Large sheet metal parts inspected rapidly to high accuracy

Citation

(2007), "Large sheet metal parts inspected rapidly to high accuracy", Aircraft Engineering and Aerospace Technology, Vol. 79 No. 6. https://doi.org/10.1108/aeat.2007.12779fab.002

Large sheet metal parts inspected rapidly to high accuracy

Large sheet metal parts inspected rapidly to high accuracy

There can be few sectors where modern manufacturing methods have had greater beneficial impact than in checking the accuracy of sheet metal parts for aircraft. The UK's leading independent aerospace contractor, Cambridge-based Marshall Aerospace, is one company that is leading the way, having done away with the arcane, time- consuming and inaccurate process of inspecting flat components visually using foil lofts.

In its place, the company is using the latest InspecVision planar machine to be installed in the UK. Manufactured in Northern Ireland and supplied through UK agent, Press & Shear, the machine checks the profile and hole positions in sheet aluminium components to within ^25m. Other 2D parts made from titanium, steel and plastic are similarly inspected.

The need to find a more accurate way to check the 22 to 10 gauge (0.7 to 3.0mm thick) components after they have been routed and drilled is being driven by increasing use in aircraft manufacture of pre-determinate assemblies.

They require components whose machined features are so precise that they fit together perfectly, without having to use jigs on the factory floor and ream out pilot holes. Advantages include avoiding the expense and lead- time of producing jigs, faster assembly and less risk of damage, especially if the operator is working in a confined space.

Said Kevin Patterson, Manufacturing support manager at Marshall Aerospace, “To support the manufacture of pre-determinate assemblies, we need to be able to inspect to positional tolerances of 5,000 of an in.

“This is not possible for an operator to do by the conventional method of checking a sheet metal component by eye against a foil loft, which is essentially a semi – transparent sheet of film with the profile of the component traced onto it”.

Ever since copy routing of a component using a template produced from the loft profile had given way to CMC routing in the late 90s, Mr Patterson had been looking for a way to modernise the inspection process as well. It was surprisingly difficult to find a solution.

The existing co-ordinate measuring machines (CMM) at Cambridge had the precision, but it would have been difficult to fixture the sheet component off the surface of the granite to allow a touch probe to access the profile and holes. In any case, the inspection cycle would have been slow.

Complex go/no-go gauges were considered, but such a procedure would have been expensive and inflexible, especially with the large number of one-offs. A vision system seemed to be the best way forward, but a scanning solution on a CMM would have been too costly, and standard optical profile projectors on the market could not accommodate the large component sizes produced by Marshall Aerospace.

Then at the MACH 2006 show in Birmingham, Mr Patterson saw the ideal solution in the Planar machine (Figures 1-3). It could accommodate a majority of sheet metal parts produced by Marshall Aerospace, as not only is the light table large, but the control software is able to snap together two sets of results from an oversize component that has been repositioned and measured again.

Figure 1 The InspecVision Planar P65 measuring machine from Press & Shear inspecting an aircraft component at Marshall Aerospace, Cambridge. Here the machine is pictured in the inspection department, but it will soon move to the shopfloor to be used by the router operators

Figure 2 As the Planar camera is calibrated to the face of the table, a laser beam detects the thickness of the component, such as this rib system bracket for an Airbus A400M wing, and corrects for triangulation error. Alternatively, the thickness of the part can be entered manually

Figure 3 Captured by the InspecVision measuring machine at Marshall Aerospace, the profile of a sheet aluminium component (white trace) is being inspected against the Catia CAD model (green trace) that was used to route the part. Out of tolerance areas are shown in red

The machine was also relatively inexpensive, quick to use and is of simple, robust construction with no moving parts, allowing it to be used on the shop floor by the router operators. InspecVision says that, as ordinary glass and fluorescent lights are used in the machine construction, it costs only a few hundred Euros to get the system up and running again even if a component is dropped right through the table.

As the Planar machine provides a dimensional check rather than a visual comparison, first-article inspection reports can be generated to ensure that the part produced matches the design intent, which is especially useful if there has been a change in the production process. Customers can be supplied with copies of the report for their own quality audit purposes.

“It takes just a fraction of a second for the overhead digital camera to take a picture of the part and a few min for the Planar software to compare the measured data against the Catia CAD model from which the CNC routing cycle was derived” enthused Mr Patterson. “We have virtually eliminated the need for drawings, resulting in savings throughout the production process”.

The first job on the measuring machine was the cover that forms the vertical, leak-proof corners of a fuel tank. It was used to test the effectiveness of the system before the Planar equipment was purchased. Other parts have quickly followed, such as cockpit console components, wing rib brackets and leading-edge panels, and more are being put on all the time.

An additional benefit of the Planar machine is its ability to reverse-engineer replacement legacy parts when no CAD data is available, the original foil lofts are no longer available, and perhaps only undimensioned drawings exist.

The machine is able to digitise a drawing in seconds by converting it automatically into a DXF file, enabling the part to be remanufactured on a CNC router. The system even includes optical character recognition and neural networks to extract any text and dimensions if they appear on the drawing. To create an accurate DXF manually can take hours, or even days. If a drawing is not available, the legacy component itself can be flattened and measured.

Details available from: Press & Shear Machinery Ltd; Tel: +44 (0)1827 250000; Fax: +44 (0)1827 250022; E-mail: sales@pressandshear.com; web site: www.pressandshear.com>