Patent Description:
Conventionally, there is known a machine tool having mutually opposite two spindles, such as a lath, wherein an axial end portion of a workpiece gripped by a gripping portion of one of the spindles and an axial end portion of another workpiece gripped by a gripping portion of the other spindle are joined to each other by a joining means, such as friction welding, welding, press-fitting or the like, to transform these workpieces are into a single joined workpiece (refer, for example, to <CIT>(PTL <NUM>)).

<CIT> (PTL <NUM>), on which the preamble of appended claim <NUM> is based, discloses a machine tool comprising first and second clamps for holding first and second workpieces, the axial ends of which are to be joined by diffusion welding to form a joined workpiece. The machine tool further comprises a position deviation eliminating means for eliminating a positional deviation between the first and second workpieces.

In the above-mentioned conventional machine tool, in order to accurately join the two workpieces so as to be coaxial with each other, before the joining, the axial end faces of both workpieces are typically processed into a shape suitable for joining, by means of a tool such as a cutting tool. However, due to various factors, such as wear or tear of the tools, deterioration of the cutting oil supplied to the machining part, aging deterioration of the machine tools, etc., misalignment (eccentricity) may occur unexpectedly, between the two joined workpieces.

However, even when misalignment occurs between the two joined workpieces, the conventional machine tool is unable to detect such misalignment.

The present disclosure has been accomplished in view of the above problems, and it is an object of the present invention to provide a machine tool that is capable of easily detecting misalignment between that two workpieces.

According to the present invention there is provided a machine tool comprising: a first spindle for gripping a first workpiece: a second spindle for griping a second workpiece; and a joining means for joining an axial end of each workpiece gripped by each spindle to form a joined workpiece from the first workpiece and the second workpiece, characterized in that the machine tool further comprises: an electric servomotor for moving the first spindle in a direction intersecting the axis of the first spindle; a current value detecting means for detecting a current value of the electric servomotor; and a misalignment detecting means for detecting misalignment between the first workpiece and the second workpiece of the joined workpiece, based on the current value detected by the current value detecting means when the joined workpiece is gripped by both spindles and rotated by the first spindle or the second spindle.

Preferably, in the machine tool of the present disclosure with the configuration as described above, the misalignment detecting means detects the misalignment between the first workpiece and the second workpiece in the joined workpiece, when the amplitude of the current value detected by the current value detecting means reaches or exceeds a predetermined value.

Preferably, the machine tool of the present disclosure with the configuration as described above further comprises a rotation angle detecting means for detecting a rotation angle of the first spindle with reference to a predetermined rotation position, and the misalignment detecting means is configured to detect the direction of misalignment of the second workpiece with respect to the first workpiece, based on the rotation angle detected by the rotation angle detecting means and the fluctuation cycle of the current value detected by the current value detecting means.

Preferably, in the machine tool of the present disclosure with the configuration as described above, the joining means is configured to carry out friction-welding of the axial end of the first workpiece gripped by the first spindle and the axial end of the second workpiece gripped by the second spindle.

According to the present invention it is possible to provide a machine tool capable of easily detecting misalignment between two joined workpieces.

The machine tool <NUM> according to the present invention will be described below in detail with reference to the drawings.

The machine tool <NUM> illustrated in <FIG> is configured as a lathe, such as a CNC lathe, and includes a first spindle <NUM> and a second spindle <NUM> mounted on a base <NUM>.

The first spindle <NUM> and the second spindle <NUM> are arranged so that the axis of the first spindle <NUM> and the axis of the second spindle <NUM> are parallel to each other and separated from each other in the axial direction. Hereinafter, the direction parallel to the axes of the first spindle <NUM> and the second spindle <NUM> is defined as the Z-axis direction, the direction orthogonal to the Z-axis direction is defined as the X-axis direction, and the directions orthogonal to the Z-axis direction and the X-axis direction is defined as the Y-axis direction.

