FACING HEAD HAVING HIGH PERFORMANCES AND HIGH
This invention relates to a facing head adapted for application on machine tools for facing and reaming, even with continuously variable diameter, and characterized by high performances and high machining accuracy.
Existing facing heads are equipped with a system for driving and measuring the position of the slide which is integral with the fixed part of the head and which therefore drive and measure the position of the slide through the agency of a differential mechanism based on prior art solutions. Owing to the elasticity of the mechanical transmission, however, this mechanism introduces backlash and lost motion each time the radial movement of the slide is reversed, resulting in errors in the measuring and positioning of the slide. This backlash and lost motion may lead to positioning in only one direction in order to obtain acceptable levels of accuracy.
The differential system present in the above mentioned solution, besides producing a backlash in the transmission, limits the maximum speed attainable by the head and thus also limits the cutting speed during machining of relatively small diameters.
Currently existing heads transmit the rotational movement of the rotating part of the head directly from the spindle of the machine tool. That means the maximum torque available on the head is equal to the maximum torque made available by the spindle of the machine tool which, in the
case of small and medium-sized milling machines, is often insufficient for reaming large diameters. In large machine tools designed to be equipped with a facing head, it is, in most cases, precisely the high torque required by the facing head which makes it necessary to adopt satisfactory machine design solutions to guarantee the availability of such high torque on the spindle, thereby increasing the costs.
Existing large size facing heads are inherently balanced because they are equipped with two symmetrical slides which, when driven, move in two opposite directions. This balance is important because it allows limiting the extent of the centrifugal forces which depend on the speed of rotation and which affect machining accuracy. The solution with two slides is not used in small and medium-sized heads, however, and this has the disadvantage of producing unbalance on account of the eccentricity of the single slide during its movement. This generates forces which may be quite considerable and which depend on the radial position of the slide and on the speed of rotation.
Existing heads thus suffer from the above mentioned disadvantages which, to sum up, entail: reduced accuracy in positioning the slide, limitations on the speed attainable, limitations on the torque available to the head itself and unbalances which reduce machining accuracy when working with a single slide.
Disclosure of the invention
This purpose of this invention is to overcome the above mentioned disadvantages suffered by existing facing heads.
The main aim of this invention is to increase slide positioning accuracy, allowing positioning in both directions and continuous interpolation machining.
A further aim of the invention is to increase the available rotation speed of the head.
Another aim of the invention is to increase the torque available on the head in relation to the torque made available on the spindle of the machine tool.
A further aim of the invention is to provide, along the axis of the facing head, the space needed for placing the electrical and fluid collectors which are small in diameter and therefore have high reliability and reduced cost. A further aim of the invention is to reduce the extent of the unbalancing forces in single-slide heads.
Further aims are achieved according to the invention by a facing head equipped with one or more reduction gear trains which transmit motion from the spindle to the rotating part of the head.
These aims and others, which shall become apparent from the description that follows, are achieved, in accordance with the invention, by a facing head for machine tools with one or two slides, characterized in that:
- the radial position of the slide or of the two slides is measured by a transducer mounted on the rotating part of the facing head,
- the slide is moved radially by a drive system mounted on the rotating part of the facing head,
- the rotational motion from the spindle of the machine tool to the rotating part of the facing head is transmitted by one or more gear trains.
Brief description of the drawings
The invention is clarified in further detail below in the description of a non- limiting example embodiment of it with reference to the accompanying drawings, in which:
- Figure 1 is a schematic axial cross section of the facing head with direct rotational drive and with measuring and drive systems mounted on the rotating part of the facing head, which is equipped with annular electrical and fluid collectors;
- Figure 2 is a schematic transversal cross section of the rotating part of
the facing head showing the drive and measuring systems mounted on the rotating part of the facing head;
- Figure 3 is a schematic axial cross section of the facing head showing two gear trains for transmitting rotation to the facing head;
- Figure 4 is a schematic axial cross section of the facing head showing the annular electrical and fluid collectors mounted in the central part of the facing head;
- Figure 5 is a schematic transversal cross section of the rotating part of the facing head showing the slide balancing solution;
- Figure 6 is a schematic side view of the rotating part of the facing head showing the slide balancing solution.
Detailed description of preferred embodiments of the invention
Figure 1 shows a cross section through the axis of rotation of a facing head 1 applied to the head of a machine tool 2 by means of the interface plate 14, the facing head essentially comprising the fixed part 3 and the rotating part 4 which supports the radial slide 5, the rotary part being supported by the bearing 6.
Transmission of rotation to the facing head is direct and is accomplished by the machine spindle 15 through the keys 16 to the facing head flange 8 which is integral with the central shaft 7 which sets in rotation the rotating part 4 of the facing head.
The electrical connection between the fixed part and the rotating part is made by means of an annular electrical collector whose rotating part 12 is fixed to the rotating shaft 7 of the facing head and which is connected through the electrical connections 13 to the electric motor, to the transducer, to the limit switches and to all the other electrical rotating parts of the facing head.
The fixed part 0 of the collector is fixed to the fixed body 3 of the head and is connected through the electrical connections 11 to the apparatus
which feeds and controls the movements of the slide according to the prior art.
Through the rings of the rotating collector are transmitted all the signals for driving and controlling the radial movement of the slide and the signals of the limit switches, while the power supplies are connected through the collector to the motor and, when necessary, also to the brake. The electrical collector is provided with suitable protections to prevent grease or other substances from adversely affecting operation.
Also schematically represented in the drawing are the screw 25, supported by the fixed support 24 anchored to the rotating part of the facing head, and the lead nut 26 fixed to its support 27 which is fixed to the slide 5. Rotation is imparted to the screw through the gear 23.
