US3012380A

US3012380A - Table extension for a grinding machine

Info

Publication number: US3012380A
Application number: US814475A
Authority: US
Inventors: Miles M Bass; Ramsay M Bell; Lee C Hammond; Philip J Robischung
Original assignee: Hammond Machinery Builders Inc
Current assignee: Hammond Machinery Builders Inc
Priority date: 1956-03-29
Filing date: 1959-05-20
Publication date: 1961-12-12
Anticipated expiration: 1978-12-12

Description

Dec. 12, 1961 M. M. BASS ETAL 3,012,380

TABLE EXTENSION FOR A GRINDING MACHINE Original Filed March 29, 1956 5 Sheets-Shet 1 I mmvrons MILES M- 81458 RAMJAY M 8!! y 1:: c. Hanna/v0 PHIUF n7. ROBISCHUNG' (QWMM Wall *9 QQ M.

ATTORNEYS Dec. 12, 1961 M. M. BASS ETAL 3,012,380

TABLE EXTENSION FOR A GRINDING MACHINE Original Filed March 29, 1956 5 Sheets-Sheet 2 Z INVENTORS MILES M. BAGS 5 RAMJAY M BEZL BY 1!! C. HAMMOND PHILIP i ROS/SCHUNG w ATTORNEY? Dec. 12, 1961 M. M. BASS. ETAL 3,012,380

TABLE EXTENSION F OR A GRINDING MACHINE Original Filed March 29, 1956 5 S heet 3 INVENTORS MILEJ M. an. BAMJAY M. BELL By Li: C. HAMMOND PH/UP J. ROBIJ'CHUNG (.AJOBQ- m 39AM W g a ATT OR NEY S 1961 M. M. BASS EI'AL TABLE EXTENSION FOR A GRINDING MACHINE 5 Sheets-Sheet 4 Original Filed March 29, 1956 m Y m My n I zua MSENO om 8 mm wummn 3 a d W 5 J CF r MAIH MRI-F mm... I .1

.v: B +m t 02 Q 1mm! NM m 6 7 //A mm I'll-J III. llln'lnIAl-IL II I .1 l1 Ii! on t. 1 v0 5. 1 1 a mm N u 1 4 mm X mk +9 5 wmfim o m9 6 m9 m9 ATTORNEYS Dec. 12, 1961 M. M. BASS ETAL TABLE EXTENSION FOR A GRINDING MACHINE Original Filed March 29, 1956 5 Sheets-Sheet 5 I I 5l ---77 14 7 III 5| 59d 50 6B 68a. .l b\\\ I 53 49 /4 s 57 55 54 623 67a. V643 V661 INVENTORS MILES M- BAJ'J RANJAY M Bill y LEE c. HAMMONO PHIL/P J: ROBIJ'CHUNG ATTORNEYS 3,012,389 TABLE EXTENSION FGR A GRHJDING MACHINE Miles M. Bass, Kalamazoo, Ramsay M. Bell, Schoolcraft,

and Lee C. Hammond and Philip J. Robischung, Kalamazoo, Mich, assignors to Hammond Machinery Builders, Inc, Kalamazoo, Mich, a corporation of Michigan Ori 'nal application Mar. 29, 1956, Ser. N 574,723, now Patent No. 2,922,258, dated Jan. 26, 1960. Divided and this application May 20, 1959, Ser. No. 814,475

2 Claims. 'Ji. 51=55) This invention relates to a grinder and particularly to one in which the grinding wheel may be caused to oscillate at a controllable frequency and amplitude. This application is a division of our application entitled Grinder Vr/ith Oscillating Head, Serial No. 574,728, filed March 29, 1956, now Patent No. 2,922,258, granted Jan. 26, 1960.

In conventional tool grinding equipment, it is customary to provide a rotatable grinding wheel adjacent a work-supporting table. The table is normally movable for adjustment relative to the axis of the grinding Wheel, so that the plane defined by the Working surface of the wheel and the plane defined by the table top will intersect at whatever angle is desired to facilitate grinding the work in a desired manner. However, in all of these grinders, insofar as we are aware, it is necessary for the workman who is utilizing the machine, such as for grinding a tool point, to manually move the tool point across the working face of the grinding wheel. The reasons for such movement of the tool point across the face of the grinding wheel are well known to the industry and may be summarized here by pointing out that such movement of the tool tends both to speed the grinding operation and to work out any irregularities which may exist in the face of the grinding wheel, i.e., dress the wheel This latter tends to secure a more nearly fiat and a more smoothly ground surface than would be possible if no such movement of the tool point were employed. Further, this movement of the tool point effects a desirable amount of so called cross-grain grinding and prevents the wheel grit from channeling the work.

