GB2158377A - Grinding screw-threads - Google Patents

Abstract

The present invention relates to a method of thread grinding and an apparatus therefor, in which a thread having at least one flank B2 disposed at right angles or at an acute angle in relation to the axis of the screw is precisely ground without producing interferences such as wheel marks. The grinding is carried out by the use of a grindstone having tapering surfaces T1, T2 on both sides, and the grindstone axis X2, in addition to being inclined at the thread lead angle, is inclined to intersect a plane P1 containing the axis of the screw. Apparatus for forming internal and external coupling threads on oil well pipes by this method is described. <IMAGE>

Description

SPECIFICATION Method of thread grinding and apparatus therefor The present invention relates to a method of thread grinding and an apparatus therefor in which at least one flank angle of buttress or square thread, which are formed at the end portions of the oil well pipes, coupling for connecting the oil well pipes with each other in order to connecting oil well pipes with each other or thread guages for inspecting abovementioned thread, is 0, or negative.

Description of the Prior Art Oil well pipes, which are deeply inserted into the earth, are extended by connecting them with each other. But the connection of oil well pipes is carried out by directly engaging a external screw formed on the periphery of the end portion of one oil well pipe with a internal screw formed on the inside circumferential surface of another oil well pipe or engaging the end portions of oil well pipes, which are provided with external screws formed on the peripheries of both end portions thereof, with a internal screw formed in a short cylindrical coupling with the external screw formed at the end portion of each oil well pipe.

In addition, the ordinary triangular thread as shown in Fig. 1 (a) has not been used but a buttress thread and a square thread, in which at least one flank angle O2 of one flank B2 is 0, or negative, as shown in Fig. 1 (b) have been used for the thread formed at the end portions of an oil well pipe or in the coupling for the reasons that the load in the longitudinal direction of oil well pipe is remarkably large, an oil must be prevented from leaking and the like.

Hereat, the flank angle in the ordinary triangular thread as shown, for example, in Fig.

1 (a) designates an angle 01, 8 2 made by the surface meeting at right angles with a center line (axis) x1 of screw thread and each of both flanks B1, B2 of screw thread. The relation of 8 = 02 ( > 0) holds good for the ordinary triangular thread. On the other hand, a thread according to the present invention has flank angle of 0, or negative, as described later. it is noted, that flank angle is negative designates, for example, a top of thread of one flank B2 overhangs a bottom of thread, as shown in Fig. 1(b).

Next, the conventional method of thread grinding is described at first with reference to Figs. 2 to 4. Fig. 2 is a plan view showing the state of a trapezoidal thread which is being ground, Fig. 3 being a sectional view of Fig.

2 taken along the axis of screw thread, and Fig. 4 being an enlarged side sectional view of the principal parts taken along the lead angle of thread. The flank B,, B2 of a work W, on which a trapezoidal thread is formed by grinding, makes an angle a'1 a'2 to a plane A, meeting at a lead angle ss of thread with the center line (work axis), x1 of work W, respectively. In addition, a grindstone T is formed in a disc-like shape having a grindstone working surface T1, T2 making an angle a'1, a'2, which is same as the inclination of the flank B1, B2 respectively, to a plane J, meeting at right angles with a rotational center (axis of grindstone) x2 thereof.The screw thread is ground by rotating the grindstone T and the work W with moving the grindstone T in the direction of center line x1 of work W synchronizing the rotation of the work W, inclining the center line x2 of grindstone to the center line x1 of work W in a plane parallel to the center line x1 of work W by the lead angle ss, and giving the predetermined depth of cut to the work W to grind the work W. Hereon, the grindstone T grinds the work W with contacting with merely a line (which is designated by "a" in Fig. 3) meeting at right angles with the center line x2 of grindstone thereby the screw thread can be precisely formed in the desired trapezoidal profile.

However, in the case where a thread, in which the flank angle of the flank B2 is 0 or negative, such as buttress thread is ground, as shown in Fig. 5, also the grindstone working surface T2 corresponding to the flank B2 must be formed perpendicular to the center line x2 of grindstone since the flank B2 meets at about right angles with the center line x1 of the work W. Also, as shown in Fig. 6, in the above-mentioned thread whose flank angle is O" or negative, the flank B2 has a tortion whose direction changes in the reverse direction at the both sides of a portion (designated by "a" in Fig. 2) of a line perpendicular to the center line x2 of grindstone T since the flank B2 is formed in continuous helical flute (curve line) and top of thread is slightly smaller than bottom of thread in lead angle because of difference of diameter.When the grinding is carried out by use of conventional method, the grindstone T grinds not only the portion "a" to be properly ground but also the portion "b" which is one side part of the remaining portion of the flank B2. As a result, the portions of the flank B2 are marked with wheel-marks and when interference is heavy, the portion "C" is removed from the work W.

