US3707956A

US3707956A - Apparatus and method for forming relief portions in the periphery of a grinding wheel

Info

Publication number: US3707956A
Application number: US00116806A
Authority: US
Inventors: H Swenson
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-02-19
Filing date: 1971-02-19
Publication date: 1973-01-02
Anticipated expiration: 1990-01-02

Abstract

This invention pertains to the dressing of face-formed grinding wheels and in particular diamond-faced grinding wheels wherein certain face portions may be at an acute angle to a plane normal to the axis of the grinding wheel. To drastically reduce the probability of burning the workpiece while contour grinding a selected portion of this workpiece, apparatus is provided to alter the periphery of the grinding wheel so that determined portions of said periphery of the grinding wheel are relieved. The grinding wheel face so contoured presents a series of faceforming segments by which the workpiece is ground. The dressing apparatus is selectively movable to and from a dressing condition of the periphery of the grinding wheel.

Description

United States Patent m1 Swenson APPARATUS AND METHOD FOR FORMING RELIEF PORTIONS IN THE PERIPHERY OF A GRINDING WHEEL Jan. 2, 1973 1,700,634 1/1929 Hopf ..5 1/206 R Priniary Examiner-Harold D. Whitehead Attorney-Ralph R. Roberts [57] ABSTRACT This invention pertains to the dressing of face-formed grinding wheels and in particular diamond-faced grinding wheels wherein certain face portions may be at an acute angle to a plane normal to the axis of the grinding wheel. To drastically reduce'the probability of burning the workpiece while contour grinding a selected portion of this workpiece, apparatus is provided to alter the periphery of the grinding wheel so that determined portions of said periphery of the grinding wheel are relieved. The grinding wheel face so contoured presents a series of face-forming'segments by which the workpiece is gr0und.'The dressing apparatus is selectively movable to and from a dressing condition of the periphery of the grinding wheel.

Claims, 2 Drawing Figures PATENTED M' 2 m 3 707 956 INVENTOR.

HENRY F SWENSON "'7 f v ((141.4. 4,5, AGENT APPARATUS AND METHOD FOR FORMING RELIEF PORTIONS IN THE PERIPHERY OF A GRINDING WHEEL CROSS REFERENCE TO A RELATED APPLICATION The apparatus for forming relief portions in the periphery of a'grinding wheel may be, and preferably is, used with apparatus for contour dressing a grinding wheel as shown and described in U.S. Patent Application entitled, Grinding Wheel Dressing Apparatus" and given Ser. No. 63,060 when filed Aug. 12th, 1970 in the name of Henry F. Swenson who is also the applicant for this application.

BACKGROUND OF THE INVENTION development began more than a century ago. Contour grinding by precisely shaping the face of the grinding wheel before and during the grinding of portions of the workpiece has become an established machining method developed within the last few decades. A problem which has plagued this contour grinding method of machining has been the tendency of burning the workpiece when the angle of a face por-' tion of the grinding wheel approaches a condition near right angles to the axis of the grinding wheel rotation. This condition occurs when there is a tendency for the face of the grinding wheel to rub the workpiece surface being ground. There is also a tendency for burning the work when the grit of the grinding wheel is quite fine and tends to glaze during the grinding of the workpiece as when it is necessary for a micro" finish of '8 r.m.s. (root means square) micro-inches or better to be formed on the workpiece.

- This problem of burning is largely solved or at the least is greatly reducedby the method of forming a series of recessed contours into the periphery of a grinding wheel by the apparatus provided by this invention. Whereas, a contour-faced grinding wheel conven tionally has its entire periphery precisely formed and radially disposed around the axis of the grinding wheel, this invention includes an additional forming step.

After initially forming the face of the grinding wheel to grinding residue to be dispersed and distributed particularly at the impingement nip of the grinding wheel and workpiece. The engagement of the face of the grinding wheel with the workpiece, instead of being continuous is by the relieved grinding of this invention reduced to intermittent engagement periods which are preferably regular and which enable the workpiece, at the area of grinding, to be kept at a substantially cooler temperature.

2 BRII-EF SUMMARY OF THE INVENTION This invention may be summarized at least in part with reference to its objects.

It is an object of this invention to provide, and it does provide, apparatus pivotally supported so that in response to a predetermined cam arrangement a previously form-shaped grinding wheel may have its peripherial grinding surfaces portion formed with a plurality of relief portions which are at least a few thousandths of an inch below the previously dressed surface of the grinding wheel.

