GB2557866A

GB2557866A - Apparatus and method for machining a thin-walled component

Info

Publication number: GB2557866A
Application number: GB1518458.3A
Authority: GB
Inventors: Geng Zunmin; Mccleay Tom; Morgan Gareth
Original assignee: Advanced Manufacturing Sheffield Ltd
Current assignee: Advanced Manufacturing Sheffield Ltd
Priority date: 2015-10-19
Filing date: 2015-10-19
Publication date: 2018-07-04
Also published as: GB201518458D0

Abstract

A dynamic damping device for machining a thin-walled component 10 is disclosed. Beneath the component is a support platform 12 and between the platform and the component is an attenuator 14 which maybe in the form of a porous damping medium. A negative pressure source exerts a negative pressure on the interface between the component and the attenuator, indicated by arrows A. The effect of the negative pressure is to allow the ambient air pressure to push the component firmly against the attenuator. The pores of the attenuator act as Helmholtz resonators to dampen vibration in the component during machining. Helmholtz resonators including apertures of differing or varying diameters and/or depths may be provided. Damping may be provided by a fluid or liquid moving or swinging through or in an aperture in the resonator.

Description

(54) Title of the Invention: Apparatus and method for machining a thin-walled component

Abstract Title: A vibration damping support for supporting a component during machining (57) Adynamic damping device for machining a thin-walled component 10 is disclosed. Beneath the component is a support platform 12 and between the platform and the component is an attenuator 14 which maybe in the form of a porous damping medium. A negative pressure source exerts a negative pressure on the interface between the component and the attenuator, indicated by arrows A. The effect of the negative pressure is to allow the ambient air pressure to push the component firmly against the attenuator. The pores of the attenuator act as Helmholtz resonators to dampen vibration in the component during machining. Helmholtz resonators including apertures of differing or varying diameters and/or depths may be provided. Damping may be provided by a fluid or liquid moving or swinging through or in an aperture in the resonator.

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Apparatus and Method for Machining a Thin-Walled Component

The present invention relates to a support device for machining a thin-walled component, and to a method for supporting a component during machining.

Thin-walled components, such as disc components, certain examples of which are used in the aerospace industry, are machined to a fine degree of accuracy at a final manufacturing stage. The term thin-walled may refer to a component having at least one dimension that is of the order of 0.5 mm to 5mm in comparison with another dimension which may be between 100 and 1000 mm, for example. Machining of the thin part of the component, for example by turning the component on a lathe, presents challenges to the manufacturing process.

For example, dynamically, the component may experience vibration, known as chatter which can affect the quality of the surface being machined.

Other problems that may arise include a static deflection of the part, and residual stress distortion (RSD). As the part is weak, it deflects away from the tool leaving errors on the surface. This is another imperfection that may lead to rejection of the sample as unsuitable. With residual stress distortion, as the component is removed from forging, in the disc machining process, it relaxes. Inherent stress from the forging process and, sometimes, surface stresses put into the surface during machining, can lead to an imbalance of tensile and compressive stress on the component. If the stresses are unbalanced, the component will· deflect when the clamps are removed.

This can be engineered out by initially leaving in excess material and then performing extra machining operations to remove the deflection error.

Figures 5a and 5b show, respectively, a thin walled disc and schematic representation of the static deflection of the disc during machining.

Figure 5a shows a thin-walled disc component DC with typical dimensions 100-1000mm in outer diameter and 0.3-5mm wall-thickness on its diaphragm section.

Figure 5b shows schematically the thin-walled disc of Figure 5a, rotating in the direction of Arrow Al being machined, with the machining causing deformation and vibration. A thin-wailed diaphragm section DC' is very liable to deform (at X) under a light-load L or force action on it, moving in direction of Arrow A2. As a typical symmetric round disc acoustic structure, it is also very easily to be excited into a wide frequency band vibration or vibro-acoustic response throughout the whole disc structure even by a very small disturbance from the moving cutting-force of the cutting-tool. This phenomenon is scientifically termed as 'Moving-load caused structural and/or dynamic problems'’ , and a typical engineering example is the structural performance when a car passes over a flexible suspension bridge, where time-space-varying moving-load, time-space-varying deformation and time2 varying vibration/acoustic response are the main features of the phenomenon.

The above two 'moving-load'' caused structural problems (time-varying deformation/vibration) are also two key problems in machining a thin-walled disc component.

