to form an annular track having two parallel rectilinear portions which are joined at the ends by curved portions 107 along the opposite faces of the guides 103 and 105 are located rollers 109 mounted in stationary positions to form tracks for workpiece conveying members 110 which form carriages the carriages 110 are arranged in an endless loop and may be entrained by an endless chain 112 or other flexible endless members the chain being suitably guided along a path located between the guides 103 105 and the tracks formed by the rollers 109 means such as those indicated at 114 116 118 120 122 124 and intended to effect the same operation or different operations on the workpieces are provided along a rectilinear portion a of the annular track the above described arrangement of these means is similar to that of the working stations in the previous embodiment the means 118 120 122 may operate in parallel to effect the same operation in a slower manner than those operated for instance by the means 114 116 124 the means 118 120 122 may each be associated with two mechanical grippers or hands 126 for transferring the workpieces to and from the conveyor portion a by angular movement in a horizontal plane a similar mechanical gripper is associated with each of the other means 114 116 124 a carriage 110 can be engaged by the mechanical grippers and transferred together with the workpiece p transported thereby and which still has to undergo an.