# A Mini spot welder from scrap



## skyline1

I have recently seen on the net several designs for spot welders made from old Microwave Oven Transformers these ranged from dangerous looking contraptions to some really quite professional units. So I decided to have a go at one and hoped that I could produce something in the latter category. Whilst this is not strictly machining I hope it will prove interesting.

First, a few words about safety

Some parts of this unit operate at mains voltage, 240V in the UK, so it should be properly insulated and earthed, the cables should be mains rated.

Microwave Transformers as they stand are very dangerous and no attempt should be made to use one unmodified. The secondary operates at 3000Volts and can produce enough current to kill you. They also have a large high voltage capacitor connected to them, most of them have safety discharge resistors across them but they could fail so it is a good idea to discharge it with a well insulated screwdriver.

The output from the unit is only a very low voltage (About 2 Volts) so is not dangerous from a shock point of view but the current is many hundreds of amps so it could give some very nasty burns like any welding equipment.

These things are experimental, and I cannot give any guarantees about performance or even if it will work at all if anyone decides to build one (But mine did). As they say your mileage may vary.

They are not intended for continuous use and can overheat quite easily. After about 5 - 10 welds it will be quite hot and should be allowed to cool for a few minutes.

Having got the be careful bit out of the way on to the start of the build.

The first step is to obtain an old microwave. I was lucky with this as a friend was replacing an old mechanical one (A Toshiba ER669) with something more modern which meant that it was actually working if very elderly. But a dead one will work if the transformer is O.K.

Having taken the lid off and made sure the capacitor was discharged as above I set about dismantling it. Being a mechanical one it yielded an Aladdin's Cave of goodies gears, sprockets, motors,switches, and loads of other stuff. 

It also yielded the required transformer, Material for a case and the inner shell was stainless of just the right thickness for parts of the minijet I am considering for the next project, (the reason I needed a spot welder).

Modifying the Transformer

The first step is to remove the transformer secondary. This is the thinner of the two windings and it has a couple of turns of thick gauge wire over the top. It will also be heavily insulated so it is quite easy to spot which one.
The best way to do this is to saw off one side of the winding and drive the whole thing out of the core with a flat punch. Great care should be taken not to damage the primary winding doing this. A protective strip across the top of it will prevent any inadvertent damage with the hacksaw. It does take a fair bit of brute force to remove it. Do not remove the magnetic shunt between the two windings I shall explain why later.
If the core insulation should come away with the winding, no problem, Insulating tape is a good substitute as we are only talking about low voltages. (mine didn't luckily).
I haven't got any pics of this sorry but search "homemade spot welder" you should find examples of the method.
You should now have a transformer with the primary intact and a hole where the secondary was.
You now need to find out how many turns you need on the secondary. For this I used an AC voltmeter. Carefully connect the transformer to a test lead with a plug on the end and insulate the terminals on the transformer. Wind about 4 turns of insulated wire around where the secondary was, power up the transformer and measure the voltage across the ends of this temporary secondary winding. adjust the number of turns until you get about 2 Volts. This bit needs to be done with great care as the transformer primary is at mains voltage, and the ends of the secondary should not be allowed to touch as the winding will burn out very quickly. Once you know how many turns you need you can remove this temporary winding.

You then need some heavy gauge wire for the secondary and some terminal lugs for it. I used two windings of 16mm Sq Extraflex welding cable. each of 3 turns. Wind these in the secondary holes in the same direction. (takes a bit of effort to squeeze them in.) leave the ends long enough to reach the welding heads and cable tie them neatly in place. your transformer should now look like this.





As I am using two windings in parallel to keep the current up it is important that they are "In phase" with each other or they will simply cancel each other and burn out.
To do this strip a little of the insulation from one end of both windings and clamp them together ensuring that they do not touch anything . In my case I clamped the two inside bottom ends together (See photo).

Power up the transformer as before having first made sure the secondary ends are not touching anything and measure the voltage between the two joined ones and each of the free ends both should read about 2 Volts. Finally check between the free ends this should be zero. If you get 4Volts this means one winding is out of phase and needs to be reversed.
This again needs to be done with care as the transformer can now deliver it's full welding current and can make quite a flash and/or burn out the winding.

