US2787868A

US2787868A - Means and method of grinding thread cutting tools

Info

Publication number: US2787868A
Application number: US255757A
Authority: US
Inventors: Schifando Fred
Original assignee: Individual
Current assignee: Individual
Priority date: 1951-11-02
Filing date: 1951-11-02
Publication date: 1957-04-09
Anticipated expiration: 1974-04-09

Description

F. SCHIFANDO April. 9, 1957 MEANS AND METHOD OF GRINDING THREAD CUTTING TOOLS Filed NOV. 2 1951 5 Sheets-Sheet 1 Sn w QM W \A wwt 3 QM .wm Mm Q 3 Q A i! w I 2 SM n llll l 41 T {.5 4 8 m g R. N. Y K N mm INVENTOR. Fred Sch/7471140 April 9, 1957 F. SCHIFANDO 2,787,868

MEANS AND METHOD OF GRINDING THREAD cu'rmc TOOLS Filed Nov. 2, 1951 5 Sheets-Sheet 2 c \HUI xii 12 F/gz INVENTOR. Fred Jc/wfimdo ATTORNEY April 9, 1957 F. SCHIFANDO 2,787,868

MEANSAND METHOD OF GRINDING THREAD CUTTING TOOLS Filed Nov. 2, 1951 5 She ets-Sheet 3 INVENTOR.

- Fred $C/7/f0/7Q'0 ATTORNEY April 9, 1957 F. SQHIFANDO 2,787,868

MEANS AND METHOD OF GRINDING THREAD CUTTING TOOLS Filed Nov. 2, 1951 5 Sheets-Sheet 4 a? n2 a4 86 80 IN VEN TOR.

' m0 [50 c 55d Fred scmfa/zao ZED/WM AT TORNEY April 9, 1957 F. SCHIFANDO 2, 8 ,8

MEANS AND METHOD OF GRINDING THREAD CUTTING TOOLS Filed Nov. 2, 1951 5 Sheets-Sheet 5 INVENTOR. Fred sm/fafld 4 ATTORNEY ,only two of its lands.

United States Patent 6 MEANS AND METHOD OF GRINDING THREAD CUTTING TOOLS Fred Schifando, New York, N. Y.

Application November 2, 1951, Serial No. 255,757

11 Claims. (CI. 51-96) This invention relates to means and the method of grinding thread cutting tools such as taps, dies and chasers.

The leading ends of the lands on conventional taps,-

dies and chasers are provided with chamfers extending lengthwise of the lands. These chamfers are produced by eccentrically grinding each land of the tool to thus form a relief or clearance extending from the cutting edge to the heel of the land.

Threads cut by the conventional threading tools are frequently defective. Some of the defects are strip threads, taper threads, eccentric threads and errors in the lead of the threads.

The object of the present invention is to produce threading tools which are ground in a novel manner, by eccentric grinding; and by novel means to avoid the above mentioned defects.

To this end the present invention provides means whereby each land of the threading tool is moved simultaneously in two directions, relatively to the grinding wheel, namely, crosswise and slightly lengthwise of the land, so that the resultant path of grinding of the various thread crests in the chamfer is in the same direction as the angles of the thread teeth cross-wise of the land.

In carrying out the invention provision is made for supporting the threading tool on a fixture relatively to the grinding wheel and then grinding the various lands in- Other features and advantages will hereinafter appear. 7

In the drawing which forms part of the specification.

Fig. 1 is a side elevation, partly in section, of a novel machine for grinding a chamfer on a thread cutting tool such as a tap;

Fig. 2 is a view, in longitudinal section, of one of various guides which may be used in the machine of Fig. 1;

Fig. 3 is a view, on a larger scale, of the forward end of the tap shown in Fig. 1;

Fig. 4 is a cross-section of the tap taken on the line 44 of Fig. 3 looking in the direction of the arrows at said line;

Fig. 5 is a developed view of the tap shown in Fig. 3 showing thefour lands with respect to each other. The view also shows the shapes of chamfer faces on the threads included inthe chamfer of each land;

Fig. 6 is a view in section, similar to Fig. 4, but of a conventional tap, illustrating the relief on the threads included in its chamfer;

Fig. 7 is a developed view, similar to Fig. 5, of a. portion of the conventional tap illustrated in Fig. 6 but shows It also shows the shapes of the chamfer faces on threads included in the chamfer of each land;

Fig. 8 is a side elevation, partly in section, of a machine for grinding of the chamfer on a chaser;

Cir

ice

Fig. 12 shows the upper left hand corner, on a larger scale, of the chaser shown in Fig. 10. Similar portionsof the other three chasers of. Fig. 9 are shown behind it for the purpose of comparison with it and each other;

Fig. 13 is a view showing the shapes of the chamfer faces on the leading threads of a conventional chaser; and

Fig. 14 an elevation showing the grinding wheel and its support.

