Welding_resistance

spot and seam machines:

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New specific Welding_resistance machines are built to perform successfully their job.

At the required quality level at the lowest cost per piece, all included.

Modern Welding resistance machines are much more advanced than older ones.

They are endowed with monitoring and real time adaptive controls of all the main parameters.

Old machines can be retrofitted with such controls to reach a new lease of continuing productive life.

Fully controllable Welding_resistance machines are worth their higher price as they help provide quality welds, repeatedly, at the required production rate.

Programmed maintenance should help in minimizing unscheduled breakdowns.

In a previous page on WELDING EQUIPMENT (click for seeing), a short review was offered of the main concerns that should be addressed when procuring new hardware as needed to perform any given welding process.

Here we are going to enter into some more details on Welding_resistance equipment. For more information on the Resistance Welding Process, including data on electrode materials, you may click on the underlined subject title.

See also Resistance Welding Tips.

Welding_resistance Spot welders

SPOT welders are a sub-class of Welding_resistance equipment that provide the capacity to process the materials as follows.

Two or more sheet metal or wire parts are overlapped and then welded in separate spots by causing a fused weld nugget to form between the faying surfaces.

That occurs in correspondence of the place where the elements are been held in place within clamping electrodes.

Force is exerted between two short copper bars called electrodes or tips.

Heat is developed at the points of contact as a consequence of resistance to the passage of an electric current.


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Precise control for every spot

The electric current needed is generally of low voltage and high amperage, obtained directly from the grid through a one phase or a three phase transformer.

For consistent and repeatable results the amount of current drawn at each cycle is exactly supplied by electronic controls.

As already pointed out elsewhere in this Site, drawing such a high current imposes a heavy load on the grid, even if of short duration, so that other nearby instruments might suffer if no precautions are taken.

The Electric Company may object to unbalanced loads (of one phase transformers) on the grid and impose corrective actions.

Voltage regulators should be considered if the voltage of the electric grid tends to fluctuate, because of the negative effect on quality of the nuggets and on uniformity of results.

Alternating current (ac) or direct current (dc) can be used to produce resistance welds.

Direct welding current is preferred when very high current levels are required.

Quite different types and sizes of Welding_resistance machines are available.

They can be fixed and portable, depending on the materials and on the maximum thickness to be welded.

Special complex installations were used in the past for automatic operations.

Modern implementations call for universal robots manipulating light weight clamp type welders.

Fixed machines are heavy and bulky. Parts must be brought to the machine and placed at the proper location between electrodes.

Conventional machines have the transformer located somewhere, far from the welding gun.

The current is brought to the electrodes through a heavy, water cooled cable.

Portable machines permit to perform spot welding on constructions too large or cumbersome to be brought to a stationary Welding_resistance machine.

They can be manually operated or mounted on a robot arm.

Mobile machines, called portable welding guns, are light and easy to move around, to reach welding spots all over large parts.

If the welding transformer is integrally mounted in the welding assembly, the machine is called a Transgun which may have less electrical losses than those present in manual welder with separate fixed transformer.

Cable-type guns or transguns have a smaller transformer mounted on the welder clamp, with short cables connected to the electrodes.

Different types exist, mainly C-type and scissors-type, each one more suited to certain types of joint to perform.

These manual, lighter transguns are ideal for being mounted on suitable robots.

The same thickness combinations of various materials need the same current for similar welds, independent from the machine type, fixed or portable.

Transguns, being more compact, are more efficient electrically, potentially resulting in power savings.

The three main elements always present in a Welding_resistance machine are:

  • the transformer and the electric circuit,
  • initiation and control devices for precise operation,
  • and the frame with the mechanical force application devices.

An Article on Automatic Resistance Welding was published in our Practical Welding Letter No. 12, issue of August 2004. To read the Article click on PWL#012.

An Article on "How to select your Resistance Welder" was published in Practical Welding Letter issue No. 22 for June 2005. To see the article click on PWL#022.

An Article on Extending Life of Copper RSW Electrodes was published in Practical Welding Letter issue No. 41 for January 2007. To see the article click on PWL#041.

An Article on MFDC Power Supplies for Resistance Spot Welders was published (2) in Issue 42 of Practical Welding Letter for February 2007. Click on PWL#042 to read it.