The first spindle <NUM> is rotatably supported by the first headstock <NUM> and is adopted to be rotationally driven by a first spindle motor. As the first spindle motor, for example, there may be used a built-in motor configured between the first spindle <NUM> and the first spindle <NUM> inside the first spindle <NUM>. A first chuck 10a is provided at the tip of the first spindle <NUM> so that the first spindle <NUM> is adopted to grip the first workpiece W1 by the first chuck 10a. The first workpiece W1 may be rotated by rotationally driving the first spindle <NUM> by means of the first spindle motor while gripping the first workpiece W1 by means of the first chuck 10a.

The second spindle <NUM> is rotatably supported by the second spindle <NUM> and adopted to be rotationally driven by a second spindle motor. As the second spindle motor, for example, there may be used a built-in motor configured between the second spindle <NUM> and the second spindle <NUM> inside the second spindle <NUM>. A second chuck 20a is provided at the tip of the second spindle <NUM> facing the first spindle <NUM>. The second spindle <NUM> is adopted to grip the second workpiece W2 by the second chuck 20a. The second workpiece W2 may be rotated by rotationally driving the second spindle <NUM> by means of the second spindle motor while gripping the second workpiece W2 by means of the second chuck 20a.

Between the base <NUM> and the first headstock <NUM>, there is provided an X-axis moving mechanism <NUM> for moving the first spindle <NUM> in the X-axis direction, and a Z-axis moving mechanism <NUM> for moving the first spindle <NUM> in the Z-axis direction.

The X-axis moving mechanism <NUM> includes an X-axis guide rail <NUM> fixed to the base <NUM> and extending along the X-axis direction, and the Z-axis moving mechanism <NUM> is slidably mounted on the X-axis guide rail <NUM>. A ball/screw mechanism <NUM> is provided between the X-axis guide rail <NUM> and the Z-axis moving mechanism <NUM>. An electric servomotor <NUM> is connected to the ball screw mechanism <NUM>. By rotationally driving the ball screw mechanism <NUM> by means of the electric servomotor <NUM>, the first spindle <NUM> may be moved together with the Z-axis moving mechanism <NUM> in the X-axis direction along the X-axis guide rail <NUM>. Furthermore, the electric servomotor <NUM> can also be operated so as to hold the position of the first spindle <NUM> in the X-axis direction.

As the electric servomotor <NUM> for moving the first spindle <NUM> in the X-axis direction orthogonal to the axis of the first spindle <NUM>, various configurations may be adopted, such as an AC servomotor, a DC servomotor, etc. capable of moving the first spindle <NUM> in the X-axis direction while holding the first spindle <NUM> at a desired position in in the X-axis direction.

The Z-axis moving mechanism <NUM> includes a Z-axis guide rail <NUM> that extends along the Z-axis direction. The first headstock <NUM> is slidably mounted on the Z-axis guide rail <NUM>. A ball/screw mechanism <NUM> is provided between the Z-axis guide rail <NUM> and the first headstock <NUM>. An electric servomotor <NUM> is connected to the ball/screw mechanism <NUM>. By rotationally driving the ball screw mechanism <NUM> by means of the electric servomotor <NUM>, the first spindle <NUM> may be moved together with the first spindle stock <NUM> in the Z-axis direction along the Z-axis guide rail <NUM>.

As the electric servomotor <NUM> for moving the first spindle <NUM> in the Z-axis direction, various configurations may be adopted, such as an AC servomotor, a DC servomotor, etc., capable of moving the first spindle <NUM> in the Z-axis direction while holding the first spindle <NUM> is moved as a desired position in the Z-axis direction can be adopted.

The machine tool <NUM> may be configured to include a cutting tool. In this case, the configuration may be such that the first workpiece W1 gripped by the first chuck 10a of the first spindle <NUM> and the second workpiece W2 gripped by the second chuck 20a of the second spindle <NUM> can be machined by the cutting tool.

