Apparatus for cutting a hole through a plastic liner in a gas or other main comprises a body forming module of a train of modules adapted to travel through the main. The body has a radially movable platform carrying a cutter. The platform has four cams engaging four cams pivotally mounted on tie-bars. Advancing movement of the platform pushes the cams outwardly causing support legs to move outwardly and to cause four slave legs carried by the support legs to engage the plastic liner. Further advancing movement of the platform causes the cutter to engage the liner and also holds the support legs in their advanced position as noses on the cams traverse the faces of the cams.