One previously considered approach to mitigating the aforementioned problems is the so-called 'Melting-Wax' technique, in which wax is used to fill the geometric cavity enclosed between the component and a supporting fixture so as to supply sufficient stiffness and damping upon the component surface to mitigate machining-induced vibration. However, there are several significant disadvantages with this method. Firstly, in the process of melting-wax solidification, the wax medium can shrink significantly and therefore can cause mechanical distortion of the thin-walled component to be machined. Secondly, this method needs special· equipment to thermally or chemically melt the wax into a liquid state, fill it into the component-fixture enclosure before machining operations, re-melt the solidified wax medium and remove it after the machining operations. This causes a toxic medium which may be harmful to the health of operatives and which can pollute the environment. Also, the increased machining cycle-time and relevant costs make the process very low in cost-efficiency, according to existing industrial standards.

Embodiments of the present invention aim to provide apparatus and a method whereby the effects of the aforementioned problems are reduced or eliminated, using 3 apparatus and a method to dynamically enhance damping and support.

The present invention is defined in the attached independent claims, to which reference should now be made. Further, preferred features may be found in the sub-claims appended thereto.

According to one aspect of the present invention, there is provided a support device for supporting a component during machining of the component, the device comprising a pressure source for applying a pressure to the component, and an attenuator arranged in use to contact the component to support the component during machining, the pressure source being arranged in use to urge the component against the attenuator, and wherein the attenuator comprises a Helmholtz resonator.

In a preferred arrangement the Helmholtz resonator includes a swinging fluid plug in an aperture and may include a connected fluid volume. The fluid plug and/or the connected fluid volume may comprise air.

The attenuator may comprise a porous material.

In a preferred arrangement the attenuator comprises a plurality of Helmholtz resonators. At least some of the pores of the attenuator material· may comprise the Helmholtz resonators. The plurality of Helmholtz resonators may include apertures of differing diameters and/or differing depths.

The pressure source may comprise a low-pressure source and preferably comprises a vacuum generator arranged in use to generate a partial vacuum.

In a preferred arrangement the pressure source comprises a vacuum pump.

The device may comprise a support platform for supporting the attenuator. The support platform may comprise one or more channels through which a gas pressure may be transmitted to the attenuator.

The one or more channels may be arranged in use for the withdrawal of gas, such as air, from the attenuator.

The one or more channels may comprise one or more grooves in an upper surface of the support platform on which the attenuator is arranged to rest in use. In a preferred arrangement the attenuator is located in use between the support platform and a component.

The device may comprise two supporting platforms and two attenuators, one being arranged to be located either side of a component in use.

According to another aspect of the invention there is provided a method of supporting a component during machining of the component, the method comprising applying a pressure to the component, and urging the component against an attenuator comprising a Helmholtz resonator.

The pressure may be applied by low-pressure source and preferably by a vacuum generator arranged in use to generate a partial vacuum.

In a preferred arrangement the pressure source comprises a vacuum pump .

The method may comprise supporting an attenuator on a support platform. Preferably the method comprises transmitting a gas pressure to the attenuator through one or more channels of the support platform.

The method may comprise the withdrawal of gas, such as air, from the attenuator.

The one or more channels may comprise one or more grooves in an upper surface of the support platform on which the attenuator is arranged to rest in use. Preferably the method includes locating the attenuator between the support platform and a component. The method may comprise locating a component between two attenuators, each supported on a support platform.

The invention may include any combination of the features or limitations referred to herein, except such a combination of features as are mutually exclusive, or mutually inconsistent.

A preferred embodiment of the present invention will now be described. By way of example only, with reference to the accompanying diagrammatic drawings, in which:

Figure 1 is a schematic representation of a dynamic damping device for use in the machining of a thin walled component;

Figure 2 is a schematic representation of a detail of an attenuator surface;

Figure 3 shows in part-sectional view a part of a support device in accordance with an embodiment of the present invention;

Figure 4 is a cross section of the device shown in Figure

1;

Figure 5a shows a thin walled disc component of the prior art; and

Figure 5b illustrates the problem of static deflection of a disc of Figure 5a, according to the prior art.

In seeking the development of methods of manufacture for machining thin walled disc, an adaptive floating-solution has been devised which ensures that when machining these thin-walled components, the regional dynamic stiffness is adaptively enhanced to avoid chatter vibration, the thinwalled structural stiffness (time-varying as a result of moving-load from the cutting-tool} is regionally enhanced to avoid excessive part deflection or unwanted part deformation when undergoing cutting forces as a moving-load across the thin-walled surface to be machined, and any distortion that occurs during machining has minimal impact on the final conformance of the machined product. These aspects are particularly challenging when attempting to turn thin walled disc components using a single sided operation (i.e. not turning both sides of the component simultaneously).

Embodiments of the present invention are concerned with the application of Helmholtz resonators to machining-induced vibration or for structure-borne acoustic control, particularly in thin-walled components such as aerofoils. More particularly, the embodiment described herein utilises a Helmholtz-resonator based perforated vibration absorber.

