MACHINE TOOL CONTROL & MEASUREMENT SYSTEM

A computer numerical control machine tool with a machine head having one or more process tools. The machine head is moveable with respect to a workpiece. A position sensor system detects the position of the machine head relative to the workpiece. A controller coupled to the position sensor system provides closed loop control of the position of the machine head relative to the workpiece. The position sensor system includes an ultrasonic probe. Also a method of controlling the position of a computer numerical control machine tool.

FIELD OF THE INVENTION

The present invention relates to a control and measurement system for a machine tool such as a computer numerical control machine tool, and a method therefor.

BACKGROUND OF THE INVENTION

Computer numerical control (CNC) machine tools are used widely in manufacturing industries to perform automated production tasks. CNC machine tools typical have a machine head which can carry process tools for a variety of processes, such as drilling, cutting etc. and these process tools may be interchangeable. Once a workpiece is installed the CNC machine tool can be programmed to perform a series of steps on the workpiece to produce a part in a highly automated process with great accuracy.

In the aircraft manufacturing industry it is known to use a CNC machine tool in the manufacture of an aircraft wing. A typical aircraft Wing construction includes joining wing covers (or skins) to longitudinal spars and transverse ribs. The covers may have longitudinal stiffeners (known as stringers) attached or integrally formed therewith prior to assembly with the ribs and spars to form a “wing box”.

In the process of assembling the wing box all of the ribs and spars needed to form the complete wing box are typically arranged in a jig to ensure dimensional accuracy. The covers are then offered up to the ribs and spars in the jig and then drilled before final fasteners are installed to fix the covers to the ribs and spars. To avoid unwanted pre-stresses and to ensure that the completed wing box conforms to the design shape the covers, ribs and spars are not pre-drilled prior to assembly. Also, the cover must be drilled from the side that will form the aerodynamic surface due to machine tool accessibility constraints and, in the case the cover comprises laminate composite materials, to ensure that any drill hole exit delamination is not on the outer aerodynamic surface.

Thermal expansion and manufacturing tolerances make it difficult to locate the CNC machine tool for performing the drilling operations to align with the rib feet and spar flanges hidden behind the cover. The rib feet particularly may have only a small land around the intended fastener hole location so accurate fastener positioning is crucial.

A CNC machine tool in current use for wing box assembly uses a magnetic field sensor to align the machine tool with magnets manually installed in blind holes formed at the desired fastener locations on the inner surface of the wing cover, i.e. the side opposite the outer surface that will form the aerodynamic surface. This solution provides the required positioning accuracy but has a drawback in that the installation and subsequent removal of very many magnets on the wing cover makes use of the CNC machine tool a two-man operation.

The present invention aims to solve this problem to facilitate one-man operation of the machine tool.

SUMMARY OF THE INVENTION

The invention relates to a computer numerical control machine tool comprising a machine head having one or more process tools and moveable with respect to a workpiece, and an ultrasonic probe. The ultrasonic probe may be used to measure material thickness of the workpiece for positioning the machine head relative to the workpiece and/or for measuring exit delamination of a hole formed by the machine tool.

A first aspect of the invention provides a computer numerical control machine tool comprising a machine head having one or more process tools and moveable with respect to a workpiece, a position sensor system for detecting the position of the machine head relative to the workpiece, and a controller coupled to the position sensor system for closed loop control of the position of the machine head relative to the workpiece, wherein the position sensor system includes an ultrasonic probe.

A further aspect of the invention provides a method of controlling the position of a computer numerical control machine tool, the machine tool comprising a machine head having one or more process tools and moveable with respect to a workpiece, a position sensor system including an ultrasonic probe, and a controller coupled to the position sensor, and the method comprises detecting the position of the machine head relative to the workpiece with the position sensor system, and closed loop control of the position of the machine head relative to the workpiece with the controller.

The invention is advantageous in that the ultrasonic position sensor system can detect the location of a feature of the workpiece on a side of the workpiece opposite to that where the machine tool is located but without requiring positioning devices, such as magnets or machine readable sensors, to be installed by a second operator. In this way the machine tool facilitates single operator operations. The absence of positioning magnets such as those currently used in the art may enable machine processing of materials such as Titanium which are known to distort the magnetic field of such magnets and until now have required alternative processing techniques.

The position sensor system may be configured to use the ultrasonic probe for measuring material thickness of the workpiece.

The position sensor system may be configured to measure the material thickness in the vicinity of a predetermined location on the workpiece.

The position sensor system may be configured to measure the material thickness of the workpiece by scanning along at least one axis.

The scan may be performed along two orthogonal axes.

The position sensor system may be configured to determine a geometric centre of a workpiece feature having a material thickness different to that of the workpiece feature environs. For example, the position sensor system may be used to determine the centre of a blind hole formed in the workpiece. Alternatively the position sensor may be used to determine the centre of a land around an intended fastener location.

The position sensor system may be configured to determine the material thickness profile of a workpiece feature. For example, where the workpiece feature is a blind hole the material thickness at the blind hole will be reduced compared to the surrounding material thickness.

The machine tool may be operable to automatically perform a process machining operation on the workpiece once the machine head has been automatically aligned with workpiece using the position sensor system.

The machine tool may be a multi-axis machine tool for fully automated drilling of holes for final fasteners in aircraft wing box components in an aircraft wing box assembly jig.

