GB2387800A

GB2387800A - High precision machine tool structure

Info

Publication number: GB2387800A
Application number: GB0209493A
Authority: GB
Inventors: Frank Peter Wardle; John Franks Webb Sweet
Original assignee: Loadpoint Ltd
Current assignee: Loadpoint Ltd
Priority date: 2002-04-25
Filing date: 2002-04-25
Publication date: 2003-10-29
Also published as: GB0209493D0

Abstract

The present invention concerns the improvement of the structure of machine tools and comprises a square or rectangular base (5) and four inclined struts (1-4) attached to a hub (6) positioned over the centre of the base. This structure increases the static stiffness and dynamic stiffness of a machine tool in comparison to conventional post and bracket or cantilevered machine tool structures, thereby improving the dimensional precision and surface finish achieved by the machine tool. The incorporation of four struts into the machine tool structure provides sufficient working volume and access for the use of this structure in practical machine tool applications.

Description

Title: Ultrahigh precision machine-tool structure This invention relates

to the field of machine tool structures and structures requiring a high

degree of dimensional precision.

It is well known that the frame or structure of a machine tool should have a high static stiffness to maintain dimensional precision and a high dynamic stiffness to achieve a fine surface finish and geometry. In these respects it has been demonstrated that a tetrahedral shaped structure (UK Patent No. G 2 194 182) improves upon conventional machine tools structures in which work spindles or tool posts are effectively cantilevered. Furthermore it has been reported in the technical press that a grinding machine utilising a tetrahedral structure has achieved exceptional surface finishes in the range 1 - 10 rim Ra and extremely low levels of subsurface damage on materials known to be difficult to machine.

A basic tetrahedral structure comprises six struts and four pin joints arranged as shown in Figure 1. Its static stiffness is high because forces may only be transmitted around the structure by compression or tension of its members; i.e. there are no bending deflections.

Its dynamic stiffness can also be high by arranging for damping at the pin joints, across which the forces must be transmitted. However, these advantages are to some extent compromised in a complete machine tool. Here it is necessary to have machine elements such as slideways, rotary tables and spindles attached to the machine structure, which add mass, flexibility and damping to the tool - workpiece loop. As shown in Figure 2 the slideways in a practical machine tool are attached to the base struts and a spindle and slideway are built into the uppermost pin joint, which must now have a spherical surface of large radius. The finite size of the struts and joints means that the structure departs from a theoretical pin jointed structure and in so doing it has a lower static stiffness. Furthermore the additional mass attached to the upper pin joint has the effect of reducing the natural frequency of rocking and torsional modes of vibration, which are shown in Figure 3. In machine tool design it is good practice to keep natural frequencies as high as possible as structural damping is more effective.

Other considerations on a practical machine tool are the size of the working volume and the ease of access to the workpiece and tool, both of which are severely restricted by using a tetrahedral structure. The inclined struts restrict the size and travel of slideways, which

therefore limits the size of work pieces that can be machined and at the same time prevents them from being moved clear of the working zone for easy operator access.

An object of the machine design herein disclosed is to overcome the working volume and access limitations of the tetrahedron whilst retaining and possibly improving the static and dynamic stiffness of the machine structure when considered in relation to a complete machine tool.

Accordingly the machine design comprises of a square or rectangular base and four inclined struts attached to a hub positioned over the centre of the base. With the struts being fixed to the comers of the base the possible size or distance of travel of slideways mounted on the base is substantially improved when compared to the tetrahedral structure with three inclined struts. Having an additional strut and not being constrained by the requirements of a pin jointed structure enables a better optimization of the strut-hub arrangement with consequential improvements in static and dynamic stiffness compared to a tetrahedral structure forming the basis of a practical machine tool. In one possible embodiment of the machine structure the hub and struts may be cast together. Here the elimination of hub-strut joints will clearly enhance static stiffness and raise the natural frequencies of rocking and torsional modes of vibration. In another possible embodiment the struts may be bolted to both the hub and base.

