EP1537951B1 - Method and device for machining thin sheets and thin, single or double curved plates or shells - Google Patents

Abstract

Thin plate machining method comprises using a vacuum to generate a surface tension over surface of workpiece being clamped to fix same and then machining the workpiece whereby the cutting speed and feed speed are set so that a defined local heating occurs in the contact zone between workpiece and tool where the temperature peak is greater or equal to the melting temperature of the surface of the diffuser. Thin plate machining method comprises using a vacuum to generate a surface tension over surface of workpiece being clamped to fix same and then machining the workpiece whereby the cutting speed and feed speed are set so that a defined local heating occurs in the contact zone between workpiece and tool where the temperature peak is greater or equal to the melting temperature of the surface of the diffuser. After a complete machining of the work piece the vacuum tension is released and the workpiece lifted from the clamping device. An independent claim is included for an assembly where a vacuum table has a base plate with bores opening on top side of base plate where a diffuser is provided consisting of a thin-walled air permeable heat-reactive layer. Independent claim describes such layer having several perforations adapted to outline of workpiece.

Description

The invention relates to a method and an arrangement for processing thin sheets and thin-walled, single or double curved, three-dimensionally shaped sheets, plates or shells, in particular by material-removing machining methods, such as milling and drilling, according to the preambles of claims 1 and 3 and the use a distributor or diffuser in such an arrangement.

In the material-removing machining of thin-walled sheets, the machining forces and thus the feed rates are significantly dependent on the possibilities of fixing the parts when cutting the connection to the skeleton. In particular, this applies to the manufacture of small parts, where often the usual vacuum stress does not find enough surface to hold the part.

It is known to attempt to achieve a greater stiction caused by the vacuum by increasing the surface friction value of interchangeable papers or similar, partially air-permeable materials.
Adhesive spray or similar agents, however, are difficult to handle in terms of production, since at the same time all unwanted particles such as chips and the like adhere to the bearing surfaces and pose considerable risks for a perfect vacuum tension.

Well-known pads with suction cup character of rubber are costly and less suitable for small parts.

From the DE 201 17 390 U1 a device for clamping of plates to be processed with negative pressure is known. The device has a level support plate, which has leading openings from its top to its bottom. Between the top of the support plate and the underside of a workpiece plate to be machined a limited air-permeable layer of filter paper is arranged. Via a vacuum pump, a partial vacuum can be generated, which acts on the workpiece via Durchströmungsdrosselmittel and the limited air-permeable layer. This makes it possible, regardless of the respective outline of the plate to be processed on the underside of a sufficiently large for clamping to generate negative pressure.
In a further development, the support plate is not dimensionally stable per se, but thin and flexible like a foil. If such a support plate is arranged with the interposition of air-permeable filter material of defined thickness on the base plate, there is a defined top of the support plate, which is completely suitable for clamping plates to be processed with the interposition of the limited air-permeable material.

Disadvantage is the use of two thin-walled, film-like layers, which serve the defined generation of a vacuum. As a result of sliding friction at the boundary layers of these two films, local shifts and thus position changes of the workpiece to be clamped can occur. In addition, the technological production preparation requires the clamping of two flexible films on the vacuum table.

Another disadvantage is that especially in highly structured, z. B. skeletal or herringbone shaped workpieces and in the processing with high cutting and feed speeds, such as when milling with end mills, no secure positional fixation of the workpiece on the vacuum table can be realized.

Out DE 40 30 113 C2 For example, a device for clamping plates to be processed is known in which the base plate, on which the workpieces to be machined are received, has a plurality of closely spaced through-holes with a diameter between 0.1 and 1.0 mm. Disadvantage of this arrangement are the comparatively high production costs for the production of the base plate.
In order to rule out a contact between the main cutting edges of the rotating cutting tool and the surface of the base plate during machining, in particular during milling, a replaceable wear material is placed in a preferred development on top of the base plate.
The filter paper preferably used causes a change in the volume flow when applying a negative pressure to the base plate. The task of the filter paper is also to retain the chips produced during the processing of the plate from penetrating into the holes of the base plate so as to prevent damage to the vacuum pump.

Out DE 87 03 223 U1 a vacuum clamping plate is known in which there is an adhesive coating on the surface of the clamping surface, which preferably consists of a perforated mat made of rubber or plastic or alternatively is sprayed onto the clamping surface. The reusable adhesive coating should ensure the absorption of the acting feed forces during rolling or face milling.
The proposed use of an elastic adhesive coating, this task can not be solved technically. So it comes with the use of an adhesive pad of perforated rubber or perforated plastic under the influence of the acting cutting and feed forces to a torsion-like movement of the clamped workpiece, which inevitably leads to positional or shape deviations.

A method or arrangement for processing thin sheets according to the preambles of claims 1 and 3 is known from US 5,141,212 known. Also shown is the use of a manifold or diffuser.

The object of the invention is to eliminate the disadvantages of the prior art and to propose a method for safely receiving workpieces to be machined, in particular of highly structured, perforated parts on a vacuum table and an arrangement for carrying out the method.

