Abstract: The front end of a handgrip mounts an interchangeable tool-holder for pincer type tools whose opening and closing is controlled pneumatically through a cylinder-piston assembly to which a source of compressed air is supplied. Downstream of the compressed-air source there is a pneumatic circuit for the automatic operation, intermittently at adjustable intervals, of the cylinder-piston assembly. The placing in service and the taking out of service of the pneumatic circuit is controlled by simply obturating and freeing, respectively, an aperture provided in the handgrip and communicating with a bypass duct in parallel with the duct that supplies the compressed air to the cylinder-piston assembly.