The preferred arrangement is to have cast hub-strut joints and to maximise static stiffness of the structure as the main source of damping is not the structure but the machine elements such as slideways and spindles which make up the tool-workpiece loop. The clamping from these elements is most effective if deflections within the structure are small compared to those in the machine elements, i.e. the structure is much more rigid than the machine elements.

A preferred embodiment of the invention will now be described with reference to the accompanying drawing in which: Figure 1 shows an idealised pin jointed tetrahedral structure

Figure 2 is a longitudinal representation of a machine-tool design incorporating a tetrahedral structure Figure 3 shows a) a rocking mode of vibration produced by a machine tool incorporating a tetrahedral structure and b) a torsional mode of vibration produced by a machine tool incorporating a tetrahedral structure Figure 4 is a three dimensional representation of the preferred embodiment of a high precision machine tool structure Figure 5 is a longitudinal representation of how a machine tool might be constructed using the high precision machine tool structure As shown in Figure 4 the preferred embodiment of the aforementioned invention has four inclined struts (1-4) attached to a rectangular base (5) with a suitable jointing technique.

One strut is attached at each corner of the base. The four struts incline towards a common point above the centre of said base. It is preferable but not essential that all of the struts should have the same or substantially similar dimensions.

The length, cross sectional area and angle of inclination of the struts as well as the size of the hub are important design parameters, which are chosen to maximise either the static stiffness or resonant frequencies of the machine structure.

In an alternative embodiment the base could be of any convenient shape so long as each of the four struts is attached to the base at the corners of a rectangle described on the uppermost surface of the base.

Said struts are attached to a hub (6) at their uppermost ends with cast joints. In the preferred embodiment the hub is rectangular in shape although it could be constructed in a variety of suitable shapes.

The base, struts and hub should all be constructed of steel or other suitably rigid materials.

As shown in Figure 5 a machine tool workpiece (7) or other instrument with positioning means may be suspended through a hole in the hub, attached to the struts or attached to the underside of the hub. A work- holding means (8), including such elements as suitable slideways, turntables and chucks may be attached to the upper surface of the base.

Whilst certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes in the invention disclosed herein may be made without departing form the scope of the invention, which is defined in the appended claims.

Claims

s Title: Ultra-high Precision machine-tool structure What is claimed:

1. A machine-tool structure comprising: a base with four struts attached at points corresponding to the corners of a rectangle on the upper surface of said base, said struts being inclined towards a common point above said base and being attached to a hub.

2. A machine-tool structure as claimed in Claim 1 where work-holding means comprising horizontal slide-ways and turntables for locating workmaterials in the horizontal axis are attached to the upper surface of said base.

3. A machine-tool structure as claimed in Claim 2 where a machine-tool spindle is suspended through said hub and may be located in the vertical axis using suitable positioning means.

4. A machine-tool structure as claimed in Claim 2 where a machine-tool spindle is .., suspended from the underside of said hub and may be located in the vertical axis using suitable positioning means.

5. A machine-tool structure as claimed in Claim 2 where a machine-tool spindle is suspended from said struts and may be located in the vertical axis using a suitable positioning means.

6. A machine-tool structure as claimed in any of the preceding claims where said . struts are attached to said hub with cast joints.

7. A machine-tool structure as claimed in Claims 1, 2, 3, 4 or 5 where said struts are attached to said hub with bolted joints.

8. A machine-tool structure as Claimed in any of the preceding claims where said hub is substantially square in plan.

9. A machine-tool structure as claimed in Claims 1, 2, 3, 4, 5, 6 or 7 where said hub is of any shape suitable to equally prevent bending in said four struts as a result of structural and operational tensions.

10. A machine-tool structure as claimed in any of the preceding claims where said base is substantially square in plan.

11. A machine-tool structure as claimed in Claims 1, 2, 3, 4, 5, 6, 7, 8 or 9 where said base is of any shape suitable to equally prevent bending in said four struts as a result of structural and operational tensions.

12. A machine-tool structure as claimed in any ofthe preceding claims where said base and said struts and said hub are made of steel or any other suitably rigid material.

13. A machine-tool structure substantially as claimed above and as illustrated in the accompanying drawing.

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