Patent Description:
In a well-known NC (numerical control) lathe, a front side of a workpiece held by a front spindle is machined and the workpiece is then passed to a back spindle for back machining to be performed on a back side of the workpiece. A cut-off tool is used to cut-off the workpiece rotatably held by the opposite front and back spindles. Breakage of the cut-off tool would cause a cut-off failure and thereby hinder success of a subsequent machining. It is therefore necessary to determine whether the workpiece has been normally cut-off.

Generally, a workpiece detecting pin is advanced to a position between the front spindle and the back spindle to check whether the pin hits the workpiece. The pin is provided with a contact switch. If the switch is not activated, it is determined that the the workpiece has been normally cut-off. If the switch is activated, it is determined that a cut-off failure has happened and then the machine stops. The workpiece is rotated at high speed during the cut-off process. The spindle rotation speed needs reduction before the pin is advanced to prevent a collision with the workpiece rotating at high speed. Thereafter, the spindle rotation speed needs to be restored to a level suitable for a subsequent machining.

Patent Document <NUM> discloses a cut-off process in a lathe provided with phase synchronizing control means for two coaxially-opposite spindles. The phase synchronizing control means includes a phase difference detecting unit for detecting a phase difference between the rotating first spindle and the rotating second spindle and further a correction instructing unit for correcting a speed command given to a spindle motor according to the detected phase difference. The first spindle and the second spindle are operated synchronously and then the workpiece held by the spindles is cut-off. A feedback output of the correction instructing unit is disconnected and then different rotation speed commands are respectively given to a first spindle motor and a second spindle motor. If a phase difference detected by the phase difference detecting unit exceeds a setting value, a start of a next machining is permitted. Further lathes capable of checking the cutting-off operation of a workpiece fixed in a first spindle and a second spindle are disclosed in Patent Documents <NUM> and <NUM>.

The conventional method using the workpiece detecting pin takes a longer determining time since the spindle rotation speed is necessarily reduced before the pin is advanced. The conventional method using the phase difference takes a longer determining time since the spindle rotation speed is necessarily changed between the first spindle and the second spindle after the cut-off process ends. Such slow determining process prolongs a cycle time of workpiece machining.

Therefore it is an object of the present invention to provide a lathe capable of shortening a cycle time of workpiece machining.

This object is solved by a lathe comprising the features of claim <NUM>.

The invention provides a lathe capable of shortening a cycle time of workpiece machining.

Hereinafter, an embodiment of the present invention will be described. The invention is not limited to the exemplary embodiment and the features disclosed herein are not necessarily essential to the invention.

Technology of the invention will be described with reference to <FIG>. The drawings only schematically show an example of the invention. They may have a mismatch to each other due to different magnification in each direction. Each element denoted by a symbol is only an example. The range of values (the minimum to the maximum) means the minimum or more and the maximum or less.

The lathe of the invention includes a first spindle (a front spindle <NUM>, for example), a second spindle (a back spindle <NUM>, for example), a tool post <NUM>, and a control unit U1. The first spindle (<NUM>) holding a workpiece WO is configured to rotate on a spindle axis AX1. The second spindle (<NUM>) being opposite the first spindle (<NUM>) and holding the workpiece WO is configured to rotate on a spindle axis AX2 preferably aligned with the spindle axis AX1. A cut-off tool T1 is attached to the tool post <NUM> for use to cut the workpiece WO held by the first spindle (<NUM>) and the second spindle (<NUM>). The control unit U1 is configured to control rotation of the first spindle (<NUM>) and the second spindle (<NUM>) and further to control relative movement of the first spindle (<NUM>) and the second spindle (<NUM>) and the tool post <NUM> to cut-off the workpiece WO. The control unit U1 is further configured to add a rotation speed fluctuation to the rotation speed V1 of the first spindle (<NUM>) after rotating the first spindle (<NUM>) and the second spindle (<NUM>) synchronously and detect the rotation speed V2 of the second spindle (<NUM>). The control unit U1 determines that the workpiece WO has been normally cut-off upon determining that a detected fluctuation (a fluctuation width ΔV2, for example) in the rotation speed V2 is within a predetermined range (below a threshold value TH, for example).