According to the invention the object is achieved by a method according to claim 1, an arrangement according to claim 3 and the use of a distributor or diffuser according to claim 4. Preferably further developments are set forth in the dependent claims.

In the method, a sufficiently large holding force is generated in a first step using a vacuum on a acting as a distributor or diffuser layer on the surface of the workpiece to be clamped for holding the position.
In a second step, the machining of the workpiece, wherein the cutting parameters, in particular the cutting speed and the feed rate are adjusted so that it comes to a targeted local heating in the contact zone of the tool and workpiece, wherein the temperature peak is greater than or equal to the melting temperature of the Surface of the manifold or diffuser is. In addition to the chips, the heating particularly affects the burr. The temporary, local melting causes a positive connection and bonding of the burr on the surface of the distributor / diffuser.
After the complete machining of the workpiece, the vacuum voltage is released in a third step and lifted the workpiece from the clamping device.

The assembly consists of a vacuum table with a base plate having a plurality of air exhaust ducts or bores opening at the top of the base plate. On the upper side of the base plate, a thin-walled, film-like, layer is arranged, which is completely or partially permeable to air.
Preferably, the about 0.1 mm to 1.0 mm thick homogeneous layer uniformly or stochastically arranged perforations, which allow the passage of air. Depending on the workpiece classes to be machined and their contours, the perforations are preferably arranged in the form of a grid or a matrix. In other preferred Embodiments, the individual perforations have a meandering structure or are formed as concentric circles.
In a further preferred embodiment, the arrangement of the perforations is adapted directly to the contour of the respective workpiece or workpiece range to be processed, depending on the lot size or number of pieces to be processed. Due to the spatial arrangement of the individual elements of the perforation and their distance from each other can be generated by the vacuum clamping device defined, constant voltages on the underside of the workpiece to be machined. In order to enable a secure tension even with larger cutting forces, in particular during contour milling with high feed and cutting speeds, at least the surface of the layer facing the workpiece to be clamped wets with a plastic that melts when heated slightly. In a preferred embodiment, the homogeneous layer consists of a thin-walled polyethylene film having a number of symmetrically arranged perforations.

The layer consists of an environmentally friendly, air-permeable carrier material, such as paper flow or textile fabric, on the surface of which point or line a lightly melting plastic is applied.

In a further, likewise preferred embodiment, the top and bottom of the layer have a partial wetting or coating with a light-melting plastic. As a result, this layer can be used twice, because after any damage to the, the workpiece to be clamped surface facing by a cutting tool still the intact underside of the layer can be used for the subsequent operation.

In the machining of a workpiece, especially in the Machining of aluminum, forms on the lower edge of the workpiece a fine, with the naked eye barely noticeable ridge. This ridge, on the one hand, penetrates into the soft, yielding surface of the layer and clings to it. In addition, it was found that during the machining with high cutting and feed speeds, despite the good heat dissipation, a brief local heating of the workpiece in the area of the cutting edge occurs. The frictional heat between tool and workpiece leads locally and temporarily to a melting of the plastic on the surface of the layer in the region of the cutting edge, ie on the entire outer contour of the workpiece and in consequence to a positive, thermoplastic clamping of the underside of the workpiece with the surface of the from the vacuum clamping table in its position fixed layer.
The used, about 0.1 mm to 1.0 mm thick layer is relatively soft and has an average mechanical stability to prevent penetration of sharp edges z. B. burr allow. It is reusable and easy to clean due to its preferably smooth surface. Moreover, no unwanted particles (chip fragments) adhere to the layer.

• Ausführungsbeispiel 1:
  • Für das Konturenfräsen und -bohren eines Aluminium-Dünnbleches (Figur) mit einem Schaftfräser wird auf dem Vakuumtisch einer Portalfräsanlage eine 0,2 mm starke Polyethylenfolie mit gleichförmig angeordneten Perforierungen aufgelegt. Darauf wird ein Aluminiumblechzuschnitt positioniert und nachfolgend über die Vakuumspanneinrichtung das Halbzeug in seiner Lage fixiert. Die Schnitttiefe a ist geringfügig größer als die Blechdicke s des zu bearbeitenden Halbzeuges. Drehzahl n bzw. Schnittgeschwindigkeit v_s und Zahnvorschub s_z bzw. Vorschub s sind so gewählt, dass es auf Grund der Reibungswärme zwischen den Hauptschneiden des Schaftfräsers und dem Werkstück zu einer lokalen Erwärmung von >= 120 °C kommt. Durch das Abfließen der Reibungswärme in Richtung des Vakuumtisches kommt es zu einem Erweichen der oberflächennahen Zone der Polyethylenfolie und in der Folge zu einer Anhaftung und formschlüssigen Verklammerung des sich beim Schaftfräsen an der Unterseite des Aluminiumbleches bildenden Grates. Durch diese Verbindung wird ein axiales Verschieben des Aluminium-Bleches auf der als Verteiler oder Diffusor wirkenden dünnwandigen Polyethylen-Schicht, ebenso wie eine etwaige Torsionsbewegung wirksam verhindert.
• Ausführungsbeispiel 2:
  • Für die Fräsbearbeitung eines fischgrätenartig geformten Aluminium-Dünnbleches mittels Formfräser wird auf der Vakuumspanneinrichtung einer Kreuzschiebtisch-Fräsmaschine als Verteiler/Diffusor eine dünnwandige, luftdurchlässige Vliesschicht aufgelegt, die an ihrer, dem zu spannenden Halbzeug zugewandten Oberseite eine Vielzahl wärmereaktiver Kunststoffnoppen aufweist. Die Schichtdicke des Papiervlieses beträgt etwa 0,1 mm.
  • Für die Bearbeitung großer Stückzahlen bzw. Losgrößen ist die Anordnung der Kunststoffnoppen an den Umriss der zu fräsenden Kontur des Werkstückes angepasst. Dadurch kann zum einen die Menge des auf dem Vlies aufzubringenden Kunststoffs aus ökologischer Sicht begrenzt werden. Zum Zweiten kann die Anordnung der Kunststoffnoppen für die leichtere Positionierung und Justierung der zu spannenden Werkstücke auf dem Vakuumtisch genutzt werden. Dazu werden vorzugsweise farbige Kunststoffnoppen appliziert.

