Numerical control walking center machine for preventing workpiece resonance in machining of elongated shaft

Abstract

The present application relates to the technical field of lathe, specifically to a numerical control lathe for preventing workpiece resonance in slender shaft machining, which comprises a spindle seat, a rotating spindle rotatingly arranged in the spindle seat, an extrusion cylinder arranged at one end of the rotating spindle, and a resonance suppression mechanism arranged in the extrusion cylinder; the resonance suppression mechanism comprises an elastic sleeve capable of clamping and fixing the workpiece under the extrusion of the extrusion cylinder; a filling cavity coaxial with the clamping opening of the elastic sleeve is arranged in the elastic sleeve; a piston is movably arranged in the filling cavity, and particles are arranged in the filling cavity; when the piston moves axially along the filling cavity, the particles in the filling cavity can be extruded, so that the particles are wrapped outside the free overhanging part of the workpiece penetrating into the filling cavity; the free overhanging part of the workpiece is supported, the damping characteristics of the workpiece are changed, the resonance of the workpiece in the machining process is effectively suppressed, and the machining quality of the workpiece is improved.

Description

Technical Field

[0001] This invention relates to the field of Swiss-type lathe technology, and more specifically to a CNC Swiss-type lathe for machining slender shafts to prevent workpiece resonance. Background Technology

[0002] CNC Swiss-type lathes are specialized machines used for machining slender shafts. After machining one end of the slender shaft, it needs to be cut off before machining the other end. Because the length-to-diameter ratio of the machined end is relatively large, existing CNC Swiss-type lathes simply hold the slender shaft close to the machined end with a chuck, leaving the unmachined end in a free-hanging state. This causes the high-speed rotation of the spindle during machining, combined with chip vibration, to easily cause resonance in the free-hanging part of the workpiece. Consequently, tool marks are generated on the workpiece surface due to resonance, reducing the surface finish and affecting the machining quality. Summary of the Invention

[0003] The purpose of this invention is to overcome the above-mentioned shortcomings and provide a CNC Swiss-type lathe for machining slender shafts that prevents workpiece resonance.

[0004] To achieve the above objectives, the specific solution of the present invention is as follows: A CNC Swiss-type lathe for machining slender shafts to prevent workpiece resonance, comprising a spindle seat, a rotating spindle rotatably disposed within the spindle seat, an extrusion cylinder disposed at one end within the rotating spindle, and a resonance suppression mechanism disposed within the extrusion cylinder. The resonance suppression mechanism includes an elastic sleeve that can clamp and fix the workpiece under the extrusion action of the extrusion cylinder; the elastic sleeve has a filling cavity coaxial with the clamping port of the elastic sleeve; a piston is movably installed in the filling cavity, and the filling cavity is filled with particles; when the piston moves along the axial direction of the filling cavity, it can squeeze the particles in the filling cavity, so that the particles cover the outside of the free overhanging part of the workpiece that penetrates into the filling cavity.

[0005] Furthermore, the CNC Swiss-type lathe also includes a power component located on one side of the spindle seat and a push rod connected at one end to the output end of the power component; the other end of the push rod is movably inserted into the rotating spindle and coaxial with the rotating spindle; the push rod is used to drive the piston to move axially along the filling cavity.

[0006] Furthermore, the resonance suppression mechanism also includes a magnetic cylinder coaxially disposed within the elastic sleeve, a sealing assembly coaxially disposed at one end of the magnetic cylinder, and a permanent magnet ring coaxially disposed at the other end of the magnetic cylinder; the particles are ferromagnetic particles. The piston is slidably disposed inside the magnetic cylinder; the sealing assembly, the magnetic cylinder, and the piston together enclose the filling cavity; the piston is axially movable and has a push pin inserted through it; a first spring extending axially is connected between the push pin and the piston; one end of the push pin can be axially movable and inserted into the filling cavity, and the other end is fixedly connected to the other end of the push rod.

[0007] Furthermore, the sealing assembly includes a sealing ring and a plurality of sealing blocks slidably arranged around the central hole of the sealing ring; a second spring is connected between one of the sealing blocks and the sealing ring; the sidewall of each sealing block is movably abutting against the sidewall of the adjacent sealing block; each sealing block has a driving inclined surface on the side facing the workpiece insertion direction; when the workpiece contacts the driving inclined surface of the sealing block, the sealing block slides relative to the sealing ring under the squeezing action of the workpiece, overcoming the elastic force of the second spring, so as to open the central hole of the sealing ring, thereby allowing the workpiece to enter the filling cavity.