The machine 15 illustrated in Fig. 1 includes a fixture 16 for grinding a chamfer 18 on the thread cutting teeth 20 at the leading end of each

land

22, 22a, 22b and 220 of a tap 24. The lands are formed by flutes 25 which extend axially of the tap. The fixture 16 includes a base plate 26 to which are secured brackets 28 and 34 the object of the bracket 30 being described later. bracket 28 has a housing or hub portion 32 having a horizontal bore 34 in which is located a gmide as in the form of a bushing. The guide 36 has a slot 38 in its upper portion into which the lower, reduced end 40 of a thumb screw 42 extends. The thumb screw 42 is threaded through the upper portion of the housing 32 and may be screwed down against the guide 36 to prevent accidental axial displacement of the guide. The screw 42 also prevents circumferential displacement of the guide 36, because its end 40 engages in the slot 38. It will be understood that after backing off the screw 42 the guide 36 may be adjusted lengthwise in the housing bore 34.

The guide 36 has an axial bore :4 into which the tap 24 may be inserted from the right hand end. The guide 36 has, at the left hand end of its bore 44 screw threads 46 with which the tap threads engage to guide the tap 24.

After assembling the tap 24 and guide 36 the latter may be inserted into the housing 32 from its left hand end.

A grinding wheel 47, is located at the leading end of v The grinding wheel 47 is normally 7 the tap 24 (Fig. 1). clear of the tap and is mounted to rotate on a suitable bracket 47a (Fig. 14), which bracket may be pivotally I supported at 47b on the base plate 26 so that the wheel 47 may be moved bodily toward and away from the tap 24. The wheel 47 has a grinding face 48 which is angularly disposed with reference to the tap 24 so as to grind the chamfer 18 on the thread teeth at the leading end of each tap land.

After the tap 24 is inserted into the fixture 16 it is advanced towards the grinding wheel, which in the meantime has been lowered into grinding position. The tap is then positioned about its axis for grinding. For this purpose there is provided a stop pin 52 supported for vertical movement in :1 lug 54 on housing 32. The pin 52 is pushed downwardly to project its lower end into one of the tap flutes 25. The tap is then rotated until the land 22 engages the stop pin 52 thus causing the land 22 to extend upwardly towards the grinding wheel. The tap with its guide or bushing 36 is then advanced axially until the first thread just touches the rotating grinding wheel. This position may be observed by sparks coming from the work. The thumb screw 42 is then screwed down to secure the guide 36 to the fixture 16.

After the guide 36 has been so secured an end stop 56 supported in the upper end of the fixed bracket 30 is moved into engagement with the right hand end or shank 58 of the tap 24. The end stop 56 is then locked in this position by a set screw 69. The pin 52 is subse- The quently raised clear of the land 22 where it may be held temporarily by a spring pressed ball 61 in the lug 54 and adapted to engage in an annular groove 62 in pin 52. The tap 24 is then rotated back-and-forth in its guide 36 relatively to the grinding wheel, from the cutting edge 63 to the heel 64- (Figs. 3, 4 and 5) of the land 22, to grind the chamfer 18.

To rotate the tap back-and-forth there is provided a knurled finger wheel 62:: demountably secured to the shank or square end 58 of the tap 24 by a set screw 65. Thus it will be understood that by rotating the fingerwheel 62a back and-forth the tap is rotated about its axis in the fixed guide 36 and relatively to the wheel 47. During the back-and-forth rotation transversely of the land, the tap is caused to move back-and-forth axiall i of the guide by the thread 46 and relatively .to the grinding wheel 47.

During the grinding operation, the grinding wheel 47 may be moved in small increments towards the tap, by a thumb screw 47c (Fig. 14), until the desired depth of chamfer has been reached.