The cables will need some solder lugs to connect to the welding heads but I left this until the heads were made.

Next the welding heads, (bit of actual lathework and some primitive carpentry).

Regards Mark


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## basement_guy

This is most interesting.  I see the big ones everyday.  Stationary or mounted on a robot.  
There is also an electrical cabinet to regulate the current.  
They are liquid cooled and work all day long.

I'll follow your build.


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## modeng2000

I'm interested as well Mark.

John


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## skyline1

Thanks for the interest fellas, Yes the big ones are watercooled, they need to be, the currents involved are massive, thousands of amps not hundreds but at £20,000+ a little out of my budget. 

There are no real  mechanical drawings for this project as it sort of evolved as it went along. (As many of these things do) and was built mostly from scrap and bits and pieces I had lying around. As I am starting to learn Solidworks I shall be doing some in due course but in any event most of the bits are pretty simple and the dimensions are not critical. 

On with the build,

The Welding Heads







I now had a power source so I needed some "tongs" to do the actual welding. After a little head scratching Decided to try timber supports and came up with the Idea in the pictures. The supports are pieces of 1 3/4" x 2 1/4" softwood cut to 2 3/4" long. The dimensions are not critical but I had a length this size (2" x 2 1/2" planed). They do need to be cut square and and exactly the same length though. so I cut them a little oversize and faced them to length in the lathe just like a piece of metal. (run the lathe at top speed).
I then marked the exact centre of the widest face and drilled a 1/2" hole through both at the same time to make sure the arms would align. Two M6 clearance holes in each were next to take the clamp bolts you can see them in the picture on the top block. The two blocks were then carefully sawn across the centreline of the 1/2" hole with a hacksaw, it's a little thinner than a woodsaw and gives just about the right clearance for the blocks to grip the welding arms. The inside faces of the clamp bolt holes are counterbored on both blocks to allow the bolt heads to sit flush. The lower two mounting bolts pass right through the casing and secure the whole assembly to it. A small brass hinge connects the two blocks and allows the top block to pivot on the bottom one. This should be a fairly strong tight one but needs to be clear of the welding arms so they don't short.

Next more on the welding arms the start of the casing and first test.

Regards Mark


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## Dunc1

You wrote: "...16mm Sq Extraflex welding cable..."
I am not familiar with this wire. Please specify a wire gauge equivalent (or closest).
Thanks.


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## skyline1

Hi Dunc1

5 AWG is closest but I guess that's not a standard size so 4AWG is probably your best bet this is about 21mm Sq if it will fit in your transformer or you could go for a single winding of 2AWG which would give you about the same CSA.

I will be continuing with the build post soon but it might be a couple of days as I may be away over the weekend

Regards Mark


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## skyline1

First an apology for the delay in updates but it's been a busy couple of weeks and having a close friend in hospital with a heart attack didn't help. He is recovering well now though.

A couple of pictures of the nearly complete unit.







And it's control box




Back to the build

On assembling the welding arm supports I thought the distance between the welding arms was bit high so the inside edges of the support blocks were shaved down to make the blocks 2" high.





This is easily done with a wood plane or saw it then skim it in the lathe as before. It is personal preference and would most likely work fine with other dimensions. but you should try to keep the welding current "path" as short as possible as at these currents even tiny extra resistances matter.

The clamp pressure adjustment is taken care of by a piece of black mild steel strip (1/2" x 1/8")  bent to to a "U shape and a snug sliding fit on the outside of the blocks. It was tapped M6 in the centre for an adjusting screw. A small spring goes below it held in place by a stepped cup at the top and a plate of the same steel strip at the bottom to stop the spring sinking into the blocks.




The Welding Arms

The welding Arms are made from pieces of 1/2" copper, mine were actually bronze but only because I happened to have a piece of bronze about the right size they seem to work just fine. I am just learning Solidworks so please excuse the poor drawing quality (Another reason for the delay)





Note that the Dimensions are in metric as I used metric threads but Imperial could be used I simply found M6 taps and dies closest to hand at the time.

These are a fairly simple turning and drilling job and presented no problems, other than making the threads needed care (copper isn't the easiest material to tap) they could possibly be made from brass at a pinch but I have not tried it.