This movement, while universally practiced where tool points and grinders are used, is nevertheless tedious, time consuming and tiring to the workman. Also, it is difiicult to grind a tool accurately while manually moving it along a table and with respect to a grinding surface. Further, where the tool is moved relative to the table, the table becomes Worn, and such wear is often sumcient to alter the positional relationship between a tool being held on the table and the wheel.

These disadvantages of the technique previously employed have long been recognized, but no acceptable provision for solving them has, insofar as we are aware, been offered to the industry. it is recognized that some machines have been provided which hold the tool mechanically and provide a relative oscillation between the wheel and the tool, but these machines are relatively expensive, are often difiicult to utilize, and their use is limited to conditions involving high production operation.

it is therefore desirable to provide a tool grinding machine adaptable to general-use, by which a workman can matintain all of the precision and individual attention to the exact shaping of the tool point, which is customarily obtained where the tool point is guided by hand, while avoiding the fatigue, inconvenience and occasional inaccuracy wiu'ch sometimes occurs where the relative motion of the tool with respect to the axis of the grinding wheel is provided by manually moving the tool across the face of the grinding wheel.

3,912,380 Patented Dec. 12, 1961 ice Accordingly, a principal object of the invention has been the provision of a grinding machine effective for obtaining the individual shaping of the work, such as a tool point, which is customarily obtainable by hand guiding of said work with respect to the grinding surface, while eliminating the fatigue, inaccuracy and inconven ience of manually moving the work back and forth across the face of the grinding wheel.

A further object of the invention has been to provide a machine, as aforesaid, in which the workman can hold the work, especially a tool point, stationary with respect to a supporting table, either with or without assistance of a mechanical holder or guide, and the relative motion of the work with respect to the axis of the grinding wheel is provided by the machine.

A further object of the invention has been to provide a machine, as aforesaid, having a device by which the axis of the grinding wheel is caused to oscillate with respect to the work-supporting table.

A further object of the invention has been to provide a device, as aforesaid, by which the foregoing-named objectives may be accomplished in a machine capable of relatively general use.

A further object of the invention has been to provide a device, as aforesaid, which will also be adaptable to use with grinding wheels employing a technique commonly identified as electrolytic grinding.

A further object of the invention has been to provide a machine, as aforesaid, which can be employed by any ordinarily skilled personnel without requiring the use of techniques diiferent from those which are already familiar.

A further object of the invention has been to provide a machine, as aforesaid, having portions of its worksupportiug table extending substantially across, and past, the periphery of said grinding wheel for supporting work, such as a tool having a reverse angle, of such shape that it must lie across, or past, the periphery of the wheel in order to present the surface to be ground to the working surface of the wheel, and yet such portions of the table are arranged so that said portions can be moved toward and away from the axis of said grinding wheel to accommodate the table to variations in amplitude of oscillations of said wheel.

A further object of the invention has been to provide a device having a work table, as aforesaid, with end portions extending through the extended plane of the working surface of the grinding wheel and including means associated with said grinding wheel for contacting and moving said portions away from said wheel prior to the striking thereof by said wheel, thereby protecting said wheel from contact with said portions.

A further object of the invention has been to provide a device, as aforesaid, including a guard extending around a substantial portion of said grinding wheel adjacent to its grinding face for preventing the tool, the operators hands, or the like, from accidentally slipping past the periphery of the wheel and being damaged or injured thereby.

A further object of the invention has been to provide a machine, as aforesaid, which will be adaptable for grinding tools of a variety of shapes.

Other objects and purposes of the invention will be apparent to persons acquainted with equipment of this general type upon reading the following disclosure and inspecting the accompanying drawings.

In the drawings:

FIGURE 1 is a side eleva'tional view of the machine to which this invention relates, with the adjacent side sheets thereof broken away.

FIGURE 2 is a sectional view taken on the line IIII of FIGURE 1.

7 cent to said table.

FIGURE 3 is a sectional view taken on the line II1III of FIGURE 1.

FIGURE 4 is a sectional view taken on the line IVIV of, FIGURE 3.

- FIGURE 5 is a sectional view taken on the line V-V of FIGURE 1.

FIGURE 6 is a sectional view taken on the line VIVI of FIGURE 3.

FIGURE 7 is a sectional view taken on the line VII- VII of FIGURE 3.