Accordingly, the thread grinding using the conventional method and the conventional apparatus therefor can not precisely grind the above described threads, such as buttress threads and square threads, in which the flank angle is 0 or less and only a method of finishing by cutting has been able to be used for the fine working of threads, such as thread gauges for use in the inspection of such threads, which require the high precision.

After all, the above described conventional method has been remarkably dissatisfied in working precision, useful lifetime and the like.

OBJECTS OF THE INVENTION The present invention was achieved in respects of the above described disadvantages incidental to the prior art.

It is the first object of the present invention to provide a method of thread grinding and an apparatus therefore which is capable of precisely thread grinding, such as buttress threads, in which a flank angle is 0 or less, that is to say, a flank of screw thread meets at right angles with a work axis as a center line of screw or a top of thread overhangs a bottom of thread and screw threads make an acute angle to a center line of screw, without wheel-marks, interferences and the like.

It is the second object of the present invention to provide a method of thread grinding and an apparatus therefor which is capable of grinding buttress threads formed at the end portions of oil well pipes for connecting them with each other, buttress threads formed on a coupling for connecting the oil well pipes or thread gauges of hard materials for inspecting these threads in high precision and prolonging the useful life time of the thread gauges.

It is the third object of the present invention to provide a method of thread grinding and an apparatus therefor which is capable of improving the profile of buttress threads in uniformity and precision and producing highly interchangeabie members.

It is the fourth object of the present invention to provide an apparatus for thread grinding which is capable of easily accommodate to different profile, taper angles and the like of screw thread.

The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.

Figure 1 is an explanatory diagram of a flank angle; Figure 2 is a plan view showing the grinding state of the conventional thread grinding; Figure 3 is a sectional view of Fig. 2 taken along a work axis as a center line of a screw; Figure 4 is an enlarged sectional view showing the principal parts of Fig. 2 taken along the direction of a lead angle; Figure 5 is a sectional view showing the principal parts of buttress threads under the grinding conditions by conventional method; Figure 6 is a plan view of Fig. 5; Figure 7 is a sectional view showing the principal parts under the practical conditions of a method of thread grinding of the present invention; Figure 8 is a schematic diagram showing an apparatus of the present invention; and Figure 9 is a schematic diagram showing another preferred embodiment of an apparatus of the present invention.

A method of thread grinding of the present invention is fundamentally characterized by that an axis (center line) of a grindstone is inclined to a work axis (center line of the work) by a lead angle in a projection on a plane parallel to the work axis and further, the extension thereof is inclined so as to intersect with a plane which is parallel to said plane and includes the work axis.

At first, this will be below described with reference to the drawings.

Fig. 7 is a sectional view showing the principal parts under the practical conditions of a method of thread grinding of the present invention taken along the lead angle of the side of the grindstone T.

A flank B2 of the work W is at right angles with the center line (work axis) x1 of the work (a flank angle is 0 ) or a top of the flank B2 overhangs a bottom thereof (a flank angle is negative). In addition, a flank B1 opposite to the flank B2 is inclined to the center line x1 of the work toward the side apart from the opposite flank B2 by the predetermined angle.

A grindstone T used for the thread grinding of the work W has a center line axis) x2 of grindstone, which is a rotational center, inclined to the center line x1 of the work in the plane P0 (whose section is shown by x0 in Fig.

7) parallel to the center line x1 of the work by the lead angle P (as shown, for example, in the above described Fig. 6) and the extension of the center line x2 of grindstone is inclined so as to intersect with a plane P1 which is parallel to above-mentioned plane P0 and includes the center line x1 of the work. As a result, the working surfaces Tg, T2 of the grindstone T requiring the same inclinations as the flanks B1, B2 can be formed in the tapered shape on both sides of a plane J, which meets with the center line X2 of grindstone at right angles.