The welding Electrodes.







These are made from 1/4" copper. I had some I had bought from R.S.Components in the U.K. many years ago http://uk.rs-online.com/web/c/abras...als/copper-rods-sheets-bars/copper-rods-bars/

This should be easily available and many will have some suitable material in the scrap box as I did. The only slightly critical bit is that the end diameter of the top one should be slightly smaller than the bottom one this allows for very slight misalignment when they come together . After some experimentation I used 1.4mm and 1.6mm respectively. They should have a very slight radius on the ends easily achieved with a fine stone or crocus paper.

I found out about this through a close friend (and brother in law) who has used spot welders a great deal in the Aerospace industry, his help was invaluable on this project.

I found that I needed to skim a few thou' off the threaded portion to thread them properly. A couple of brass nuts secure the electrodes to the welding arms (must be brass or copper not steel).




That just about completes the welding head so next making a casing to put it all in (a bit of brutality with an angle grinder), the start of putting it together, and does it actually work.

Regards Mark


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## gus

Great work> When can we see the video? 

Gus


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## TNvolute

Mark-

I designed industrial automated spot welding equipment for about 20 years.  You have an excellent grip on what is needed in a light-duty, intermittent use welder.  For us a "small" welder started at around 20,000 amps capacity and went way up from there.  The components and current demands are far too expensive for the small shop.  Machinists have always been recyclers and "repurposers".  You are 'spot on' on your approach to this project.  For those who may have no experience with this field and wish to duplicate this approach, I would point out a couple practical matters.  Mark has sized the electrode tips properly to get a good trade-off of current density vs weld nugget size.  With use the tips will tend to mushroom and need to be kept dressed to original size.  Twice the diameter at the tip requires 4X the current which would probably smoke the transformer before getting a weld performed.  A good alloy in the USA for the electrodes is known as "Class 2" copper which is an alloy of copper and tungsten.  I'm not sure what it is called in other locales.  Class 2 has somewhat less conductivity but time between dressings will be much less as its mechanical properties are close to mild steel.  I can't wait to see the final package and am tempted to go trolling for an old microwave myself.

-Jerry


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## skyline1

Hi All

Thanks for your kind comments,

Gus,

I will be posting some video of it at sometime later in the build when I can scrounge a camera to take it. My Nikon DSLR doesn't do full motion video strangely, only time lapse.

Jerry

The theoretical max current on this one is 660A based on the transformer turns ratio and primary current. Much less than your 20,000A "monsters" but in practice has proved sufficient.  Within it's limits of course, it will weld 2 pieces of 0.6mm mild steel together quickly and with ease. Takes about 2 Seconds per weld. plus a few seconds for the weld to cool. I can't measure the actual current as I haven't got an Ammeter that will go that high but I estimate around 450 to 500A possibly a little more

The test welds when peel tested showed all the signs of a good weld, There was very little heat spread evidenced by the fact that the oxide ring around the welds was quite small. They pulled away cleanly leaving a hole in one part and the weld nugget attached to the other and they took quite a force to break them. Exactly how much I don't know as I haven't  got a tensile tester but it surprised me how strong they were.

Tensile tester, Cannibalise some old bathroom scales, some bits of angle and... Hmm... But I digress.

Your comments on electrode wear were much appreciated. I was expecting this which is why I went for the screw in approach making replacement of the Electrodes easy. I made a couple of spare sets at the same time for this very reason.

During testing the unit has done about a hundred or so welds with no appreciable wear as yet so I guess time will tell on that one.

This class 2 Copper/Tungsten alloy sounds interesting I shall have to do some research on that. Do you have any sources of info on it. I would imagine that with tungsten in it it might be fairly tough to machine threading especially.

More to follow on the build shortly.