DETAILED DESCRIPTION OF EMBODIMENT(S)

FIG. 1shows a side view of an aircraft wing box jig1with a plurality of ribs2(only one of which is visible inFIG. 1) coupled to front and rear spars3installed in the jig1and ready to receive wing covers4. The wing covers4are attached to rib feet2aof the ribs2by an automated process using a computer numerical control (CNC) machine tool5, shown inFIG. 2.

The machine tool5is a 5-axis machine tool configured for fully automated drilling and slave bolt insertion for assembling the wing covers4to the rib feet2a. The machine tool is designed to produce holes for final fasteners to fasten the wing covers4to the ribs2. The machine tool may perform additional operations, such as drilling off holes for engine pylon reinforcings, etc. The machine tool is in essence a travelling column post mill with a 5-axis machine head6. The machine head is configured to provide a gap closing pressure, e.g. to press the wing cover4onto the rib feet2a,during machine processing, and also has the capability to bring one of a variety of process tools to the tool point for drilling and fastening operations.

The machine tool5is mounted upon rails (not shown) for movement along the X-axis so that it is able to travel the full length of the wing assembly jig1. The machine tool5includes a machine column7having rails8for vertical movement of the machine head6along the Y-axis. Mounted to the Y-axis rails8is a Y-axis sled9having a rotary table bearing having an axis parallel to X and providing a rotation in A to accommodate cord-wise curvature of the wing cover4. A similar arrangement provides for rotation in the B-axis, around an axis parallel to Y. The machine head6sits below the A-B pivot and includes a plurality of interchangeable process tools on a shuttle table. The machine head has a thrust axis U, which is parallel to the z-axis when A is zero. Movement along all axes is servo-controlled.

The machine head includes a pressure foot10and a through-skin sensor (TSS)11. The TSS11is called automatically by the CNC machine tool5. The TSS11forms part of a position sensor system and allows an operator to locally re-zero the machine tool as desired. The TSS11may also form part of a measurement system for measuring the profile of a workpiece feature. The process tools of the machine head6may include a drill spindle, a slave bolt detection tool, a hole diameter probe, a bolt insertion tool, or any of a variety of process tools, as required. Finally, the machine tool5includes an operator platform12with an interface console13.

FIG. 3shows a partial view of one of the wing covers4having a predrilled blind locating hole14formed in its inner surface15, this being opposite its outer surface16which will form the outer aerodynamic surface of the completed aircraft wing.FIG. 4shows the wing cover4having the blind locating hole14when installed in the jig1(not visible inFIG. 4) adjacent a rib foot2aand with the machine head6in close proximity such that the TSS11is approximately aligned in the X and Y directions with the position of the blind locating hole14.

The TSS11includes an ultrasonic probe17which emits an ultrasonic wave towards the wing cover4from the side nearest the outer surface16of the wing cover4. The ultrasonic wave travels through the thickness of the wing cover4and reflects back a reflected ultrasonic wave that is distorted by the blind locating hole14due to the differing material thickness of the wing cover at the location of the blind locating hole14as compared with the surrounding material thickness. The reflected ultrasonic wave is received at the ultrasonic probe17.

The output from the TSS11is coupled to a controller of the machine tool position sensor system which analyses the reflected ultrasonic wave to compute the precise location of the blind locating hole14in the X-Y-Z coordinate system by closed loop control. In the preferred embodiment the position sensor system uses the TSS11to measure the material thickness of the wing cover by scanning along the X and Y axes so as to determine a geometric centre of the blind locating hole14. Since the blind locating hole14is predrilled in the wing cover4prior to assembly in the jig1the machine tool5can be precisely locally re-zeroed off the blind locating hole14.

Turning next toFIG. 5the machine tool6, having been locally re-zeroed, is aligned such that one of the process tools, in this case the drill spindle18, is precisely aligned with the desired fastener hole location for fastening the wing cover4to the rib foot2a.As shown inFIG. 6the machine head6is advanced such that the pressure foot10contacts the outer surface16of the wing cover4to apply a gap closing pressure to the wing cover4. This causes the inner surface15of the wing cover4to bear against the rib foot2a.The drill spindle18is then advanced along U to drill a hole through the wing cover4and the rib foot2a.

Following retraction of the drill spindle18and the machine head6the completed through-hole19is shown inFIG. 7. One or more further processes may be performed by the machine tool, such as countersinking, deburring, etc., before a final fastener20is installed to fasten the wing cover4to the rib foot2a,as shown inFIG. 8. The deburring may be performed manually after the cover4has been removed and prior to fastener installation.

The machine tool may also use the TSS11to measure exit delamination of the hole drilled by the drill spindle18where the rib foot2a,the cover4, or other workpiece component(s), are laminate components, e.g. comprising fibre reinforced composite. The machine tool may position the TSS11adjacent the drilled hole19, measure the profile of the drilled hole, and output a measurement of the extent of any exit delamination around the periphery of the hole. Further processing, by the machine tool5or manually, may be performed to address any excessive exit delamination prior to installation of the final fasteners.

As an alternative to picking up the location of the pre-drilled blind hole, the position sensor system may use the TSS11to pick up the edge profile of the rib foot2a,or other workpiece feature. This may obviate any need for the pre-drilled blind holes.

The machine tool5may use the TSS11to scan in orthogonal directions, or may scan in one direction only, or may scan in multiple directions. The position sensor system may use the TSS11to locate the centre of a thinnest part of the workpiece, or a workpiece feature edge.