When such grindstone T is forced its way into the work W along the plane J, meeting at right angles with the center line x2 of grindstone and the work W is rotated around the center line x, of the work as the rotational center thereof and rotating the grindstone T, the working surfaces T1, T2 of the grindstone T contact with the flanks B1, B2 only on the line of radial direction positioned on the side of work W even in the case where the inclination of the flank B2, that is to say, the flank angle is 0 or negative, so that it makes an acute angle to the center line x1 of the work, whereby the aimed profile of screw threads can be precisely ground without contacting other portions of the grindstone T with the flanks B1, B2 and interferences.

At this juncture, thread having the desired pitch of screw thread can be ground by giving a relative movement along a pitch line Px by a precise pitch width every time of one rotation of the work W to the grindstone T in a synchronized relation with the work W.

In addition, in the present invention, also a method, in which a working surface of a grindstone is preliminarily positioned at the starting side of the threaded portion and then the thread grinding is carried out with giving a relative movement along the pitch line Px to the grindstone, may be adopted. At any rate, various kinds of modification may be given to the present invention within the spirit thereof.

Fig. 8 is a schematic diagram showing a preferred embodiment of an apparatus for thread grinding of the present invention. This apparatus is one for grinding an external tapered thread comprising a work carriage 2 movably mounted on a machine base 1 along a center line x1 (the work axis) and a grindstone carriage 4 arranged sideways the work carriage 2.

The work carriage 2 is provided with a holding member 1 a for rotatably holding a work W (an external tapered thread member) with the center line x1 of the work as the center of rotation thereon and a driving means 3 consisting of an electric motor 3a and a gear train 3b at one side of the holding member 1a in the direction of center line x1, a rotation being given to the work W held by the work carriage 2 by rotating the driving means 3, and the work carriage 2 being transferred in the direction along the center line x, in a synchronized relation with the rotational speed of the work W.

In addition, a cam member 6 provided with a linear cam surface 6a formed on the side opposite to the grindstone carriage 4 is provided sideways the holding member 1 a of the work carriage 2, the cam member 6 being adapted to be able to angularly dislocate against the center line x1, and the linear cam surface 6a being positioned in parallel to the direction of pitch line Px of the work W.

On the other hand, the grindstone carriage 4 is provided with a pressing apparatus 8 against the cam surface 6a and adapted to be reciprocally movable on a guide axis Y meeting at right angles with the center line x1. In addition, the grindstone carriage 4 is provided with a grindstone inclining means 5 rotatable on a plane parallel to a plane including both the center line x1 and the guide axis Y.

Furthermore, the grindstone inclining means 5 is provided with a driving head 5a by a supporting member 5b and a disc-like grindstone T is rotatably mounted on the driving head 5a.

Accordingly, the center line x2 (axis of grindstone) of the head 5a of the grindstone driving means 5 is adapted to be able to incline with respect to the center line x, of the work carriage 2 in a projection on a plane including the center line x, of the work carriage 2 and the guide axis Y and incline with respect to the center line x, of the work carriage 2 in a projection on a plane which meets at right angles with a plane including the center line x1 of the work carriage 2 and the guide axis Y and includes the center line x2 af grindstone inclined with respect to the center line x1 of the work carriage 2. The tilting angle of the center line x2 of grindstone with respect to the center line x1 of the work carriage 2 in the latter plane corresponds to a lead angle P of thread.

The driving head 5a of the grindstone inclining means 5 is provided with a driving source for giving a rotation to the grindstone T included therein and a cam follower 7, which can appear and disappear against the driving head 5a of the grindstone inclining means 5, protruded on a wall opposite to the cam surface 6a, so that the pointed head of the cam follower 7 is contacted with the cam surface 6a of the work carriage 2 side by moving the grindstone carriage 4 along the guide axis Y.

In addition, the cam follower 7 is provided with an in-feed screw 9 for regulating the quantity of advance of the cam follower 7 from the driving head 5a connected with the rear end thereof, so that the depth of cut of the grindstone T in the work W can be regulated by changing the quantity of advance of the cam follower 7 from the driving head 5a of the grindstone inclining means 5 by means of the in-feed screw 9 whereby the position of the grindstone T against the work W can be regulated in the case where the diameter of grindstones T is changed in various sizes.

