QMT Features: September 2007
Visionary measurement
Large sheet metal parts for aircraft parts inspected rapidly and with high accuracy against CAD drawings


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 shop floor to be used by the router operators.
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 ±  25 microns.  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  3.0 mm thick) components after they have been routed and drilled is being driven by increasing use in aircraft manufacture of predeterminate 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 predeterminate assemblies, we need to  be able to inspect to positional tolerances of five thousandths of an  inch.

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 CNC 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.  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.

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 shopfloor 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 minutes 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.l

E-mail: sales@pressandshear.com 

www.pressandshear.com
  
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Rob Tremain Photographer
www.4exposure.co.uk
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