GB2317354A

GB2317354A - Workpiece positioning apparatus

Info

Publication number: GB2317354A
Application number: GB9619637A
Authority: GB
Inventors: Mervyn Paul Head
Original assignee: DALPO Ltd
Current assignee: DALPO Ltd
Priority date: 1996-09-20
Filing date: 1996-09-20
Publication date: 1998-03-25
Anticipated expiration: 2016-09-20
Also published as: GB2317354B; GB9619637D0

Abstract

A workpiece positioning apparatus comprises a turntable 12, gripping means, drive means 32 for turning the table (in the plane of the table) about a first axis and a tilt actuator 49 for tilting the table about a second axis perpendicular to the first axis. In use, as the orientation of the table changes from the horizontal (Fig 1) to a more inclined position (Fig 3) and thus the eccentric load exerted by the workpiece increases, the tilt actuator 49 exerts a greater moment/torque due to the increase in effective lever arm. Preferably the tilt actuator comprises a screw jack actuator 49, a mounting 48 and an extendable arm 50 pivotally coupled to a lever plate 54. The increasing torque of the tilt actuator is designed to enable the apparatus to handle workpieces of a substantial mass.

Description

APPARATUS FOR POSITIONING A WORKPIECE The present invention relates generally to apparatus for positioning a workpiece.

Workpiece positioning apparatus is known as such. Prior art workpiece positioners include those having a rotatable table with gripping means by which the workpiece may be retained in place on the table and drive means by which the table may be rotated about an axis generally orthogonal to the plane of the table whereby to achieve orientation adjustments of the workpiece about this axis, which will be referred to hereinafter as the first axis of adjustment.

Prior art workpiece positioners of this type find particular utility in a range of engineering applications, particularly in connection with automatic welding apparatus where it is necessary not only to locate a workpiece in a convenient orientation for access thereto by a welding head of automatic welding apparatus, but also to be able to move the workpiece, particularly to rotate it, in a regular and predictable manner to allow automatic welding to proceed. Such welding positioners must be capable of retaining a high degree of precision in use in order to be able to perform the mechanical and/or welding operations accurately and repeatably. They must also be capable of withstanding considerable forces in view of the fact that the workpieces may be of substantial size. Typically, in engineering works, workpieces to be welded may weigh up to 5 tonnes (5,000 Kg) or more. In addition to rotation about its first axis, the welding positioner table must be turnable about a second axis parallel to the plane of the table and perpendicular to the first axis with a range of movement extending over at least 900 between a first position in which the table is substantially horizontal and the first axis is substantially vertical and a second position in which the plane of the table is substantially vertical and the first axis is substantially horizontal. In this latter position the moment about the second axis exerted by a workpiece of substantial weight may be very considerable and it is important that the drive means, and the drive transmission by which movement of the positioner about the second axis is achieved, should be capable of withstanding these forces and, moreover, of adjusting the position of the workpiece table about this axis with a high degree of accuracy.

One known workpiece positioner of this type used for automatic welding operations has a drive transmission for rotation about the first axis comprising a ring gear engaged by a pinion driven by a motor the axis of rotation of which is parallel to but offset from the said first axis. The ring gear is located on the rear face of the workpiece table. Backlash between the pinion and the ring gear inevitably introduces positioning errors, and this type of transmission has disadvantages in being slow to react to command signals and is inherently inaccurate.

Rotation of the workpiece table about the second axis is achieved, in the known prior art positioner, by providing a sub-frame on which the table is mounted with a toothed quadrant the teeth of which are engaged by a pinion driven by a separate motor. Again, the forces exerted by such a mechanism are not well suited to withstanding the loads imposed by a heavy workpiece and, in particular, the moment applied by the drive transmission is the same regardless of the orientation of the workpiece table about the second axis, so that when the workpiece table lies in a substantially vertical plane the forces transmitted between the quadrant and the pinion are at a maximum and very often exceed the force which can be applied by the motor. This places constraints on the range of movement available from such positioners when heavy workpieces are involved and is an unacceptable limitation.

The present invention seeks to provide a workpiece positioner capable of a full range of adjustment motion regardless of the weight of the workpiece, and which will provide a high degree of accuracy in its movement so that high precision operations can be performed reliably.

According to a first aspect of the present invention there is provided apparatus for positioning the workpiece, of the type comprising a workpiece table turnable about a first axis generally perpendicular to the plane of the table driven by drive means selectively to turn in one directional sense or the other about the said one axis, workpiece gripping means for securing a workpiece in place on the table, and table mounting means by which the table is tiltable about a second axis generally parallel to the plane of the table and perpendicular to the said first axis, in which the drive means for turning the table about the said first axis includes drive transmission means operable to apply torque to a shaft the longitudinal axis of which is substantially coincident with the said first axis.

