provided with a spherical mounting nut is double safe. Not only does such wheel pro vide an obvious and umnistakable indication that it has been over-stressed, but also, since the nut is relatively free to rotate in the corresponding socket in the molded wheel, there is much less tendency for any such stress applied to the mounting nut to create a cracked wheel.
In order to prove the effectiveness of the new mounting a series of test wheels were made identical in every way except for the mounting used. In addition to wheels made with both versions of the new mounting a number of conventional mountings were employed. These include a hexagonal nut similar to that shown in Gregory US. Patent 2,250,580 but modified to the extent that four upstanding axial projections are provided at equally spaced intervals along the groove between the end flanges. All of the wheels provided with such a hexagonal nut were additionally provided with an independent conventional safety ring. Another series of wheels were made using a conventional prong-anchor bushing type of mounting. A third series was made using a modification of the conventional prong-anchor bushing type of mounting wherein the back member is continued beyond the prongs to terminate in an upturned edge about the outer periphery of the wheel thereby providing in one piece a pronganchor bushing insert and a molded in place rotary cup guard. Such a mounting is referred to as a safety back type.
Five wheels of each type, each 6"/4%" x 2" x /3" in size were tested by attaching the cup wheel to an 18 pound steel cylinder (to approximate the weight and size of a portable grinder) by means of a threaded stud. This assembly was then dropped from a height of four feet onto a steel plate on a concrete floor. Each wheel was thereafter mounted on a speed tester and spun to a speed of 16,000 s.f.p.m. The results were as follows:
Condition after test The five wheels with the new type insert vibrated excessively when spun to 16,000 s.f.p.m., but did not disintegrate or show any sign of visible cracking whatsoever. Both versions of the new type of mounting were tested with no discernible differences. In no instance was any secondary safety device such as a rotary cup guard used.
In the development of a mounting it is customary to perform torque tests to determine if the mounting has an adequate safety factor against turning or slipping within the wheel body. A secondary factor to be considered in this respect is whether under torque the mounting itself transmits forces which tend to disintegrate the abrasive body comprising the wheel. Most commonly used grinders are driven by a two horsepower motor with a no load speed of 5 to 6,000 rpm. The maximum stall torque under load for such a grinder has been calculated to be approximately inch pounds. In the static test employed for determining torque induced damage a torque of 7,000 inch pounds has been selected as a desired standard. Such test gives a safety factor of 70 over the calculated maximum actual stall torque of a portable grinder.
Such torque tests were run on a group of wheels provided variously with the new insert, the hexagonal nut insert and the prong-anchor insert. In each instance the torque at failure was measured and the reason for the failure recorded. The results were as follows:
Torque Type (inch lbs.) Remarks at failure 7, 700 Stud failed.
720 Do. 7, 380 Do. 7, 930 Do. 8, D0. 7, 380 Do. 8,020 Nut turned-wheel cracked. o 8, Stud failed. Hexagonal nut 7, 960 Stud failedwheel cracked.
Do 4, 300 Do.
In every instance with wheels using the new insert the mounting stud failed and there was no evidence of slippage of the ball nut in the wheel body or of cracks in the wheel.
As a further test a group of wheels containing the new insert, and a corresponding group containing the conventional prong-anchor bushing insert was speed tested to destruction. As has been mentioned above the ultimate strength of a wheel under such tests is that of the molded abrasive mass and the nature of the insert has little effect on the ultimate strength under a centrifugal stress unless, for some reason, the insert itself is of such a nature as to reduce this strength. The speed of destruction (the average of five wheels tested) for the pronganchor type insert was 21,130 s.f.p.m. compared to 20,- 970 s.f.p.m. for the average of five wheels with the new insert. Since this difference in ultimate strength is well within the range of error inherent in the test it is apparent that the new insert does not in any way reduce the ultimate strength of the wheel. It is to be noted that in each instance the speed at destruction is substantially more than twice the maximum permissible operating speed for the type of wheel tested.
In all of the tests both types of the new insert were used with no discernible difference in result. In other words, it appears to make no difference whether the safety ring is welded to the tips of the upturned prongs of a conventional prong-anchor type insert, or whether the similar structure is formed by punching and stamping as disclosed.
The essential features of the mounting unit of the present invention include a mounting nut having a spherical or substantially spherical exterior configuration (such that there will be substantial force tending to pry the wheel apart if nut is tilted for any reason), a flat generally circular backing plate, upturned prongs upstanding at an oblique angle located at spaced intervals about the periphery of the backing plate in such position relative to the wheel as to extend toward the walls of the cup beyond the recess and a continuous band of metal connecting the prongs and with an effective radius greater than that of the bottom of the recess in the cup and located in the wheel at a level slightly below the level of the bottom of the cup. A suflicient amount of space should be left between adjacent prongs and between the base and the back to permit a substantial bonding between the molded abrasive mass located on both sides of the upstanding portion of the insert. An arrangement wherein the prongs occupy only about one-half of the periphery of the base appears to be adequate.
While the invention has been described in terms of the modification of a conventional prong-anchor bushing type of insert, it is obvious that the same modifications can be made to the safety back type of insert described above by substituting a spherical nut for the conventional cylindrical mounting nut and mounting a safety ring on the upturned prongs.
1. A mounting insert for molded grinding wheels of the resinoid cup type comprising a backing plate, a mount 8 ing nut upstandingly mounted on said backing plate, said mounting nut having an exterior configuration substantially that of a spherical segment, a plurality of spaced upturned prongs upstanding from said plate and a metal band connecting said prongs.
2. A mounting insert as claimed in claim 1 wherein said metal band is a ring attached to the ends of said prongs.
3. A mounting insert as claimed in claim 1 wherein 10 said backing plate, said prongs and said metal band comprise a single punched and shaped element.
4. A molded grinding Wheel of the resinoid cup type, having as the mounting element thereof an integrally molded threaded mounting nut having an exterior configuration substantially that of a spherical segment.
5. A molded grinding Wheel as claimed in claim 4 wherein said mounting nut is provided with a peripheral exterior groove.
References Cited by the Examiner UNITED STATES PATENTS 1,672,573 6/28 Maynard 51168 2,250,580 7/41 Gregory 51 209 2,278,301 3/42 Bauer 5l209 2,316, 161 4/43 Harvey 51209 X 2,324,377 7/43 Fischer 5l-209 2,351,129 6/44 Jockel 5l209 2,771,719 11/56 Fuglie 51168 2,806,331 9/57 Hoye 51-168 2,950,582 8/60 Beauchaine 51l68 LESTER M. SWINGLE, Primary Examiner.
JOHN C. CHRISTIE, Examiner.