The use of Helmholtz resonators as a classical method for air-borne acoustic noise reduction and control is well known. Typically multiple holes, such as can be found in porous material, act as tiny resonators. The design of anechoic chamber is one example of this.

Helmholtz resonators are acoustic systems that consist of a swinging air plug and a connected air volume. They can have a variety of forms, including an empty wine bottle, the corpus of a string instrument, bass reflex enclosures of loudspeakers and wall coverings made of perforated wood or gypsum boards. A simple way to explain the geometry and the functionality of a Helmholtz resonator is the example of an empty wine bottle. The air in the bottleneck creates the said air plug, and the air in the rest of the bottle functions as the connected volume. The air plug has an acoustic mass, which results from its geometry and specific air density. It piles on the springy air pillow of the rest of the bottle volume. Together they comprise a swinging system with a specific resonance frequency.

Helmholtz resonators are frequently exploited to amplify sound. For example, in case of string instruments, too little sound energy would be emitted through the swinging of strings alone. Sufficient loudness is only achieved after connection of strings to the corpus with its openings .

In order for a Helmholtz resonator not to amplify sound, but to absorb it, the swinging air in the opening must be slowed down through friction. This is mostly done by means of a thin fleece glued behind the opening, sometimes also equipped with an additional layer of mineral wool or foam.

In the embodiment described below, concerning particular structure-borne acoustics, porous damping material is used with uniformly spread micro-holes in OD= = 0.1mm to 0.5mm and depth >= 5.0mm where the holes work as Helmholtz resonators so as to absorb the component vibration energy rather than amplify it.

For air-borne acoustic control, porous absorber materials include all porous and filamentary materials such as textiles, fleece, carpets, foam, cotton and special acoustic plaster. They all absorb sound energy by slowing down swinging of the air particles through friction.

By illustration, if a plane sound wave is considered, which incidents perpendicular on a massive wall, because there is no absorption, it is fully reflected. The incident and the reflected wave superimpose according to the principle of superposition. Therefore we always measure the sum of the sound pressures or sound velocities of both waves and the result is a standing wave. Air cannot move in the immediate vicinity of the wall, which is assumed to be immovable. The sound velocity of a plane wave directly at the wall must be zero. In contrast, high sound pressure can develop there. The sound velocity reaches its first maximum at a distance of I'M of the wavelength from the wall.

A porous absorber can slow down the air particles in the most effective way, if they have high sound velocity. If a porous absorber is mounted directly onto the wall, it must have a certain thickness, in order to be able to absorb sound waves up to a certain lower limiting frequency. On the other hand, if it is mounted at a certain distance from the wall, it can be made correspondingly thinner. Manufacturers currently make use of this effect in acoustic ceilings. The absorbing material is of equally high importance. The most important parameter is the flow resistance. In this particular structure-borne acoustics, porous damping material with uniformly spread micro-holes in OD= = 0.1mm to 0.5mm and depth >= 5.0mm is used,

Turning to Figure 1, this shows schematically a dynamic damping device for machining a thin-walled component. The component is represented at 10. Beneath the component 10 is a support platform 12 and between the platform 12 and the component 10 is an attenuator 14 in the form of a porous damping medium. A negative pressure source (not shown} exerts a negative pressure (partial vacuum} on the interface between the component 10 and the attenuator 14, indicated by arrows A. The effect of the negative pressure is to allow the ambient air pressure to push the component 10 firmly against the attenuator 14.

The pores of the attenuator act as Helmholtz resonators, to damp vibration in the component during machining.

Figure 2 is a schematic representation of one of the pores of the attenuator and shows schematically the effect of vibrations caused by the action of a tool (not shown) as it works on the component.

The attenuator/damper is porous, and at the componentfacing surface the pores 20 comprise a plurality of recesses 22 each of which has a neck portion 24 that leads to a cavity 26. In the neck 24 air forms a fluid plug 28. A reservoir of the fluid in the cavity 26 acts upon the plug 28 as a spring, represented schematically as S, so that the plug swings inwards and outwards with the vibrations induced in the surface of the component 10 by the action of the tool. The direction of the swinging is indicated by arrows B. Walls of the cavity act as dampers, represented schematically as D.

There are a great number of these tiny Helmholtz resonators across the component-facing surface of the attenuator 12 and in use they have the effect of damping surface vibrations induced in the component by the action of the tool as it machines the component.

Turning to Figures 3 and 4, these show a dynamic support device 100 supporting a thin-walled component 120 that is to be machined. Typical dimensions for this type of component are 100-1000mm in outer diameter and 0.5-5 mm wall thickness at its thinnest point.