The machine tool <NUM> includes a control unit <NUM>. The control unit <NUM> has a function as a microcomputer provided with a CPU (central processing unit) and a storage means, such as a memory, and is connected to the spindle motor of the first spindle <NUM>, the spindle motor of the second spindle <NUM>, the electric servomotor <NUM> and the electric servomotor <NUM>.

The control unit <NUM> integrally controls the operation of the spindle motor of the first spindle <NUM>, the spindle motor of the second spindle <NUM>, the electric servomotor <NUM>, the electric servomotor <NUM> and the tool, so that the tool is operated to process first workpiece W1 gripped by the first chuck 10a of the first spindle <NUM> or the second workpiece W2 gripped by the second chuck 20a of the second spindle <NUM>.

Furthermore, the control unit <NUM> integrally controls the operation of the spindle motor of the first spindle <NUM>, the spindle motor of the second spindle <NUM>, the electric servomotor <NUM> and the electric servomotor <NUM>, so that the axial end of the first workpiece W1 gripped by of the first chuck 10a of the first spindle <NUM>. and the axial end of the second workpiece W2 gripped by the second chuck 20a of the second spindle <NUM> can be friction-welded to form one joined workpiece W3. That is, the control unit <NUM> has a function as a joining means <NUM> for friction welding the axial end of the first workpiece W1 gripped by the first chuck 10a of the first spindle <NUM> and the axial direction of the second workpiece W2 gripped by the second chuck 20a of the second spindle <NUM> to form a single joined workpiece W3.

Hereinafter, explanation will be made of the procedure or method in which, by means of the function of the control unit <NUM> as the joining means <NUM>, the machine tool <NUM> friction-welds the axial end portion of the first workpiece W1 and the axial end portion of the second workpiece W2.

First, as illustrated in <FIG>, the first spindle <NUM> is made to grip the first workpiece W1 by means of the first chuck 10a, and the second spindle <NUM> is made to grip the second workpiece W2 by means of the second chuck 20a. The first spindle <NUM> and the second spindle <NUM> are arranged coaxially so that the axial end of the first workpiece W1 and the axial end of the second workpiece W2 are opposed to each other.

As the first workpiece W1 and the second workpiece W2, for example, a round bar made of a steel material may be used, though one made of another metal material or having another shape may also be used. Before the first workpiece W1 and the second workpiece W2 are friction-welded, it is preferred that the first workpiece W1 and the second workpiece W2 are processed into a shape suitable for friction welding by using, for example, the above-mentioned tool.

Next, the second spindle <NUM> is rotated at a predetermined rotation speed while the rotation of the first spindle <NUM> is stopped, so that the first workpiece W1 gripped by the first spindle <NUM> and the second workpiece W2 gripped by the second spindle <NUM> are rotated relative to each other by a predetermined rotation speed difference.

In the present embodiment, the first workpiece W1 and the second workpiece W2 are rotated relatively to each other, by rotating only the second spindle <NUM> at a predetermined rotation speed while stopping the rotation of the first spindle <NUM>. However, the first workpiece W1 and the second workpiece W2 may be rotated relatively to each other, by rotating only the first spindle <NUM> at a predetermined rotation speed while stopping the rotation of the second spindle <NUM>. Alternatively, the first workpiece W1 and the second workpiece W2 may be rotated relatively to each other, by rotating the first spindle <NUM> and the second spindle <NUM> in the same direction at different rotation speeds. Further alternatively, the first workpiece W1 and the second workpiece W2 may be rotated relatively to each other, by rotating them in opposite directions at different rotation speeds or the same rotation speed.