FIGURE 8 is a sectional view taken on the line VIII- VIII of FIGURE 7.

For the purpose of convenience in description, the terms upper, lower, left, right, and derivatives thereof, will have reference to the machine and parts thereof as appearing in FIGURE 2. The terms front and rear will have reference to the left and right sides, respectively, of the machine as appearing in FIGURE 1. The terms inner, outer, and derivatives thereof, will have reference to the geometric center of said machine and parts thereof.

General Description In meeting the objects and purposes above set forth, we have provided a machine having an adjustable, but relatively stationary, table with which a manually operable tool holder may be used, if desired, said machine having a structure for supporting a rotatable grinding wheel adja- The grinding wheel is mounted for oscillatory movement transversely of its rotational axis. During such movement, the center of said wheel traces an arcuate path which is generally parallel to the surface of the grinding table, that is, the tangent at the midpoint of said path is parallel both to the surface, and to an adjacent edge 50, of said table. This result is obtained by securing the grinding wheel 11 (FIGURES l and 4) to a shaft 12, which is rotatably mounted adjacent to the table 14 upon a shaft support bracket 13. The support bracket 13 is in turn pivotally supported by the bearing assembly upon the platform 15 of the machine frame 19. The

shaft 12 is continuously rotated by a belt 16 driven by the m gtor 17.

Oscillation of the bracket 13 about its pivotal support 20 is, in the preferred embodiment here shown, effected by a lever 18 (FIGURES 2 and 5) pivotally supported by the pillow block 19 on the upper wall 34 of frame 16 and connected to the bracket 13 by the pit-man 21. The lower end of the lever 18 is caused to oscillate, or is alternatively held against oscillation, by a roller 22, which is mounted upon the shaft 26, rotatable at a controllable speed with therpulley 25. The shaft 26 maybe moved radially of the axis of the pulley 25 by a hand wheel 23 working through a'mechanism 24 for adjusting the amount of eccentricity of the shaft 26 with respect to said pulley. The amplitude of oscillation of said wheel 11 is, therefore, controlled by the DETAILED CONSTRUCTION A. Frame and table I As shown particularly in FIGURES 1 and 2, the base frame 10 of the grinding machine to which this invention relates is upright, substantially rectangular, in this partichlarembodiment, andfabricated from any sturdy, conventional material, such as steel plates. A platform or bed plate 15 is mounted upon the upper wall 34 of the frame 10 and extends forwardly of the front wall 123 of the machine frame 10. The forward end of the platform engage and slidably support the base 36 of the table 14 for slidable, adjustable movement rearwardly and forwardly of the machine frame 16. Such movement is obtained by means of the threaded engagement between the screw 37 (FIGURES 3 and 4) and the double or split nut 38, which has front and rear portions 4% and 41, respectively, through both of which the screw 37 is threadedly received. One portion, here the rear portion 41, is rigidly secured to the platform 15 between the ways 35. The screw 37 is rotatably supported upon the front end of the table base 36 and held against axial movement with respect thereto in a substantially conventional manner. A hand wheel 45 is' secured to the front end of the screw 37 for effecting rotation thereof, hence movement of the base 36 along the ways 35. g

The table base 36 (FIGURES 3 and 4) extends laterally of, and above, the ways 35 and supports the bottom of a hood 4'6, which substantially surrounds the table 14 and the grinding wheel 11. Left and right table support members 47 and 48 (FIGURE 7) are mounted upon, and extend upwardly from, the table base 36 adjacent to its lateral edges (FIGURE 3) within the hood 46. The members 47 and 43 are preferably hollow and have concave, arcuate upper surfaces 49 having a substantially horizontal common axis 50 (FIGURES 4 and 7) lying within the plane defined by the upper surface 51 of the table14. A pair of table brackets 52'and '53 (FIGURE 5) are secured to, and depend from, the lower surface of the table 14 near its lateral edges. Said

brackets

52 and 53 have arcuate, convex lower surfaces 54, which are snugly, slidably and respectively receivable into the upper, concave surfaces 4 9. Side plates 55 are secured to, and extend downwardly beyond, the lateral sides of the

brackets

52 and 53. Said side plates 55 embrace the lateral sides of the

support members

47 and 48 when they are engaged by the

brackets

52 and 53, respectively, in order to prevent dirt from entering between the brackets and their corresponding support members. The

brackets

52 and 53 are slidable with respect to the

support members

47 and 48 for effecting tilting of the table 14 about the axis 59.

