An end effector for a surgical robotic system, comprising a main body, one or members, and an adjustment mechanism. The main body has an interior sidewall and an opening extending through the main body. The one or more members are located within the main body and extend into and retract from the opening to vary a diameter of the opening through which a tool may be inserted. The adjustment mechanism coupled to the one or more members for varying the diameter of the opening, and comprises a collar coupled to the one or more members such that rotation of the collar in a first direction causes the one or more members to move away from the interior sidewall to reduce the diameter of the opening, and rotation in a second direction causes the one or more members to move towards the interior sidewall to increase the diameter of the opening.