Movement of the wheel 47 towards the tap may be limited or stopped in its extreme position by suitable means including a stop 47d. The stop 47d may engage the bracket 47:! which supports the grinding wheel.

After the chamfer has been ground on the first land 22 the grinding wheel 47 is raised slightly by the screw 47c and the tan is then rotated about its axis until the land 22 passes the stop pin 52. The latter is then lowered into the path of the next land 22a in order to position the latter at the grinding wheel. During the rotation of the tap, to bring the second land 22a into engagement with the stop pin 52, the tip is advanced axially a slight distance due to the threads 46 in the guide 36. This caused the shank 58 of the tap to move away from the end stop 56. Before proceeding to grind the chamfer on the second land 22a the tap is repositioned, axially, against the end stop 56. This is elfected by first backing off the thumb screw 4-2 and then pushing the tap with its guide 36 rightwnrdly (Fig. 1) until the shank 58 of the tap engages the end stop 55. The thumb screw 42 is then screwed down to secure the guide bushing 36. The stop pin 52 may then be raised out of engagement with the land 22a so that the tap 24 may be freely rotated backand-forth by the finger wheel 62a to grind the chamfer 18 on the land 22a. The grinding wheel 47 is then lowered by the screw 47c to grind the chamfer on the land 22a.

The above describedoperations are repeated for grinding the chamfer on each of the lands 22b and 220 and include the repositioning of the tap against the end stop 56 before each grinding operation. Because of the repositioning operation of the tap against the end stop 56 and limiting the movements of the grinding wheel toward the tap the chamfers on all of the

lands

22, 22a, 22b and 22c are ground to the same depth. In other words the chamfer on all of the lands will be the same distance, radially, from the axis of the tap.

Provision is made for handling taps of various sizes, smaller or larger than the one 245, shown in Fig. 1. To this end other bushing guides are used in the housing 32. which have the same outside diameter as the guide 36 to fit into the bore 34- of housing 32, but in which the bore 44 varies to fit the diameter of the larger or smaller tap. A guide 36a for a tap of larger diameter is shown in Fig. 2. This guide 36o fits the bore 34 of housing 32. The guide 36:! is similarly provided with a bore 44a which is larger in diameter than the one, 44, shown in Fig. l. The guide 36:: has thread 46a at its left hand end which is the same size as that of the larger tap. The guide 36:1 is also provided with a slot 38a in cooperation with the end 40 of the lock screw 42.

The chamfers 18 on the various lands after grinding 1 width from the cutting edge to the heel of the land.

appear as in Fig. 3 where the crowns of the leading thread sections or teeth on the :four lands have been re moved thus leaving the chamfer faces 66 011 the leading thread cutting teeth. This is the result of concentric grinding since the tap 24 is rotated back-and-forth about its axis from the cutting edge 63 to the heel 64 of the land during the chamfer grinding operation. These chamfer faces extend in the same direction as the thread cutting teeth. Each complete chamfer face is of uniform Fig. 5 shows the chamfer faces 66 on all of the lands. It shows how the two sides of all of the complete chamfcr faces 66 are of uniform width from the cutting edge to the heel of each land 'thus making the side edges of each complete face 66 parallel and of smaller width on the teeth of each land from the leading end of the tap inward. The faces of the various sections of the same thread as it extends over the four lands are also of smaller width from top to bottom of Fig. 5.

For the purpose of comparison the conventional tap 24x is illustrated in Figs. 6 and '7 where parts similar to those of .Fig. 3 have the same reference numerals with the subscript x added. In the tap 24x each thread tooth included in the chamfer has a relief 74 behind the cuting edge 63x of each land. This relief is usually pro duced by mechanism including a cam which causes the grinding wheel to move towards the axis of the tap MA: as the grinding proceeds from the cutting edge to the heel of the land. Grinding the relief 74 produces faces, on the thread cutting teeth, with sides which are not parallel but are inclined towards each other. The present invention, however, provides comparatively simple means to grind the chamfer because of the method employed,

namely that of concentric grinding in the direction of the thread crosswise of the lands.

