A tool assembly comprising: an anvil assembly defining a first longitudinal axis, the anvil assembly including an inner surface positioned to engage tissue and a proximal portion, the proximal portion including a base portion defining a longitudinal channel and a pivot member slot, the pivot member slot extending in a direction substantially perpendicular to the longitudinal channel and including an open end and a closed end; a cartridge assembly defining a longitudinal axis, the cartridge assembly including a channel member and a staple cartridge, the channel member defining a cavity and a knife slot and having a proximal portion and a distal portion, the proximal portion of the channel member including a ramp and a pivot pin positioned proximally of the ramp, the staple cartridge received within the cavity of the channel member and including a cartridge body, staples, pushers, and an actuation sled, the cartridge body defining a knife slot, the actuation sled movable through the cartridge body into engagement with the pushers to eject the staples from the cartridge body; and a drive assembly having a proximal portion and a distal portion and including a flexible drive beam and a working end, the flexible drive beam having a proximal portion and a distal portion and being movably received within the longitudinal channel of the base portion of the anvil assembly, the working end secured to the distal portion of the flexible drive beam, the proximal portion of the drive assembly defining a cam slot having a proximal portion and a distal portion, the proximal and distal portions of the cam slot defining axes (Z1, Z2) that intersect to define an acute angle β, the drive assembly movable from a retracted position to an advanced position to move the working end in relation to the anvil and cartridge assemblies between retracted and advanced positions; wherein the pivot member of the channel member is received in the pivot member slot of the anvil assembly and the cam slot of the drive assembly, and the drive assembly is movable from its retracted position into engagement with the ramp of the channel member to move the tool assembly from the open position in which the first and second longitudinal axes define an acute angle, to a partially clamped position in which the first and second longitudinal axes of the anvil and cartridge assemblies are substantially parallel, and to a clamped position in which the first and second longitudinal axes of the anvil and cartridge assemblies are substantially parallel. an anvil assembly defining a first longitudinal axis, the anvil assembly including an inner surface positioned to engage tissue and a proximal portion, the proximal portion including a base portion defining a longitudinal channel and a pivot member slot, the pivot member slot extending in a direction substantially perpendicular to the longitudinal channel and including an open end and a closed end; a cartridge assembly defining a longitudinal axis, the cartridge assembly including a channel member and a staple cartridge, the channel member defining a cavity and a knife slot and having a proximal portion and a distal portion, the proximal portion of the channel member including a ramp and a pivot pin positioned proximally of the ramp, the staple cartridge received within the cavity of the channel member and including a cartridge body, staples, pushers, and an actuation sled, the cartridge body defining a knife slot, the actuation sled movable through the cartridge body into engagement with the pushers to eject the staples from the cartridge body; and a drive assembly having a proximal portion and a distal portion and including a flexible drive beam and a working end, the flexible drive beam having a proximal portion and a distal portion and being movably received within the longitudinal channel of the base portion of the anvil assembly, the working end secured to the distal portion of the flexible drive beam, the proximal portion of the drive assembly defining a cam slot having a proximal portion and a distal portion, the proximal and distal portions of the cam slot defining axes (Z1, Z2) that intersect to define an acute angle β, the drive assembly movable from a retracted position to an advanced position to move the working end in relation to the anvil and cartridge assemblies between retracted and advanced positions; wherein the pivot member of the channel member is received in the pivot member slot of the anvil assembly and the cam slot of the drive assembly, and the drive assembly is movable from its retracted position into engagement with the ramp of the channel member to move the tool assembly from the open position in which the first and second longitudinal axes define an acute angle, to a partially clamped position in which the first and second longitudinal axes of the anvil and cartridge assemblies are substantially parallel, and to a clamped position in which the first and second longitudinal axes of the anvil and cartridge assemblies are substantially parallel.