WO2006043170A2

WO2006043170A2 - A machining head for machine tools with a shaft with low thermal expansion coefficient

Info

Publication number: WO2006043170A2
Application number: PCT/IB2005/003166
Authority: WO
Inventors: Stefano Motta; Piergiacomo Bongiorni
Original assignee: Jobs S.P.A.
Priority date: 2004-10-22
Filing date: 2005-10-13
Publication date: 2006-04-27
Also published as: WO2006043170A3; ITBO20040658A1

Abstract

In a machining head that comprises a supporting structure (2) attachable to a machine tool (100), a shaft (3) coupled rotatably to the supporting structure (2) and centred on a relative longitudinal axis (J), a drive motor (6) by which the shaft (3) is set in rotation about the longitudinal axis (J), and a tool-holding or work-clamping device (5) mounted to the shaft (3), at least the shaft (3) is fashioned from a low thermal expansion material.

Description

A machining head for machine tools

Technical Field

The present invention relates to a machining head for machine tools with features identifiable as in the preamble of claim 1.

Background Art

The function of a machining head is conventionally to induce relative motion between a tool and a workpiece, in such a way as to removal material from the piece or to bring about its deformation. To this end, the head includes a shaft by which rotary cutting motion is transmitted to the workpiece, or alternatively to the tool .

Preferably, though not exclusively, the invention is applicable to numerically controlled multi-axis machine tools used for high speed milling operations. In this type of application, the machining head carries the tool . Machine tools of the type in question are utilized particularly in the aircraft building industry for contouring and drilling parts made of aluminium and composite materials, and in the automobile sector for producing master models, and models for bodywork, internal parts and dies. In machining heads representative of the prior art, both the shaft that carries the tool, or the work, and the structure supporting the shaft, are made of metal, generally steel, and the supporting structure and the shaft are coupled by way of bearings.

Disadvantageously, the rise in temperature of the assembled components aforementioned, occasioned by the machining operation, has the effect of inducing thermal expansion in metal parts, and particularly in the shaft . The elongation of the shaft generates internal forces and affects the precision of machining.

These problems are overcome in part by devices of conventional type associated with or incorporated into the bearings, such as will allow the shaft to elongate freely at the end opposite from the tool holder. Such devices tend however to increase the complexity of the machining head, and to impact negatively on its weight and cost . The drawbacks in question are particularly evident in the case of heads fitted to numerically controlled multi-axis machine tools, which must guarantee ultra high precision; in these machines, the shaft is driven by an electric motor installed in the head. The motor comprises a stator mounted to the supporting structure, and a rotor mounted to the shaft. The passage of current through the stator and the rotor generates a significant amount of heat, which is transmitted to the shaft and brings about its elongation. The applicant finds also that machining heads of the prior art are improvable in a number of ways, particularly as regards their complex construction and the precision they are able to guarantee. The object of the present invention, accordingly, is to design a machining head for machine tools such as will allow of substantially overcoming the above noted drawbacks.

It is also an object of the present invention to provide a machining head offering greater precision in machining than heads of the prior art.

Disclosure of the Invention

The stated objects are realized, according to the present invention, in a machining head for machine tools incorporating one or more of the technical solutions claimed hereinafter.

The invention will now be described in detail, by way of example, with the aid of the accompanying drawings, in which:

-figure 1 shows a longitudinal section through a machining head for machine tools in accordance with the present invention;

-figure 2 shows a multi-axis machine tool equipped with the head of figure 1.

With reference to the accompanying drawings, numeral 1 denotes a machining head for machine tools, in its entirety.

The head 1 is installable advantageously, though by no means uniquely, on a multi-axis machine tool 100 of the type that comprises a bed 101 over which the head 1 is capable of movement on a plurality of positioning axes. The head 1 is positioned on the various axes by drive means 102 of conventional embodiment (not described further) , connected to a processing and control unit, not illustrated, and serving also to govern the rotation of the tool U about a relative machining axis J on the basis of data programmed into the control unit . By way purely of example, the machine tool 100 shown schematically in figure 2 has five axes X, Y, W, Z and K, certain with linear motors, and is designed to drill and countersink holes in parts of appreciable dimensions such as the panels and ribs of aircraft wings. More specifically, the bed 101 of the machine 100 comprises two parallel slide ways 103 extending along a first positioning axis X, between which the work will be placed, and is surmounted by a gantry 104 traversable along the slide ways 103. The gantry 104 supports a carriage 105 traversable along the beam of the selfsame gantry 104, which coincides with a second positioning axis Y orthogonal to the first axis X and parallel to the floor. A cross slide 106 mounted to the carriage 105 is traversable along a third positioning axis Z, extending perpendicular to the floor, and carries a rotary unit 107 at the bottom end centred on a fourth axis W parallel to the third axis Z. The head 1 is mounted to the rotary unit 107 and pivotable also about a fifth axis K orthogonal to the fourth axis W. With reference to figure 1, the machining head 1 according to the invention comprises a supporting structure 2 attachable to the machine tool 100 and a shaft 3 coupled rotatably to this same structure 2, preferably by way of bearings 4a and 4b.