Another machine embodying the invention is shown in Fig. 8. This machine is used for grinding chamfer faces 82 on the thread cutting teeth 84 at leading ends of the land 86 of a chaser 88 which is one of a plurality of chasers used to cut external threads. Four of these chasers, 88, 88a, 88b and 88c are shown in Fig. 9 where they are arranged to cut an external thread 91 on a bolt or conduit .92. The machine 80 is similar, in many respects, to the machine 15 of Fig. 1 and includes a base plate 94 on which is rigidly supported a bracket 96. The bracket 96 .is provided with a housing or hub portion 98 having a horizontal bore 10!) in which is supported a guide, in the form of a shaft 102.

The guide .102, like the guide 36 of Fig. '1, also has a slot 104 in its upper portion and extends axially thereof. The lower end 106 of a thumb screw 108 engages in the slot 104 to prevent rotation of the guide 102 about its axis and to secure said guide axially of the housing 98 at 'will. The guide 102 has, on its left hand end, an external thread 110 which thread is the same size as an internal thread 112 of the chaser 83.

A chaser supporting element 114 includes a hub 116 by which it is rotata'bly supported on a bearing portion 118 of the guide shaft or threaded element 1 32. The supporting element 114 also has a knurled finger piece 120 by which it may be rotated back-and-forth about the guide 102. A lug 122 extending forwardly from the face of the finger wheel 120 is provided with a vertical shoulder 126. The chaser is positioned on the element 114 by placing it against the shoulder 126 and against a wall portion 128 of the lug 122. The chaser is detachably secured to the lug 122 by a screw 130 which passes through a washer 132, a hole 134 (Fig. 10) in the chaser, the screw being threaded into a hole 136 in the wall 128 of the lug 122. A grinding wheel 138 is supported by means which may be similar to that shown in Fig. 14 in connection with the Wheel 47.

To assemble the guide 102, chaser supporting element 114 and chaser 88 on the bracket 96, the wheel is moved up to a position above that shown in Fig. 8. The guide 102 may then be placed into the bore 100 of housing 98 from the right hand side of the bracket '96 with its bean ing portion 118 extendingheyond the left end of the housing 98 with the screw end 106 in the slot 104. The chaser supporting element 114 may next be placed on the bearing portion 118 of guide 102. The guide 102 is then positioned axially so that a few teeth, of thread 110, at its leading end are to the left of a vertical plane in which the face 126 of lug 122 is located. The chaser 88 is then placed on the lug 122 and the chaser thread cutting teeth 112 are caused to engage the thread 110 of the guide 102, the pitch of the thread 110 and teeth 112 being the same. If the thread teeth of chaser 88 and the thread of the guide 102 do not register properly an adjustment may be made by sliding the guide 102 leftwardly or rightwardly in its housing 98 until they do register. After the threads engage properly the chaser 88 may be secured to the lug 122 by the screw 132 as previously described.

After thus assembling the chaser 88, support 114 and guide 102, the entire assembly is pushed axially of the housing 98 until the hub 116 engages the left hand end 133 of the housing. The screw 108 may then be tightened "to secure the guide 102 on the bracket 96. The wheel 138 is next lowered into engagement with the chaser to grind the chamfer on it. While the grinding wheel 138 is rotating the chaser supporting element 114 is rotated back-and-forth by the finger-wheel 120 so that the leading threads are ground transversely of the land or in other words from cutting edge 140 to the heel 142 thereof.

To limit the rotation of the element 114 in both directions there is provided a stop pin 144 onthe left end of the housing 98. The pin 144 extends into a slot 146 formed in the end of the hub 116 of supporting element 114 and cooperates alternately with the opposite ends of the slot. The slot 146 is wide enough to permit grinding the full width of the land 86. Since the threads of the chaser 88 engage the threads of the guide 110 the oscillation of the supporting element 120 causes the chaser to move in the direction of the threads 110.

While the grinding of the chamfer 82 proceeds the grinding wheel 138 is moved downwardly towards the ,chaser 88. It is arrested in its lowermost position by an adjustable stop which may be similar to the stop 47d. To provide clearance for the wheel 138 the upper portion at the left hand end of the guide 102 is cut away as at 148, Fig. 8. After the grinding operation has been completed the chaser may be removed from the support 114 and the next chaser 88a of the set may be mounted on and secured to the support.