The first spindle (<NUM>) and the second spindle (<NUM>) both holding the workpiece WO are synchronously operated and then a rotation speed fluctuation is added to the rotation speed V1 of the first spindle (<NUM>). If the workpiece WO has been remained uncut between the first spindle (<NUM>) and the second spindle (<NUM>), the fluctuation added to the rotation speed V1 influences the rotation speed V2 of the second spindle (<NUM>). If the workpiece WO has been normally cut-off, the fluctuation added to the rotation speed V1 never influences the rotation speed V2. Accordingly, it can be determined that the workpiece has been normally cut-off if the detected fluctuation in the rotation speed V2 of the second spindle (<NUM>) is within the predetermined range. The rotation speed V2 may be detected and the detected fluctuation in the rotation speed V2 may be used to determine whether the workpiece has been normally cut-off. Since the cut-off success can be determined immediately after the cut-off process, the invention provides a lathe capable of shortening a cycle time of workpiece machining.

The first spindle may be the front spindle while the second spindle may be the back spindle. Instead, the first spindle may be the back spindle while the second spindle may be the front spindle. Relative movement of the first and the second spindles and the tool post may include variations. Only the tool post may move without movement of the spindles. Instead, only the spindles may move without movement of the tool post. Instead, both the spindles and the tool post may move. The control unit may determine that the workpiece has been normally cut-off when it determines that the detected fluctuation is within the predetermined range after several determining steps which have been repeated since the detected fluctuation is still outside the predetermined range. Instead, the control unit may immediately determine that the workpiece has been normally cut-off when it determines that the detected fluctuation is within the predetermined range at the first time it detects the rotation speed of the second spindle.

The control unit U1 adds a rotation speed fluctuation to the rotation speed V1 of the first spindle (<NUM>) and detects the rotation speed V2 of the second spindle (<NUM>) while moving the tool post <NUM> to the cut-off end position (an X=+Xe position, for example). The control unit U1 determines that the workpiece WO has been normally cut-off when a detected fluctuation (ΔV2) of the rotation speed V2 is within the predetermined range. The invention provides a lathe of simple configuration since there is no need of moving the first spindle (<NUM>) and the second spindle (<NUM>) in a direction perpendicular to the spindle axis AX1 (an X-axis direction, for example).

<FIG> schematically shows a configuration of an NC (numerical control) lathe <NUM> provided with a movable front spindle <NUM>. <FIG> is only a simplified example for explanation and the invention is not limited thereto. A positional relation between elements is only an example for explanation. The left and right direction may be replaced by the up and down direction or the front and back direction. The up and down direction may be replaced by the left and right direction or the front and back direction. The front and back direction may be replaced by the left and right direction or the up and down direction. The rotational direction may be inversed. If something is the same as something in direction or position, they may be the same or almost the same within an error range.

The lathe <NUM> may include an NC apparatus <NUM>, a front headstock <NUM> mounted on a fixed base <NUM>, a back headstock <NUM> mounted on a fixed base <NUM>, a tool post <NUM> mounted on a fixed base <NUM>. The NC apparatus <NUM> may control operations of the front headstock <NUM>, the back headstock <NUM>, and the tool post <NUM>.

The front headstock <NUM> may move in a Z-axis direction along the spindle axis AX1. The NC apparatus <NUM> may control a Z-axis position of the front headstock <NUM> by a driving unit such as a Z1-axis motor MZ1 as shown in <FIG>. The front spindle <NUM> mounted on the front headstock <NUM> is provided with a chuck <NUM> such as a collet to releasably hold a cylindrical (bar) workpiece W1 inserted in the Z-axis direction. The NC apparatus <NUM> may rotate the front spindle <NUM> on the spindle axis AX1 extended in the longitudinal direction of the workpiece W1 by a driving unit such as a rotation motor <NUM>. The front spindle <NUM> may thereby rotate the workpiece W1 on the spindle axis AX1.

A guide bush <NUM> may be provided in a position on the front side of the front spindle <NUM> as shown by a two-dot line in <FIG>. The guide bush <NUM> may slidably support the longitudinal workpiece W1 inserted in the front spindle <NUM> in the Z-axis direction. The guide bush <NUM> may be rotatable on the spindle axis AX1 in synchronization with the front spindle <NUM>.