Next, as illustrated in <FIG>, in a state where the first workpiece W1 gripped by the first spindle <NUM> and the second workpiece W2 gripped by the second spindle <NUM> are relatively rotated with each other by a predetermined rotation speed difference, the electric servomotor <NUM> of the Z-axis moving mechanism <NUM> is operated to move the first spindle <NUM> in the Z-axis direction so as to approach the second spindle <NUM>, so that the axial end portion (axial end face) of the first workpiece W1 is brought into contact with the axial end portion (axial end face) of the second workpiece W2. When the axial end portion of the first workpiece W1 comes into contact with the axial end of the second workpiece W2, due to the predetermined rotation speed difference between the first workpiece W1 and the second workpiece W2, friction heat is generated between the axial end portion the first workpiece W1 and the axial end portion of the second workpiece W2, so that the first workpiece W1 and the second workpiece W2 are frictionally heated.

The rotation speed difference between the first workpiece W1 and the second workpiece W2 that rotate relative to each other may be a rotation speed difference that can generate frictional heat required for friction-welding the first workpiece W1 and the second workpiece W2.

When the axial end portion of the first workpiece W1 and the axial end portion of the second workpiece W2 reach a predetermined temperature due to the frictional heating, the rotation of the second spindle <NUM> is stopped for stopping the relative rotation between the first workpiece W1 and the second workpiece W2 and the first spindle <NUM> is then further moved in the Z-axis direction so as to approach the second spindle <NUM>. As a result, the axial end portion of the second workpiece W2 is pressed against the axial end portion of the first workpiece W1 at a predetermined pressure (upset pressure) in the direction along the Z-axis direction, so that the first workpiece W1 and the second workpiece W2 are friction-welded to form one joined workpiece W3, with their axial end portions as joint surfaces.

The procedure of the friction-welding by means of the joining means <NUM> is not limited to what has been described above, and may be variously changed as long as the first workpiece W1 and the second workpiece W2 can be joined by friction-welding.

The machine tool <NUM> includes misalignment detecting means <NUM> for detecting the misalignment when misalignment (eccentricity) occurs between the axes of the first workpiece W1 and the second workpiece W2 of the joined workpiece W3, which has been formed by joining the first workpiece W1 and the second workpiece W2 by means of the function as the joining means <NUM>. The misalignment detecting means <NUM> is configured as a function of the control unit <NUM>. That is, the control unit <NUM> is configured to function as the misalignment detecting means <NUM> for detecting misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3 formed by joining the first workpiece W1 and the second workpiece W2 by the joining means <NUM>.

Furthermore, the control unit <NUM> is provided with an ammeter <NUM> as a current value detecting means. The ammeter <NUM> is connected to the electric servomotor <NUM> and is adopted to detect the current value of the current (i.e., the current value of the electric servomotor <NUM>) supplied from the control unit <NUM> to the electric servomotor <NUM>.

As illustrated in <FIG>, in the joined workpiece W3 formed by joining the first workpiece W1 and the second workpiece W2 by means of the machine tool <NUM>, there may be caused unexpected misalignment between the joined first workpiece W1 and the second workpiece W2 due to various factors, such as wear or loss of the tool, deterioration of the cutting oil supplied to the machine tool and aged deterioration of the machine tool <NUM> itself. The machine tool <NUM> according to the present embodiment is capable of detect such misalignment by means of the misalignment detecting means <NUM>.

Hereinafter, explanation will be made of the procedure or method for detecting the misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3, by means of the misalignment detecting means <NUM>.

First, as illustrated in <FIG>, in a state where the first workpiece W1 and the second workpiece W2 have been joined by the joining means <NUM> of the machine tool <NUM> to form the joined workpiece W3, which is then gripped by the first chuck 10a and the second chuck 20a, only the second spindle <NUM> is operated to rotate the joined workpiece W3. On this occasion, the position of the first spindle <NUM> in the X-axis direction is held by the electric servomotor <NUM> that is controlled by the control unit <NUM>. When the second spindle <NUM> operates and the joined workpiece W3 rotates, the rotation of the second spindle <NUM> is transmitted to the first spindle <NUM> via the joined workpiece W3, and the first spindle <NUM> is also driven by the second spindle <NUM> to rotate.