The tilting mechanism includes an adjustment screw 75, which is threadedly received through the adjustment block 74 and rotatably supported in a substantially horizontal position upon the support member 48 and the front wall of the hood 46,.through which said screw extends forwardly. A hand wheel 75 (FIGURE '1) is mounted upon the front end of the screw 75, said screw preferably being substantially parallel with the base adjusting screw 37. Accordingly, rotation of the hand wheel 76, hence rotation of the adjustment screw 75, effects forward or rearward movement of the adjustment block 74, hence similar movement of the lower end of the post 57, whereby said table 14 is caused to tilt about its axis 50.

The table 14 may include a plurality of removable and replaceable wear plates 77 (FIGURE 6) and a recess 78 (FIGURE 5) in its rearward edge, into which the grinding wheel 11 is received. The portion of the table 14 on opposite lateral sides of'said recess 78 are provided with

wear plates

79 and 80, which are movable toward and away from each other to vary the lateral extent of the recess 78 for reasons appearing hereinafter.

B. Grinding wheel and wheel support The bearing assembly 20, which pivotally supports the support bracket 13 (FIGURES 1 and 4) includes a pivot shaft 82. which is substantially parallel with the extended axes of the

screw

37 and 75 and is secured at its opposite ends in the blocks 83 and 84 (FIGURE 1) mounted upon the rearward end of the platform 15. The shaft 82 is preferably located substantially directly behind the cen ter line of the table base 36. The support bracket 13 (FIGURE 4) includes a cylindrical housing 85 at its lower J I end, which encircles the pivot shaft 82 and is rotatably 15 1s provided with ways 35 on its upper surface, which 75 mounted upon said shaft by means of the bearings 86.

Said bracket 13 also includes a support plate 37, which is secured to, and spaced from, the housing by a pair of webs 38. The shaft housing 89, in which the grinder shaft 12 is rotatably supported by means of the bearings 91, is mounted upon the plate 37. The grinder shaft 12 is axially parallel with the pivot shaft 82 and extends both forwardly and rearwardly from within the shaft housing 89.

A grinding wheel mount 92 (FIGURE 4) is rigidly secured to the front extension 93 of the shaft 12 by means of a bolt 94 in a substantially conventional manner. The rearward face of the wheel mount 92 and the forward, annular end wall 95 of the shaft housing 89 are provided with a suitable sealing device for preventing the waste materials of the grinding operation from getting into the bearings 91. The grinding wheel 11 is secured to the front surface of the wheel mount 92, as by means of bolts 96 (FIGURE 4), so that the planar grinding face of said wheel 11 lies in a substantially vertical plane and said wheel can be disposed adjacent to, and parallel with, the edge 78 1 (FIGURE 5) of the table 14 within the recess 78.

C. Wheel guard and table extensions A sleeve 1% (FIGURE 4) is adjustably supported upon the forward end of the shaft housing 39 for controlled movement about the shaft 12 in both rotational directions. A detent mechanism 1'31 tends to hold the sleeve 16!) in selected positions with respect to the shaft housing 89. Said sleeve 190 is disposed within the hood 46 and supports a forwardly extending, arcuate flange 102, upon which the arcuate guard element 103 is secured. In this particular embodiment, the tilting axis 54) of the table 14 is disposed somewhat below the axis of the grinding shaft 12. However, the opposite ends of the arcuate guard 103 are capable of extending simultaneously below said table upper surface 51 in one selected position of said guard, effected by appropriate positioning of the sleeve 1'09 upon the shaft housing 89. The guard 193 is spaced slightly radially outwardly of the grinding wheel 11, and the front surface of said arcuate guard 103 is preferably substantially coplanar with the front, or grinding, surface 104 of the wheel 11. Said sleeve 100 also has an annular flange 90 (FIGURES 4 and 6), preferably near its forward edge, which extends radially beyond the periphery of the grinding wheel 11. The flange 9 prevents the wheel 11 from striking those portions of the table 14 on opposite sides of the recess 78 when said wheel 11 is oscillated and the guard 103 is removed from, or re-positioned on, the housing 89. As shown in FIGURES 3, 5 and 6, the end wear plates 79 and 81 which are disposed on the table 14 radially outwardly from the grinding wheel 11, are in their normal, or extended, position when the arcuate guard 103 is centered. However, when said guard 193 is moved, for purposes appearing hereinafter, to its broken line position 103a, as shown in FIGURE 5, the end plate 79 adjacent to that end of the guard 103 which is raised above the level of the table may be moved in adiacent to the peripheral edge of the grinding wheel 11, as indicated at 79a, or as close as the flange 99 on the sleeve 109 will permit.