It is a further object of this invention to provide, and it does provide, a pivotally mounted grinding wheel dressing apparatus disposed to carry shaping means for forming the face of the grinding wheel, said apparatus further having a follower arm adapted to ride on a rotary cam carried by and rotated with the spindle upon which is mounted the grinding wheel; said cam being contoured so that the engaged and supported follower arm is raised and lowered by the contour of the cam to form a plurality of relief portions in the peripherial face of the grinding wheel.

It is a still further object of this invention to provide, and it does provide, a method for forming determined relief portions of a grinding wheel including the steps of supporting grinding wheel dressing apparatus so as to be movable toward and to the grinding wheel carried on the grinding machine; attaching a follower arm on the dressing apparatus; mounting a lobed circular cam on the spindle ofthe grinding machine, and arranging a follower roller on the follower arm so as to ride on the lobes of the cam to limit movement of the dressing apparatus toward the grinding wheel while the movement which provides the means for limiting the movement of the dressing apparatus into the grinding wheel is manipulated to permit the roller, after leaving a lobe, to

- lobes and recesses is removably mounted on the retain-' ing nut mounted on the outer end of the spindle of the grinder. The cam follower is adjusted to rest upon the periphery of the cam after the dressing mechanism has been used to complete the dressing of the grinding wheel. The cam follower is caused to bear on the lobes of the cam to lift the dressing means from the grinding wheel and when the cam follower is brought in way of and enters the recesses of the cam the grinding wheel is permitted to dress these local areas of the grinding wheel to provide recesses in the grinding wheel surface. The depth of the several recesses is usually each alike and of several thousandths of an inch.

In addition to the above summary the following disclosure is detailed to insure adequacy and aid in understanding of the invention. This disclosure, however, is not intended to prejudice that purpose of a patent which is to cover each new inventive concept therein no matter how it may later be disguised by variations in form or additions of further improvements. For this reason there has been chosen a specific embodiment of the apparatus for forming relief portions in the periphery of a grinding wheel as adopted for use with contoured grinding wheel dressing and showing a preferred means for mounting a lobed cam on the spin-. 7

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 represents a side view of a lobed cam and a follower support arm of this apparatus in mounted condition on a pivoted dressing apparatus as shown in phantom outline, and

FIG. 2 represents an end view of the apparatus of FIG. 1 with the grinding wheel shown with its periphery contoured and in solid outline prior to its being relieved by the apparatus of the invention.

In the following description and in the claims various details will be identified by specific names for convenience; these names, however, are intended to be generic in their application. Corresponding reference characters refer to like members throughout the two figures of the drawing.

The drawing accompanying, and forming part of, this specification discloses certain details of construction forthe purpose of explanation of the broader aspects of the invention, but it should be understood that structural details may be modified in various respects without departure from the concept and principles of the invention and that the invention may be incorporated in other structural forms than shown.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now in particular to the drawing and the apparatus depicted in FIGS. 1 and 2, there is to be seen in phantom outline a frame 10 of a grinding wheel dressing apparatus such as that shown in my application Ser. No. 63,060, filed Aug. 12th, 1970. To the extent necessary to show and describe contour dressing of grinding wheels, application, Ser. No. 63,060, is incorporated by reference into the instant application. This frame 10 or upper housing is pivotally supported on a base 12 attached to the grinding wheel machine. Base 12 is also shown in phantom outline and may be removably attached to the grinding machine by means disclosed in the reference patent. A micrometer screw 14 is disposed to limit and control the downward movement of frame 10 toward base 12 and is precisely adjusted to enable shaping wheel to form and dress the periphery 22 ofa grinding wheel 24 (FIG. 2).

A follower arm 30 has a roller or ball bearing type cam follower 32 rotatably carried on its lower end. The upper end of arm 30 is formed with

slots

34 and 35 through which the shank and threaded portion of a turnscrew 36 is passed for mounting the arm 30 on frame l0. The threaded end 38 of turnscrew 36 is retained in a threaded hole formed in frame 10 while the headed end portion of the turnscrew 36 is disposed to engage the outer face of the follower arm. When the turnscrew is tightened in the threaded hole it retains the follower arm 30 in a selected fixed condition and position on the frame.

