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Welding-control is one of the means employed to obtain successful welds.
Of the different levels of quality checks devised for ensuring acceptable production results:
- Welding control is applied before and during welding (and may be manual or automatic),
- TESTING, usually destructive, is performed on a test-piece before starting production runs and at regular intervals thereafter, and
- INSPECTION is carried out after welding operations once the manufactured items are completed.
A special case is that, implemented sometimes for spot welds, where a load test is performed on actual welds, with a force just capable of opening up defective welds but insufficient to damage good welds.
Certification Tests are performed on completed constructions like hoisting devices, bridges or pressure vessels.
These are documented Load Tests with an actual load, established by relevant codes, exceeding by a definite percentage the nominal rated capacity of the structure.
Welding control as used here is intended as meaning two different things, both having influence on the outcome of a welding operation.
Welding control systems...
The first applies to those Welding control systems designed to maintain an automatic operation working within a preset range of parameters in order to ensure repetitive good welding results.
They are usually based on electronic instruments and sensors coordinated by suitable microprocessor technology.
If the required correction is excessive, beyond the scope of action of the equipment, the system either sounds an alarm, to alert the operator to intervene, or stops automatically all operations before much damage is done.
The second meaning refers to the collection of data obtained after testing and inspecting several production runs.
Welding control is performed through statistical analysis of the data in order to detect questionable trends.
These could signify drifting of the process, potentially endangering the continuing production outcome.
By knowing what means are available, and by asking for manufacturers' assistance, the attentive person responsible for a welding shop may investigate further into the Welding-control subject.
This may help him/her decide if something of what described here might be applicable and useful to the projects on hand.
At the beginning of technological development, Welding control was performed by simple devices to monitor a single parameter, like time, or to regulate another, like pressure.
But even when the process parameters were selected with care, instantly differing conditions, like a mains voltage sudden drop or local metal surface dirty condition could have had a major influence on the outcome of the process.
That is why with time Welding control evolved to the present state where many different parameters are individually monitored by independent sensors.
All results are integrated together by a software driven microprocessor.
The system becomes capable of modifying some of those parameters in real time, by closed loop feedback, in order to restore the correct conditions, to provide at all times acceptable welds.
This Welding-control trend is especially visible in RESISTANCE WELDING.
This is a high production class of processes quite suited to be automated.
The operator intervention, if not superseded by automatic equipment, is now mostly limited to feed new work and to operate the Start command.
Among the parameters checked by adaptive Welding-control sensors one can find:
- time (measured in the number of cycles of alternating current),
- force exerted by the electrodes on the workpiece,
- electrical resistance,
- workpiece thickness,
- thermal expansion and
- expansion rate.
The use of adaptive Welding-control has improved considerably the level of performance enabling better quality with less inspection and testing, some destructive, other nondestructive.
This means that the cost of specialized systems can be easily covered by increased savings.
Controllers are available where single Welding-control parameters can be entered by the operator either following a prepared schedule or in response to specific queries supplied by the system.
Some allow easy and self explanatory programs to be set up as schedules to be called for each job in turn.
Certain Welding-control systems perform automatic acquisition of running data to help in further statistical analysis of performance.
Others can remind to the operator (after a preset number of welds) that time has come to perform simple maintenance jobs like electrode dressing.
In some mechanized arc welding operations, an arc voltage Welding-control provides real time correction.
The voltage tends to vary following distance changes between electrode and work.
Automatic control restores the preset distance, to keep heat input constant, in order to maintain uniform weld properties.
Another popular Welding-control, useful for automatic arc welding and for electron beam welding, is called Seam Tracking.
It provides guidance and real time correction to follow the joint and produce the seam where it is needed and not adjacent to it.
Statistical Process Control (SPC) identifies sources of parameter variation in the welding process and allows corrective measures to be implemented in time, before the process runs out of control.
As mentioned before, a statistical study needs a database of past inspection results in order to monitor the present performance in the light of the historical record.
SPC is usually implemented by preparing a set of charts, that visually display in a quite intuitive way some useful resulting parameter, i.e. the percentage of defective welds as a function of time and other data as feasible and useful.
Not too much...
One should remember that a significantly better than needed quality by Welding-control is not only wasted, it drives costs up and may negatively affect productivity and economic performance.
It must be recognized that serious statistical analysis of any process builds on a specialized body of knowledge that is not within the common tools of the trade of an industrial welding operation.
Experts in the field should be hired for obtaining a guide in establishing routine statistical controls.
The purpose of these increased Welding-control is to save on costly production inspection, by keeping records of actual processing conditions.
It is up to the manager to decide if the current resistance welding operation warrants the costs involved in upgrading the machine Welding-control and/or of establishing a statistical program to improve performance and benefits.
An Article on Intelligent Process Controls was published (2) in Issue 91 of Practical Welding Letter for March 2011.
Click on PWL#091 to see it.
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How to Weld With a MIG Welder With Pulse Control
* * *
Find some Online Resources in a special
Mid Month Bulletin Page of our PRACTICAL WELDING LETTER.
It is Bulletin No.83 for Mid March 2013 on the subject of
Adaptive Welding Process Control.
We urge you to explore this rich source of essential knowledge.
Click on Bulletin 83 to see it.
For more Online References on additional welding subjects
Click on Welding Resources.
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Welding-control for real time monitoring and compensation of parameters. Gains and savings by improving quality performance and reducing inspection. More info...