The working surface formed on the circumference of the grindstone T is arranged so as to face to a flank of the work W; as described above, the shape of the working surface of the grindstone T is adjusted so that the working surface of the grindstone T is tapered on both sides of the plane J, of grindstone meeting at right angles with the center line x2 of grindstone and the sectional shape thereof may coincide with or similar to the sectional shape of a thread groove of the work W under the condition that the center line x2 of grindstone is inclined with respect of the center line x1 of the work carriage 2.

Next, the operation of such apparatus for thread grinding is described. At first, as to the preparatory process, upon rotation of the grindstone inclining means 5, the center line x2 of grindstone is inclined with respect ot the center line x1 of the work carriage 2 by an angle of a in a projection on a plane including the center line x, of the work carriage 2 and the guide axis Y and further inclined with respect to the center line x, of the work carriage 2 in a projection on a plane including the center line x2 of grindstone inclined by the angle of a and meeting at right angles with a plane including the center line x1 and the guide axis Y of the work carriage 2 by an angle equal to a lead angle. In addition, the cam follower 7 is extended by the in-feed screw 9 by the quantity corresponding to a depth of cut of the grindstone T.Then, after the grindstone T was moved to the starting end side of thread of the work W, the cam follower 7 is contacted with the cam surface 6a of the cam member 6 arranged on the work carriage 2 side with moving ahead and back the grindstone carriage 4 in the direction of the guide axis Y and the working surface of the grindstone T is positioned at the predetermined position in the thread groove of the work W.

After the completion of such preparatory process, the grindstone T is rotated by means of the driving head 5a and when the driving means 3 of the work carriage 2 side rotates the work W, the flank of the work W is ground with transferring the work carriage 2 at the predetermined speed in the direction of the center line x, in a synchronized relation with the rotation of the work W.

That is to say, the grindstone carriage 4 is transferred along the cam surface 6a of the cam member 6 arranged in parallel to the pitch line P, in the direction of guide axis Y by means of the cam follower 7 and simultaneously relatively transferred along the center line x, of the work carriage 2, so that also the grindstone T fixedly mounted on the grindstone carriage 4 by the driving means 5 is advanced along the pitch line P, and the predetermined lead in the thread groove of the work W in a synchronized relation with the rotation of the work W with holding the inclined relation between the center line x1 of the work carriage 2 and the center line x2 of grindstone, whereby threads, in which a flank angle is 0' or less, can be ground precisely without producing interferences and the like.

Fig. 9 is a schematic diagram showing an apparatus for grinding internal taper thread as another preferred embodiment of the present invention. This embodiment is mainly described with the differences from the above described embodiment. A work W' (internal taper thread member) rotatably held by a holding member 1 a' of a work carriage 2 has a thread portion formed on the internal wall surface thereof, so that a center line x2' of grindstone is inclined with respect to a center line x1 of the work carriage 2 by an angle of a' in the direction opposite to that in the embodiment as shown in Fig. 8 in a projection on a plane including the center line x1 of the work carriage 2 and a guide axis Y and a grindstone T is held on the center line x2' of grindstone.

On the other hand, a cam member 6' is provided on the side of the work W' opposite to that in the above described embodiment so that the linear cam surface 6a' thereof may be inclined in the direction opposite to that in the above described embodiment to be parallel to a pitch line of the work W'.

Such construction makes the precise grinding of internal taper threads, in which a flank angle is 0, or less without producing interferences and the like by means of a method similar to that as shown in Fig. 8.

In addition, the present invention is not limited to the above described embodiments.

Various kinds of modification may be given to the present invention.

For example, although the grindstone carriage 4 is adapted to rotate in order to make the center line x2 of grindstone incline with respect to the center line x1 of the work carriage 2 in the abovementioned embodiment, the grindstone carriage 4 may be fixed and the work carriage 2 may be adapted to rotate in order to make the center line x2 of grindstone incline with respect to the center line x1 of the work carriage 2.In addition, although the grindstone carriage 4 is adapted to be able to move only in the direction of guide axis Y and the work carriage 2 is adapted to be able to move only in the direction of center line x1 thereof in the above described embodiment, the construction, in that the grindstone carriage 4 is movable in the direction of guide axis Y and the center line x1 of the work carriage 2 while the work carriage 2 is fixed, or the construction, in that the grindstone carriage 4 is fixed while the work carriage 2 is movable in the direction of center line x1 of the work carriage 2 and guide axis Y, may be adopted.