An Article on Proper Water Cooling of Spot Welding Electrodes was published (3) in Issue 87 of Practical Welding Letter for November 2010.
Click on PWL#087 to see it.

An Article on Automated tip dressing for robotic Resistance Spot Welding was published (7) in Issue 94 of Practical Welding Letter for June 2011.
Click on PWL#094 to see it.

An Article on Spot Weld Lobes was published (2) in Issue 104 of Practical Welding Letter for April 2012.
Click on PWL#104 to see it.

An Article on Spot Weld Quality for Autos was published (11) in Issue 116 of Practical Welding Letter for April 2013.
Click on PWL#116 to see it.

An Article on Inserted strips in Resistance Spot Welding was published (3) in Issue 136 of Practical Welding Letter for December 2014.
Click on PWL#136 to see it.

An Article on Resistance Welding Tips was published (3) in Issue 156 of Practical Welding Letter for August 2016.
Click on PWL#156.

An Article on Low Impedance Resistance Welding (*) was published (11) in Issue 162 of Practical Welding Letter for February 2017.
Click on PWL#162.

(*) - Important for leak-free sealing of packages

An Article on Welding of Ultra-Thin Steel was published (2) in Issue 163 of Practical Welding Letter for March 2017.
Click on PWL#163.

To receive at no cost all the issues of PWL as they are published, please subscribe.

Specifications...

Visit the 2008 Membership Directory (84 pages) of
the Resistance Welding Manufacturing Alliance (RWMA)
(http://files.aws.org/rwma/docs/2008dir-final.pdf).
See also http://www.aws.org/rwma/publications.html

Controls are regulated by the National Electrical Manufacturers' Association,
to be visited at NEMA.

Single phase spot welders are simpler and less expensive than three phase ones but their influence on the balance of the electric grid is more drastic.

This should be addressed as an objective concern, because of the penalty that the Electric Company might decide to impose.

Three phase Welding_resistance machines draw a more balanced load from the power grid.

In most cases they deliver high current to the secondary windings of alternate current transformers.

Single-phase direct current, three-phase direct current, three-phase frequency converter, and medium frequency direct current power systems are also used.

For Welding_resistance machines developed to weld aluminum, one should remember that current level requirements are higher than for other materials, and controls must be capable of exact timing to ensure consistent high quality.

The force for clamping is usually produced by pneumatic actuators when compressed air supplies are available.

Mechanical force application was used in older Welding_resistance machines.

Hydraulic machines employ cylinders operated by high pressure hydraulic oil supplied by a motor driven pump.

Best quality with best controls...

Controls establish the current intensity and duration but also of the timing of the successive phases.

More recently, adaptive controls monitor constantly variables like voltage and actual resistance, and provide, through feedback, corrected welding parameters designed to result in acceptable welds.

Controls are specialized instruments of Welding_resistance machines.

They are designed to permit the passage of precise amounts of current by establishing the exact timing.

That is done by catching the position of firing on the curve (from zero to maximum and back to zero) describing the voltage as a function of time (or of phase).

See also Welding Control.

They initiate and terminate the exact timing of current flow to the transformer.

They control the current magnitude measured in amperes, and establish the timing and sequence of mechanical motions and force application in Welding_resistance equipment.

More modern controllers provide modulated pulses whose current level, polarity, timing and duration are individually established and initiated by microprocessor technology.

Medium Frequency, Direct Current (MFDC) welding control, permit the use of much lighter transformers, and provide very precise welding power control by pulse width modulation or by frequency modulation.

All Welding_resistance machines are operated by unskilled workforce, which contributes to the economy of operation, after the proper welding schedules have been developed for each job and set up by competent technicians.

Equipment selection

Equipment selection presumes complete and precise knowledge of the planned joint configurations, but also of quality required, production programs and economic evaluation.

A frank discussion with a few knowledgeable manufacturers will reveal critical questions, many of which will get similar answers from most suppliers.

Materials and conditions should be known exactly, because some materials need that the weld nugget be forged during the welding cycle. Plated material may require special attention.

The thinnest and thickest combinations, as well as the maximum number of sheets in a stack must be known.

Future needs should be anticipated if the same equipment is intended to process also new production.

