This invention concerns apparatus for generating instructions for machines of a manufacturing chain used to manufacture a workpiece. The apparatus comprising a processor arranged to receive a model based definition (MBD) of a workpiece including geometric dimensions and tolerances; receive inputs setting an additive build design for building the workpiece based upon the geometric dimensions; generate additive instructions for an additive manufacturing machine of the manufacturing chain based upon the additive build design; determine a prospective intermediate workpiece product expected from an additive build in accordance with the additive build design; determine differences between the prospective intermediate workpiece product and the model based definition of the workpiece; and generate further instructions for at least one further machine of the manufacturing chain based upon the differences.