[0008] Furthermore, the number of the closing blocks is set to six; all six closing blocks are in the shape of an equilateral triangle; the closing ring has a regular hexagonal groove on the outer periphery of its central hole; each closing block has a slide that is movably embedded in the corresponding side groove of the regular hexagonal groove; the second spring is connected to one of the slides.

[0009] Furthermore, a limiting platform is provided at the end of the ejector pin that penetrates the filling cavity.

[0010] Furthermore, the power component is a cylinder or an electric actuator.

[0011] Furthermore, the outer wall of one end of the elastic sleeve is provided with an outer conical surface; the inner wall of one end of the extrusion cylinder is provided with an inner conical surface adapted to the outer conical surface.

[0012] Furthermore, the end of the rotating spindle near the workpiece insertion end is threaded with a cap; the cap cooperates with the extrusion cylinder to confine the elastic sleeve within the space formed by the two.

[0013] The beneficial effects of this invention are as follows: By setting a filling cavity filled with particles inside an elastic sleeve and setting a piston inside the filling cavity, after the workpiece enters the filling cavity, the piston compresses the effective volume of the filling cavity, thereby squeezing the particles inside the filling cavity and making the particles cover the outside of the free overhanging part of the workpiece, thus supporting the free overhanging part of the workpiece, thereby changing the damping characteristics of the workpiece, effectively suppressing the resonance of the workpiece during the processing, and improving the processing quality of the workpiece. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the overall structure of the CNC Swiss-type lathe of the present invention; Figure 2 This is a cross-sectional schematic diagram of the second spindle mechanism in the CNC Swiss-type machine of the present invention; Figure 3 yes Figure 2 A magnified view of a section at point A in the middle; Figure 4 This is a cross-sectional schematic diagram of the resonance suppression mechanism in the second main shaft mechanism of the present invention; Figure 5This is a schematic diagram of the structure of the closed component in the resonance suppression mechanism of the present invention; Figure 6 This is an exploded schematic diagram of the closed component in the resonance suppression mechanism of the present invention; Explanation of reference numerals in the attached drawings: 1. Machine base; 2. First spindle mechanism; 3. First X-axis displacement mechanism; 4. Y-axis mechanism; 5. Cutting mechanism; 6. Second X-axis displacement mechanism; 7. Z-axis displacement mechanism; 8. Second spindle mechanism; 10. Spindle seat; 20. Rotary spindle; 30. Extrusion cylinder; 40. Resonance suppression mechanism; 41. Elastic sleeve; 42. Piston; 43. Particle; 44. Magnetic guide cylinder; 45. Sealing assembly; 451. Sealing ring; 4511. Regular hexagonal groove; 452. Sealing block; 4521. Driving inclined plane; 4522. Slide table; 453. Second spring; 46. Permanent magnet ring; 47. Ejector pin; 471. Limiting stage; 48. First spring; 49. Filling cavity; 50. Power component; 60. Ejector rod; 70. Cap. Detailed Implementation

[0015] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments, but this is not to limit the scope of the invention to this.

[0016] like Figures 1 to 6 As shown in the figure, the CNC Swiss-type lathe for machining slender shafts to prevent workpiece resonance described in this embodiment includes a machine base 1, a first spindle mechanism 2 disposed at one end of the machine base 1, a first X-axis displacement mechanism 3 disposed above the first spindle mechanism 2, a Y-axis mechanism 4 disposed at the output end of the first X-axis displacement mechanism 3, and a cutting mechanism 5 disposed at the output end of the Y-axis mechanism 4. The first spindle mechanism 2 clamps the workpiece and drives the workpiece to move along the Z-axis direction. The first X-axis displacement mechanism 3 and the Y-axis mechanism 4 work together to adjust the displacement of the cutting mechanism 5 in the X-axis and Y-axis directions, thereby performing the first machining on the workpiece clamped by the first spindle mechanism 2. The CNC Swiss-type lathe also includes a second X-axis displacement mechanism 6 located at the other end of the machine base 1, a Z-axis displacement mechanism 7 located at the output end of the second X-axis displacement mechanism 6, and a second spindle mechanism 8 located at the Z-axis displacement mechanism 7. After the first spindle mechanism 2 drives the workpiece to complete the machining, the second X-axis displacement mechanism 6 and the Z-axis displacement mechanism 7 work together to drive the second spindle mechanism 8 to adjust the displacement in the X-axis and Z-axis directions, so that the second spindle mechanism 8 clamps and fixes the workpiece after the first machining. The first spindle mechanism 2 releases the clamp on the workpiece, and then, under the joint operation of the second X-axis displacement mechanism 6 and the Z-axis displacement mechanism 7, the workpiece is driven to perform the second machining.