Regards Mark


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## skyline1

Hi All

The Casing




I was originally going to to make a sheetmetal box but then decided to use parts of the original microwave casing. This had several advantages, It was at least partially ready made as it already had the mounting studs for the transformer fitted and a nice solid metal plate to mount it on. It also had a handily placed connector block by the side of the transformer. and a cable gland for the mains cable (in fact I reused the original cable). But more on that later

Initial Tests

Before I committed to further work I Wanted to be sure that it stood a chance of actually working. So I mounted the transformer in the bottom of the now empty casing. I stripped down the bottom welding head support block and clamped it to the front of the casing in front of the transformer. I spotted the holes through it with a long drill into the casing. and drilled them. The welding heads were then assembled into the case, the welding arms were lined up by eye and the clamping bolts tightened. The alignment of the electrodes was found to be quite good considering that the mounting blocks are timber. but I may change the arrangement at some later date.

Next step was to put some mounting lugs on the ends of the output cables, these are usually crimped on but I do not have a crimping tool big enough They can be soldered on so this is what I did I use a small propane "pencil torch" to do it and some resin based flux. You need to be quite careful as it is quite easy to melt or burn the insulation. It is a good idea to practice of a piece of scrap cable first.

The cables were cut to length (pair of heavy duty tinsnips) the lugs soldered on and connected to the back of the welding arms thus :-




Two things to watch for when doing this. that the two windings are in phase and the cables don't short against the hinge.

On connecting the mains cable I was now able to test it. and in fact the welder could be used like this but there is no way of controlling the weld time accurately. The initial testing was done outside with a fire extinguisher handy just in case (suitable for electrical fires) It is a good idea to double check your wiring at this stage and make sure everything is earthed just in case your transformer is duff.

The first test was done with the electrodes insulated from each other any thin strip of insulation will do I used an old credit card. Slip the insulator between the two electrodes and switch on. The transformer buzzed gently but not too loud as I expected, If It emits a very loud buzzing, grinding noise switch off immediately, something is amiss. I checked the voltage across the electrodes and it was about 2 Volts A.C. as it should be then switched off.

Now it was the moment of truth, I prepared some pieces of 0.5mm steel and cleaned them well  then placed two between the electrodes. I blipped the mains on and off for about a second, the transformer buzzed a little louder than the first time but not unduly, and the test pieces seemed to have got hot where the weld should be. On taking them out they seemed to have partially welded but the "weld" was very weak.

I carried out further tests increasing the "weld" time a little each time and adjusting the clamp pressure until at about 6 seconds I could get fair if not good quality welds. By then it was getting a bit too hot for my liking but I was being cautious.
The "Jaws of death machine" as a friend called it was obviously working though not too well.

I had made two mistakes, I had originally used steel nuts on the weld arms (I couldn't find any brass ones) so now I knew it worked (sort of) I made some brass ones and fitted them

The second one was that my electrodes were all wrong so after talking to my friend Jon they were corrected to the profile in the drawings my original ones were too big. This has been confirmed by TNvolute (thanks mate handy to have the pros with me).

The difference was quite dramatic It was now producing super welds in a couple of seconds and the heat was concentrated right where it was needed. Much to my surprise I now had a workable spot welder but I was getting a bit tired of counting one thousand, two thousand.  So a timer was needed.

This will be next along with the rest of the casing saga and some pics of the test welds. (and a video for Gus if I can borrow a camera)

Regards Mark


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## TNvolute

The mantra in spot welding is "squeeze, weld, hold, release".  This is the differing amounts of time for a cylinder or other actuator to reach welding pressure, X cycles of weld current, time at pressure to allow the weld nugget to solidify and cylinder retraction.  I could get into a lond dissertation of all the theory but we are using materials on hand and do not have a production schedule or car crash-worthiness standards to meet so we'll skip all of that.  There are rules to follow in design and you seem to have already hit upon the most important, whether by accident or design.  One of the most important is that you want to keep any magnetic materials out of the "throat" of the welder (arms, electrodes and secondary wiring) and not very near.  We had a rule of thumb that nothing ferrous was to come withing 2" of any part of the throat.  At these lower power levels, that can probably be reduced but the throat becomes an induction heater to such items, robbing weld efficiency.  Also, keeping the loop area of the throat as small as possible consistant with the job requirement increases efficiency.  The means keeping the weld arms close as practical and running the cables as short as practical will help.  this will reduce impedance (resistance to AC flow).