It is to be noted that the joined workpiece W3 may be rotated by operating the first spindle <NUM>, instead of the second spindle <NUM>.

When the joined workpiece W3, with the first workpiece W1 and the second workpiece W2 misaligned relative to each other, is gripped by the first chuck 10a and the second chuck 20a and rotated, as illustrated in <FIG>, vibration occurs in the X-axis direction on the first spindle <NUM>, about the axis of the second spindle <NUM>, which is fixed to the table <NUM> and cannot move in the X-axis direction, and with an amplitude that is twice the amount of misalignment (eccentricity) between the first workpiece W1 and the second workpiece W2. This vibration is applied to the electric servomotor <NUM> via the ball/screw mechanism <NUM>, so that the electric servomotor <NUM> is applied with a load corresponding to the misalignment between the first workpiece W1 and the second workpiece W2 on the first spindle <NUM>, at a predetermined cycle according to the rotation angle of the first spindle <NUM>.

In this instance since the electric servomotor <NUM> of the X-axis moving mechanism <NUM> is controlled by the control unit <NUM> to hold the position of the first spindle <NUM> in the X-axis direction, a reaction force acting against the load is generated on the first spindle <NUM> to hold the position of the first spindle <NUM> in the X-axis direction. Thus, the current value of the electric current supplied from the control unit <NUM> to the electric servomotor <NUM> increases or decreases according to the rotation angle of the first spindle <NUM>. That is, the current value of the electric servomotor <NUM> increases or decreases according to the rotation angle of the first spindle <NUM>, for example, as illustrated in <FIG>. The range of such increase/decrease, i.e., the fluctuation range, of the current value of the electric servomotor <NUM> increases as the amount of misalignment (eccentricity) between the first workpiece W1 and the second workpiece W2 increases.

The correlation between the amount of misalignment (eccentricity) of the first workpiece W1 and the second workpiece W2 and the fluctuation range of the current value of the electric servomotor <NUM> differs depending on the structure of the machine tool <NUM>, the structure of the control unit <NUM>, and the like. Therefore, the storage means of the control unit <NUM> stores the correlation between the eccentricity and the amplitude of the current value obtained in advance by experiments or the like. For example, as illustrated in <FIG>, the correlation may be stored as a function describing the correlation between the eccentricity and the amplitude of the current value, or the correlation between the eccentricity and the amplitude of the current value may be stored by another method, such as a numerical table indicating a plurality of numerical values.

Since the current value of the electric servomotor <NUM> is detected by the ammeter <NUM>, the misalignment detecting means <NUM> is capable of detecting the misalignment between the first workpiece W1 and the second workpiece W2 of the joined workpiece W3, based on the fluctuation range of the current value detected by the ammeter <NUM>. In the present embodiment, the misalignment detecting means <NUM> is configured to detect the misalignment between the first workpiece W1 and the second workpiece W2 of the joined workpiece W3, based on the amplitude of the current value (i.e., the difference between the maximum value and the minimum value of the fluctuating current value) detected by the ammeter <NUM>.

More specifically, the misalignment detecting means <NUM> is adopted to detect the mount of misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3, by applying the amplitude of the current value obtained from the fluctuation of the current value detected by the ammeter <NUM>, as illustrated in <FIG>, to the correlation as illustrated in <FIG>. Then, when the amplitude of the current value obtained from the current value detected by the ammeter <NUM> reaches or exceeds a predetermined value, i.e., a predetermined threshold value, the misalignment detecting means <NUM> determines that there is misalignment between the workpiece W1 and the second workpiece W2 of the joined workpiece W3 and detects the misalignment.

The threshold value for determining the presence or absence of misalignment can be arbitrarily set in consideration, for example, of the dimensional tolerance required for a product manufactured by further processing the joined workpiece W3. Furthermore, the threshold value is set in advance and input to a program or the like stored in the storage means of the control unit <NUM> in advance.

