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Features

Special Asie
Aerospace
Sport & Leisure

28 May 2008  -  United Kingdom

Software to produce high-accuracy parts

© Delcam

Switching to Delcam’s CADCAM software has helped Crosby Composites to produce composite components to levels of accuracyy. Owner Paul Crosby now uses this ability to finish machine every part to tolerances between 0.1 and 0.25mm as the key differentiator for his business.

Mr. Crosby founded his company 25 years ago and has spent all his working life in the autosport industry. He began his career as an engineer with the March F1 team, when the use of composites in racing cars was first introduced. “It was obvious that this was an opportunity that would get bigger and bigger so I started my own company,” he remembered. “We began with very basic wet lay-up methods, and moved to using pre-preg and autoclave curing as the company developed.”

Mr. Crosby introduced 3D modelling and machining around eight years ago. Before then, he had thought the systems were too expensive but CADCAM quickly became the normal way to do things. “I started with PowerSHAPE,” he recalled. “The user-friendly interface was what sold it to me. I could use it immediately for basic tasks and found it very easy to learn how to do more complex modelling operations. I then took PowerMILL on a thirty-day trial. At the end of that time, I didn’t need to think twice about adding it.”

“Since we swapped over to Delcam software, we have gone from strength to strength,” claimed Mr. Crosby. “The software is so powerful and so easy to use that it is difficult to criticise it.”

“PowerMILL is the only practical way to machine repeatably to the level of accuracy we need,” he added. “The F1 market generally requires a maximum run of sixteen parts. Unless you can be very efficient, it is difficult to justify the cost of machining everything.”

The latest addition to the range of Delcam software at Crosby has been the PowerINSPECT On-Machine Verification software. “I saw a demonstration and immediately decided we needed it,” said Mr. Crosby. “The software has been a great investment.”

With composites, machined holes and pockets tend to be undersize because the material relaxes when it is cut. This effect is difficult to predict because it is impossible to cut all the fibres in the same orientation. To overcome this problem at Crosby, the initial machining operation is followed by an inspection on the machine tool with PowerINSPECT. This shows how much more material needs to be removed and enables the required extra toolpaths to be generated in PowerMILL.

For a typical component, with between 20 and 30 holes, a further cycle of inspection and machining may be needed to produce all the dimensions to the required tolerance. However, for subsequent parts, the complete machining sequence can be repeated and the results checked with a final inspection.

Apart from the increase in accuracy possible with this approach, another big benefit is that all the machining and inspection can be completed on the machine tool on a single fixture. According to Mr. Crosby, it is impossible to maintain the necessary tolerances when moving between a series of fixtures, while using multiple set-ups on different fixtures would also take much longer.

The first set of seventeen components produced with this method was supplied to one of the F1 teams and fitted onto the car with no clashes or re-work. It was the first time in the team’s history that this had happened with any set of composite parts from any supplier.

“Since then we have used On-Machine Verification as much as we possibly can,” said Mr. Crosby. “It ensures that we can catch any mistakes before they reach our customers. In the six months since we started with this approach, we have only had one part rejected and that was because of just one undersize hole.”

Crosby has recently added its sixth CNC machine, a five-axis CMS router, and is already looking into expanding further. “We will continue to use technology to do things better and more accurately,” concluded Mr. Crosby. “Once the teams realise the level of accuracy that we can provide, they will soon switch from other suppliers that regularly send boxes of bits that don’t fit together.”

Source : Delcam