•            A positioner is a Mechanical device which is used to hold the Job and move them according to their required position to weld and do other operations.
•            A fixture can be mounted on the positioner in order to hold and move the Job in an effective manner to obtain advantageous quality welds.
•            The job can be repositioned according to the requirement of the process to be done on it.
•            The movement and the positioning of the job can be classified into three different types with different variants under each type,

• Rotation about an axis is possible using turn table, turning rolls, head stock and tail stock arrangements, tilting is possible with the positioner having a tilting table and the vertical movement is possible by using an elevating device which can move the component accordingly.

http://www.arcraftplasma.com/welding/Welding-Positioners.htm


•            Turning Rolls are generally used for cylindrical shaped jobs which rotates about horizontal axis, non-cylindrical jobs can also be mounted on rollers but it requires special roll fixtures according to the shape of the job used.
•            They can be used in manual, semi-automatic and fully automated welding machines in a flat position. 
•            They basically consist of wheels, steel frame, drive train, drive motor and other controls. Generally in turning rollers one will be a powered roll and the others will be an idle rollers
•            Longitudinal welds can also be performed by attaching a manipulator or a carriage along with the setup.
•            They have low initial cost as well as low maintenance and repair cost. An example for turning roll positioner is shown below,
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•           It is a single axis positioning device in which the table is fixed along vertical direction and the rotation is along horizontal direction. It gives easy access to all the parts of large work piece in flat welding
•           The head stock is always connected to a power source which is responsible to rotate the object in the particular axis where in tail stock it is not powered instead a free roller and a fixture or a support structure is fixed to hold the work piece and rotate it along with the headstock.
•           The alignment of the headstock and tail stock is very much essential because misalignment may cause damage to the work piece or the welding equipment.
•           Generally the concept of head stock and tail stock resembles a common lathe machines used in the industries. An example for welding machine with headstock and tailstock is shown below.
•           The can be used to weld various components like truck and machinery frames, large transformers tanks, structural girders, armored tank hulls, earth moving and farm equipment components.
•            An example for headstock and tail stock positioner is shown below.

•            It is single axis rotating device which rotates the table along horizontal axis either in clockwise or anti-clockwise direction. The load carrying capacity and size varies with respect to the work piece mounted on the table.
•            Their size can be varied from a range of 20kg’s to 500 tons. These turn tables can be attached with other positioners for various applications
•            Some of them are listed below

•            These turn table can be widely used for various applications such as machine welding, scarfing and cutting, cladding, grinding, polishing, assembly and NDT. An example for turn table is shown below,

•            Generally tilting rotating positioners are recommended only when a versatile in positioning of the work piece is required. The tilting mechanism can be constructed according to the limitations required by the user. These positioners can perform two functions

•            Tilting is carried out through gear mechanism which is mounted on the setup with a turn table on the top which allows the work piece to rotate in clockwise or anticlockwise direction.
•            The base of the positioner is fixed with two legs which can allow to adjust the height of the positioner vertically in order to provide proper clearance to the table.
•            The capacity rating of the positioner is based on

•          The total load
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•          The location of Center of Gravity (CG) of the combined load

•            There are two dimensions used to locate the center of gravity of the

1. The distance from the positioner table to the CG of the combined load.
2. The distance from axis of rotation to the CG of the combined load referred to as Eccentricity (ECC).

•            The mini positioner which are used for micro welding processes, heavy positioner with a load carrying capacity of maximum 1 ton and their tilting capacity varying from 90 degree to 180 degree including both horizontal and vertical position are shown below,

• In this type of positioner the tilting mechanism is placed outside rather than beneath the table. And so the size of the work piece is limited according to the distance between the vertical side members of the base. The offset distance between both the tilting mechanism is called inherent overhang. The distance may vary according to the positioner size. Its application is especially for a work piece which is supposed to travel along the direction with a constant welding head. Example for drop center positioners are hemispherical components which requires the movement of the work piece rather than the movement of the welding head. A schematic representation for drop center tilting positioner mounted with a hemispherical work piece is shown below,

• Generally the height of the positioner is increased and lowered using a shop crane.  But when powered elevation positioner is used it can be avoided an instead it can be raised and lowered under power using a rack and pinion or a screw jack mechanism. They are the most versatile among all the other processes but it is very expensive. They are available with capacities up to 60 tons and the axis of rotation is along horizontal direction.

• The basic construction of the machine is, a long arm is attached to the headstock which rotates without any restrictions and the turn table is attached at the end of the long arm. This type of machine provides unlimited rotation along both the tilt and rotation axes. These kind of positioners are used only for specific application. A schematic representation for a skyhook positioner is shown below,

•           Reduces welder fatigue.
•           Increases welder safety.
•           Improves weld quality.
•           Increases productivity when compared to manual positioning of the parts.
•           Assists welders in maneuvering and welding large work pieces.
•           Ensures smooth welding table rotation.
•           Faster welding speeds especially for obtaining X-ray quality welds.

      See a video of Circumferential MIG welding automation here 

      https://www.youtube.com/watch?v=8w1pXP0xVyk