It is to be noted that the misalignment detecting means <NUM> may be configured to determine the occurrence of misalignment between the workpiece W1 and the second workpiece W2 of the joined workpiece W3 and to detect the misalignment when the amplitude of the current value obtained from the fluctuation of the current value detected by the ammeter <NUM> reaches or exceeds a preset predetermined value, i.e., the predetermined threshold value, without using the correlation between the eccentricity amount and the amplitude of the current value.

Furthermore, the misalignment detecting means <NUM> may also be configured to determine the occurrence of misalignment between the workpiece W1 and the second workpiece W2 of the joined workpiece W3 and to detect the misalignment when, within the fluctuation range of the current value detected by the ammeter <NUM>, the absolute value of the increase/decrease amount relative to the center value of the fluctuation of the current value, instead of the amplitude of the current value, reaches or exceeds the predetermined threshold value.

When the misalignment detecting means <NUM> detects that there is misalignment between the first workpiece W1 and the second workpiece W2 with an amount reaching or exceeding the predetermined misalignment amount (eccentricity), the machine tool <NUM> makes an error determination for the joined workpiece W3. The machine tool <NUM> upon an error determination may be configured to notify an operator, etc., of such an error. The error notification may be made in various manners, for example, by automatically stopping the operation of the machine tool <NUM>, issuing an alarm, turning on the warning light, and displaying an error on a monitor or the like.

In this way, according to the machine tool <NUM> of the present embodiment, the joined workpiece W3 gripped by the first chuck 10a and the second chuck 20a is rotated by the first spindle <NUM> or the second spindle <NUM>. On this occasion, misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3 can be easily detected based on the fluctuation range of the current value detected by the ammeter <NUM>.

Furthermore, based on the correlation between the eccentricity amount and the amplitude of the current value obtained in advance, the misalignment amount between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3 can be easily detected.

Furthermore, according to the machine tool <NUM> of the present embodiment, the misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3 is detected based on the fluctuation range of the current value of the electric servomotor <NUM>, which fluctuates depending upon the misalignment amount. Thus, the amount of misalignment can be detected regardless of the mass and shape of the first workpiece W1, the second workpiece W2, or the joined workpiece W3. Therefore, it is possible to improve the versatility of the machine tool <NUM> by eliminating the need to use a misalignment detecting means <NUM> with a setting or configuration corresponding to each of a plurality of types of workpieces having different masses, shapes, and the like.

The machine tool <NUM> may further include a rotation angle detecting means <NUM> for detecting the rotation angle of the first spindle <NUM> with respect to a predetermined rotation position, and may be configured so that the misalignment detecting means <NUM> detects the direction of misalignment of the second workpiece W2 with respect to the first workpiece W1, based on the rotation angle detected by the rotation angle detecting means <NUM>, and the fluctuation cycle of the current value detected by the ammeter <NUM>.

In this case, as the rotation angle detecting means <NUM>, there may be used a rotary encoder or the like.

In the misalignment detecting means <NUM>, for example, when the misalignment of the first workpiece W1 with respect to the second workpiece W2 occurs in the positive direction of the X axis, the current value increases in the positive direction so that, as illustrated in <FIG>, the rotation angle detecting means <NUM> detects the rotation angle of the first spindle <NUM> with reference to the upper spindle angle detection position along the Y-axis of the first spindle <NUM>. In this instance, if the current value reaches the maximum value when the rotation angle of the first spindle <NUM> is θ (see <FIG>), it can be detected that the misalignment is in the direction of <NUM> ° + θ in terms of the rotation angle of the first spindle <NUM>.

As described above, in the machine tool <NUM> of the present embodiment, even upon occurrence of misalignment between the first workpiece W1 and the second workpiece W2 not only in the X-axis direction but also in any direction. it is possible to detect such misalignment. However, by adopting the configuration including the rotation angle detecting means <NUM>, it is possible also to easily detect the direction of the misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3.

