Technical field

The present invention relates to the field of machining closed envelopes with curvilinear contours, especially thin-walled ones, made of high-strength aluminium alloys. Such structures are used primarily in the aviation industry.

Background art

In the aviation industry, there are relatively stringent requirements as to the quality of the components used. The accuracy of machining and parameters of elements used in aviation are determined by and subject to the strength and aerodynamic considerations. Such requirements also apply to machining thin-walled closed envelopes with curvilinear contours, made of high-strength aluminium alloys. Such structures, used for aviation purposes, should have walls with a minimum thickness g=0.8-l mm, with tolerance +/-0.1 mm.

Machining such surfaces usually meets the above-mentioned wall thickness requirements, although it requires that proper machining conditions and proper workpiece clamping conditions are ensured, and machining conditions are adjusted depending on whether it is a shaping or finish machining.

In the finish machining, the most important one from the point of view of the objective of this application, cutting conditions and parameters should be used to allow for obtaining: the surface roughness at the level of Sa = 0.16 micrometers, where Sa is the arithmetic mean deviation of the height of surface unevenness from the reference plane; the shape tolerance (especially convexity) at the level +/-0.05 mm, and the envelope thickness tolerance at the level +/-0.1mm from the shape and nominal size. Chinese patent application CN103639655 A describes a method for machining and clamping surfaces of curvilinear semi-open envelopes in a device, where the workpiece is supported on one of the finished surfaces, and the free surface is subject to machining. In order to increase the envelope rigidity and reduce the risk of its vibrations during machining, the free volume of the clamping device is filled with liquid gypsum, which, after solidification, supports the walls of the workpiece and prevents their deformation. This method has, however, the following disadvantages: it requires the removal of gypsum and cleaning of its particles adhering to the surface, and the gypsum reduces its volume when it cools down, which affects the dimensions of the envelope after machining.

Chinese patent document CN105522206B describes a machining method for thinwalled outer and inner surfaces made of aluminium alloys, comprising steps of rough and finish machining of outer surfaces having variable curvature radii in three planes, and rough and finish machining of the inner surface. A technological issue is the finish machining of inner surfaces with wall thicknesses up to 1 mm, while maintaining the appropriate machining accuracy as well as avoiding vibrations of the UPON (Chuck-Machine Tool-Workpiece-Tool) system during machining. To enable machining of the inner surface, the workpiece must be clamped on the outer surface in the device. The device for machining is made in the form of a rectangular metal box, in which the blank workpiece is positioned and supported, and then, in order to clamp it, the free space is filled with paraffin at temperature 95°C. After solidification, the paraffin fixes and clamps the workpiece in a suitable position, thus enabling the machining. This method of clamping is functional for finish machining where the cutting forces and cutting temperature are relatively low. The disadvantage of this solution is that paraffin must be heated to 95°C, whereby also the workpiece is heated to approximately 95°C. Moreover, the temperature varies during the course of the process. This causes the dimensions of the workpiece to change and paraffin to detach from it when it cools down.

Vacumsystem Polska offers devices for clamping and machining thin-walled objects made of metal and plastics, wherein these devices are tables operated with the use of vacuum and dedicated to flat surfaces, not envelopes.

Schmalz offers devices for clamping thin workpieces with flat surfaces subjected to machining, wherein these are vacuum devices in which flexible gaskets, usually rubber, adhere to the surface to be clamped and press the workpiece against the supporting surface of the device. The company also offers a system for point clamping and positioning of elements such as open envelopes through the use of an adjustable support and pressure. This system cannot, however, be used for clamping closed envelopes with machined inner surfaces. Moreover, it requires the use of metal islands for clamping devices, which necessitates the use of additional machining of them before applying pressure.

The aforementioned vacuum clamping systems using air vacuum reduce the positioning accuracy and rigidity of the workpiece clamping, especially in the perpendicular direction to the surface, due to the presence of a thin air layer between the interacting parts. The air layer, due to its relatively high susceptibility (as compared to, for example, metal or liquid) causes the surfaces to displace, and thus reduces the accuracy of the reproduced shape and reduces the frictional forces between the support and the clamped object, which affects its tangential displacements.