A cam 40, as depicted, is a disc-like member having a bore 42 sized to slide on and be retained'on the outer diameter of a retaining nut 44 which conventionally provides the means for retaining the grinding wheel 24 on a spindle 46 of the grinding machine. A set screw 48 is carried in a threaded transverse hole in cam 40 and is tightened in the hole to retain cam 40 on nut 44. As

shown, the cam 40 is formed with four lobes 50 of like radius and each is of an extent equal to about one? eighth of the diameter of the cam.

Between lobes 50 are formed recess portions 52 which are also preferably of a like configuration. These recesses are madeat least about one-thirty-second to one-eighth of an inch radially less than theouter surface of lobes 50. The width or thickness of the cam 40 is made sufficiently ample so that roller 32 may be positioned to easily ride on the periphery of the cam 40.

USE AND OPERATION OF THE RELIEF FORMING APPARATUS To use the relief forming apparatus of this invention it is necessary that there be provided a grinding wheel dressing device which preferably is similar if not identical to the apparatus shown in my US. Patent application, Ser. No. 63,060. During the dressing or forming of the periphery 22 of grinding wheel 24 by shaping wheel 20 the follower arm 30 is either removed from frame 10 or is slid outwardly in

slot

34 or 35 so that cam follower 32 is away from any possible engagement with cam 40. Micrometer screw 14 is manipulated to progressively lower wheel 20 into dressing condition until the grindingwheel 24 hasbeen brought to the desired dressed condition. Screw 14 is then manipulated to lift wheel 20 a few thousandths of an inch from this final dressing condition.

With shaping .wheel 20 in this lifted condition, the turnscrew 36'is loosened so that follower arm 30 may he slid along the

slot

34 or 35 and toward cam 40. The follower arm 30 is advanced until follower roller 32 engages the peripherial surface of a lobe 50 whereupon turnscrew 36 is tightened to retain arm 30 in this adjusted condition. To insure that this tightened condition is at the desired setting the screw 14 is manipulated to lower frame 10 a couple of thousandths of an inch toward base 12 whereupon the spindle 46 and mounted grinding wheel 24 are rotated a couple of timesv to insure that roller follower 32 engages and rests on each lobe 50 as the cam 40 is rotated with the wheel 24 and that when the follower rests on a lobe the dressing apparatus does not engage the grinding wheel just dressed.

Having establishedthat the arm 30 and the follower roller 32 are at the desired setting, the grinding wheel is once more rotated at the determined dressing speed and the shaping wheel 20 is again rotatedat its desired dressing speed. Micrometer screw 14 is now rotated so that shaping wheel 20 moves into and dresses the grinding wheel 24 to provide recess portions every time follower roller 32 leaves lobe 50 and drops into one of the recess portions 52. The micrometer screw 14 is continued to be manipulated to permit shaping wheel 20 to form recesses until about fifteen thousandths of an inch is removed from the grinding wheel at the four arc portions corresponding to the four recesses 52. At the portions corresponding to the lobe portions of the cam the grinding wheel remains as formed since the roller engages each lobe to lift shaping wheel 20 from forming or dressing contact with grinding wheel 24.

After the desired depth of the recesses has been established in the periphery of the grinding wheel, the frame is lifted to disengage shaping wheel 20. Arm 30 may be removed, if desired, after which the shaping and finishing of the workpiece is accomplished by grinding in the usual manner. The recesses in the face of the grinding wheel 24 permit the grinding of the workpiece to be accomplished at a much cooler temperature and with a greater flooding of coolant on the workpiece surface being ground. This grinding method is particularly effective where the grinding grit of wheel 24 is quite fine and/or the surface of the workpiece being ground is at an acute angle to a plane normal to the axis of spindle 46.

There has been shown two

slots

34 and 35 in the upper portion of follower arm 30. Only one slot is used at one time but slot 35 permits the cam follower 32 to be moved leftwardly to engage the periphery of cam 40 when cam 40 has been moved leftwardly or outwardly when retaining nut 44 is moved outwardly to retain a thicker grinding wheel 24.

