(By BISHAM MALKANI )

Modular automatic welding and cutting systems are finding increasing applications due to their " building block " type flexibility, and the advantage of the flexibility of " all position" welding, not to mention the feature of ease of transportation of the system to the job instead of the vice versa arrangement in most fabrication shops. Today, the focus is on increasing the arc-time, which is directly proportional to weld deposition rates. In fabrication shops the weldments are heavy and cumbersome to move from one place to other. At construction sites and shipyards, specially, during the erection stage the weldments can not be moved or positioned for welding in the downhand position. Here the modular welding systems play very important role due to their inherent characteristics of flexibility due to their small size. Secondly, the most important feature, they can be used for "in-position" welding at high arc-times, at the same time producing excellent welds satisfying the rigid radiography inspection norms. It is always mentioned that labour in India is cheap. Today with higher wages, higher level of supervision, low outputs coupled with the expenses of reworking, has encouraged fabricators to go in for automation in welding and cutting. Competition is more fierce, and there is a general complaint that the prices at which contracts are procured makes the business not very profitable. The only way the profits can be maintained is by lower cost and a quick turnaround times... in other words high productivity and minimum labour cost. BENEFITS OF WELDING AUTOMATION USING OSCILLATION WITH MIG WELDING: Since we are laying an emphasis on the "in-position" welding oscillation process, we have taken a typical example of welding in the 3G (vertical-up) position. SMAW- average consumption of 60 electrodes per 8 hours shift = 2 kgs/shift. MIG- 1.2 mm dia FCAW wire burn off rate at 180 amps = 2.8 kg/hour 1.2 mm dia FCAW wire deposition (semiautomatic at 23%) = 5.1 kg/shift Average arc-time for SMAW in our country is 18-19 %. Average arc-time for MIG?FCAW in the country is 23 % There is only one way of increasing the arc time. It is with the use of automatic welding processes, using continuous wire (solid or flux cored, wire and flux, strips etc.) i.e. CO2 / MIG, TIG, SAW, etc. These processes, if used semi-automatically, have the human limitation, which limit the arc-time to approximately 23 %, a common norm in India. In western countries, this norm is around 30 %. If, with the help of automaton, the arc-time can be improved to 80% to 90%, weld deposition would increase four-fold, compared with semi-automatic MIG / FCAW welding, and by over six-fold as compared to SMAW. Weld productivity would further increase with the use of higher diameter wires and higher amperages, since the welder fatigue and discomfort is minimised due to the deployment of weld automation. The welder basically does the job of a " supervisor" making minor changes to the welding parameters, position of the welding head/torch etc. (as and when required) during the welding process. Automation not only increases productivity but also reduces the need for a skilled welder. One can think in terms of utilising trained operators instead of skilled welders. With automation, arc-time of MIG /FCAW can be comfortably increased to 60% -70%, or even higher depending on the jog. Let us take the arc-time of MIG/FCAW when automated, to be 60% Weld deposition in 3G position with automated MIG/FCAW process = 14.4 kg/shift. The final comparison per welder in an 8-hour shift is as follows: SMAW 4 mm electrode with 19% arc-time in 8 hour shift = 2.4 kg/ welder MIG 1.2 mm FCAW wire with 60% arc-time in 8 hour shift = 14.4 kg per welder A reasonable arc-time in semi-automatic MIG/FCAW is 40% = 9.6 kg per welder. One can see from the above, 2.4 kg of weld deposition per welder can be increased to 9.6 kg of weld deposition per welder at 40% arc-time, or to 13.4 kg of weld deposition per welder at 60% arc-time, by automating to MIG process and using FCAW wire. This shows a tremendous jump in weld deposition. One can see the gains in production levels with lesser man-power by co-relating the increase in deposition rates through automation, to the well known empirical formula of electrode core wire weight as a percentage of weight of fabrication to be 1 to 2% depending on the type of fabrication. Modular automatic welding and cutting equipment and accessories can be categorised as follows: A range of microprocessor abased, programmable, variable speed carriages, which run on rigid or flexible tracks. These tracks are magnetically held on to the weldments. The flexible tracks have the advantage of following the contour of the weldments. These carriages have a wide range of accessories; i.e. oscillators, which can be programmed to weave the torch in "all position welds", mechanical and electronic seam trackers and height sensors, indexers, vertical and horizontal motorised arms etc. Cutting torch holders to provide excellent cuts and bevels using oxy-acetylene, plasma, and water-jet processes.   