The invention will be described in more detail with reference to two exemplary embodiments:

• Embodiment 1
  • For the contour milling and drilling of an aluminum thin sheet (Figure) with an end mill, a 0.2 mm thick polyethylene film with uniformly arranged perforations is placed on the vacuum table of a portal milling machine. Then an aluminum sheet blank is positioned and subsequently fixed on the vacuum clamping device, the semi-finished in its position. The cutting depth a is slightly larger than the sheet thickness s of the semi-finished product to be processed. Speed n or cutting speed v _s and tooth feed s _z or feed s are chosen so that due to the frictional heat between the main cutting edges of the end mill and the workpiece local heating of> = 120 ° C occurs. By the If the frictional heat in the direction of the vacuum table flows away, the near-surface zone of the polyethylene film softens and, as a consequence, adhesion and positive clamping of the ridge forming at the bottom of the aluminum sheet during end milling. By this connection, an axial displacement of the aluminum sheet on the distributor or diffuser acting thin-walled polyethylene layer, as well as any torsional movement is effectively prevented.
• Embodiment 2:
  • For the milling of a herringbone-shaped aluminum thin sheet by means of form a thin-walled, air-permeable nonwoven layer is placed on the vacuum chuck of a Kreuzschiebtisch milling machine as a distributor / diffuser, which has on its, the exciting semi-finished top facing a variety of heat-active plastic knobs. The layer thickness of the paper web is about 0.1 mm.
  • For processing large quantities or batch sizes, the arrangement of the plastic knobs is adapted to the outline of the contour of the workpiece to be milled. As a result, on the one hand, the amount of plastic to be applied to the nonwoven can be limited from an ecological point of view. Secondly, the arrangement of the plastic knobs can be used for the easier positioning and adjustment of the workpieces to be clamped on the vacuum table. For this purpose, preferably colored plastic knobs are applied.

Claims (4)

1. Method for machining thin sheets and thin, single or double curved, three-dimensionally formed sheets, plates or shells, particularly by material removing machining methods, such as milling and drilling, wherein in a first step using a vacuum a surface tension sufficient for fixing the position is produced over a layer acting as a distributor or diffusor on the surface of the workpiece to be clamped and in a second step, machining of the workpiece is performed, characterized by that the cutting parameters, particularly the cutting speed and feed rate are set such that a defined local heating in the contact region of the tool and workpiece is created, whereby the peak temperature being higher than or equal to the melting point of the surface of the distributor or diffusor, and after machining of the workpiece has been completed, vacuum clamping is released and the workpiece is removed from the clamping device.
2. Method to claim 1, characterized by that the workpiece is removed from the clamping device using an air cushion.
3. Device for performing the method to claim 1 or 2, essentially comprising a vacuum table provided with a base plate which has a plurality of air exhausting channels or holes exiting at the top surface of the base plate, wherein at the top surface of the base plate a distributor or diffusor comprising a thin, totally or partially air-permeable, heat-reactive layer is arranged, characterized by that the layer comprises an ecologically degradable carrier material whereby an air-permeable, low-melting plastic material is applied on one or both sides of the carrier material or the carrier material is made of a paper web or textile fabric whereby a low-melting plastic material is applied at the surface of the carrier material in punctiform or linear manner.
4. Use of a distributor or diffusor in a device to claim 3 comprising a thin, totally or partially air-permeable, heat activated layer established as a homogeneous layer, with the number, arrangement and size of the perforations being adapted to the contour or projected area of the workpiece, or workpieces to be machined, for the machining of thin sheets and thin, single or double curved three-dimensionally formed sheets, plates or shells by material removing machining methods, preferably milling and drilling.

Images