Although the preferred apparatus for forming the grinding wheel 24 is the apparatus shown and described in my application, Ser. No. 63,060, it does not follow that this is the only apparatus with which the present invention may be used. Other apparatus may be provided wherein the dressing or shaping wheel or means is pivotally supported so that it is freely movable by gravity toward the grinding wheel and is readily lifted by arm 30 as follower 32 engages a lobe 50.

It is also contemplated that the dressing means 20, instead of being pivotally retained, may be carried on a substantially vertical slide or guide means and by gravity be advanced to and into the grinding wheel 24. Relief portions are similarly made in the grinding wheel by the dressing means as it is lifted from the grinding wheel by follower roller 32 as it engages a lobe 50.

Although primarily directed toward use in apparatus.

for contour shaping of the periphery of the grinding wheel, this relief forming of a grinding wheel may also be used with grinding wheels whose faces are straight or angle dressed. Desirably the recesses are formed generally symmetrical so as to keep the grinding wheel generally in balance, although quite a variation in the balance of the wheel can be made without vibrations developing in most grinding machine spindles. Ultra precision fit of bore 42 on nut 44 of the forming and disposition of lobes 50 and recesses 52 is not necessary as the unbalance of the recessed grinding wheel is generally so slight that this small unbalance of the grinding wheel does not affect the grinding accuracy or finish.

It is noted that instead of a separate cam 40 the nut 44 may have lobes and recesses formed on its periphery so while an integral member, it still provides a lobe cam.

The above-described apparatus also provides a method for forming determined relief portions in the periphery of a grinding wheel while and as it is carried on the spindle of a grinding machine, this method including the steps of supporting a grinding wheeldressing apparatus on a grinding machine so that the dressing apparatus is pivotally movable toward and away from the grinding wheel as it is carried on the spindle of a grinding machine; mounting a circular cam having a plurality of lobes and relief portions on the spindle of the grinding machine so that the cam is rotated as the spindle is turned; attaching a follower arm to the grinding wheel dressing apparatus so that a roller follower carried by and on the distal or lower end of said follower arm is disposed to engage and ride on the lobes of the circular cam to limit the movement of the dressing apparatus toward and to the grinding wheel as the roller engages a lobe, and finally there is provided the step of selectively limiting the movement of the dressing apparatus into the grinding wheel to dress a plurality of recesses into the periphery of the grinding wheel at local extents corresponding to the recesses formed in the periphery of the cam at those portions between lobes.

Terms such as left, right, up, down, bottorn, top, front, back, in, out", and the like are applicable to the embodiment shown and described in conjunction with the drawing. These terms are merely for the purposes of description and do not necessarily apply to the position in which the recess forming apparatus may be constructed or used.

While aparticular embodiment of the recess forming apparatus has been shown and described. it is to be understood the invention is not limited thereto since modifications may be made within the scope of the accompanying claims and protection is sought to the broadest extent the prior art allows.

What is claimed is:

1. Apparatus for forming determined relief portions in the periphery of a grinding wheel as used on a grinding machine, said relief portions providing discontinuations in the prepared peripheral grinding surfaces in and on the face of said wheel, said relief forming apparatus including: (a) a grinding wheel dressing ap paratus adapted to dress the periphery of a grinding wheel as said grinding wheel is carried on and rotated with a spindle of a grinding machine, said dressing apparatus adapted to form the periphery of the wheel to a selected diameter and configuration; (b) means for supporting the dressing apparatus in a hinged manner so that the movable portion thereof is movable toward and away from the wheel and so that a dressing portion may be brought selectively into engagement with the grinding wheel; (c) adjustable means for limiting the movement of said dressing apparatus toward the grinding wheel for forming and dressing a continuous. peripheral surface on said grinding wheel; (d) a circular cam having a plurality of recesses formed on its outer periphery to provide lobes on said cam; (e)

means for mounting said cam on the spindle carrying I the grinding wheel so as to turn with said spindle and grinding wheel with the mounted cam disposed so that to engage each of the lobes and when in engagement therewith to support the dressing apparatus at-a determined disengaged position above the dressed periphery of the grinding wheel, and (g) means for adjusting said limiting means and said follower arm to positions whereat when the roller of the follower arm is engaged by a lobe of the rotating cam the dressing apparatus is supported so as to be out of contact with the grinding wheel and when the cam is further rotated and the roller is brought from a lobe and in way of a recess, the dressing apparatus to the limit controlled by the adjusted limiting means moves toward the grinding wheel permitting the dressing apparatus to locally engage and further dress the grinding wheel to form a relieved surface of the grinding wheel periphery which is below the previously dressed periphery of the grinding wheel, the length of the relief corresponding to the length of the recess in the cam.