Microprocessor based, variable speed portable fillet welding carriages, which have a range of accessories i.e. twin torch holding devices, welding oscillators etc. These portable fillet-welding carriages are also available with features for "stitch welding". One person can operate two carriages with two arcs each. By operating four arcs simultaneously, a single operator can deposit upto cumulative of over 20-kg weld metal per 8-hour shift using 1.6-mm dia. wire at 300 amps. (At 300 amps the deposition rate of 1.6 mm FCAW wire is 4.6 kgs per 8 hour shift.) There are fabrication shops where six variable speed carriages are controlled by a PLC and operated by a single operetta. Hence, increasing weld deposition per operator tremendously. Portable edge bevellers. A range of models of high-speed portable edge beveling machines are available for beveling carbon steel, stainless steel, aluminium, titanium, etc. at speeds of three meter per minute providing excellent and accurate levels, which are necessary for good fit-ups for weld automation. Ceramic backing strips: FLEXIBLE MODULAR AUTOMATION SYATEMS – AN OVERVIEW Modular welding systems consist of programmable variable speed, lightweight carriages, and a host of accessories, which can be retrofitted on them for upgradation. The salient feature of these carriages is the sophisticated controls, which provide accurate and stable speeds. The carriages have programmable microprocessor based tacho feedback, close-loop control circuitry. These controls ensure constant speeds in all positions of welding, i.e. vertical, inclined, overhead etc.... that too, with a provision to carry a load upto 45 kgs (weight of a wire feeder and a spool.) The variable speed, all position carriages run on tracks by a pinion gear drive mechanism that engages the rack on special tracks. A wheel guide system grips each side of the track, enabling the carriage in any position at pre-determined speeds. These tracks are rigid or the flexible type (the latter made from spring steel) facilitating movement at the carriage on circular jobs, fabrications with curvatures, giving the welding carriage tremendous flexibility. The tracks are fixed onto the weldments by means of magnets or vacuum activated mounting devices, in the case of non-ferrous fabrications. Welding guns MIG/TIG/SAW etc. or cutting torched can be readily mounted on to these welding carriages to facilitate automatic welding and cutting of a high quality. It is important that there should be provision for X, Y and Z adjustments of the torch while welding. To facilitate this, a range of rack arm and rack boxes is readily available. These rack boxes can be fitted on the front, back or side of the carriage. Four motion holding devices can be clamped on to the rack arms to facilitate positioning of the torch n any direction and to make minor adjustments to the arc during welding. The rack arms and rack boxes are also available in motorised versions with remote control pendants. A range of special accessories are alas available, i.e. mechanical height sensors, seam trackers, fillet welding height sensors, clamping arrangements for two or more torches, etc. 2 TRACK TYPE WELDING CARRIAGES AND CONTROLS Standard carriage: forward/reverse variable speed... With and without welding, preset speed facility through potentiometer with digital readout. Arc start-delayed carriage start (delay with variable timing), - carriage stop. Provision for limit switch actuation. Emergency stop switch. Free wheel and clutch engaged drive. Compact Oscillator Carriage Combination: all features of the standard carriage plus added features of the electronic oscillation sopped control and oscillation width control 9 independent dwells 0-5 seconds " left-centre-right’), electronic selection of arrange of oscillation patterns, independent head tilt feature of oscillator for exact positioning and correct angle for multipass welding, tangential and pendulum motions etc. All controls are in simple, user-friendly units displayed in windows in digital units. The oscillator carriage combination can be used in any position – even in the overhead position to produce excellent x-ray quality welds. It can carry a vertical load of 50 kg without any variation in the speed. The oscillator carriage combos are widely used in shipyards, all over the world. In most of the shipyards and construction sites, mainly for "out-of-position", continuous multipass welding operations using the MIG process with solid or flux-cored wires. They produce excellent x-ray quality welds with high deposition rates (as cited earlier in the paper). The services of very skilled welders are not necessary for welding with the oscillator. Once the parameters are set, only minor adjustments are required to be performed by the operator during the welding. The operator has to make slight adjustments to keep the arc-length constant, and ensure that the torch follows the weld seam accurately by making minor adjustments in the X and Y-axis. Several oscillator carriage combos can be used on a weldment. For example, in shipyards, several