The above data will determine the maximum needed electrode force so that a suitable machine be selected.

Furthermore the duty cycle has to be determined depending on the weld time vs. total time.

A seam welding machine firing with higher frequency than a spot welding machine needs correspondingly a higher duty cycle.

The electrode force needed for any material and thickness combination shall be found in manuals, handbooks or manufacturers publications.

These data will help determine which characteristics should have the equipment needed for the production in question.

Sometimes it may be difficult (and expensive) to accommodate in the same machine requirements for widely different thicknesses, like 0.125 to 0.125" (3.18 to 3.18 mm) aluminum alloy, and also 0.016 to 0.016" (0.4 to 0.4 mm).

If the whole needed range cannot be covered by a single machine, with the required production quality, one should better know before purchasing.

The purchaser is entitled to request from the supplier an equipment acceptance test program, to demonstrate the required quality at both extremes of the operating range, for all the materials involved.

The frame of the fixed machine should have a throat capable to accommodate the largest production parts.

Rocker arm type, so called because of the pivoting movement of the upper electrode holder, are smaller and simpler machines easy to set up, usually operated with compressed air.

Press type machines for spot and projection welding, have a welding head moving in a straight line, guided by suitable bearings.

Machines up to 500kVA are pneumatically operated, generally, while above that their actuators are hydraulic.

Normal machines may be incapable of keeping always the electrodes in proper contact with the workpiece, because of a certain inertia of the moving parts.

If nugget forging is needed (i.e. for aluminum), the full forging force should be reached within 6-12 milliseconds to avoid internal cracks or voids.

That is true also for projection and upset welding in which a large displacement occurs in the workpiece as projections collapse.

To improve the dynamic response of the upper electrode holder, special construction is needed.

It should include low mass springs, a diaphragm air cylinder between the ram and the electrode or an accumulator with a pre-charged fluid for instant supply of the actuator.

Now for seam...

SEAM welding is performed on Welding_resistance equipment similar to the preceding one except that the electrodes are here rotating copper wheels.

These grip the overlapping sheets and move progressively the workpiece while producing a sequence of overlapping spot weld nuggets.

The wheel axis can be parallel or perpendicular to the throat of the machine to provide for circular or longitudinal welding.

Normally there are provisions to change configuration from one to the other as needed.

The design of the machine should be carefully studied because it limits the dimensions (diameter and length) of the workpiece that can be welded.

The shape of the electrode wheels can be adapted to accommodate protrusions which interrupt the regular continuous thickness of the joint.

One of the axes of the wheels can be angled in order to accept wheels shaped to avoid interference with a wall when a narrow flange is welded.

Electrode wheels of Welding_resistance equipment must be effectively cooled in order to control the geometry of the spots and prevent damage to the wheels.

This can be done by having cooling water circulate within the wheels or externally by flooding, or by a combination.

As for projection...

PROJECTION Welding_resistance machines of the press type tend to be custom made or at least must be capable of accommodating the design of the workpieces.

As such they are useful for planned mass production. Projections are small protrusions obtained in sheet metal by forming the material with a die under a press.

Welding occurs at the same time at once on all the projections pressed by the top platen, against a plain sheet, also held by the bottom platen.

These machines essentially include all of the main elements of other Welding_resistance machines reviewed above except for some differences as follows.

The movement of the platens relative to one another must be parallel (that is no rocker arm) in order not to move the elements.

The upper electrode must follow immediately the collapse of the projections, otherwise the necessary force is relieved too soon with dangerous consequences on the quality of welds.

This requirement demands special provisions like low inertia welding head as briefly remarked above.

The establishment of the correct force needed and its adequate control is essential to the production of consistent high quality welds.

The weld time is determined only after the proper electrode force and the required current have been determined, and its duration should be sufficient to produce nuggets of suitable size, while avoiding expulsion of molten metal.

A special Issue of Practical Welding Letter, the Mid June Bulletin No.74, full of Resources on Resistance Welding was published on June 14, 2012.
Click on PWL#106B to see it.

Watch the following Video on

Resistance Welding Machine

http://www.youtube.com/watch?v=uffVD2U8yyk

* * *

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Useful for hard to weld materials.

To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter,
click on Welding Topics.

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