2. Apparatus for forming determined relief portions in a grinding wheel as in claim 1 in which the grinding wheel dressing apparatus includes a base and a frame maintained in a pivotal relationship to eachother and with the follower arm removably mountable on the movable member of the dressing apparatus.

3. Apparatus for forming determined relief portions in a grinding wheel as in claim 2 in which the cam is removably mountable on a retaining nut providing means for retaining the grinding wheel on the spindle.

4. Apparatus for forming determined relief portions in a grinding wheel as in claim 2 in which the movable member is the upper frame member and there is provided a micrometer screw carried in a threaded hole in the frame and disposed to engage the base to provide the means for limiting the movement of the dressing apparatus toward the grinding wheel.

5. Apparatus for forming determined relief portions in a grinding wheel as in claim 4 in which the follower arm is provided with at least one slot formed in its upper extent, and in which there is provided a turnscrew carried in a threaded hole in the frame, said turnscrew having its shank disposed to be slidably retained in said slot while its head portion is adapted to engage the outer surface of the follower arm to retain said arm against the frame when the turnscrew is tightened in the threaded hole in the frame.

6. Apparatus for forming determined relief portions in a grinding wheel as in claim 5 in which there is provided in the follower arm a pair of slots in substantially parallel arrangement with one slot aligned so that its axis passes through the roller on thearm.

7. Apparatus for forming determined relief portions in a grinding wheel as in claim l in which the lobes on the circular cam form arcs which are at least one-sixteenth of an inch greater in radius than are the radial dimensions of the arcs forming the recesses of the cam.

8. A method for forming relief portions of determined extent and depth in the periphery of a grinding wheel while and as it is carried on the spindle of a grinding machine, said relief portions providing discontinuations in the prepared peripheral grinding surfaces in and on the face of said wheel, said method including the steps of: (a) supporting a grinding wheel dressing apparatus adapted to dress the periphery of the grinding wheel on a grinding machine so that this dressing ll l mll vfifiiefilii d lti l'ivg'll'fir til glllltimg wheel; (b) mounting a circular cam having a plurality of lobes and relief portions therebetween on the spindle of the grinding machine so that the axis of this cam coincides with the axis of the grinding wheel and retaining said'cam on the spindle so that this cam is rotated as the spindle is turned; (c) supporting the grinding wheel dressing apparatus by an adjustable follower arm having a roller carried on its distal end and with said roller disposed to engage and ride on the periphery of the circular cam; (d) adjusting said arm so that when the roller is brought into supporting engagement of said lobes the dressing apparatus is not in dressing engagement with the wheel, and (e) adjusting an adjustable stop provided on the dressing apparatus to limit the movement of the follower roller into the relief portions of the cam and when the roller is in a relief portion between lobes to dress a recess into the peripheral grinding surface corresponding to the extent of relief portion formed on the cam and with the depth of dressed recess corresponding to the adjustment of the adjustable stop. i

9. A method for forming determined relief portions on a grinding wheel as in claim 8 in which the step of adjusting an adjustable stop for limiting the movement of the dressing apparatus toward the grinding wheel is by manipulating a micrometer screw rotatably carried by the dressing apparatus and disposed to support a movable frame portion of the dressing apparatus above a fixed member of the dressing apparatus.

10. A method for forming determined relief portions on a grinding wheel as in claim 9 in which the mounting of the circular cam is on the periphery of a retaining nut carried on the end of the spindle of the grinding machine, and in which the attaching of the follower arm to the dressing apparatus includes arranging the follower arm so that a slot in said arm is disposed-to be engaged by a shank of a turnscrew which is loosened for adjustment and removal of the follower arm and is tightened to maintain saidarm in a determined position on said frame.

a a k =0:

Claims

1. Apparatus for forming determined relief portions in the periphery of a grinding wheel as used on a grinding machine, said relief portions providing discontinuations in the prepared peripheral grinding surfaces in and on the face of said wheel, said relief forming apparatus including: (a) a grinding wheel dressing apparatus adapted to dress the periphery of a grinding wheel as said grinding wheel is carried on and rotated with a spindle of a grinding machine, said dressing apparatus adapted to form the periphery of the wheel to a selected diameter and configuration; (b) means for supporting the dressing apparatus in a hinged manner so that the movable portion thereof is movable toward and away from the wheel and so that a dressing portion may be brought selectively into engagement with the grinding wheel; (c) adjustable means for limiting the movement of said dressing apparatus toward the grinding wheel for forming and dressing a continuous peripheral surface on said grinding wheel; (d) a circular cam having a plurality of recesses formed on its outer periphery to provide lobes on said cam; (e) means for mounting said cam on the spindle carrying the grinding wheel so as to turn with said spindle and grinding wheel with the mounted cam disposed so that the peripheral surfaces of the lobes are substantially equally distant from the axis of the spindle; (f) an adjustable follower arm removably retained on the grinding wheel dressing apparatus, said arm having a roller rotatably mounted on its distal end, this roller adapted to ride on the lobes of the cam when the arm is mounted on the dressing apparatus and is moved toward and into engagement with a lobe of the cam, and after the arm is fixed in the condition for the roller to engage each of the lobes and when in engagement therewith to support the dressing apparatus at a determined disengaged position above the dressed periphery of the grinding wheel, and (g) means for adjusting said limiting means and said follower arm to positions whereat when the roller of the follower arm is engaged by a lobe of the rotating cam the dressing apparatus is supported so as to be out of contact with the grinding wheel and when the cam is further rotated and the roller is brought from a lobe and in way of a recess, the dressing apparatus to the limit controlled by the adjusted limiting means moves toward the grinding wheel permitting the dressing apparatus to locally engage and further dress the grinding wheel to form a relieved surface of the grinding wheel periphery which is below the previously dressed periphery of the grinding wheel, the length of the relief corresponding to the length of the recess in the cam.

2. Apparatus for forming determined relief portions in a grinding wheel as in claim 1 in which the grinding wheel dressing apparatus includes a base and a frame maintained in a pivotal relationship to each other and with the follower arm removably mountable on the movable member of the dressing apparatus.

6. Apparatus for forming determined relief portions in a grinding wheel as in claim 5 in which there is provided in the follower arm a pair of slots in substantially parallel arrangement with one slot aligned so that its axis passes through the roller on the arm.

7. Apparatus for forming determined relief portions in a grinding wheel as in claim 1 in which the lobes on the circular cam form arcs which are at least one-sixteenth of an inch greater in radius than are the radial dimensions of the arcs forming the recesses of the cam.

8. A method for forming relief portions of determined extent and depth in the periphery of a grinding wheel while and as it is carried on the spindle of a grinding machine, said relief portions providing discontinuationS in the prepared peripheral grinding surfaces in and on the face of said wheel, said method including the steps of: (a) supporting a grinding wheel dressing apparatus adapted to dress the periphery of the grinding wheel on a grinding machine so that this dressing apparatus when carried on the grinding machine is pivotally movable toward and away from the grinding wheel; (b) mounting a circular cam having a plurality of lobes and relief portions therebetween on the spindle of the grinding machine so that the axis of this cam coincides with the axis of the grinding wheel and retaining said cam on the spindle so that this cam is rotated as the spindle is turned; (c) supporting the grinding wheel dressing apparatus by an adjustable follower arm having a roller carried on its distal end and with said roller disposed to engage and ride on the periphery of the circular cam; (d) adjusting said arm so that when the roller is brought into supporting engagement of said lobes the dressing apparatus is not in dressing engagement with the wheel, and (e) adjusting an adjustable stop provided on the dressing apparatus to limit the movement of the follower roller into the relief portions of the cam and when the roller is in a relief portion between lobes to dress a recess into the peripheral grinding surface corresponding to the extent of relief portion formed on the cam and with the depth of dressed recess corresponding to the adjustment of the adjustable stop.

10. A method for forming determined relief portions on a grinding wheel as in claim 9 in which the mounting of the circular cam is on the periphery of a retaining nut carried on the end of the spindle of the grinding machine, and in which the attaching of the follower arm to the dressing apparatus includes arranging the follower arm so that a slot in said arm is disposed to be engaged by a shank of a turnscrew which is loosened for adjustment and removal of the follower arm and is tightened to maintain said arm in a determined position on said frame.