By contrast with the prior art positioner, in which the rotation of the table about its first axis is achieved by an offset motor pinion driving a ring gear, the direct application of torque to a central shaft of the turntable makes it possible for this to be turned accurately and rapidly about the first axis to orientate a workpiece in any desired position about this axis.

The workpiece table may be turned about the said second axis by a tilt actuator orientated and positioned such that the moment exerted thereby about the said second axis increases as the orientation of the workpiece table varies to increase the eccentric load exerted by a workpiece carried thereby.

This configuration overcomes the disadvantage of prior art positioners which, because a toothed quadrant and pinion drive transmission is used, offer no variation in the moment. Therefore the risk of stalling the motor at high load orientations is avoided due to the natural increase in moment at appropriate orientations. This allows an actuator of modest force to resist even the large turning moments exerted when the table is turned to lie in a substantially vertical plane whilst carrying a heavy load.

In a preferred embodiment of the invention the said tilt actuator is a linear actuator. In one embodiment the linear actuator is a screw actuator having a lead screw and nut driven by an electric motor. Alternatively the linear actuator may be a fluid pressure actuator.

The drive means for turning the workpiece table may comprise motor means carried by a sub-frame on which the workpiece table is carried, the motor means being located in a position offset from the first axis in a direction along the said second axis.

This has a particularly advantageous consequence that the motion of the motor and the drive transmission between the motor and the table shaft is minimised upon turning of the whole sub-frame assembly carrying the workpiece table about the said second axis.

Indeed, it is preferred for this reason that the workpiece table is carried on a sub-frame pivotally supported on a base frame for turning movement about the said second axis, and that the drive means comprise a motor and drive transmission means carried on the said sub-frame, the drive means being turned about the said second axis with the workpiece table. By mounting the entirety of the drive transmission and motor in this way it is possible to achieve a compact mechanism which can nevertheless apply sufficient force to rotate the table about the first axis in any orientation without risk of stalling the motor.

In embodiments such as those outlined above in which the workpiece table is turned about the said second axis by a tilt actuator in the form of a screw jack it is preferred that the actuator has a body pivotally mounted on the said base frame or a component carried thereby, for turning movement about an axis parallel to the said second axis, and an extendable rod or arm pivotally connected to the said sub-frame or a member carried thereby. Typically, the member carried thereby constitutes a short lever arm.

In a preferred embodiment of the invention the said linear actuator is located in a position offset from the first axis in a direction along the said second axis and on the opposite side of the said first axis from the said drive means. The moment applied about the second axis by the actuator is then determined by the length of the short lever arm in the orientation with the work table horizontal, namely with a minimum moment about the second axis, whilst when the workpiece table is turned through 90" to lie in a vertical plane, and thus when any workpiece carried thereby exerts a maximum turning moment, the moment exerted by the actuator is determined by the distance between the axis and the location of the short lever arm which is substantially greater than the length of the short lever arm thereby increasing the moment about the second axis exerted by the linear actuator for the same force.

The drive means for rotating the workpiece table about the said first axis preferably comprises a motor and drive transmission in the form of a reduction gearbox.

Other possible drive transmissions may include bevel gears, a train of pinions or a belt of chain transmission. The motor and reduction gearbox configuration is preferred, however, because it more directly applies torque to the shaft.

The present invention also comprehends, according to another aspect, apparatus for positioning a workpiece, of the type comprising a workpiece table turnable about a first axis generally perpendicular to the plane of the table driven by drive means to rotate selectively in one directional sense or the other about the said one axis, workpiece gripping means for securing a workpiece in place on the table, and table mounting means by which the table is tiltable about a second axis generally parallel to the plane of the table and perpendicular to the said first axis, in which the orientation of the workpiece table about the said second axis is controlled by means of a tilt actuator linked to the workpiece table in such a way that the moment about the second axis exerted by the tilt actuator is greater when the workpiece table is tilted to an orientation in which the moment about the second axis exerted by a workpiece carried thereby is greater, and lesser when the workpiece table is in an orientation in which the corresponding moment exerted by the workpiece is lesser.

One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of the main structural components of a workpiece positioning apparatus formed as an embodiment of the present invention; Figure 2 is a sectional view taken on the line II-II of Figure 1; and Figure 3 is a view similar to that of Figure 1 showing the workpiece table tilted to a different orientation.

Referring now to the drawings, the workpiece positioner is generally indicated with the reference numeral 11 and comprises a workpiece support in the form of a planar generally circular support table 12 which is provided with gripping means (not shown) by means of which a workpiece may be secured in place on the table. The gripping means have been omitted for clarity and may by of any conventional type suitable for retaining a workpiece to a flat table, and in particular may include radially displaceable clamping jaws mounted captive in slots in the table or its support.