In Fig. 12 the

chasers

88, 88a, 88b and 880 are shown in superposed relation and the thread cutting teeth of each chaser are ofiset with respect to those of the preceding chaser. Consequently, in order to properly assemble the chaser 88a on the support and with respect to the guide 102 it is necessary to loosen the guide 102 so that it may be moved slightly lengthwise of the housing 98 to have the thread 110 thereof engage properly with the thread teeth 112 of the chaser. After this has been accomplished the supporting element 114 is pushed against the face 133 of the housing 98 and the guide 102 is then secured to the bracket 96 by the screw 108. The grinding wheel 138 which has been raised temporarily may now be lowered against the corner of the chaser 88a to grind the chamfer thereon. During the grinding operation the chaser 88a is oscillated back-and-forth by the fingerwheel 120 of supporting element 114. The chasers 88b and 88c may be mounted on the element 114 and ground in a manner similar to that described in connection with the

chasers

88 and 88a.

When all the

chasers

88, 88a, 88b and 880 of the set have been ground the chamfers on the chasers appear as in Figs. 11 and 12, where the sides of the complete chamfer faces 150 are parallel from the cutting edge 14% to the heel 142 of each land. Their curvature extends about the axis 152 (Fig. 9) of the set of chamfers, this axis 152 coinciding with the axis of the guide 102 of Fig. 8. Because of the fact that each chaser is caused to follow the threads of the guide 102 the amount ground from each complete thread cutting tooth is equal throughout its length. The grinding is efiected in the direction of the thread cutting teeth on the land.

In the conventional chaser 88x (Fig. 13) each thread in the chamfer is provided with a relief. Consequently more material is removed from the thread at the heel than at the cutting edge x to produce the relief. This results in tapered chamfer faces x. This curvature of each chamfer face is eccentric with the axis 152 of the chaser 88x; the conventional method of grinding the chamfer is known as eccentric grinding.

Briefly, the threading tool of my invention consists of means including a land having thread cutting teeth extending cross-wise thereof, the complete leading threads being provided with concentric chamfer faces extending in the direction of the thread teeth.

My method and device for grinding a chamfer on the threading tool includes means having the same size threads as the threading tool whereby the tool is guided while it is being moved back-and-forth and in contact with a stationary grinding wheel to produce a chamfer on the leading threads of the threading tool.

In the form of the invention (Fig. 1) for grinding the chamfer on the tap 24 the tap is placed within the guide 36. The guide with tap therein is then placed into the housing 32. After the finger-wheel 62 has been secured to the tap shank the stop 52 is placed into one of the flutes 25 and the tap is rotated until the cutting edge of land 22 engages the stop 52 which has been lowered into the flute 25. Thus the tap is positioned with one of the lands up. The tap is then placed against the end stop 56. The screw 42 is next screwed down to secure the guide 36 and the stop 52 is pulled up to its ineffective position out of the path of the land 22. The grinding wheel 47 may then be lowered into contact with the tap for grinding the chamfer. While the grinding is taking place the tap is rotated back-and-forth a distance sufiicient to expose the entire width of the land to the grinding wheel. As the grinding proceeds the wheel 47 is moved bodily closer to the tap, by the screw 470 until the desired depth of the chamfer has been reached, the bodily movement of the grinding wheel being arrested in this position by the stop screw 47d.

When the next land of the tap is to be ground the tap is given a fraction of a rotation until the land previously ground passes the stop 52. The stop is then lowered into the succeeding flute 25. The tap is turned farther until the cutting edge of the next land to be ground engages the stop. This causes the tap to advance axially in the guide 36 due to the threads 20 of the tap engaging with the threads 46 of the guide 36. Consequently the tap shank 64 moves away from the end stop 56. The tap 24 is therefore repositioned on the bracket 28 and against the end stop 56 each time a land has been positioned for grinding.

To reposition the tap against the end stop 56 the screw 42 is backed 01f to loosen the guide 36 which guide is then moved axially of the housing 32 until the tap shank 58 engages the end stop 56. The screw 42 is subsequently screwed down to secure the guide 36 and thus hold the guide and tap axially, in their readjusted position.

The stop 52 may then again be raised to its ineifective position so that the oscillation of the tap to grind the chamfer, on the newly positioned land, may be proceeded with. The above described operations are repeated for the positioning and grinding of each land.

It will be understood because of the repositioning of the tap against the end stop 56 before grinding each land that the chamfers on all of the lands are alike. This causes the lands to cut uniformly and at the same time when the tap enters a hole to be tapped.

