FOR CONTROLLING VIBRATION OF A LATHE SYSTEM

TECHNICAL FIELD

The present invention generally relates to lathe vibration control. The invention relates particularly, though not exclusively, to active vibration control of a lathe.

BACKGROUND ART

This section illustrates useful background infornnation without admission of any technique described herein representative of the state of the art.

Turning process of lathe machining suffers from vibration problems especially when the machined shaft is long and slender. The vibrations arise, for example, from the interaction between the turning tool and the machined shaft. Vibrations occur particularly if the excitation frequency meets the natural frequency of the system.

The vibration system in a lathe is rather complex and depends on properties ranging from the body and its damping against the floor to the driving and supporting of the machined shaft or work piece, machining parameters and the cutting tool itself.

Traditionally, it is attempted to avoid or reduce the vibrations by changing the machining parameters. However, changing the parameters typically decreases the efficiency of the process, extends the machining time, and increases the wear of the turning tool. SUMMARY

According to a first aspect of the invention there is provided a live centre for a tailstock of a lathe, comprising:

a body;

a rotating part; and

a radial bearing configured to rotatably support the rotating part to the body; characterized by the radial bearing being stiffness adjustable in the radial direction for control of vibration characteristics of the lathe during machining. The radial bearing may be stiffness adjustable sufficiently to enable an effective change of natural frequency of a system formed by the lathe and a machining shaft being machined in the lathe. The effective change may be at least 1 , 2, 2.5, 3, 3.5, 4, 4.5 or 5 per cent.

The adjustability of the bearing may enable on-the-fly control of vibration properties of the lathe system and mitigation of vibration-originated problems.

The stiffness may be adjustable by varying an axial compression force that effects over the live centre. By adjusting the stiffness by the axial compression, the vibration properties of the lathe system may be controlled with robust and relatively simple live centre system using a non-rotating actuator such as a hydraulic cylinder.

The axial compression force may be varied between a minimum and maximum force. The maximum force may be 1 , 2, 3, 4, 5, 6, 8 or 10 kN. The minimum force may be 0 to 90 % of the maximum force, such as 10, 20, 30, 50 or 60 per cent of the maximum force.

The radial bearing may reside near a head part of the live centre. The live centre may comprise a sealing. The sealing may reside between a head of the live centre and the radial bearing. The sealing may be configured to protect the radial bearing from entry of dirt. The sealing may be an SKF HMSA10 sealing.

The live centre may comprise rear bearing. The radial bearing or the rear bearing or both may comprise a thrust bearing.

The radial bearing may be an adjustable bearing. The radial bearing may be chosen from a group consisting of a ball bearing; an angular ball bearing; a tapered roller bearing; and a spherical roller bearing.

The live centre may comprise in total two or more radial bearings configured to rotatably support the rotating part to the body. The bearings may be similar or different. According to a second aspect of the invention there is provided a tailstock for a lathe, comprising the live centre of the first aspect. According to a third aspect of the invention there is provided a lathe comprising the live centre of the first aspect.

According to a fourth aspect of the invention, there is provided a method of controlling vibration of a lathe system during machining of a shaft, comprising: rotatably supporting by a radial support a rotating part of a live centre of a tailstock to a body of the live centre;

characterized by:

adjusting stiffness of the radial support during the machining of the shaft. The adjusting of the stiffness may comprise adjusting axial compression of the radial support.

The axial compression may be varied sinusoidally as a function of time. The axial compression may continuously compress the live centre with variable force. The axial compression may be varied over time so as to avoid exciting the system to resonate even when passing a resonance point of the system.

The natural frequencies of the system can be estimated in advance. The estimation may be based on simulation of a vibration model for the lathe system. The method may be fine-tuned to avoid estimated natural frequencies during the turning process. The vibration properties may be additionally or alternatively calculated in real time during the turning process. The estimated natural frequencies may be refined based on measurement data obtained during the turning process. The lathe may be a manual lathe. Alternatively, the lathe may be a computer controlled lathe. Sinusoidal or otherwise varying axial compression may be particularly useful in manual turning where the process control may be difficult or impossible to simulate simultaneously with the machining. The rotatable supporting may be implemented using one or more bearings. The one or more bearing may be selected based on desired stiffness range. The desired stiffness range may depend on the dimensions and other properties of the machined shaft. Different live centres of different vibration properties may be stored and a suitable one selected depending on the machined shaft and / or the intended machining.

The live centre may be automatically replaceable. The live centre may be arranged in a revolver comprising a plurality of different tailstock systems that differ by at least the live centre.

Different non-binding example aspects and embodiments of the present invention have been illustrated in the foregoing. The embodiments in the foregoing are used merely to explain selected aspects or steps that may be utilized in implementations of the present invention. Some embodiments may be presented only with reference to certain example aspects of the invention. It should be appreciated that corresponding embodiments may apply to other example aspects as well. BRIEF DESCRIPTION OF THE DRAWINGS

Some example embodiments of the invention will be described with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic picture of a live centre of a tailstock for a lathe, according to an embodiment;

Fig. 2 shows a sectional view of the live centre of Fig. 1 ;

Fig. 3 shows a flow chart of a method of controlling vibration of a lathe system during machining of a shaft, according to an embodiment; and

Fig. 4 shows a lathe, according to an embodiment.