Means, including the tube 195, is supported by the flange 192 for directing cooling and/or grinding fluid at the center of the wheel 11.

The support bracket 13 and that portion of the shaft 12 and the shaft housing 89 disposed outside of the hood 46 are substantially enclosed by an enclosure 97 (FlGURE 4), which is mounted upon the frame and has an opening 98 in its front wall, through which said shaft housing 89 extends. The opening 98 is laterally elongated to permit oscillation of said housing 89 with respect to the enclosure 97. The space between the housing 89 and the enclosure 97 is closed by an annular sheet 99 having a flanged, internal edge 99:1, which snugly embraces the housing 39. Said flanged edge 99a, hence the sheet 99, is secured to the housing 89 by a resilient ring 81 and said sheet 99 slidably and snugly engages the front wall of the enclosure 97 to provide a dust seal.

D. Wheel drive mechanism The rear extension 106 of the shaft 12 supports a pulley 1 37 (FIGURES l and 4), which is connected to a pulley 1198 by the belt 16, said pulley 108 being mounted upon the shaft of the motor 17. The motor 17 is supported below the platform 15, in this particular embodiment, upon a motor mount 199 so that the axis of said motor is parallel with the shaft 12 (FIGURE 2). The motor mount 1199 is pivotally supported along one edge thereof, parallel with said motor axis, upon a pivot shaft 111, which is supported upon the frame 10. An arm 112 extends from the opposite edge of said motor mount 1G9 and supports a substantially horizontal pin 113 near its free end, which pin slidably extends through a lengthwise slot 114 in the support bar 115 near the lower end thereof.

The upper end of said bar 115 (FIGURES l and 2) is pivotally mounted upon the upper wall 34 of the machine frame 10.

E. Oscillatory drive mechanism The pitman 21 is pivotally supported at one end upon the front web 88 between the support plate 87 and the cylindrical housing 85. The other end of said pitman 21 is pivotally secured to the upper end of the lever 13, which lever is pivotally supported between its upper and lower ends by a pivot shaft 116 and a pillow block 19 mounted upon the top wall 34 adjacent to the bearing assembly 20. The lower end of the lever 18 is provided with a slot 113 which is elongated lengthwise of said lever 18. The roller 22 (FIGURE 5) is rotatably disposed within the slot 113 and is engageable with the side walls of said slot.

The pulley 25 is connected by the belt 27, in a substantially conventional manner, to the output of the speed reducer 28, which is controlled by the hand wheel 31 mounted thereon, and which is connected to the motor 29, also in a substantially conventional manner.

A pump 136 (FIGURE 2) may be provided for forcing a coolant or lubricant through the tube and returning said fluid to the pump from its point of collec tion in the hood 46 by conventional means, not shown.

an OPERATION After the grinding wheel 11 (FIGURE 4) is secured in place with respect to the shaft or spindle 12, the table 14 is then moved rearwardly until the rearward edge 78a (FIGURES 4 and 6) of said table 14 within the recess 78 and between the end wear plates 79 and 80 (FIGURE 5) is spaced a relatively slight distance from the front face :104 of the grinding wheel 11. Where the tool, such as that indicated in broken lines at 137 in FIGURE 4, is of the type which may be supported upon the wear plates 77 in front of the grinding wheel 11, the horseshoe-shaped guard 103 (FIGURES 3 and 5), which encircles the exposed portions of the periphery of the grinding wheel 11, will be disposed so that its opposite ends are below the surface of the table 14, as shown in FIGURE 5. In such case, the end wear plates 79 (FIG- URES 3 and 6) will be moved laterally outwardly of the edges of the other wear plates 77. However, if it becomes desirable to work on a hook-type tool of the type shown in broken lines at 138 in FIGURES 6, then the guard 103 may be rotated in one direction or the other around the axis of the shaft 12 into a position to expose the periphery of the wheel 11, as indicated in broken lines at 103:: in FIGURE 5. The end wear plate 79 adjacent to the exposed portion of said wheel is then moved into a position, as shown in broken lines at 79a I in FIGURE 6, so that it can hold the tool 138, which must extend partially behind the plane defined by the grinding face 104.

When the proper adjustment of the table has been obtained, the motor 17 can then be energized, thereby causing the shaft 12 and grinding wheel 11 to rotate.

