Abstract: A support arm for supporting a power screwdriver or similar device. The support arm prevents the transmission of the rotational force generated by the screwdriver to the hands and wrists of the operator. The support arm includes an internal lock plate secured to an extensible rod, which is disposed within a generally square outer housing. The internal lock includes a pair of generally square lock plates mounted on either side of a Teflon.RTM. guide sleeve, which acts as a slide bearing. The sides of the lock plates are flat and are used to abut against the interior flat surfaces to prevent the extensible rod from rotating inside the housing. A pair of universal joints are connected to the housing and rod to permit limited pivotal movement. The support arm may be mounted to a shiftable carriage, which rides on a transverse railing.