oscillator carriage combos are simultaneously used in one area during welding of the hull. Mr. D.W.Miller, Welding Engineer Manager, Kvaener Govan Ltd, Glasgow succeeded in decreasing man hours by 72% on maximising positional welding using flexible tracks and the oscillator compact combos with ceramic backing strips, in the ships hull. This has been documented and published in several international welding journals. Electronic seam trackers: These are high precision tracking systems that maintain the torch in optimum operating position regardless of the variations in the weld seam. They take care off the variations due to material warpage, misalignment, and irregular edge fit-up. Difference in material thickness and other conditions that cause variations in the seam. A shockproof mechanical probe is interfaced with the electronic controls. Its signals actuate sensitive independent set off sophisticated precision-engineered motorised X-Y slides. The seam trackers can be used independent of the carriage. A wide range of seam trackers are available from the simple mechanical probe to the laser probe with the adaptive system where the laser probe moves ahead of the arc tracking the joint sending signals of the fit-up to the electronic controls which in turn vary the welding current, arc voltage and speed of the carriage appropriately to ensure sound welds. There are lot more functions these laser seam trackers can carry out, i.e. follow complex geometries, control multipass beads for high quality welds, increase the speed of welds, etc. the laser seam trackers are more commonly used with robots. Seam tracking is a very vast subject by itself, hence, only the basic working principle has been described here. There are various other accessories; i.e. electronic height sensor, indexer etc. are available. PORTABLE VARIABLE SPEED FRICTION DRIVE FILLET WELDING CARRIAGES: These portable light weight variable speed friction drive carriages have carves a niche for themselves in shipyards (specially for welding long T-joints and stiffeners) and the construction industry (welding of fabricated girders, columns etc.) and are better classified as portable fillet welders, since they are widely used for fillet welding applications. They are very simple to use and controls are user friendly. Controls consist of a switch each for arc start and forward/reverse, and a potentiometer for presetting thee speed, which appears as the digital display on the window. An arc start- carriage delay (0-3 sec) provision is incorporated in the design. Limit switches are mounted on the front and back of the carriage to stop all functions on their actuation. These trackless variable speed carriages have sophisticated microprocessor-based controls through taco feedback, closed loop circuits since precise speed control is mandatory. Some carriages also have controls for stitcch-welding applications. The trackless friction drive carriages take the reference from the vertical plane of the weldment by means of idler rollers mounted onto adjustable arms (front and back of the carriage). One arm is made approximately one inch longer than the other is in order to make the longitudinal axis of the carriage 5 degrees towards the vertical plane of the weldment in the direction of the line of travel of the carriage. This way, the carriage is forced to follow the contour of the vertical plane, which it literally "tracks". These carriage can also run on simple fabricated grooved tracks for butt welding applications, or an angle can be temporarily tacked along the length of the butt joint to provide the vertical plane for the idler wheels to follow in the absence of a retrace. Accessories are available for mounting of twin torches, oscillators, cutting torches etc. The control of the leg length is important for several factors i.e. economics, control of distortion etc. Taking economics alone, e.g. if a 6 mm leg length required is increased to 8 mm dude to negligence of the welder, there is a wastage of over 54% in the welding consumables due to over welding... automation ensists a consistent speed, maintaining accuracy of a leg length, provided the fit-up is reasonably good. HIGH SPEED PORTABLE MECHANICAL PLATE BEVELLERS: All the advantaged of increased arc time and high deposition rates that the variable speed carriages "family" can provide, will be "undone" due to poor edge preparation leading to poor joint fit-ups. High-speed portable beveling machines are commercially available. They can bevel carbon steel, alloy steel, stainless steel, titanium, aluminium, etc., at high speeds upto 10 feet/minute very accurately at different beveling angles on plates upto 50 mm thick, allowing bevels for double "V" (25 mm on either side of the plate.). Beveling is carried out by means of a shearing process. The process is noiseless and vibration free. These machines are used in plate shops to remove the heat-affected zones from plasma cut edges. Mechanical beveling, being a non-thermal process, has the important metallurgical advantage of not inducing a heat affected zone, nor causing thermal distortion. Other