The back headstock <NUM> may be movable in the Z-axis direction along the spindle axis AX2 and a Y-axis direction perpendicular to the Z-axis direction. The NC apparatus <NUM> may control a Z-axis position and a Y-axis position of the back headstock <NUM> by a driving unit such as a Z2-axis motor MZ2 and a Y2-axis motor MY2 as shown in <FIG>. The back spindle <NUM> mounted on the back headstock <NUM> may be provided with a chuck <NUM> such as a collet to releasably hold a workpiece W2, whose front side has been machined, inserted in the Z-axis direction with the spindle axes AX1 and AX2 are aligned. The NC apparatus <NUM> may rotate the back spindle <NUM> on the spindle axis AX2 by a driving unit such as a rotation motor <NUM>. The back spindle <NUM> may thereby rotate the workpiece W2 on the spindle axis AX2. The back spindle <NUM> may be referred to as an opposite spindle since it is opposite the front spindle. The workpiece W1 and the workpiece W2 whose front side has been machined may be collectively called the workpiece WO. The workpiece held by the chucks <NUM> and <NUM> of the spindles <NUM> and <NUM> may be called the workpiece WO.

The tool post <NUM> to which a plurality of tools T0 are attached may be movable in the X-axis direction and the Z-axis direction. The X-axis direction may be a direction perpendicular to the Z-axis direction and the Y-axis direction. The NC apparatus <NUM> may control an X-axis position and a Z-axis position of the tool post <NUM> by a driving unit such as an X3-axis motor MX3 and a Z3-axis motor MZ3 as shown in <FIG>. The plurality of the tools T0 may include a cut off tool T1 for use to cut-off the workpiece WO held by the chucks <NUM> and <NUM> of the spindles <NUM> and <NUM>. The tool post may include a turret tool post and a gang tool post. The lathe may be provided with a plurality of the tool posts. The moving direction of the front headstock <NUM>, the back headstock <NUM>, and the tool post <NUM> may not be limited to those shown in <FIG>.

<FIG> schematically shows a configuration of an electrical circuit in the NC lathe <NUM>. The NC apparatus <NUM> may include an operation panel <NUM>, the Z1-axis motor MZ1, the Y2-axis motor MY2, the Z2-axis motor MZ2, the X3-axis motor MX3, the Z3-axis motor MZ3, the rotation motor <NUM> for driving the front spindle <NUM>, the rotation motor <NUM> for driving the back spindle <NUM>, an actuator <NUM> for opening and closing the chuck <NUM> of the front spindle <NUM>, and an actuator <NUM> for opening and closing the chuck <NUM> of the back spindle <NUM>. The NC apparatus <NUM> may be a computer including a CPU (Central Processing Unit) <NUM>, a semiconductor memory including a ROM (Read Only Memory) <NUM> and a RAM (Random Access Memory) <NUM>, a timer circuit <NUM>, and an I/F (Interface) <NUM>. The I/F <NUM> may include interfaces for the operation panel <NUM>, the servo motors MZ1, MY2, MZ2, MX3, and MZ3, the rotation motors <NUM> and <NUM>, and the actuators <NUM> and <NUM>. The ROM <NUM> may store an interpretive execution program P1 for use in interpreting and executing a machining program P2. The RAM <NUM> may rewritably store the machining program P2 prepared by an operator. The machining program P2 may be also called an NC program. The CPU <NUM> may use the RAM <NUM> as a work area to execute the interpretive execution program P1 stored in the ROM <NUM>. Part or all of the functions achieved by the interpretive execution program P1 may be executed by another means such as ASIC (Application Specific Integrated Circuit).

The operation panel <NUM> may include an input <NUM> and a display <NUM> serving as a user interface of the NC apparatus <NUM>. The input <NUM> may include a button and a touch panel for accepting an input by the operator. The display <NUM> may display various information relating to settings ordered by the operator and various information relating to the NC lathe <NUM>. The machining program P2 may be stored in the RAM <NUM> by the operator via the operation panel <NUM> and an external computer.

The Z1-axis motor MZ1 may drive the front headstock <NUM> in the Z-axis direction upon receipt of a command from the NC apparatus <NUM>. The Y2-axis motor MY2 may drive the back headstock <NUM> in the Y-axis direction upon receipt of a command from the NC apparatus <NUM>. The Z2-axis motor MZ2 may drive the back headstock <NUM> in the Z-axis direction upon receipt of a command from the NC apparatus <NUM>. The X3-axis motor MX3 may drive the tool post <NUM> in the X-axis direction upon receipt of a command from the NC apparatus <NUM>. The Z3-axis motor MZ3 may drive the tool post <NUM> in the Z-axis direction upon receipt of a command from the NC apparatus <NUM>.

