This paper presents the largest hydraulic press for open-die forging ever made in the Czech Republic. Its capacity of 170 MN combines with a workspace of 8.5-metre height and 9.8-metre width. In each case, the result is a unique machine, a prototype made to customer requirements. Designing machines of this size always involves a great deal of ingenuity and experience which cannot be taught. The paper describes designs of mechanical parts, namely methods of joining individual parts into structural details which comprise the machine frame. The designs were developed with the use of numerical modelling. The two purposes included finding the stress distribution and perform weight optimization of individual parts. The weights were near manufacturability limits.