Provided is an expandable planar sheet (142) provided with a pattern of cuts arranged such that planar expansion of the sheet provides a longitudinal member, LM, assembly (140) having a set of LMs (110) for a mechanical transmission system, MTS, (100) for a steerable tool (500), which MTS (100) has a proximal end (20), a distal end (40), a transmission shaft region, TSR (132), and transmission bendable distal part (130, TBDP) adjacent to the TSR (132) that moves omnidirectionally responsive to actuation of the MTS (100) at the proximal end (20), wherein the set of longitudinal members, LMs (110) is configured to transmit actuating movement along the TSR (132) to the TBDP (130), wherein, in the expandable planar sheet (142) the set of LMs (110) are arranged mutually parallel, each LM (110) attached at a first end to a common end stop (150) that has an undulating pattern and is expandable.