Summary of the invention

The subject matter of the invention is a device for clamping and machining thin-walled closed envelopes with curvilinear contours comprising:

- a base for fastening on a machine tool table;

- an intermediate plate attached to the base;

- a body placed on the intermediate plate; characterized in that the body has a supporting surface being an exact representation of the outer surface of the envelope to be machined; the body has a through hole in its wall, into which is screwed a stub pipe for air suction from the device, intended for connection to the air vacuum system, wherein the dimensions of the intermediate plate are matched to the dimensions of the body formed; the supporting surface has a stepped structure with vertical faults in the range 0.1 - 0.2 mm, arranged in the direction perpendicular to the vertical axis of the body; circumferential recesses are formed in the lower and upper parts of the supporting surface - a lower channel and an upper channel, in which sealing rings are placed; mutually intersecting grooves connected to the through hole of the body are formed on the supporting surface, wherein the supporting surface is divided by the grooves into patches with dimensions not greater than 15 mm x 15 mm, and wherein these grooves intersect at an angle of at least 45°. Preferably, the grooves intersect at an angle of 90°.

Preferably, the grooves are latitudinal grooves running in the horizontal plane of the body and meridional grooves running in the vertical plane of the body.

Preferably, the grooves have a depth between 3 mm and 3.5 mm, and width between 4 mm and 5 mm.

Preferably, the intermediate plate is fastened to the base with screws.

Preferably, the body is fastened to the intermediate plate with screws.

Preferably, the body is made in 3D printing technology based on a 3D model of the envelope to be machined.

Preferably, the body is made of material used in 3D printing.

Preferably, the body is made of PET-G.

The subject matter of the invention is also a method of preparation for machining thin-walled envelopes in the device as described above, which comprises the following steps:

- the position of the body, formed earlier based on the envelope shape and dimensions, is fixed, and the body fastened on the intermediate plate which is fastened to the base;

- a layer of mineral oil is applied to the supporting surface of the body, wherein the mineral oil has a minimum viscosity of 25 cP;

- the envelope is placed inside the body and pressed against the supporting surface equipped with sealing rings until the outer surface of the envelope adheres tightly to the supporting surface;

- an air vacuum of about 0.8 bar is set in a vacuum system connected to the stub pipe whereby the supporting surface of the body and the outer surface of the envelope are pressed against each other.

Preferably, the body is created by 3D printing using a 3D model of the envelope. Preferably, the stepped structure of the supporting surface is created by stacking 0.1 - 0.2 mm thick layers of material during 3D printing in the direction perpendicular to the vertical axis of the body.

Advantageous effects of the invention

The present invention allows for accurate positioning of the workpiece and its clamping in a way that guarantees a sufficiently high clamping rigidity, which is extremely important in the case of not very rigid envelopes (especially made of aluminium alloys). This allows the inner surfaces to be finish machined by milling at a cutting speed v _c up to 1000 m/min (preferably in the range 750 do 1000 m/min) and cutting depth a _p in the range 0.05 - 0.2 mm at a cutting width b up to 1 mm (for curvilinear surfaces). The invention allows the workpiece to adhere well to the device, and the use of a wetting oil increases the adhesion forces of the device body and the machined envelope, which further improves the clamping conditions. The layered shape of the body surface of the device ensures good adhesion of the oil to the surface, and its appropriate viscosity prevents it from flowing down.

Brief description of Drawings

The invention will be now explained in more detail in a preferred embodiment with reference to the accompanying drawing, wherein:

Fig. 1 shows the device in top view;

Fig. 2 shows the device in front view;

Fig. 3 shows a cross-sectional view of the complete device with the workpiece clamped in cross section, along the plane A-A in Fig. 1;

Fig. 4 shows the workpiece in a perspective view;

Fig. 5 shows the device body in a perspective view;

Fig. 6 shows a cross section of the inner fastening wall of the device body, in the plane B-B marked in Fig. 5.