The rotation motor may include an encoder <NUM>. The encocer <NUM> may generate a reference angle pulse of an interval according to the rotation speed of the front spindle <NUM>. The rotation motor <NUM> may drive the front spindle <NUM> in response to a command from the NC apparatus <NUM>. The rotation motor <NUM> may include an encoder <NUM>. The encoder <NUM> may generate a reference angle pulse of an interval according to the rotation speed of the back spindle <NUM>. The rotation motor <NUM> may drive the back spindle <NUM> in response to a command from the NC apparatus <NUM>. The rotation speed may be also called a number of rotations meaning the number of rotations per unit time.

The actuator <NUM> may open and close the chuck <NUM> of the front spindle <NUM> through a power transmission mechanism such as a sleeve under control of the NC apparatus <NUM>. Opening the chuck <NUM> may allow the workpiece to move in the Z-axis direction. Closing the chuck <NUM> may unmovably hold the workpiece. The actuator <NUM> may open and close the chuck <NUM> of the back spindle <NUM> through a power transmission mechanism such as a sleeve under control of the NC apparatus <NUM>. Opening the chuck <NUM> may allow the workpiece to move in the Z-axis direction. Closing the chuck <NUM> may unmovably hold the workpiece. The actuators <NUM> and <NUM> each may include a servo motor such as a linear motor, an air cylinder, and a hydraulic cylinder. The actuators may include a deceleration mechnism such as a ball screw mechanism.

The front spindle <NUM> may be an example of the first spindle. The back spindle <NUM> may be an example of the second spindle. The NC apparatus <NUM>, the rotation motors <NUM> and <NUM>, and the X3-axis motor MX3 may be examples of the control unit U1.

<FIG> schematically shows how the workpiece is normally cut-off with the cut-off tool T1 attached to the tool post <NUM> in the lathe having no guide bush. The cut-off process may be similarly performed in a lathe having a guide bush. The left diagram shows that a cutting edge of the cut-off tool T1 is advanced to an X=-D/<NUM> position when the workpiece WO of diameter D is held by the chuck <NUM> of the front spindle <NUM> and by the chuck <NUM> of the back spindle <NUM> approaching the front spindle <NUM>. The tool post <NUM> may further advance in the +X direction to further cut the workpiece WO while the both spindles <NUM> and <NUM> are rotated at rotation speed set for the cut-off process. The workpiece WO may be normally cut off at an X=<NUM> position corresponding to the center of the workpiece WO. Actually, the tool post <NUM> may move to an X=+Xe (Xe><NUM>) position corresponding to a cut-off end position beyond the X=<NUM> position as shown in the right diagram.

<FIG> schematically shows how a cut-off failure happens by a broken cut-off tool. The left diagram shows a broken part of the cut-off tool T1 by a two-dot line. The right diagram shows that the workpiece WO is remained uncut between both the spindles <NUM> and <NUM>. Such cut-off failure would influence a subsequent machining such as a back machining. It is therefore necessary to determine whether the workpiece has been normally cut-off before starting a subsequent machining.

<FIG> schematically shows a detecting apparatus <NUM> in a comparative example to determine whether the workpiece WO is normally cut-off. The detecting apparatus <NUM> is mounted on a tool post (not shown) movable in the X-axis direction. The detecting apparatus <NUM> has a workpiece detecting pin <NUM> movable in a direction perpendicular to the spindle axis AX1 of both the spindles <NUM> and <NUM> and a contact switch <NUM> attached to the end of the pin <NUM>. If the advancing pin <NUM> does not touch the workpiece WO and the contact switch <NUM> is thereby not activated, it is determined that the workpiece has been normally cut-off and a start of a subsequent machining is permitted. If the advancing pin <NUM> touches the workpiece WO and the contact switch <NUM> is thereby activated, it is determined that a cut-off failure has happened and the machine stops. The workpiece WO is rotated at high speed during and immediately after the cut-off process. The rotation speed of both the spindles <NUM> and <NUM> needs reduction before the pin <NUM> is advanced to prevent a collision with the workpiece WO rotating at high speed.

<FIG> schematically shows the rotation speed of the spindles <NUM> and <NUM> with a lapse of time in the comparative example. The horizontal axis of the graph shows the time (t) and the vertical axis shows rotation speed (rpm: rotations per minute) of the spindles <NUM> and <NUM>. The upper graph shows the rotation speed V1 of the front spindle <NUM> with a lapse of time. The lower graph shows the rotation speed V2 of the back spindle <NUM> with a lapse of time.