The workpiece table 12 is carried on a central shaft 13 which is borne in a main bearing generally indicated 14 for rotation about a first axis A-A (see Figure 2).

The bearing 14 is carried in a sub-frame generally indicated 15 comprising two parallel pivoted frame beams 16, 17 joined by transverse sub-frame cross members 18, 19 to form a rectangular frame. The sub-frame cross members 18, 19 are spanned by two subsidiary beams 20, 21 from which depend two struts 23, 24 extending orthogonally of the plane defined by the sub-frame 15 and spanned at their free ends by a plate 25 which carries an end bearing 26 for the shaft 13 on which the workpiece table 12 is carried.

The workpiece table 12 itself is reinforced by a plurality of radial arms 27, 28; 29, 30 which lie beneath the workpiece table 12 in the space between the table and the sub-frame 15.

Suspended from the struts 23, 24 is a drive and transmission device generally indicated 31, comprising an electric motor 32 and a reduction gearbox mechanism represented by the casing 33 housing gear wheels for transmitting drive from the motor 32 to the shaft 13 with a suitable gear reduction to allow appropriate torque and speed control. The motor 32 may be a DC stepping motor or an AC motor allowing indexing of the shaft 13 and therefore of the workpiece table 12 to be effected so that accurate positioning of a workpiece carried by the table 12 about the axis A-A can be effected be electronic control. If appropriate the motor drive shaft may be provided with an encoder.

The entirety of the sub-assembly comprising the sub-frame 15 and the drive/transmission mechanism 31 is pivotal about a second axis B-B defined by pivotal connections of the beams 16, 17 of the sub-frame 15 to a main support frame generally indicated 33. The main support frame 33 comprises four upright legs 34 joined by two transverse lower frame members 37 and two longitudinal cantilever frame members 35, 36. Pad-like feet 38 at opposite ends of the cross members 37 and cantilever members 35, 36 support the fixed frame assembly 33 on the ground.

At their upper ends the legs 34 carry two horizontal cantilever beams 39, 40 which at their free ends 41 (only the free end of cantilever beam 39 is visible in Figure 1) are reinforced by diagonal struts 42, 43 extending from the adjacent legs 34.

A further transverse beam 45 spans the cantilever beams 39, 40 at a location between the front and rear legs 34 (see Figure 1), from which beam depend two parallel support plates 46, 47 between which is pivoted a mounting 48 carrying a screw jack actuator 49 the extendable arm 50 of which is pivotally coupled at its free end 51 by a pivot 52 to a bifurcated end 53 of a lever plate 54 securely bolted or otherwise fixed to the cross member 18 of the sub-frame 15. The screw jack actuator 49 is a lead or ball screw mechanism operated by an electric motor. The electrical connections and electronic control components have not been shown.

In use of the welding positioner 11 described hereinabove a workpiece is placed on the workpiece table 12 and secured thereto by the gripping means (not shown) in a desired orientation. When it is desired to operate on the workpiece in an orientation other than that in which it was originally placed the electric motor 32 is energised to turn the workpiece table 12 about the axis A-A in either directional sense to a desired orientation and/or the motor controlling the screw jack 49 is appropriately operated to shorten or extend the arm 50 to cause the workpiece table 12 carried on the sub-frame 15, together with the motor and transmission 31 about the axis B-B. Appropriate control of the command signals allows the workpiece to be oriented in any convenient orientation for the operation to be performed. Should it be appropriate the command signals may be continuous, allowing controlled displacement of the workpiece, especially, for example, rotation about the axis A-A during the operation to allow work to be performed around the entire surface of the workpiece. As can be seen in Figure 3, when the table 12 is tilted from its horizontal position to an inclined position such that a workpiece secured thereon would exert a moment about the axis B-B, the moment or torque applied about the axis B-B by the actuator 49 is greater, for a given force applied by the actuator 49 than in the horizontal orientation due to the increase in the effective lever arm represented by the dimension L in Figure 3 in comparison with the corresponding dimension 1 in Figure 1.

This positioner allows high precision movements to be imparted to the workpiece table 12 rapidly and effectively even with workpieces of substantial mass.

Claims

1. Apparatus for positioning a workpiece of the type comprising a workpiece table turnable about a first axis generally perpendicular to the plane of the table driven by drive means selectively to turn in one directional sense or the other about the said one axis, workpiece gripping means for securing a workpiece in place on the table, and table mounting means by which the table is tiltable about a second axis generally parallel to the plane of the table and perpendicular to the said first axis, in which the drive means for turning the table about the said first axis includes drive transmission means operable to apply torque to a shaft the longitudinal axis of which is substantially coincident with the said first axis.