Detailed description The object of the invention is a vacuum device for clamping, for the purpose of machining, thin-walled closed envelopes with curvilinear cross sections in the main planes of symmetry.

The device according to the invention enables the positioning and clamping of the workpiece, namely a low-rigid envelope made, for example, of an aluminium alloy, and carrying out finish machining of the inner surfaces of the workpiece by milling at high cutting speeds v _c = 750 - 1000 m/min, at a cutting depth a _p = 0.05 - 0.2 mm and cutting width b up to 1 mm (for curvilinear surfaces).

Fig. 1 and Fig. 2 show the device according to the invention in the top view and in the front view, respectively.

The device has a universal base plate 1 which is positioned and fastened on a machine tool table (e.g. a CNC machine tool, not shown in the drawing). The base 1 is used for fastening the device on the machine tool table and for this purpose it has side recesses and dowel pins which enable the formation of multiplied clamping units. The base 1 can be also equipped with an air vacuum supply to allow its use for vacuum clamping.

On the base 1 an exchangeable intermediate plate 2 is arranged, attached to this base 1 using fastening means - here screws 3. On the intermediate plate 2 the body 4 is arranged, attached with fastening means - in an embodiment it is tightened with screws 5. The intermediate plate 2 (its shape and dimensions) is selected individually depending on the dimensions of the body 4.

Fig. 3 shows a cross section of the device with the workpiece placed in it. The body 4 has a through hole (cylindrical channel) in its wall, arranged, for example, in its plane of symmetry, into which hole is screwed a stub pipe 6 intended for connection to the air vacuum system, through which stub pipe 6 air is drawn and discharged (sucked under pressure). In the assembled state of the envelope 8 in the device, the outer surface of the envelope 8 to be clamped is adjacent to the supporting surface 7 of the body 4. The matching is achieved by subtracting the body 4 and the envelope 8 in the 3D structure. As mentioned above, the base 1 can be also equipped with an air vacuum supply to allow its use for vacuum clamping of workpieces directly on the base 1. To this end, in an embodiment, the base is equipped with a stub pipe 16 and a channel system to allow for air flow. In an embodiment, the stub pipe 16 is screwed into a horizontal channel 17, as can be seen in Fig. 3. Fig. 4 shows an example of an object to be machined with the present device - an envelope 8 to be clamped in the device according to the invention. The envelope 8 has an outer surface 15 representing a pattern for the shape of the supporting surface 7 (the working surface) of the body 4 shown in Fig. 5.

Fig. 5 shows the body 4 of the device in a perspective view, with the details of the construction of this body 4. The body 4 is made, for example, of material selected from the group of materials used for 3D printing of handles and devices, for example, of PET-G (and other used, for example, in the Rapid Tooling technology). The working surface (the supporting surface 7) of the body 4 is an exact representation of the outer surface 15 of the workpiece, namely the envelope 8 clamped in it, generated on the basis of a 3D model. The body 4 is shaped using 3D printing, wherein the stacked layers of material should lie in planes perpendicular to the vertical axis of the inner surface of this body 4.

On the inner surface of the body 4 there are provided, preferably shaped by printing, circumferentially (latitudinally) arranged on this surface, circumferentially closed recesses 9, 10 (approximately semicircular in cross section), in which sealing rings 11, 12, made of silicone, are placed. The inner surface of the body 4 has two such closed circumferential grooves arranged in the lower and upper part of the supporting surface 7 - the lower recess 9 and the upper recess 10, respectively.