[0017] It should be noted that the first spindle mechanism 2, the first X-axis displacement mechanism 3, the Y-axis mechanism 4, the cutting mechanism 5, the second X-axis displacement mechanism 6, and the Z-axis displacement mechanism 7 are all existing structures. Those skilled in the art can use existing technologies to implement the functions of the above structures, and they will not be described in detail here. The main improvement of this invention is the internal structure of the second spindle mechanism 8.

[0018] Specifically, such as Figures 2 to 4 As shown, the second spindle mechanism 8 includes a spindle seat 10, a rotating spindle 20 rotatably mounted in the spindle seat 10, an extrusion cylinder 30 mounted at one end of the rotating spindle 20, and a resonance suppression mechanism 40 installed in the extrusion cylinder 30; the spindle seat 10 is fixedly mounted on the output end of the Z-axis displacement mechanism 7; wherein, the resonance suppression mechanism 40 includes an elastic sleeve 41 capable of clamping and fixing the workpiece under the extrusion action of the extrusion cylinder 30; the elastic sleeve 41 is preferably made of 65Mn spring steel and has an axial expansion and contraction groove; the elastic sleeve 41 is provided with a filling cavity 49 coaxial with the clamping port of the elastic sleeve 41; a piston 42 is slidably provided in the filling cavity 49, and the filling cavity 49 is filled with particles 43; when the piston 42 moves axially along the filling cavity 49, it extrudes the particles 43 in the filling cavity 49, so that the particles 43 cover the outside of the free overhanging part of the workpiece that penetrates into the filling cavity 49.

[0019] Specifically, the elastic sleeve 41 clamps and fixes the workpiece after the first processing under the squeezing action of the extrusion cylinder 30. The workpiece after the first processing passes through the clamping port of the elastic sleeve 41 and then through the clamping port into the filling cavity 49. At this time, the piston 42 moves in the filling cavity 49, causing the piston 42 to squeeze the particles 43 in the filling cavity 49. This causes the particles 43 in the filling cavity 49 to cover the free overhang of the workpiece, thereby changing the damping characteristics of the workpiece. Then, the second X-axis displacement mechanism 6 and the Z-axis displacement mechanism 7 work together to drive the second spindle mechanism 8 to adjust the displacement in the X-axis and Z-axis directions. The spindle 20 rotates to drive the workpiece to rotate, thereby performing a second processing on the workpiece through the cutting mechanism 5, effectively suppressing the resonance of the workpiece during the processing.

[0020] It should be noted that the axial power source of the extrusion cylinder 30 is achieved using existing technology. This is not the point of invention of this invention. Anyone skilled in the art can achieve the axial displacement of the extrusion cylinder 30 by using existing structures, thereby achieving the clamping action of the elastic sleeve 41. Therefore, this invention will not describe it in detail.