At the time t91, the workpiece whose front side has been machined is held by the chuck <NUM> of the back spindle <NUM> and synchronous rotation of the spindles <NUM> and <NUM> start. A command value for the rotation speed V2 of the back spindle <NUM> is set to match with a command value for the rotation speed V1 of the front spindle <NUM>. At the time t92, the synchronous high speed operation ends. At the time t93, the tool post reaches the X=+Xe position (the cut-off end position). The workpiece WO still rotates at high speed (<NUM>,<NUM> to <NUM>,<NUM> rpm, for example). In the period from the time t93 to the time t94, the NC apparatus <NUM> decreases the rotation speed of the front spindle <NUM> and the back spindle <NUM> each to low speed (<NUM> to <NUM> rpm, for example) suitable for workpiece detection. In the period from the time t94 to the time t95, the workpiece detecting pin <NUM> is advanced to determine whether the workpiece is normally cut-off. If the pin <NUM> does not touch the workpiece WO, the NC apparatus <NUM> restores high speed rotations suitable for a next machining in the period from the time t95 to the time t96. Accordingly, the determining process in the period from the time t93 to the time t96 is necessarily executed every machining after the tool post reaches the X=+Xe position (the cut-off end position).

In the invention, the front spindle <NUM> and the back spindle <NUM> are synchronously operated and then a rotation speed fluctuation is added to the rotation speed V1 of the front spindle <NUM>. The rotation speed V2 of the back spindle <NUM> is detected to determine whether a fluctuation in the rotation speed V2 of the back spindle <NUM> is within a predetermined range. It is determined that the workpiece is normally cut-off before the tool post reaches the cut-off end position upon determining that a detected fluctuation in the rotation speed V2 of the back spindle <NUM> is within a predetermined range. The invention provides a lathe capable of immediately determining that the workpiece is normally cut-off and further eliminates a need for a space for the detecting apparatus <NUM>.

<FIG> is a graph schematically showing fluctuations in the rotation speed of the spindle with a lapse of time when the workpiece is normally cut-off. The horizontal axis shows the time (t) and the vertical axis shows rotation speed (rpm) of the front spindle <NUM> and the back spindle <NUM>. The upper graph shows fluctuations in the rotation speed V1 of the front spindle <NUM> with a lapse of time. The lower graph shows fluctuations in the rotation speed V2 of the back spindle <NUM> with a lapse of time.

At the time t11, the workpiece whose front side has been machined may be held by the chuck <NUM> of the back spindle <NUM> and a synchronous operation of the front spindle <NUM> and the back spindle <NUM> may start. A command value for the rotation speed V2 of the back spindle <NUM> may be set to match with a command value for the rotation speed V1 (without a fluctuation) of the front spindle <NUM>. At the time t12, the synchronous operation at high speed (<NUM>,<NUM> to <NUM>,<NUM> rpm, for example) for the cut-off process may end. The NC apparatus <NUM> may then start moving the tool post <NUM> in the +X direction. At the time t13, the tool post <NUM> may reach the X=-Xs (Xs><NUM>) position corresponding to the rotation speed fluctuation start position. The NC apparatus <NUM> may start adding a rotation speed fluctuation of a fluctuation width ΔV1 to the rotation speed V1 of the front spindle <NUM>. The fluctuation added to the speed V1 may be also added to the speed V2 of the back spindle <NUM> through the workpiece WO held by the front spindle <NUM> and and the back spindle <NUM>.

The fluctuation width ΔV1 may be a difference between the maximum value and the minimum value of the rotation speed. It may be the double of the amplitude representing the maximum displacement from the center value. Assuming that fluctuation in the rotation speed is ± A (rpm), the amplitude may be A (rpm) and the fluctuation width ΔV1 may be 2A (rpm). The amplitude A may be set to <NUM> to <NUM> rpm, for example, which is a value smaler than (one to three figures short) the rotation speed V1 (rpm). Frequency f1 of the rotation speed V1 may be <NUM> to <NUM>.