2. Apparatus as claimed in Claim 1, in which the workpiece table is turned about the said second axis by a tilt actuator oriented and positioned such that the moment exerted thereby about the said second axis increases as the orientation of the workpiece table varies to increase the eccentric load exerted by a workpiece carried thereby.

3. Apparatus as claimed in Claim 2, in which the said tilt actuator is a linear actuator.

4. Apparatus as claimed in Claim 3, in which the linear actuator is an electrically driven screw type actuator.

5. Apparatus as claimed in any preceding claim, in which the drive means for turning the workpiece table comprise motor means carried by a sub-frame on which the workpiece table is carried, the motor means being located in a position offset from the said first axis in the direction along the said second axis.

6. Apparatus as claimed in any preceding claim, in which the workpiece table is carried on a sub-frame pivotally supported on a base frame for turning increment about the said second axis, and the drive means comprise a motor and drive transmission means carried on the said sub-frame, the device means being turned about the said second axis with the workpiece table.

7. Apparatus as claimed in Claim 6 when dependent on Claim 3 or Claim 4, in which the screw type linear actuator is mounted on the said base frame or a component carried thereby, for turning movement about an axis parallel to the said second axis, and an extendable rod or arm pivotally connected to the said sub-frame or a member carried thereby.

8. Apparatus as claimed in Claim 7, in which the said linear actuator is located in a position offset from the said first axis in a direction along the said second axis and on the opposite side of the said first axis from the said drive means.

9. Apparatus for positioning a workpiece, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

9. Apparatus as claimed in any preceding claim, in which the said drive means comprise a motor, and drive transmission to the said workpiece table comprises a reduction gearbox transmission.

10. Apparatus for positioning a workpiece, of the type comprising a workpiece table turnable about a first axis generally perpendicular to the plane of the table driven by drive means to rotate selectively in one directional sense or the other about the said one axis, workpiece gripping means for securing a workpiece in place on the table, and table mounting means by which the table is tiltable about a second axis generally parallel to the plane of the table and perpendicular to the said first axis, in which the orientation of the workpiece table about the said second axis is controlled by means of a tilt actuator linked to the workpiece table in such a way that the moment about the second axis exerted by the tilt actuator is greater when the workpiece table is tilted to an orientation in which the moment about the second axis exerted by a workpiece carried thereby is greater and lesser when the workpiece table is in an orientation in which the corresponding moment exerted by the workpiece is lesser.

11. Apparatus for positioning a workpiece, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

Amendments to the claims have been filed as follows 1. Apparatus for positioning a workpiece of the type comprising a workpiece table turnable about a first axis generally perpendicular to the plane of the table driven by drive means selectively to turn in on directional sense or the other about the said one axis, workpiece gripping means for securing a workpiece in place on the table, and table mounting means by which the table is tiltable about a second axis generally parallel to the plane of the table and perpendicular to the said first axis, in which the drive means for turning the table about the said first axis includes drive transmission means operable to apply torque to a shaft the longitudinal axis of which is substantially coincident with the said first axis, and in which the workpiece table is turned about the said second axis by a tilt actuator oriented and positioned such that the moment exerted thereby about the said second axis increases as the orientation of the workpiece table varies to increase the eccentric load exerted by a workpiece carried thereby.

2. Apparatus as claimed in Claim 1, in which the said tilt actuator is a linear actuator.

,+ 3. Apparatus as claimed in Claim 2, in which the linear actuator is an electrically driven screw type actuator.

4. Apparatus as claimed in any preceding claims, in which the drive means for turning the workpiece table comprise motor means carried by a sub-frame on which the workpiece table is carried, the motor means being located in a position offset from the said first axis in the direction along the said second axis.

5. Apparatus as claimed in any preceding claim, in which the workpiece table is carried on a sub-frame pivotally supported on a base frame for turning increment about the said second axis, and the drive means comprise a motor and drive transmission means carried on the said sub-frame, the device means being turned about the said second axis with the workpiece table.

6. Apparatus as claimed in Claim 5 when dependent on Claim 3 or Claim 4, in which the screw type linear is mounted on the said base frame or a component carried thereby, for turning movement about an axis parallel to the said second axis, and an extendable rod or arm pivotally connected to the said sub-frame or a member carried thereby.

7. Apparatus as claimed in Claim 6, in which the said linear actuator is located in a position offset from the said first axis in a direction along the said second axis and on the opposite side of the said first axis from the said drive means.

8. Apparatus as claimed in any preceding claim, in which the said drive means comprise a motor, and drive tr nsmission to the said workpiece table comprises a reduction gearbox transmission.