[0021] In some implementations of this embodiment, such as... Figures 2 to 4As shown, the second spindle mechanism 8 also includes a power component 50 and a push rod 60; the power component 50 is fixedly installed on the side wall of the spindle seat 10, and the power component 50 can be a cylinder or an electric push rod; one end of the push rod 60 is connected to the output end of the power component 50, preferably through a rotary joint, and the other end of the push rod 60 is movably inserted into the rotating spindle 20 and coaxial with the rotating spindle 20; the push rod 60 is used to drive the piston 42 to move axially along the filling cavity 49, thereby squeezing or releasing the particles 43 in the filling cavity 49. In some embodiments of this CNC Swiss-type lathe, the resonance suppression mechanism 40 further includes a magnetic cylinder 44, a sealing component 45, and a permanent magnet ring 46. The magnetic cylinder 44 is coaxially mounted inside the elastic sleeve 41, preferably made of pure iron or low-carbon steel magnetic material with a wall thickness of 1-2 mm, and is fixed with the elastic sleeve 41 by an interference fit. The sealing component 45 is coaxially mounted at one end of the magnetic cylinder 44, and the permanent magnet ring 46 is coaxially mounted at the other end of the magnetic cylinder 44. It is preferably made of neodymium iron boron permanent magnet with a surface magnetic field strength of 800-1500 Gs, ensuring that the ferromagnetic particles are stably adsorbed on the inner wall of the magnetic cylinder 44 under non-extrusion conditions, and can be pushed by the piston 42 to cover the workpiece under extrusion conditions. The particles 43 are ferromagnetic particles, preferably spherical carbon steel particles. The particle size is 0.1-0.5mm, and the surface can be covered with a 0.02-0.05mm thick polytetrafluoroethylene wear-resistant protective layer. The filling amount is 60%-80% of the rated volume of the filling cavity 49. The piston 42 is slidably disposed in the magnetic cylinder 44. The sealing component 45, the magnetic cylinder 44 and the piston 42 together enclose and form the filling cavity 49. The piston 42 is axially movable and has a push pin 47 inserted through it. A first spring 48 extending axially is connected between the push pin 47 and the piston 42. One end of the push pin 47 can be axially movable and inserted into the filling cavity 49, and the other end is fixedly connected to the other end of the push rod 60. The permanent magnet ring 46 makes the magnetic cylinder 44 magnetic, so that the ferromagnetic particles 43 in the filling cavity 49 are magnetically attracted to the inner wall of the magnetic cylinder 44.

[0022] Specifically, the Z-axis displacement mechanism 7 drives the second spindle mechanism 8 to feed along the Z-axis, so that the workpiece after the first processing passes through the elastic sleeve 41 and the sealing component 45 and enters the filling cavity 49. Then, the extrusion cylinder 30 extrudes the elastic sleeve 41, so that the elastic sleeve 41 clamps and fixes the workpiece. Then, the power component 50 drives the push rod 60 to move axially along the rotating spindle 20. The push rod 60 pushes the ejector pin 47 to move axially synchronously. Since the ejector pin 47 is connected to the piston 42 by the first spring 48, and the piston 42 still has a degree of freedom in the axial direction at this time, the ejector pin 47 pushes the piston 42 to move axially synchronously through the first spring 48, so that the filling cavity... The effective volume of 49 decreases, thereby pushing the ferromagnetic particles 43 on the inner wall of the magnetic cylinder 44 to move and squeeze the ferromagnetic particles 43 in the filling cavity 49. As the piston 42 moves, the effective volume of the filling cavity 49 further decreases, so that the ferromagnetic particles 43 cover the part of the workpiece that is suspended in the filling cavity 49 under the compression of the piston 42 until the piston 42 is blocked by the ferromagnetic particles 43 and no longer moves, so as to suppress the resonance of the workpiece during the processing. At this time, the second X-axis displacement mechanism 6 and the Z-axis displacement mechanism 7 work together to drive the workpiece to move along the X-axis and Z-axis directions for the second processing. After the second processing is completed, the extrusion cylinder 30 gradually releases the pressure on the elastic sleeve 41, causing the elastic sleeve 41 to gradually loosen the workpiece. The power component 50 drives the push rod 60 to move further along the axis of the rotating main shaft 20. At this time, the piston 42 cannot continue to move axially due to the obstruction of the ferromagnetic particles 43. At this time, the push rod 60 drives the push pin 47 to continue to move axially relative to the piston 42, so that the first spring 48 is compressed. Thus, the workpiece after the second processing is pushed out of the elastic sleeve 41 through the push pin 47, realizing the unloading of the workpiece. After the material is unloaded, the power component 50 drives the push rod 60 to move in the opposite axial direction. The push rod 60 drives the push pin 47 to move synchronously. The first spring 48 is reset. After the first spring 48 is reset, the push pin 47 drives the piston 42 to move synchronously, thereby releasing the compression on the ferromagnetic particles 43. The ferromagnetic particles 43 in the filling cavity 49 return to a relaxed state to wait for the secondary processing of the next workpiece.