At the time t14, the tool post <NUM> may reach the X=-D/<NUM> position corresponding to the cut-off tool contact position where the cutting edge of the cut-off tool T1 touches the workpiece WO. The rotation speed V1 may decrease due to applied cutting load. The fluctuation may be continuously added to the rotation speed V2 through the workpiece WO held by the front spindle <NUM> and the back spindle <NUM>. As the cut-off process progresses, rigidity of the cut-off portion of the workpiece WO may be lowered to thereby decrease the fluctuation width ΔV2 of the rotation speed V2. As cutting load becomes smaller, the rotation speed V1 of the front spindle <NUM> and the rotation speed V2 of the back spindle <NUM> may be gradually restored. At the time t15, the tool post <NUM> may reach the X=+Xe position corresponding to the cut-off end position.

<FIG> is a graph showing fluctuations in the rotation speed of the spindle with a lapse of time when a cut-off failure happens. The horizontal axis shows the time (t) and the vertical axis shows rotation speed (rpm) of the front spindle <NUM> or the back spindle <NUM>. The fluctuations in the rotation speed of the spindle with a lapse of time in the period from the time t11 to the time t14 may be the same as that of <FIG>. At the time t15, however, the fluctuation width ΔV2 in the rotation speed V2 of the back spindle <NUM> may be still maintained since the workpiece WO remains uncut between the front spindle <NUM> and the back spindle <NUM>.

The fluctuation width ΔV2 in the rotation speed V2 of the back spindle <NUM> may decrease when the workpiece is normally cut-off as shown in <FIG>. When the decreasing fluctuation width ΔV2 falls within the predetermined range, it may be determined that the workpiece has been normally cut-off. The cut-off success can be determined immediately after the cut-off process, which eliminates the need of detection in the period from the time t93 to the time t96 in <FIG> and thereby shortens a cycle time of workpiece machining. Further, the rotation speed fluctuation start position (the X=-Xs position) may follow the cut-off tool contact position (the X=-D/<NUM> position). The time t13 may follow the time t14 in <FIG> and <FIG>.

<FIG> is a flow chart showing a single cut-off process under control of the NC apparatus <NUM> executing the interpretive execution program P1. The process may be initiated when a cut-off command in the machining program P2 is read by the NC apparatus <NUM>. It is difficult for the operator to write a machining program enabling such rotation speed fluctuations as shown in <FIG> and <FIG>. It is therefore executed under control of the NC apparatus executing the interpretive execution program P1 in <FIG>. The cut-off process is being explained also referring to <FIG>.

First, the NC apparatus <NUM> may drive the Z2-axis motor MZ2 to move the back headstock <NUM> toward the front headstock <NUM> and then drive the actuator <NUM> to allow the back spindle <NUM> to hold the workpiece WO by the chuck <NUM> thereof (Step S102). Then, the NC apparatus <NUM> may output a command value for the rotation speed V1 for the cut-off process to the rotation motors <NUM> and <NUM> to start synchronous operation of the front spindle <NUM> and the back spindle <NUM> both holding the workpiece WO (S104). The front spindle <NUM> may be a master while the back spindle <NUM> may be a slave. A command value for the rotation speed V2 of the back spindle given to the rotation motor <NUM> may be set to match with the command value for the rotation speed V1 given to the rotation motor <NUM>. Step S104 may start at the time t11 as shown in <FIG> and <FIG>. The NC apparatus <NUM> may output a synchronous operation end signal when the rotation speed V1 (V2) of the spindle <NUM> (<NUM>) is detected to reach the high speed set for the cut-off process (at the time t12 in <FIG> and <FIG>).

The NC apparatus <NUM> may acquire an X=-Xs position from the machining program P2 (S106). The X=-Xs position may correspond to the rotation speed fluctuation start position with respect to the X-axis position of the tool post <NUM>. Then the NC apparatus <NUM> may drive the X3-axis motor MX3 to move the tool post <NUM> having the cut-off tool T1 along a cut-off way in the +X direction (S108).

When the tool post <NUM> reaches the X=-Xs position, the NC apparatus <NUM> may start adding a fluctuation to the rotation speed V1 of the front spindle <NUM> (S110). The NC apparatus <NUM> adds a fluctuation width ΔV1 corresponding to a fluctuation component of frequency f1 to the command value given to the rotation motor <NUM>. An electric current or a torque applied to the rotation motor <NUM> may be controlled to change the rotation speed V1 for the cut-off process to a speed increased by the added fluctuation component. The fluctuation component may not be added to the command value given to the rotation motor <NUM> for the back spindle <NUM>. S110 may start at the time t13 as shown in <FIG> and <FIG>. The tool post <NUM> may be in the middle of movement to the cut-off end position (the X=+Xe position). The rotation speed V1 of the front spindle <NUM> may continue to fluctuate. The rotation motor <NUM> may detect the rotation speed V2 of the back spindle <NUM> according to a pulse generated by the encoder <NUM>.

