Achieving Metal-Like Tolerances In Composite Parts

On-machining probing compensates for uncertainty in machining operations in composites

CAD/CAM software supplier Delcam and F1 composite components supplier have worked together to develop processes for machining composites which, according to Delcam, can produce composite components to levels of accuracy rarely seen in the industry. Crosby Composites owner Paul Crosby is using the new technology to finish machine CFRP parts to tolerances between 0.1 and 0.25 mm.

Many composite components are made to approximate sizes and are then trimmed to their final size manually during assembly. This process is both time-consuming and inaccurate. Delcam says composite manufacturers will eventually need to produce parts to levels of accuracy similar to metal components, in order for composites to achieve their full market potential.

One of the key problems with composites is that 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. In the new Delcam process, the initial machining operation on the component is followed by inspection using a probe on the machine tool and the company’s PowerInspect “On-Machine Verification” software. This system shows how much more material needs to be removed and so enables the required extra toolpaths to be generated in the PowerMill CAM system.

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 benefit is that all the machining and inspection can be completed on the machine tool on a single setup. 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 by Crosby Composites with this method was supplied to one of the F1 teams. The parts all fit 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.

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