Positioning arrangement on a frame member and on a part having to be secured thereto

A positioning arrangement on a frame member and on a part to be secured thereto, comprising a plane supporting face on the frame member to receive said part laid thereon, a securing means removably anchored to said frame member and having an axis perpendicular to said plane supporting face and along which said securing means is movable relative to said frame member, an opening in said part for the passage therethrough of said securing means, said opening having an edge engageable by said securing means at a point located on a predetermined radius of the securing means, a conical surface on one of the securing means and the edge of the opening in the part at the point located on said predetermined radius, said conical surface being inclined in such manner that the securing means, upon moving along its axis toward said frame member, exerts a thrust on said part urging the same in the direction of said predetermined radius relative to said frame member, holding means on said frame member preventing said part from sliding on said plane supporting face under the action of the thrust exerted thereon by said securing means, said holding means comprising three abutting surface portions on said frame member, said part comprising three surface portions, each surface portion of said part corresponding to one of said abutting surface portions of the frame member, each one of said three surface portions of said part abuttingly engaging the corresponding abutting surface portion of the frame member under the action of the thrust of said securing means when said part is secured to said frame member, said abutting surface portions of the frame member being arranged in such manner that every two of them constitute a guide-way along which said part may move under the action of the thrust of said securing means, the two surface portions of said part, corresponding to the two of the abutting surface portions of the frame member, sliding on said two abutting surface portions of the frame member until the third one of said surface portions of said part butts against the third one of said abutting surface portions of the frame member.