Furthermore, since the machine tool <NUM> of the present embodiment is adopted, as described above, to detect the misalignment amount and the misalignment direction of the misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3. it is possible easily to prevent misalignment by adjusting the machine tool <NUM> based on such detection results.

It goes without saying that the present disclosure is not limited to the above-described embodiment, and various modifications may be made without departing from the spirit of the present disclosure.

For example, in the above-described embodiment, it is determined that there is misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3, when the amplitude of the current value of the electric servomotor <NUM> detected by the ammeter <NUM> reaches or exceeds a predetermined value. However, occurrence of misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3 may be determined based on other parameters derived from the current value of the electric servomotor <NUM> detected by the ammeter <NUM>.

Furthermore, in the above-described embodiment, upon determination of the misalignment, the first spindle <NUM> is moved by the electric servomotor <NUM> in the direction perpendicular to the axis of the first spindle <NUM>. However, the moving direction may be changed in various manner as long as the direction intersects the axis of the first spindle <NUM>.

For example, in the above-mentioned embodiment, it is determined that there is misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3, when the amplitude of the current value of the electric servomotor <NUM> detected by the ammeter <NUM> reaches or exceeds a predetermined value. However, it is also possible to carry out processing of the fluctuation in the current value of the electric servomotor <NUM> detected by the ammeter <NUM> by means of FFT (Fast Fourier Transform) and determine that there is misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3, when a peak waveform appears in the frequency component based on the rotation speed of the first spindle <NUM>. Furthermore, it is also possible to use means based on the frequency upon of rotation of the first spindle <NUM>, such as a bandpass filter, for extracting only the frequency component from the fluctuation in the current value of the electric servomotor <NUM> detected by the ammeter <NUM>, and to determine that there is misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3 based on the extracted components. According to these methods, even when the fluctuation range of the current value of the electric servomotor <NUM> detected by the ammeter <NUM> is minute, it is possible to accurately determine the occurrence of misalignment between the first workpiece W1 and the second workpiece W2 in the joined workpiece W3.

Furthermore, in the above-described embodiment, the joining means <NUM> is configured to weld the axial end portion of the first workpiece W1 and the axial end portion of the second workpiece W2 by friction welding. However, the present disclosure is not limited to this, and the joining method may be variously changed. For example, the axial end of the first workpiece W1 and the axial end of the second workpiece W2 may be joined by welding means, such as laser welding. Alternatively, the axial end of the first workpiece W1 and the axial end portion of the second workpiece W2 may be joined by pressing and fitting unevenness provided therein.

Furthermore, in the above-described embodiment, the first spindle <NUM> is configured to be movable in the Z-axis direction by the Z-axis moving mechanism <NUM>. However, so long as an X-axis moving mechanism <NUM> is provided for moving the first spindle <NUM> in the X-axis direction, the Z-axis moving mechanism <NUM> may be arranged on the side of the second spindle <NUM>, or the Z-axis moving mechanism <NUM> may not be provided.

Claim 1:
A machine tool (<NUM>) comprising:
a first spindle (<NUM>) for gripping a first workpiece (W1):
a second spindle (<NUM>) for griping a second workpiece (W2); and
a joining means (<NUM>) for joining an axial end of each workpiece gripped by each spindle to form a joined workpiece (W3) from the first workpiece (W1) and the second workpiece (W2),
an electric servomotor (<NUM>) for moving the first spindle (<NUM>) in a direction intersecting the axis of the first spindle (<NUM>); characterised in that the machine tool (<NUM>) further comprises
a current value detecting means (<NUM>) for detecting a current value of the electric servomotor (<NUM>); and
a misalignment detecting means (<NUM>) for detecting misalignment between the first workpiece (W1) and the second workpiece (W2) of the joined workpiece (W3), based on the current value detected by the current value detecting means (<NUM>) when the joined workpiece (W3) is gripped by both spindles and rotated by the first spindle (<NUM>) or the second spindle (<NUM>).