On the inner surface of the body 4, i.e. on the supporting surface 7, which is also visible in Fig. 3, a system of mutually intersecting grooves 13, 14 connected directly or indirectly (a given groove 13, 14 through another groove 13, 14) to the through hole of the body 4, is formed. In an embodiment, the grooves 13, 14 intersect at the right angle, and these are latitudinal grooves 13 running latitudinally on the circumference of this supporting surface 7, and meridional grooves 17 running meridionally on this supporting surface 7. The grooves 13, 14 do not reach the upper or lower edges of the body 4 (they end above the lower recess 9 and below the upper recess 10, respectively). Accordingly, the grooves 13, 14 do not come to the end of the envelope 8 to be clamped when it is fixed in the device - the top edges of the clamped envelope 8 usually extend beyond the edges of the device. The grooves 13, 14 are connected to a through hole passing through the wall of the body 4, in which hole the above- mentioned stub pipe 6 is screwed. As mentioned, the groves 13, 14 intersect with each other (they are connected with each other directly or indirectly), but in no embodiment they are connected to or intersect with the lower recess 9 or the upper recess 10. In an embodiment, the grooves 13, 14 have a depth in the range of 3 to 3.5 mm and a width in the range of 4 to

5 mm.

The body 4, and more specifically its supporting surface 7, is divided by the grooves 13, 14 into patches with dimensions not greater than 15 mm x 15 mm.

As already mentioned, the body 4 is made by 3D printing, wherein the layers of material are arranged in the direction perpendicular to its axis and have a thickness of about 0.2 mm, which results in obtaining a supporting surface 7 with a stepped structure with vertical faults 0.1 - 0.2 mm, as shown in Fig. 6 (dimension A). This structure prevents the oil from excessively flowing down from the supporting surface 7.

A layer of mineral oil with a minimum viscosity of 25 cP is applied on the supporting surface 7 of the body 4 before the envelope 8 is placed on it. The use of a wetting oil increases the adhesion forces of the supporting surface 7 of the body 4 and the envelope 8, which improves the clamping conditions and the tightness of the system. The layered shape of the supporting surface 7 of the body 4 ensures in turn good oil adhesion to this surface, and appropriate oil viscosity prevents it from flowing down.

In another embodiment, the latitudinal grooves 13 and meridional grooves 14 need not be positioned exactly in the horizontal and vertical planes of the device, respectively. This means that their arrangement may deviate from that determined by these planes, so that they will intersect at an angle other than the right angle, as long as their arrangement allows the correct operation of the device, which is possible up to the angle of intersection of the grooves 13, 14 of 45° (the angle of intersection of minimum 45° should be maintained due to the air extraction efficiency during suction).

The method of clamping and preparation for machining thin-walled envelopes in the device described above is described below.

First, based on the shape and dimensions of the workpiece (envelope), a body 4 with a shape matching it should be produced. The body 4 is produced by 3D printing using a 3D model created on the basis of the workpiece - so that the shape of the body 4, and more specifically the shape of its supporting surface 7, corresponds to the shape of the outer surface of the workpiece. The methods of producing objects by 3D printing are known from the prior art, and one skilled in the art, having them at their disposal, will have no problem to perform this step. In order to fix and clamp the workpiece - the envelope 8 - intended to be machined, the following steps are carried out:

- the position of the body 4 is fixed and it is fastened to the base 1 and the intermediate plate 2,

- a layer of mineral oil is applied to the supporting surface 7 of the body 4, wherein the mineral oil should have a minimum viscosity of 25 cP;

- the workpiece is placed inside the body 4 and pressed against the supporting surface 7 until they adhere to each other;

- an air vacuum of about 0.8 bar is set in the stub pipe 6 with connected vacuum system.

The air under vacuum of about 0.8 bar acts through the stub pipe 6 on the supporting surface 7 of the body 4 and the outer surface 15 of the clamped envelope 8, pressing them against each other. The oil layer secures them to each other, preventing them from mutual displacement, and the contact of the silicone sealing rings 11, 12 and the surface 15 of the workpiece 8 ensures the tightness of the vacuum system.

In order to ensure good adhesion of the supporting surface 7 and the outer surface 15 of the workpiece (envelope 8), a supporting surface 7 should be used, the partial surfaces of which (as defined by the latitudinal grooves 13 and the meridional grooves 14) have dimensions maximally 15 mm x 15 mm, while keeping the width of the grooves from 4 to 5 mm.

The device developed herein can be used primarily for finish machining, but its use in shaping machining is not excluded.