The support device 100 comprises a support platform 130 and an attenuator, or damping member 140 in the form of a vacuum-adhered floating constraint. The component 120 is in the form of a disc and comprises inner and outer radial edge portions 150 and 160, and a thinner, annular diaphragm portion 170, which is the portion to be machined. The component 120 is supported at its inner and outer radial edges by static supports - one {at the outer edge) being represented by 180. The floating constraint 140 is placed behind the thin-walled diaphragm portion 170 opposite the place where the machining is to be carried out.

The vacuum-adhered constraint-plate may need a floating link or support based on the existing work-piece-holder or fixture.

The vacuum-adhered constraint-plate should be able to adaptively adjust and fit on to the thin-walled diaphragm surface.

The vacuum support platform may be height-adjustable to move it closer or further from the component, and may need to be flexible so that it can 'dish' to match the shape of the component.

A plurality of vacuum support platforms may be provided, at least some of which are height and/or orientation adjustable .

The or each vacuum support platform will need an attachment for a vacuum pump. A spiral groove G may be used for the vacuum channel.

The vacuum support platform may utilise seals, such as Orings, to help retain the vacuum pressure

The dimensions of the Helmholtz damper should be in the range of 20mm-30mm in its layer-thickness and lmm-5mm in the porous Helmholtz-hole diameters. The hardness of the damping material must be selected so that it has adequate compliance given the thin wall may be a different shape or orientation to the nominal geometry. For example the part may have flatness error or a taper error.

The thickness of the attenuator/damping member must be selected so that is has adequate compliance given that the thin-walled component may be a different shape or orientation to the nominal geometry. For example the part may have flatness error or a taper error.

The damping media should also be stiff enough to support the thin wall under cutting forces.

The vacuum should successfully hold the wail so that it cannot vibrate

The vacuum may in some cases need to be sufficiently strong to hold the component when under cutting loads.

In this embodiment, the damping material is made of viscous-plastic material such as a porous polymeric material. Porous holes having an outside diameter in the range 1mm to 5mm, and depth of between 20mm and 30mm are uniformly spread over the material. These pores act as Helmholtz resonators.

Whereas the example shown in the drawings is of a disc-like component, one side of which is supported, and the other side to be machined on a lathe, other geometries and machining operations may be accommodated. Either the tool or the component may move, with the other being static.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance, it should be understood that the applicant claims protection in respect of any patentable feature or combination of features referred to herein, and/or shown in the drawings, whether or not particular emphasis has been placed thereon.

Claims

1. A support device for supporting a component during machining of the component, the device comprising a pressure source for applying a pressure to the component, and an attenuator arranged in use to contact the component to support the component during machining, the pressure source being arranged in use to urge the component against the attenuator, and wherein the attenuator comprises a Helmholtz resonator.

2. A device according to Claim 1, wherein the Helmholtz resonator includes a swinging fluid plug in an aperture.

3. A device according to Claim 1 or 2, wherein the attenuator comprises a porous material.

4. A device according to any of the preceding claims, wherein the attenuator comprises a plurality of Helmholtz resonators .

5. A device according to Claim 4, wherein the plurality of Helmholtz resonators includes apertures of differing diameters and/or differing depths.

6. A device according to any of the preceding claims, wherein the pressure source comprises a low-pressure source such as a vacuum generator arranged in use to generate a partial vacuum.

7. A device according to any of the preceding claims, wherein the device comprises a support platform for supporting the attenuator.

5

8. A device according to Claim 7, wherein the support platform comprises one or more channels through which a gas pressure may be transmitted to the attenuator.

9. A device according to Claim 8, wherein the one or more 10 channels are arranged in use for the withdrawal of gas, such as air, from the attenuator.

10. A device according to Claim 8 or 9, wherein the one or more channels comprise one or more grooves in an upper

15 surface of the support platform on which the attenuator is arranged to rest in use.

11. A device according to any of Claims 7 to 10, wherein the attenuator is located in use between the support

20 platform and a component.

12. A method of supporting a component during machining of the component, the method comprising applying a pressure to the component, and urging the component against an

25 attenuator comprising a Helmholtz resonator.

13. A method according to Claim 12, wherein the pressure is applied by low-pressure source such as a vacuum generator arranged in use to generate a partial vacuum.

14. A method according to Claim 12 or 13, wherein the method comprises supporting an attenuator on a platform.

5

15. A method according to any of Claims 12 to 14, the method comprises transmitting a gas pressure attenuator through one or more channels of the platform.

10

16. A method according to any of Claims 12 to 15, the method comprises the withdrawal of gas, such from the attenuator.

support wherein to the support wherein as air,

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Application No: GB1518458.3