Figure 2 is a transversal cross section of the rotating part of the facing head, showing the devices for driving and measuring the radial translation of the slide 5.
Fixed in the rotating part 4 of the facing head is the motor 20 which, through the reduction unit 21 and the gears 22 and 23, transmits rotation to the screw 25 which is supported by the support 24 which is also integral with the rotating part of the facing head.
The lead nut 26 of the screw 25 is integral with the lead nut support 27 fixed to the slide 5, the screw being driven by the servomotor 20 through the reduction unit 21 , and to the gears 22 and 23. The radial position of the slide is measured by the linear transducer 29 whose ruler is fixed to the rotating part 4 and whose reading head is fixed to the lead nut support which is integral with the slide 5. Also fixed to the rotating part of the facing head is the two-button limit switch which is actuated by two cams integral with the slide.
Figure 3 shows a cross section along the axis of rotation of the facing head whose rotating part 4 is supported by the bearing 6 and where
rotation is not imparted directly by the spindle but through the agency of gears according to the invention.
The spindle 15 of the machine tool 2, interfaced with the facing head through the interface plate 14, sets in rotation the input shaft 8 supported by the oblique contact bearings 33, and the input shaft is integral with the input gear 34. The input gear meshes with the two gears 35 fixed to the shafts 39 which are integral with the gears 36, both of which mesh with the output gear 37 which is made integral with the rotating part 4 of the facing head through a flange screwed thereto.
Inside are the fluid collector 39 and the electrical collector 40, which are both located along the axis of the facing head and through which all the fluid conduits are connected and all the signals for controlling the radial movement of the slide are transmitted.
Figure 4 shows the fluid collector, comprising the rotating part 41 which is integral with the facing head and which has inside it the holes allowing connection of the fluids (refrigerant, air, grease, ...) to the collector, and the fixed part 42 coupled to the internal fluid conduits and which connects them to the fixed fluid conduits 45, which are connected by way of holes to the fixed ring 43. The fixed ring 43 is linked to the fixed part of the facing head by way of a rod which has holes inside it to carry all the connections to the machine interface plate.
The rotating part of the fluid collector is internally perforated so as to allow the passage of the electrical cables 49 which are connected to all the electrical devices (motor, transducer, limit switches, ...) located on the rotating part of the facing head.
The electrical collector comprises the rotating part 47 which rotates with the facing head and which receives all the electrical connections, and the fixed part 48 which is fixed and therefore anchored to the fixed ring 43. Through the fixed ring and the cables 51 connected thereto are made all
the electrical connections up to the fixed connections to the interface plate of the facing head.
Figures 5 and 6 show the facing head slide balancing system comprising pinions 55 which mesh with the teeth 52 made on the two sides of the slide 5 and which also mesh with the teeth 54 made on two cylinders 53, which, when the slide is retracted, remain inside the facing head for their full length. When the slide moves outwards, the two pinions 55 cause the two cylinders 53 to move in the opposite direction. Since the total weight of the two cylinders is approximately equal to the weight of the slide, the centrifugal forces acting on the slide as it rotates are balanced.
It will now be shown that the invention overcomes the disadvantages initially mentioned.
The main aim of this invention is to considerably increase slide positioning accuracy, allowing slide positioning in both directions and interpolation machining. This is achieved by adopting a slide position transducer which is applied directly between the slide and the rotating part of the facing head and also thanks the fact that the motor is also located on the rotating part of the facing head, thereby minimizing backlash that would, instead, be produced in a long transmission system.
A further aim of the invention is to increase the speed of the head. This aim is achieved by avoiding use of the differential mechanism which is usually used to drive the slide and which limits the maximum rotation speed of the facing head. The higher speed is also made possible for small and medium-size facing heads when they too are equipped with this slide balancing system.
Another aim of the invention is to increase the torque available on the facing head in relation to the torque made available on the machine tool. This aim is achieved when the facing head is applied to a machine with a relatively low torque at the spindle. In this case, the invention allows
adopting a suitable reduction ratio, for example 2:1 , which allows the torque available on the facing head to be doubled.
A further aim of the invention is to prevent unbalancing of single-slide facing heads and this aim is achieved by adopting the balancing weights. Slide balancing may or may not be applied to the facing head, meaning that it is an optional feature.
There are many alternative embodiments of the invention which can be adopted without departing from the scope of the invention and some of which are described below.
In one alternative embodiment, the rotating part of the facing head is driven in rotation directly by the spindle of the machine tool by way of a central transmission shaft, the facing head is supported by one or two bearings and the annular collector is located on the outside of this shaft. In another alternative embodiment of the invention, a rotary transducer is coupled, directly or indirectly by means of gears, a toothed belt or other transmission system, to the rotation of the radial drive screw of the slide. In another alternative embodiment, a single transmission gear train is used instead of two gear trains, or more than two gear trains, for example three or four, are used. Obviously, the use of at least two gear trains makes the loads acting on the input and output gears symmetrical.
In yet further embodiments of the invention, the radial drive system of the slide comprises a rack and pinion system or a motor applied directly to the slide.
All these embodiments can also be adapted to facing heads with two slides, with some considerations and adaptations which are simple to apply. In particular, the solutions where drive motion is transmitted directly by the spindle or by means of gear trains are applied in the same way. In facing heads with two slides, the fluid and electrical collectors are annular in shape in facing heads where rotational transmission is direct and located on the outside of the transmission shaft, whilst the collectors are
placed in the central zone in facing heads where rotation is transmitted through one or more gear trains.
The two slides are driven radially by a single motor which moves both of the screws for transmitting the radial motion of the two slides, whereas the measuring system may be rotary and coupled to the rotation of the two screws or it may be applied directly to one of the two slides.
Further alternative embodiments of the invention are imaginable without departing from the scope of protection of the invention.