Furthermore, it is not always required that the moving direction of the work carriage 2 (direction of center line x,) meets at right angles with the moving direction Y of the grindstone carriage 4, that is to say, the angle, at which the moving direction of the work carriage 2 meets with the moving direction Y of the grindstone carriage 4, can be optionally set so that both of them may not coincide with each other. For example, provided that the moving direction of the grindstone carriage 4 is set so that it may meet at right angles with the center line x2 of grindstone, the grindstone T can travel with giving the predetermined depth of cut in the thread groove of the work without aid of cam member 6.

Furthermore, although a method of grinding taper threads is described in the above described embodiments, the present invention is not limited to it. It is also possible to grind parallel thread and multiple thread using a grinder according to the above described embodiments. In the case when the parallel thread is ground, a pitch line of a work is arranged in parallel to the center line of the work, so that the cam member and contact element can be omitted.

Furthermore, the simultaneous replacement of a gear train 3b of the driving means 3 and a cam member 6 in the above described embodiment by simultaneous two or three axis numerically controlled driving apparatus can be very easily carried out on the basis of the known art. For example, the moving distance of the grindstone T in the direction of pitch line Px per one rotation of the work W can be easily controlled by numerically controlling the movement of the work W in the direction of center line x1 of the work carriage 2, rotation of the work W and the movement of the grindstone carriage 4 in the direction of guide axis Y by means of a stepping motor.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiment is therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within meets and bounds of the claims, or equivalence of such meets and bounds thereof are therefore intended to be embraced by the claims.

Claims (9)

1. A method of thread grinding, in which thread, at least one of whose flank angles defined as an angle of flanks of said thread to a plane perpendicular to the axis of screw thread is 0, or less, is ground by a grindstone with rotating a work on which said thread is formed, characterized by that, the axis of said grindstone as the rotational center thereof is inclined with respect to the center line of said work by the lead angle in a projection on a plane parallel to a work axis as the rotational center thereof, further the extension of the axis of said grindstone is inclined so as to intersect a plane being parallel to said plane and including said work axis, and said grindstone and said work are relatively moved.

2. A method of thread grinding as set forth in Claim 1, wherein said relative movement is a straight movement proportional to the rotation of said work in the direction of the pitch line of said thread.

3. A method of thread grinding as set forth in Claim 1, wherein said thread is one formed at the end portions of oil well pipes for connecting them with each other or one formed at thread gauges for inspecting the thread of said oil well pipes.

4. A method of thread grinding as set forth in Claim 1, wherein said thread is one formed at couplings for oil well pipes or one formed at thread gauges for inspecting the threads of said couplings.

5. An apparatus for thread grinding, in which thread, at least one of whose flank angles defined as an angle of flanks of said thread to a plane perpendicular to the axis of screw thread is 0 or less, is ground by a grindstone with rotating a work on which said thread is formed, characterized by providing with:: a work carriage for supporting said work rotatably with the work axis as the rotational center thereof; means for rotating said work; a grindstone carriage for supporting said grindstone rotatably with an axis of said grindstone as the rotational center thereof; means for rotating said grindstone; means for making the work axis incline with respect to the axis of said grindstone in a projection on a plane parallel to said work axis; means for making the extension of the axis of said grindstone incline so as to intersect a plane being parallel to said plane and including said work axis; and means for relatively moving said grindstone and said work in proportion to the rotation of said work in the direction of pitch line.

6. An apparatus for thread grinding as set forth in Claim 5, wherein said thread is one formed at the end portions of oil well pipes for connecting them with each other or one formed at thread gauges for inspecting the thread of said oil well pipes.

7. An apparatus for thread grinding as set forth in Claim 5, wherein said thread is one formed at couplings for oil well pipes or one formed at thread gauges for inspecting the threads of said couplings.

8. An apparatus for thread grinding as set forth in Claim 5, wherein said means for relatively moving said grindstone and said work is provided with a cam surface parallel to the pitch line of said thread and a cam member which is movable in the direction of said work axis.

9. An apparatus for thread grinding as set forth in Claim 5, wherein said means for relatively moving said grindstone and said work comprises means for controlling the relative movement of said grindstone and said work in the direction of said work axis, means for controlling the relative movement of said grindstone and said work in the direction perpendicular to said work axis and means for controlling the rotation speed of said work.