The NC apparatus <NUM> repeatedly acquires a detected value of the rotation speed V2 from the rotation motor <NUM> to determine whether the fluctuation width ΔV2 of the detected value of the rotation speed V2 is smaller than a predetermined threshold value TH before the tool post <NUM> reaches the cut-off end position (the X=+Xe position) (S112). The threshold value TH may be a positive value smaller than the fluctuation width ΔV2 added in the Step <NUM>. If the fluctuation width ΔV2 of the detected value of the rotation speed V2 is smaller than the predetermined threshold value TH (ΔV2<TH), it may be determined that the detected fluctuation in the rotation speed V2 falls within the predetermined range. The threshold value TH may be stored in the ROM <NUM> or the RAM <NUM>.

If the workpiece has been normally cut-off as shown in <FIG>, the fluctuation width ΔV2 becomes smaller than the threshold value TH (ΔV2<TH) before the tool post <NUM> reaches the cut-off end position (the X=+Xe position in <FIG>). The NC apparatus <NUM> may drive the Z2-axis motor MZ2 to retract the back headstock <NUM> away from the front headstock <NUM> to proceed to a subsequent machining (S114). The subsequent machining may include a back machining applied on the workpiece W2 held by the chuck <NUM> of the back spindle <NUM> and a front machining applied on the workpiece W1 held by the chuck <NUM> of the front spindle <NUM>.

If the cut-off failure has happened as shown in <FIG>, the fluctuation width ΔV2 in the rotation speed V2 of the back spindle <NUM> may be still maintained even when the tool post <NUM> reaches the cut-off end position (the X=+Xe position in <FIG>) and therefore the fluctuation width ΔV2 may be greater than or equal to the threshold value TH (ΔV2≥TH). The NC apparatus <NUM> may output a warning and then stop machining (S116). The warning may include an alarm on the display <NUM> and a warning beep from a sounding device (not shown).

In the embodiment, a rotation speed fluctuation may be added to the rotation speed V1 of the front spindle <NUM> before initiating the cut-off process. The added fluctuation may not influence the rotation speed V2 of the back spindle <NUM> if the workpiece is normally cut-off. The cut-off success can be determined only by determining that the fluctuation width ΔV2 of the rotation speed V2 is less than the threshold value TH. The embodiment can shorten the determining time and thereby shorten a cycle time for workpiece machining. The embodiment eliminates a need for a special determining apparatus, which reduces a cost of the lathe.

Further, using the fluctuation width ΔV2 of the rotation speed V2 would increase a sampling frequency of data acquisition and therefore improves determining reliability. The determining steps of the embodiment could be further facilitated in the NC apparatus capable of adding a fluctuation to the rotation speed V1 to control resonance caused by the rotation speed V1 of the front spindle <NUM>.

The invention may be embodied in various modifications. For example, the invention may be embodied in a lathe of any type including a spindle slidable type and a spindle stationary type.

The tool post <NUM> may move in the X-axis direction to cut-off the workpiece. Instead, the front and the back spindles may move in the X-axis direction to cut-off the workpiece. Instead, both of the spindles and the tool post may move in the X-axis direction to cut-off the workpiece. The front spindle may be a master while the back spindle may be a slave. Instead, the back spindle may be a master while the front spindle may be a slave. The rotation speed fluctuation may be added to the rotation speed of the back spindle and the rotation speed of the front spindle may be detected to determine whether a detected fluctuation in the rotation speed of the front spindle is within the predetermined range.

The order of the steps as described above may be appropriately changed. For example, S106 may be executed before S102 or S104 in <FIG>. S112 may start at any time including any finish time of machining in the machining program and may end at any time.

