Robot Welding

The essential guide

Welding equipment for robots
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Dressed welding robot

The image on the below shows a fully dressed welding robot. The welding torch (water or air cooled) is mounted to a crash sensor. The purpose of this device is to prevent any damage to the robot if by mistake a fixture clamp is left open, the wrong program is called up or if the robot is moved into a wrong direction during robot programming.
The hose bundle continues over or through the upper arm to the wire feed at the back of the robot's upper arm or through the top arm in case of a hollow wrist robot. The connections continue downwards from the wire feeder to an to the welding power source.

An automatic torch cleaning station is an essential add on, which ensures that the welding shroud is kept free from any splatter build up. The robot can carry a 15 kg spool of welding wire from its lower arm although it is more common to use large bulk packs.
The advantage of using a dedicated hollow wrist arc welding robot are numerous. The hose bundle passes through the upper arm and wrist.This provides better protection to the hose bundle and there is less wear and tear. It also reduces the likelihood of the hose bundle getting caught up with external equipment such as the fixtures. This will also be an important benefit if the user intends to use off-line programming. A minor advantage, which can be critical in some applications, is that due to the shorter and better guided hose bundle the TCP (tool centre point) of the robot, for a welding robot the end of the wire with a defined stick out, is more consistent.

Welding torch

The welding torch may be air or water cooled depending on the nature of welding in the same way as this is the case for manual welding, except that the duty cycle for robot welding will be higher due to the increased arc-on time. A standard torch neck is 22.5 degrees, but there is a range of different neck configurations. The torch is precision bent to make sure that when it is replaced, the Tool Centre Point remains in the correct position. The torch is a semi consumable item and will last between one and two years. The hose bundle that connects the wire feeder to the torch body is specially designed for robotic welding and has a spatter resistant outer sleeve.


Welding power source

Most major suppliers of power sources manufacture a range of power sources with a robotic interface that is dedicated to the particular make of robot. In the majority of cases the power source will have synergic capabilities. This offers the benefit of automatic tuning of the welding process. Only the wire feed speed must be set and the corresponding voltage is automatically selected. This means that less time is spent to get optimum results. This kind of power source can also weld in pulse mode. When welding in this mode, a pulse of current gives one metal droplet per pulse resulting in virtually spatter-free welds, which can be useful when welding high alloy steels or if there is a need to control heat input. Please note though that when welding thin plate materials, the heat input will actually increase during pulse welding, which makes it harder to control burn through.

There are a number of solutions for welding very thin material, either through software or extinguishing and re-igniting the the arc in rapid successions as is the case in Fronius’ Cold Metal Transfer welding process.

The choice of a water or air-cooled welding package is simply a matter of duty cycle, but bear in mind that the arc-on time factor with a robot is much higher than for manual welding. 


Wire feed unit

The wire feeder is one of the most critical pieces of equipment of an entire arc welding cell. It must feed the welding wire consistently without slippage or else the supervision in the welding software will flag up a delay and stop the process.The quality of the weld will be adversely affected during welding if the wire feeder does not feed properly. Most wire feeders have four rollers that create a large amount of friction so the wire can be fed without slippage. The motor that drives the mechanism is controlled from the robot controller and may have tacho supervision. In some cases this motor may even be servo driven to provide even more control. 


Bulk pack

The traditional spool of welding wire carries 15 kg of welding wire and depending on the nature of your particular welding application you will need to renew this spool every shift or so. In particular for robotic welding there are many benefits associated with larger quantities of welding wire, as it will significantly increase the robot system's productivity. There are numerous suppliers of these bulk packs. ESAB's Marathon Pac comes in 250 kg and 475 kg versions and when the welding wire is spooled inside the protective drum, it is twisted using a special technique, which allows the wire to exit the welding gun straight. The advantages of the Marathon Pac for robotic welding are summarised as follows:
The larger quantity of welding wire in the Marathon Pac means that fewer spool changes need to be made and this increases the arc-on time factor and thus productivity.

The straight wire ensures that the Tool Centre Point (TCP) of the robot is consistently maintained, which means that the end of the wire will always be presented in the exactly the correct position for welding. This can be a critical factor when welding thin materials.

Traditional spool technology gives a wire with a certain cast and helix and this could result in certain weld defects because the welding wire deviates from the centre line. This is totally eliminated using ESAB's Marathon Pac. The container protects the wire from dust and moisture. This reduces maintenance to the wire feed unit and the liner.


Automatic torch cleaner

In order to allow the shielding gas to flow freely and thus eliminate weld porosity, periodic cleaning of the welding torch is essential. The robot moves to a clamp where the welding shroud is held in position. The robot then moves moves down into a rotating reamer, which will scrape any spatter build up from inside the shroud. The robot then moves to an anti-spatter fluid injector, which is mounted from the torch cleaning station and this fluid prevents premature build up of spatter. The torch cleaning cycle is automatically activated at the required intervals. On the left on the picture is a wire cutter unit, which is useful if the welding wire is used as a tactile sensor to locate the joint.


Tool Centre Calibration device

It is possible to cause the TCP (tool centre point) to go out of alignment when for instance a programming error is made, or the operator calls up an incorrect program. As a result the welds will be in the wrong position. The TCP calibration unit automatically checks and corrects the TCP. This ensures that the robot always works with an accurate reference point, which means that scrap, rework and manual set up times are reduced. Various methods exists and a common unit consist of a series of laser beams through which the welding wire is passed. Where the wire breaks the beam a reference reading is taken and if it deviates from what the system expects, off sets are made resulting in a correct TCP. These units are not essential for robot welding, but can be a useful add-on device for critical welding applications. 

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