The shaft 3 extends through the supporting structure 2, to which it is connected by way of a first bearing 4a and a second bearing 4b mounted respectively to a first end 3a and a second end 3b of the shaft 3.

The first end 3a of the shaft 3 carries a clamping device 5 by means of which to grip a tool or a workpiece. In the preferred embodiment illustrated, which refers to a head for multi-axis machine tools, the clamping device 5 serves to grip the tool U.

Finally, the head 1 comprises drive means 6 by which the shaft 3 is set in rotation about its longitudinal axis, which is one and the same as the aforementioned machining axis J. To advantage, at least the shaft 3 is made of a low thermal expansion material (LTEM) . In the context of the present specification and the appended claims, a low thermal expansion material would be one having a coefficient of linear thermal expansion (CLTE) much lower than that of steel, which is in the region of ll*10^{'6 0}C^"1. Advantageously, the material employed will have a CLTE of less than 3*10^{"6 0}C^"1, possibly between -l*10^{'6 0}C^"1 and l*10^{"6 0}C^"1, and preferably between -0.5*10^{'6 0}C^"1 and 0.5*10^{"6 0}C^"1. Among the materials possessing this characteristic, ceramic or composite materials will be preferable. Advantageously, the low thermal expansion material in question will be a composite containing carbon fibre. In particular, carbon fibre typically has negative coefficients of linear thermal expansion ranging between -l*10^{'6 0}C^"1 and -0.3*10^{~6 0}C^"1. The structure of carbon fibre composite is such that coefficients of linear thermal expansion CLTE, whether positive or negative, will in any event be close to zero. This being the case, use could also be made of silicon carbide and its derivatives.

In the example of figure 1, the shaft 3 appears as a tubular body fashioned from composite material consisting of carbon fibres in a matrix. In a preferred embodiment, drive means 6 comprise an electric motor incorporated into the head 1. The electric motor 6 presents a stator 7 associated with the supporting structure 2 and a rotor 8 associated with the shaft 3. The rotor 8 extends coaxially to the shaft 3, whilst the stator 7 is breasted with the rotor 8 and lodged in a cavity 9 delimited by the supporting structure 2.

Also incorporated into the motor, advantageously, located between the rotor 8, which will be embodied necessarily in a conductive metallic material, and the shaft 3 of low thermal expansion material, is at least one layer 10 of resilient material such as will absorb the thermal expansion of the rotor 8 and prevent internal stresses from being generated at the interface between these same components. The material in question could be a film of adhesive or a resin, both familiar to a person skilled in the art.

Similarly, the supporting structure 2 will be made of a low thermal expansion material, and preferably the same material as the shaft 3.

Alternatively, the supporting structure 2 might be embodied at least partially in a metallic material utilized typically for such applications, preferably steel, presenting a coefficient of linear thermal expansion (CLTE) greater than 10*10^{'6 0}C^'1. In this situation, a layer 10 of resilient material will also be interposed between the shaft 3 and the supporting structure 2, serving to absorb the thermal expansion of the structure. For example, if conventional steel bearings 4a and 4b are installed between the shaft 3 and the supporting structure 2, the layer 10 of resilient material will be interposed between the shaft 3 and each of the bearings 4a and 4b.

Lastly, the clamping device 5 will be made likewise of low thermal expansion material, and preferably the same material as the shaft 3. Alternatively, like the supporting structure 2, the clamping device 5 might be embodied in a metallic material utilized typically for such applications, preferably steel, presenting a coefficient of linear thermal expansion (CLTE) greater than 10*10^{'6 0}C^'1, in which case the layer 10 of resilient material will be interposed preferably between the shaft 3 and the clamping device 5, so as to absorb the thermal expansion of the selfsame device. In the preferred embodiment of figure 1, the supporting structure 2 is furnished with an external housing 11 of carbon fibre, enclosed at the ends by a first flange 12 and a second flange 13, both made of steel . The first flange 12 affords a housing for the first bearing 4a and the second flange 13 affords a housing for the second bearing 4b. Given that the carbon fibre shaft 3 interfaces with steel parts, the layer 10 of resilient material will be interposed to advantage between the shaft 3 and each of the two bearings 4a and 4b.