<FIG> shows an example of another single cut-off process executed by the NC apparatus <NUM> where S202 and S204 replace S112 (<FIG>). <FIG> collectively shows S102 to S108. The NC apparatus <NUM> may execute S102 to S110 and start adding a rotation speed fluctuation to the rotation speed V1 of the front spindle <NUM> when the tool post <NUM> reaches the X=-Xs position corresponding to the rotation speed fluctuation start position. In S202, the NC apparatus <NUM> may acquire a cut-off end time from the machining program P and determine whether the cut-off end time has come. The cut-off end time may be any time including the time when the tool post reaches the X=+Xe position. S <NUM> may be repeated until the cut-off end time comes.

When the cut-off end time has come, the NC apparatus <NUM> may repeatedly acquire a detected value of the rotation speed V2 from the rotation motor <NUM> within a predetermined period of time to determine whether an acquired fluctuation width ΔV2 is smaller than the threshold value TH (<NUM><TH<ΔV2) (S204). If a plurality of fluctuation widths have been acquired during the period, a calculated arithmetic mean may be used as the acquired fluctuation width ΔV2. Instead, among the plurality of the fluctuation widths, the maximum and the miminum values may be excluded before the calculation of the arithmetic mean. If the fluctuation width ΔV2 is smaller than the threshold value TH, a start of a subsequent machining may be permitted (S114). If the fluctuation width ΔV2 is greater than or equal to the threshold value TH, the NC apparatus <NUM> may output a warning and stop machining (S116).

In <FIG>, the fluctuation may be added to the rotation speed V1 of the front spindle <NUM> before the start of the cut-off process. The added fluctuation may never influence the rotation speed V2 of the back spindle <NUM> in the determining steps immediately after the end of the cut-off process if the workpiece has been normally cut off. The cut-off success can be determined immediately after the end of the cut-off process upon determining that the fluctuation width ΔV2 is smaller than the threshold value TH. The embodiment shortens the determining time, resulting in a shortened cycle time of machining. Further, the embodiment increases a sampling frequency of data acquisition and thereby improves determining reliability.

The NC apparatus may determine whether the fluctuation width ΔV2 is smaller than the threshold value TH when the tool post <NUM> reaches the cut-off end position (S112) in <FIG> or when the cut-off end time comes (S202) in <FIG>. Instead, the NC apparatus may determine whether the fluctuation width ΔV2 is smaller than the threshold value TH after a predetermined time period elapses since the tool post <NUM> has reached the cut-off end position or after a predetermined time period elapses since the the cut-off end time has come. A plurality of the fluctuation widths ΔV2 smaller than the threshold value TH may be continuously detected during the predetermined time period, which increases a sampling frequency of data acquisition and thereby improves determining reliability. A calculated arithmetic mean based on the plurality of data acquired during the period may be used as the acquired fluctuation width ΔV2. Instead, the maximum and the miminum values may be excluded before the calculation of the arithmetic mean.

Claim 1:
A lathe (<NUM>) comprising:
a first spindle (<NUM>) configured to rotate on a spindle axis (AX1, AX2) and to hold a workpiece (W0, W1, W2);
a second spindle (<NUM>) opposite the first spindle (<NUM>) configured to rotate on the spindle axis (AX1, AX2) and to hold the workpiece (W0, W1, W2);
a tool post (<NUM>) on which a cut-off tool (T1) is provided to cut-off the workpiece (W0, W1, W2) held by the first spindle (<NUM>) and the second spindle (<NUM>); and
a control unit (U1) configured to control rotation of the first spindle (<NUM>) and the second spindle (<NUM>) and further to control relative movement of the first and the second spindles (<NUM>, <NUM>) and the tool post (<NUM>) to cut-off the workpiece (W0, W1, W2),
wherein the control unit (U1) is further configured to add a rotation speed fluctuation to a rotation speed (V1) of the first spindle (<NUM>) after rotating the first spindle (<NUM>) and the second spindle (<NUM>) synchronously such that the fluctuation adds a fluctuation width (ΔV1) corresponding to a fluctuation component of a frequency (f1) to the rotation speed of the first spindle (<NUM>) and to detect a rotation speed (V2) of the second spindle (<NUM>),
the lathe being characterized in that
the control unit (U1) is configured to add the fluctuation during the time that the control unit moves the tool post to a cut-off end position and the control unit (U1) is configured to determine that the workpiece (W0, W1, W2) is normally cut-off upon determining, before the tool post (<NUM>) reaches the cut-off end position, by detecting that a fluctuation width (ΔV2) of the detected value of the rotation speed of the rotation speed (V2) of the second spindle (<NUM>) is smaller than a predetermined threshold value (TH).