[0023] In some implementations of this embodiment, such as... Figures 3 to 6As shown, the sealing assembly 45 includes a sealing ring 451 and a plurality of sealing blocks 452 slidably arranged around the central hole of the sealing ring 451; a second spring 453 is connected between one of the sealing blocks 452 and the sealing ring 451; the side wall of each sealing block 452 is in movable contact with the side wall of the adjacent sealing block 452; each sealing block 452 is provided with a driving inclined surface 4521 on the side facing the workpiece insertion direction; when the workpiece contacts the driving inclined surface 4521 of the sealing block 452, the sealing block 452 slides relative to the sealing ring 451 under the squeezing action of the workpiece, overcoming the elastic force of the second spring 453, so as to open the central hole of the sealing ring 451, thereby allowing the workpiece to enter the filling cavity 49. In some embodiments of this CNC Swiss-type lathe, the number of closing blocks 452 is set to six; all six closing blocks 452 are equilateral triangular in shape, forming a hexagonal linkage structure. When one of the closing blocks 452 is reset under the action of the second spring 453, the remaining five closing blocks 452 are pushed to slide and reset synchronously by the side abutment force, ensuring that after the workpiece is removed, all closing blocks 452 synchronously close the central hole of the closing ring 451, maintaining the sealing of the filling cavity 49 throughout the process; the closing ring 451 is provided with a regular hexagonal groove 4511 on the outer periphery of its central hole; each closing block 452 is provided with a slide 4522 that is movably embedded in the corresponding side groove of the regular hexagonal groove 4511; the second spring 453 is connected to one of the slides 4522.

[0024] Specifically, when the workpiece is not inserted into the elastic sleeve 41, each sealing block 452 closes the central hole of the sealing ring 451 under the elastic force of the second spring 453, and the ferromagnetic particles 43 are magnetically attracted to the inner wall of the magnetic cylinder 44, thereby preventing the ferromagnetic particles 43 from overflowing the filling cavity 49 during the opening and closing of the sealing block 452. When the workpiece is inserted, the end of the workpiece contacts the driving inclined surface 4521 of each closing block 452. The workpiece squeezes each closing block 452 through each driving inclined surface 4521, causing the slide 4522 of each closing block 452 to slide along the side groove trajectory of the regular hexagonal groove 4511. The slide 4522 compresses the second spring 453, thereby gradually opening the central hole of the closing ring 451. After the workpiece is inserted into the central hole of the closing ring 451, each closing block 452 remains in contact with the workpiece under the elastic force of the second spring 453, so as to maintain the tendency to close the central hole of the closing ring 451, thereby preventing the ferromagnetic particles 43 from overflowing the filling cavity 49 during the extrusion of the piston 42; at the same time, it can also adapt to workpieces of different sizes.

[0025] In some implementations of this embodiment, such as... Figure 4As shown, a limiting platform 471 is provided at the end of the ejector pin 47 that penetrates into the filling cavity; the limiting platform 471 can prevent the ejector pin 47 from falling off the piston 42; when the ejector pin 47 moves axially relative to the piston 42, the ejector pin 47 drives the limiting platform 471 to move synchronously, thereby ejecting the workpiece through the limiting platform 471; and during the resetting process, the ejector pin 47 drives the limiting platform 471 to move axially in the opposite direction. After the limiting platform 471 abuts against the inner wall of the piston 42, the ejector pin 47 drives the piston 42 to move axially in the opposite direction synchronously through the limiting platform 471, thereby realizing the resetting of the piston 42.

[0026] In some implementations of this embodiment, such as... Figure 3 and Figure 4 As shown, the outer wall of one end of the elastic sleeve 41 is provided with an outer conical surface; the inner wall of one end of the extrusion cylinder 30 is provided with an inner conical surface adapted to the outer conical surface. Specifically, when the extrusion cylinder 30 moves axially relative to the elastic sleeve 41, the extrusion cylinder 30 extrudes the clamping end of the elastic sleeve 41 through the transmission between the inner and outer conical surfaces, thereby clamping and fixing the workpiece with the elastic sleeve 41.

