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You only have to open the product type window and select those you wish to keep with metric measurement. Once your choices are saved, these products will be imported in metric in your next import. Since the selection is by product type, it is also possible to keep a certain product in imperial in order to ease your transactions with your suppliers using a different unit of measurement.
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However, note that it is only possible to work in metric if your CAD/CAM is set up with this unit of measurement. Cienapps does not make the conversion from imperial to metric if your CAD/CAM is already set with imperial measurement.