In the form of the invention shown in Fig. 8 for grinding the chamfers individually on the various chasers of enemas 7 a set, the guide 102 is placed in the bore 100. of the housing 98 with its left hand end protruding from the housing. The supporting element 114 is then placed on the bearing portion 118 of the guide 102. The first one, 88, of the set of chasers is then placed against the shoulder 126 and mounted on the supporting element 114. The chaser 88 and guide 102 are adjusted so that a few of its threads 112 engage properly with a corresponding number of those of the guide 162. The assembly, including the guide 102, the supporting element 114 and chaser 88, is then pushed axially of the housing 98 until the face of the hub 116 engages the face 133 of the housing. The guide 162 is then secured in position axially of the housing 98 by the screw 108. The chaser may then be oscillated relative to the grinding wheel 138 through the medium of the finger-wheel 120 While grinding the chamfer. The grinding wheel 138 may be thin with a rounded grinding edge.

After the chamfer is completely ground the chaser 38 is removed and the second chaser 88a is mounted on the supporting element 114. The screw 108 is then backed oil? to loosen the guide so that it may be moved axially of the housing until the threads 110 engage properly with those of the chaser 38a. The chaser 88 may then be secured to the supporting element 114 by the screw 130. Subsequently the assembly, including the chaser 88a, element 114 and guide 182, is placed with the hub 116 engaging the face 133. After securing the guide by thumb screw 108 the support 114 may be oscillated by its fingerwheel to move the chaser back-and-forth relatively to the grinding wheel. The supporting element 114, it will be remembered, is arrested in its extreme position by the stop 144.

Although the invention is shown and described in connection with a tool such as a tap for cutting internal threads and in connection with a tool including a set of chasers for cutting an external thread, it should be understood that the invention may also readily be embodied in other threading tools and machines for grinding the chamfers on the lands thereof. Such tools may include dies for cutting external threads and a set of Chasers for cutting internal threads.

While certain preferred embodiments of the invention have been described, it will be understood that 03. tain parts may be used with others and that changes in the form, arrangements, proportions, sizes and details thereof may be made without departing from the scope of the invention as defined in the appended claims.

I claim:

2. In a machine for grinding a chamfer on the land of a tap having thread cutting teeth, the combination of a grinding wheel, a stationary bracket, a threaded element on said bracket to engage the teeth of the tap, means associated with said tag to rotate the latter back-and-forth about its axis and relatively to the grinding wheel, means to engage each land of the tap individually to position the lands successively at the wheel, means to actuate the positioning means to an inactive position, and means to hold the positioning means in its inactive position.

2. In a machine for grinding a chamfer on the lands of a tap. each land having thread cut-ting teeth, the combination of a grinding wheel, a stationary bracket, a thread on said bracket to engage the teeth of the tap, means associated with said tap to rotate the latter back-andrfor'th about its axis and relatively to the grinding =wheel, means to engage each land of the tap individually to position the lands successiveiy in a grinding position, means to assist in repositioning the tap on the bracket after the grinding of each land, and means for securing the :tap ;to said bracket after each reposition operation thereof.

3. I n a machine for grinding achamferon the lands of a tap, each land having thread cutting teeth, the combinan o a e d a h e a station ry bracket, a housi on said bracket, sai .liousi gihavi gaa ax a ceive the tap, a thread in said bore to engage the teeth t the p. m ans ass stat t said t n o rotate t e latter batflt and forth about its axis relatively to the grinding wheel, means to engage each land of the tap individual ly to position the lands successively in a grinding posit on, means to assist in repositioning the tap with the housing axially each time a land is positioned for grinding, and means for securing the housing to said bracket after each repositioning operation of the tap and its housing.

In a machin f r g nd hem st On h d of a tap, each land having thread cutting teeth, the combination of a grinding wheel, a stationary bracket, a housing on said bracket, said housing having an axial bore to receive the tap, a thread in saidbore to engage the teeth of the tap, means associated with said tap to rotate the latter c ria-f rth bo t i ax s a rel i tq he grinding wheel, means to engage each land of the tap d ally t p siti n th ands su ces i in inding position, means to assist in repositioning the tap with h h u in axially e me a l n s sit ans r gri in s o n ha n a sl n i ut face a extending axially ofsaid housing, and a screw threaded through said bracket, and extending into said slot to prevent circumferential displacement of the housing and to secure the housing axially of said bracket after each re: positioning operation of said tap and said housing.