DETAILED DESCRIPTION

In the following description, like reference signs denote like elements or steps. Fig. 1 shows a schematic picture of a live centre of a tailstock 100 for a lathe, comprising: a body 1 10; a rotating part 120; and radial bearing 130 configured to rotatably support the rotating part to the body. The radial bearing is stiffness adjustable in the radial direction for control of vibration characteristics of the lathe during machining.

In an embodiment, the radial bearing is stiffness adjustable sufficiently to enable an effective change of natural frequency of a system formed by the lathe and a machining shaft being machined in the lathe. The effective change is, for example, at least 1 , 2, 2.5, 3, 3.5, 4, 4.5 or 5 per cent.

The adjustability of the bearing may advantageously enable on-the-fly control of vibration properties of the lathe system and mitigation of vibration-originated problems.

In an embodiment, the stiffness is adjustable by varying an axial compression force that effects over the live centre. Advantageously, by adjusting the stiffness by the axial compression, the vibration properties of the lathe system may be controlled with robust and relatively simple tailstock system using a non-rotating actuator such as a hydraulic cylinder.

In an embodiment, the axial compression force is varied between a minimum and maximum force. The maximum force may be, for example, 1 , 2, 3, 4, 5, 6, 8 or 10 kN. The minimum force may be, for example, 0 to 90 % of the maximum force, such as 10, 20, 30, 50 or 60 per cent of the maximum force.

In an embodiment, the radial bearing resides near a head part of the live centre (near the left-hand side end of the live centre 100 in Figs. 1 and 2). In Figs. 1 and 2, the live centre comprises a sealing 140 between a head of the live centre and the radial bearing. The sealing of Figs. 1 and 2 helps to protect the radial bearing from entry of dirt. The sealing can be, for example, an SKF HMSA10 sealing. In an embodiment, the live centre comprises a rear bearing 150. The rear bearing comprises, for example, a radial or an axial bearing.

In an embodiment, the radial bearing is an adjustable bearing. The radial bearing can be selected from a group consisting of: a ball bearing; an angular ball bearing; a tapered roller bearing; and a spherical roller bearing. In an embodiment, the live centre comprises two or more bearings. The bearings may be any combination of the aforementioned bearings, for example. A lathe (400 in Fig. 4) can be implemented using the live centre of any embodiment of the present document. The lathe may further comprise a controller configured to control the operation of the live centre to at least control vibrations during machining so as to avoid vibration incurred problems. Fig. 3 shows a flow chart of a method of controlling vibration of a lathe system during machining of a shaft. The method comprises:

estimating vibration properties 310 of the lathe system, by using, for example, simulation of a vibration model for the lathe system;

rotatably supporting 320 by a radial support a rotating part of a live centre of a tailstock to a body of the live centre;

adjusting 330 stiffness of the radial support during the machining of the shaft preferably based on the estimated or measured vibration properties of the lathe system; and

measuring vibration properties 340 of the lathe system and using the measurement to determine refined vibration properties for use in subsequent adjusting of the stiffness of the radial support. The method may then form a loop by resuming to step 320 or 310 (possibly variably).

In an embodiment, the adjusting of the stiffness comprises adjusting axial compression of the radial support.

In an embodiment, the axial compression is varied sinusoidally as a function of time. The axial compression may continuously compress the live centre with variable force. The axial compression may be varied over time so as to avoid exciting the system to resonate even when passing a resonance point of the system.

In an embodiment, the lathe is a manual lathe. Alternatively, the lathe is a computer controlled lathe. Sinusoidal or otherwise varying axial compression may be particularly useful in manual turning where the process control may be difficult or impossible to simulate simultaneously with the machining.

In an embodiment, the radial bearing is selected based on desired stiffness range. The desired stiffness range may depend on the dimensions and other properties of the machined shaft. In an embodiment, the different live centres of different vibration properties are stored and a suitable one is selected depending on the machined shaft and / or the intended machining. In an embodiment, the live centre is automatically replaceable. For example, the live centre can be arranged in a revolver comprising a plurality of different live centre systems that differ by at least the live centre.

Fig. 4 shows a lathe 400 of an embodiment. Fig. 4 shows a machined shaft 410 in the lathe, the live centre 100 and also an actuator 420 for adjustable axial tensioning of the live centre. The actuator is, for example, a hydraulic, electric or pneumatic actuator.

Various embodiments have been presented. It should be appreciated that in this document, words comprise, include and contain are each used as open-ended expressions with no intended exclusivity.

The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments of the invention a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented in the foregoing, but that it can be implemented in other embodiments using equivalent means or in different combinations of embodiments without deviating from the characteristics of the invention. For example, the vibration properties of the lathe model need not be estimated or simulated in advance. For instance, there may already exist measurement information or the measurement information may be obtained on producing first one or more products. Moreover, it is not necessary to measure the vibration properties during the turning process. It should be understood in general a skilled person is able to use the present description to use the invention of the appended claims also using various subsets of features of the afore-described embodiments.

Furthermore, some of the features of the afore-disclosed embodiments of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.