advantages are noiseless, vibration-free process, can be easily wheeled to the job, and no real skill required for the operation, portable, self propelled, follows the edge of while beveling without the help of the operator, spring loaded base allows the machine to run on an undulating floor, adjustment with graduated scale for setting the root face, no further grinding required for welding etc. Bevellers are also available for upside down bevels. For large-scale production, one beveller can bevel the topside of the plate while a second one follows by beveling the underside of the plate. This way a double "V" can be achieved without turning over the plate. CERAMIC BACKING STRIPS The most expensive and cumbersome operation in any fabrication shop is back gouging. For this operation the weldment has to be turned over, very often by using two or three cranes (if the job is very long), and position for gouging. Most of the fabrication shops in India use the obsolete pneumatic chipping or grinding methods. This is a very slow process, causing uneven grooves, which require more consumable than normal, for filling. These led to a host of other problems such as wider HAZ, more distortion, higher stresses, higher consumption of consumables etc. Very often fabricators use copper backing strips so that they need to weld from one side only. Copper backing requires good clamping or else the weld cut through the joint on to the other side. In many cases, where accessibility on the reverse side is limited, permanent backing strips (using the same strip as the parent material) are used, and left on the weldment after welding. Nowadays, ceramic backing strips are being freely used in many shipyards in the west and far east, they have found these ceramic backing strips to be indispensable in positional welding, using oscillator carriage combos to achieve high arc-times and high deposition rates. Besides, excellent x-ray quality welds are achieved. The ceramic backing strip consists of an aluminium foil strip, coated with an adhesive on the side where the tiles are placed lengthwise along the strip. The centerline on the grooves of the tiles is placed along the centerline of the seam, under the root of the weld. The adhesive aluminium foil holds the ceramic tiles against the weldment. The welding is carried out from the side of the "V" groove. After welding, the aluminium foil, along with the used tiles, is removed. A clean weld is formed where the tiles were placed, hence eliminating the back gouging process. These non-metallic weld backing are available in several segmented shapes and sizes, to suit any application requirement. The ceramic tiles are normally assembled in 600-mm length, with the width of the foil being 75 mm. For excellent welds and good radiography results, the inherent characteristics of the ceramic backing strips, such as halogen-free, non-hygroscopic (resistance to moisture pick-up), neutral fibrous material, granular density, the refractory nature of the material to withstand high welding heat without distorting as a heat absorbing metal bar, the quality of the adhesive tape to resist preheat temperatures, and host of other positive characteristics to increase productivity, has made this product quite indispensable. There are manufacturers who offer ceramic backing strips at throwaway prices. One has to be cautioned regarding the danger of hydrogen pick-up and halogen transfer. There are only a handful of reputable quality conscious manufacturers who provide the necessary qualify control to ensure moisture free welds, and without any halogen transfer from the ceramic backing strips. The ceramic backing tapes score over the conventional types of copper backing strip, due to their flexibility, as they can be formed into the desired shape of the weld. At the same time, they can also be adjusted for reasonably uneven fit-ups. The tiles are solid enough to support, and neatly form the molten weld puddle in its groove. The adhesive tape, combined with ceramic, provides an atmospheric shield for the joint backside. A brief summary of the advantages of using ceramic backing strip is Controlled distortion and HAZ, elimination of back-gouging and chipping, no need of removal of backing bars, excellent radiographic quality root runs with clean consistent beads etc. most of all, tremendous cost saving achieved with increased productivity, and excellent quality welds. 5 CONCLUSION In today’s age of power cuts, the focus is on increasing arc time and this is one solution. The advantages of flexible modular automation for welding and cutting have been briefly highlighted by means of description of thee equipment and accessories. Welding automation will definitely improve quality due to lesser stops and starts (continuous welding operation), x-ray quality welds, smaller HAZ, lesser distortion, no operator fatigue, and above all higher deposition rates, arc-time and productivity. About the IIW / Mumbai Branch / Other Branches / Coming Events / Technical Lectures Mumbai Weldnet / Trends in welding / Related Websites / IIW Forum / Feedback / Home