5. In a machine for grinding a chamfer on the land of a chaser having thread cutting teeth, the combination of a grinding wheel, a stationary bracket having a bore therein, a shaft in said bore, said shaft having a portion extending beyond said bracket, threads on the extending portion of said shaft, a supporting element revolubly mounted on the shaft portion, means to demountably attach the chaser to said supporting element, means to lo: cate the chaser on the supporting element so that its teeth engage the thread on said shaft, and means to rotate said supporting element back-and-forth to corresponding- 1y actuate the chaser relatively to the grinding wheel.

6. In a machine for grinding a chamfer on the land of a chaser having thread cutting teeth, the combination of a grinding wheel, a stationary bracket, means including a stationary threaded element on the bracket, said threaded element having a curvature about an axis, the threaded element having thread sections, a supporting element revolubly mounted on said threaded element, means to demountably attach the chaser to said supporting element, means for locating the chaser to said supporting element with its teeth engaging the thread sections of said threaded element, and ineans to rotate the supporting element baclg-and-forth about the axis of the threaded element to correspondingly actuate the chaser relatively to t in in whee 7.- n a m chin for ndi a s m e l nd of a chaser having thread cutting teeth, the combination of a grinding wheel, a stationary bracket having a bore therein, ,a shaft in said bore, said shaft having a portion extending beyond said bracket, thread sections on the free end of the shaft portion, a supporting element revolubly mounted on the shaft portion, means to demountably attach the chaser to said supporting element with the teeth in engagement with the thread sections, a face on said bracket against which the supporting element bears, means to prevent rotary displacement of said shaft, means to secure said shaft against accident axial displacement, n me n o rota e the su aq b cknd-t t t 9;- espondinslv aerat th chas r l t e t9 he grind n wheel.

8. in a machine for grinding a tap, the tap having a plurality of lands, each land having adjacent thread cutting teeth extending crosswise thereof, the combination of a rotatable grinding wheel arranged to grind a chamfer on each land individually at the leading end of the tap, means to rotate the tap back-andforth about its axis, .-.ea s to mandat with each lead t qs tian the t strguintereat ally so that th s en ma be a u d as each land successively by rotating the tap back-aud-forth while in contact with the wheel, means to guide the tap in the direction of the pitch of its thread cutting teeth during its back-and-forth rotative movement, and means to facilitate adjustment of the guiding means with the tap axially of the tap after the grinding operation of each land so that the next succeeding land may occupy the same initial position occupied by the previously ground land.

9. The method of forming a concentric chamfer extending over a plurality of helically arranged thread cutting teeth on the land of a threading tool, which includes presenting the crest portions of some of the teeth simultaneously to the concentric face of a rotating grinding wheel, reciprocating the tool about its axis, guiding the teeth in their helical direction from the cutting edges to the heels of the teeth while the tool is in contact with the grinding wheel so that the chamfer face formed on each complete thread tooth is concentric and of uniform width, having the active portion of the grinding surface of the wheel arranged oblique to crests of some of the teeth so that each succeeding chamfer face on each succeeding tooth of the land after grinding is narrower than the chamfer face on its adjacent preceding tooth.

10. In a machine for grinding a concentric chamfer on the land of a thread cutting tool having thread cutting teeth, the combination of a grinding wheel having a concentric grinding surface, a stationary bracket to support the tool, means to rotate the tool back and forth relatively to the grinding surface of the wheel during rotation of the latter to remove the crests of some of the teeth simultaneously, means to cause the tool to move in the direction of the slant from front to back of the thread cutting teeth while the tool is being rotated, and means to present the grinding surface of the wheel obliquely to the crests of a plurality of said teeth.

11. In a machine for grinding a concentric chamfer on the land or a tap having thread cutting teeth, the combination of a grinding wheel, a stationary bracket, a housing on said bracket and at one side of the wheel, said housing having an axial bore to support the tap with its free end exposed, a thread in said bore to engage the thread cutting teeth of the tap to guide the tap in a helical path, and a finger piece demountably attached to said tap to rotate the latter back-annd-forth about its axis while its free end is in contact with the grinding wheel.

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