The motor 29 is also energized, which drives the speed reducer 28, which in turn rotates the pulley 25 of the adjustment mechanism 24. The pulley 25, acts through suitable means within the housing 122 to drive the crank 26 and thereby efiect an oscillatory pivoting of the lever 18 about the pivotal axis provided by the lever bearing 19. Such reciprocation will be transmitted 'by the pitman 21 (FIGURE 2) to the support bracket 13 for effecting a corresponding pivotal reciprocation of said support bracket about the pivotal axis thereof provided by the bearing assembly 20. This will cause the shaft 12 to oscillate through a relatively small arc crosswise of the machine, the tangent of said are at its midpoint being substantially parallel to the pivot axis 50 of the table 14, and thereby effect a similar lateral oscillation of the grinding wheel 11. Where the grinding wheel, as shown here, has a relatively narrow grinding face 104, the oscillation of the grinding wheel will be relatively small and, as a general rule, will be somewhat less than the radial width of the grinding face on said wheel 11. Thus, the path traversed by the axis of the shaft 12 during said oscillations, although arcuate, will be relatively flat and substantially parallel to pivot axis 50 of the table 14.

The flexible and resilient annular sealing sheet 99 (FIGURE 4) permits movement of the shaft housing 89 with respect to the enclosure 97 during the oscillation of said-shaft housing 89 without impairing the seal provided thereby. The opening 98 in the enclosure 97, through which the shaft housing '89 extends, is sufliciently large to permit the desired oscillation of the shaft housing 89.

Adjustment of'the oscillation distance or amplitude of t the shaft 12 is effected by turning the hand wheel 23 associated with the adjustment mechanism 24. By appropriate adjustment of the hand wheel 23, the roller 22 can be caused to be completely concentric with the pulley 25, whereby the grinding wheel 11 can be caused to rotate without any reciprocation or oscillation whatsoever.

It will be recognized that the adjustment of the table 14, the guard 103, the

end wear plates

79 and 80, the adjustment mechanism 24 and the speed reducer 28 can be effected while the machine is in operation.

The necessary adjustments having thus been completed, the tool, either as shown at 137 in FIGURE 4 or at 138 in FIGURE 6, is placed upon the table 14 and held there, either manually or by mechanical means, while the grinding wheel 11 and the guard 103 reciprocate with respect to the table 14, as indicated by the solid line and broken line positions of these parts in FIGURE 6. The grinding wheel 11 is protected from striking those edges of the table 14 which define the lateral ends of the recess 78 by the flange 90 on the sleeve 100, which flange extends radially beyond the periphery of the wheel 11. The guard 103 provides additional protection, both for the wheel 11 and the hands ofthe workman, by closely,

8 surrounding the portion of the wheel 11 exposed above the table 14. 7

Although particular, preferred embodiments of the invention have been disclosed hereinabove for illustrative purposes, it will be understood that variations or modifications thereof, which do not depart from the scope of such disclosure, are fully contemplated unless specifically stated to the contrary in the appended claims.

We claim:

1. In a grinder having a grinding wheel movable back and forth in the plane of the radii of said wheel including means adjusting the amplitude of'movement of said wheel, a work-supporting table comprising in combination: an edge portion having a straight edge adjacent said wheel and arranged parallel with the plane of movement of said wheel and lying entirely forward of said plane of movement; at least one extension member slidably mounted on the edge portion of said table and extending from said edge portion through andbehind said plane of movement, said member being slidably arranged with respect to said edge portion for movement on and with respect to said edge portion toward and away from the center of said wheel; bumper means arranged between said grinding wheel and that edge of said extension member facing said grinding wheel, whereby electrical contact between said grinding wheel and the adjacent edge of said extension member is positively prevented by said bumper means.

2. In a grinding machine having a grinding wheel movable back and forth in the plane of the radii of said wheel including means for adjusting the amplitude of said movement of said wheel, and structure for rotatably supporting said wheel and eifecting said movement thereof, the combination comprising: a work supporting table having an edge portion with a straight edge adjacent said wheel and parallel with said plane of the radii of said wheel, said edge portion of said table lying entirely on one side of. said plane and extending substantially beyond the maximum amplitude of movement of said wheel; a table extension member slidably mounted upon said edge portion of the table and extending from said edge portion through said plane of said wheel, said extension member being movable along said straight edge of said table toward and away from the adjacent peripheral edge of said wheel; and flanged bumper means arranged on said wheel supporting structure and extending toward said table extension member for engagement thereof, whereby engagement between said wheel and said extension member is positively prevented by prior engagement of said table extension member by said flanged bumper means.