As illustrated in figure 1, the supporting structure 2 further comprises an internal tubular member 14, also of steel, housing the stator 7 of the electric motor 6. The clamping device 5 consists in a steel spindle attachable to the shank of a tool, which is conventional in embodiment and therefore not illustrated. The spindle 5 is embedded in the carbon fibre of the shaft 3 with the layer 10 of resilient material interposed between the two components. Similarly, the rotor 8 is embedded in the carbon fibre of the shaft 3 together with the interposed layer 10 of resilient material, which could also mask the rotor 8 in its entirety. As a rule, the aforementioned layer 10 of resilient material will be interposed advantageously between the shaft 3 of low thermal expansion material and any steel component, or indeed any component having a coefficient of linear thermal expansion (CLTE) greater than 10*10^{'6 0}C^'1, that may happen to be interfaced with the selfsame shaft 3.

The drawbacks mentioned at the outset are overcome with a machining head according to the invention, and the stated objects duly realized. First and foremost, the adoption of a low thermal expansion material for the construction of the shaft is instrumental in achieving greater precision in machining, and avoiding the need for mechanical devices designed specifically to compensate for the elongation of the shaft. Accordingly, the machining head according to the present invention is simpler in construction than heads typical of the prior art, and more reliable as a result.

Furthermore, by including the layer of resilient material serving to absorb the thermal expansion of metal parts such as the rotor, it becomes possible to trim production costs by adopting standard materials for the remaining parts of the head.

Finally, when adopting composites such as carbon or silicon carbide, the head can be made sufficiently light to allow the use of motion inducing motors with lower rated power, and/or to improve the response of the system.

Claims

1) A machining head for machine tools comprising: -a supporting structure (2) attachable to a machine tool; -a shaft (3) coupled rotatably to the supporting structure (2) and centred on a relative longitudinal axis (J) ;

-drive means (6) by which the shaft (3) is set in rotation about the longitudinal axis (J) ; -a clamping device (5) mounted to the shaft (3) and serving to grip a tool or workpiece; characterized in that the shaft (3) , at least, is embodied in a low thermal expansion material (LTEM) .

2) A head as in claim 1, wherein at least the shaft (3) is embodied in a material having a coefficient of linear thermal expansion (CLTE) less than 3*10^{'6 0}C^"1, preferably between -l*10^{"6 0}C^"1 and l*10^{'6 0}C^"1.

3) A head as in claim 1, wherein at least the shaft (3) is embodied in a ceramic material.

4) A head as in claim 1, wherein at least the shaft (3) is embodied in a composite material, preferably containing carbon fibre. 5) A head as in claim 1, wherein at least the shaft (3) is embodied in a composite material, preferably- containing silicon carbide or derivatives thereof.

6) A head as in claims 1 to 5, wherein the drive means (6) comprise a stator (7) mounted to the supporting structure (2) and a rotor (8) mounted to the shaft (3) .

7) A head as in claim 6 where dependent on claim 4 or 5, wherein the rotor (8) is embedded in the composite material of the shaft.

8) A head as in claim 6 or 7, further comprising at least one layer (10) of resilient material interposed between the shaft (3) and the rotor (8) and serving to absorb the thermal expansion of the rotor (8) .

9) A head as in claim 1, wherein the supporting structure (2) is embodied in low thermal expansion material.

10) A head as in claim 1, wherein the supporting structure (2) is embodied in the same material as the shaft (3) .

11) A head as in claim 1, wherein the supporting structure (2) is embodied in a material having a coefficient of linear thermal expansion (CLTE) greater than 10*10^{'6 0}C^"1.

12) A head as in claim 1, wherein the supporting structure (2) is embodied in steel.

13) A head as in claim 11 or 12, wherein the shaft

(3) and the supporting structure (2) are interfaced by way of a layer (10) of resilient material serving to absorb the thermal expansion of the selfsame supporting structure (2) .

14) A head as in claim 13, comprising bearings (4a, 4b) interposed between the shaft (3) and the supporting structure (2) , wherein the layer (10) of resilient material is interposed between the shaft (3) and each of the bearings (4a, 4b) .

15) A head as in claim 1, wherein the clamping device (5) is embodied in a low thermal expansion material .

16) A head as in claim 1, wherein the clamping device (5) is embodied in the same material as the shaft (3) .

17) A head as in claim 1, wherein the clamping device (5) is embodied in a material having a coefficient of linear thermal expansion (CLTE) greater than 10*10^{'6 0}C^"1.

18) A head as in claim 1, wherein the clamping device (5) is embodied in steel.

19) A head as in claim 17 or 18, wherein the shaft

(3) and the clamping device (5) are interfaced by way of a layer (10) of resilient material serving to absorb the thermal expansion of the selfsame clamping device (5)

20) A head as in claim 8, 13, 14 or 19, wherein the layer (10) of resilient material is a film of adhesive.

21) A head as in claim 8, 13, 14 or 19, wherein the layer (10) of resilient material is a resin.

22) A machine tool, characterized in that it comprises a machining head as in one or more of claims 1 to 20.

23) A machine tool as in claim 21, characterized in that it is a multi-axis machine tool.