[0027] In some implementations of this embodiment, such as... Figure 3 As shown, a cap 70 is threadedly connected to the end of the rotating spindle 20 near the workpiece insertion end. The cap 70 is preferably a nut. The cap 70 cooperates with the extrusion cylinder 30 to restrict the elastic sleeve 41 within the space formed by the extrusion cylinder 30 and the cap 70. In this embodiment, the cap 70 ensures that the elastic sleeve 41 is reliably restricted within the space formed by the extrusion cylinder 30 and the cap 70.

[0028] The above description is only a preferred embodiment of the present invention. Therefore, any equivalent changes or modifications made to the structure, features and principles described in the claims of this patent application are included within the protection scope of this patent application.

Claims

1. A CNC Swiss-type machining center for machining slender shafts to prevent workpiece resonance, characterized in that, It includes a spindle seat, a rotating spindle rotatably passing through the spindle seat, an extrusion cylinder located at one end of the rotating spindle, and a resonance suppression mechanism located within the extrusion cylinder; The resonance suppression mechanism includes an elastic sleeve that can clamp and fix the workpiece under the extrusion action of the extrusion cylinder; the elastic sleeve has a filling cavity coaxial with the clamping port of the elastic sleeve; a piston is movably installed in the filling cavity, and the filling cavity is filled with particles; when the piston moves along the axial direction of the filling cavity, it can squeeze the particles in the filling cavity, so that the particles cover the outside of the free overhanging part of the workpiece that penetrates into the filling cavity.

2. The CNC Swiss-type machining center for machining slender shafts to prevent workpiece resonance according to claim 1, characterized in that, The CNC Swiss-type lathe also includes a power component located on one side of the spindle seat and a push rod connected at one end to the output end of the power component; the other end of the push rod is movably inserted into the rotating spindle and coaxial with the rotating spindle; the push rod is used to drive the piston to move axially along the filling cavity.

3. A CNC Swiss-type machining center for machining slender shafts to prevent workpiece resonance, as described in claim 2, is characterized in that... The resonance suppression mechanism further includes a magnetic cylinder coaxially disposed within an elastic sleeve, a closed assembly coaxially disposed at one end of the magnetic cylinder, and a permanent magnet ring coaxially disposed at the other end of the magnetic cylinder; the particles are ferromagnetic particles. The piston is slidably disposed inside the magnetic cylinder; the sealing assembly, the magnetic cylinder, and the piston together enclose the filling cavity; the piston is axially movable and has a push pin inserted through it; a first spring extending axially is connected between the push pin and the piston; one end of the push pin can be axially movable and inserted into the filling cavity, and the other end is fixedly connected to the other end of the push rod.

4. A CNC Swiss-type machining center for machining slender shafts to prevent workpiece resonance, as described in claim 3, is characterized in that... The sealing assembly includes a sealing ring and a plurality of sealing blocks slidably arranged around the central hole of the sealing ring; a second spring is connected between one of the sealing blocks and the sealing ring; the side wall of each sealing block is in movable contact with the side wall of the adjacent sealing block; each sealing block has a driving slope on the side facing the workpiece insertion direction.

5. A CNC Swiss-type machining center for machining slender shafts to prevent workpiece resonance according to claim 4, characterized in that, The number of the closing blocks is set to six; all six closing blocks are in the shape of an equilateral triangle; the closing ring has a regular hexagonal groove on the outer periphery of its central hole; each closing block has a slide table that is movably embedded in the corresponding side groove of the regular hexagonal groove; the second spring is connected to one of the slide tables.

6. A CNC Swiss-type machining center for machining slender shafts to prevent workpiece resonance according to claim 3, characterized in that, The end of the ejector pin that penetrates the filling cavity is provided with a limiting platform.

7. A CNC Swiss-type machining center for machining slender shafts to prevent workpiece resonance according to claim 2, characterized in that, The power component is a cylinder or an electric actuator.

8. A CNC Swiss-type machining center for machining slender shafts to prevent workpiece resonance according to claim 1, characterized in that, The outer wall of one end of the elastic jacket is provided with an outer conical surface; the inner wall of one end of the extrusion cylinder is provided with an inner conical surface adapted to the outer conical surface.

9. A CNC Swiss-type machining center for machining slender shafts to prevent workpiece resonance according to claim 1, characterized in that, The end of the rotating spindle near the workpiece insertion end is threaded with a cap; the cap cooperates with the extrusion cylinder to confine the elastic sleeve within the space formed by the two.

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