An easy check of your machine is to weld a couple dozen spots, disconnect the mains and check all your connections for excess heat.  All losses show up as heat.  I am used to industrial duty design and have to dial my thinking back a bit but I really believe you are on the right track and can optimize what you have built by common sense (seems you are well-blessed in that regard) and observation/experimentation.  Lead on!

-Jerry


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## old-and-broken

If you wish to stay with wood

might I suggest Lignum Vitae.      VERY hard stuff.
http://www.rockler.com/product.cfm?page=18575&site=ROCKLER&filter=lignum vitae

Also Olive Wood is similar, and more 'showy', but a little pricey in large chucks.

Now I want one too!


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## gus

Was paying too much for wire net belt guards. Bought a Taiwan made Auto Spot welder with  " 4 inch x 2 inch wide" electrodes.
It was a so easy with dial Knobs to set the Squeeze---Weld-----Hold PLC Control. Believe timers were built-into  PLC.
Weld timing is very important. During my holiday student attachment with the British Army Vehicular workshop,I was watching the sheet metal maestros working on the Old Fashion Spot Welders. Every spot spot was perfect.They did timing by mental count!!!
They build very good filing cabinets and others.
All spot welds were uniform.
Today my Taiwan machine and the jigs and benders still operate in Nanjing making beltguards.


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## skyline1

Jerry,

Yes just the mantra I am using, squeeze and release go out of of the equation a bit on this one. As it's "Handraulic" rather than hydraulic or pneumatic these steps are accomplished by simply inserting and removing the components. the weld stage is, as far as I know, controlled by three factors current, pressure, and time. The first two are actually interrelated as increasing or decreasing the pressure will change the resistance of the "interface" between the two components and between them and the electrodes and hence the current (I = V/R). I have found that the squeeze pressure  required is in fact quite low, just sufficient to clamp the two pieces together gently. I think, and correct me if I'm wrong, that increasing it would produce poor welds as it would tend to reduce resistance and therefore the heat input (I²R). 

The second factor is weld time hence the need for a timer which I will be describing next. It makes the welds more consistent and when set right makes the thing almost "Idiot Proof" just put your components in and press the button "Simples" (Alexander the Meerkat).

The only other stage in the process is the "hold" or weld cool time, with a little welder like this not too much of a problem as this is achieved very quickly within a couple of seconds in fact but but I might at a later date include a second timer that indicates weld cool and that I can remove the components. not strictly needed for operation but possibly a handy little refinement.

As you correctly point out this being A.C. we are talking about Impedance here so Capacitance and Inductance come into play. At 50Hz I don't think stray capacitance is a problem. But with high currents stray Inductance could be and your point about keeping magnetic materials away from the output loop is well made. This is one reason why I used timber for the weld arm supports, Had I used steel with insulators I would have created a transformer core of sorts right around the weld loop ! For the same reason I screwed the pressure adjusting "hoop" to the bottom block rather than bolting it to the casing and left a small gap between it and the casing (A point I forgot to mention, oops) Had I secured it the casing I could have created a "shorted turn"  which could have robbed some of the weld current and on this I need every Amp I can get (within reason).

Your 2" rule sounds like a good maxim to follow so I am thinking about rerouting the output cables straight from the transformer to the welding heads and let them hang under their own weight  they are more than strong and rigid enough to do so. It won't look quite so tidy but might improve efficiency a little as it keeps them further from the casing and makes them slightly shorter.

Old and Broken

Lignum Vitae, (wood of life from Latin) Is a great idea if I can obtain some in the U.K. I used some once, Had to make some bearings for a flour mill. It is still used in this sort of application. You are right about it being hard it machines lovely. It's more like turning a piece of metal than timber.

Gus

Old methods can be very effective in the right hands I know a chap who can count seconds so accurately that after two minutes he's less than a second out from a digital watch. He is a drummer in a rock band which might explain it. He was always late for band practice though. It's a strange old world.

More to follow on the build shortly it gets a little bit more technical now but not too technical hopefully.

Regards Mark


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## skyline1

Hi All

First another Safety note I neglected to mention in my initial comments. Due to the high currents and magnetic fields produced anyone with a Cardiac Pacemaker fitted should not use or be in close proximity to this or any other spot welder.
Pacemaker wearers will undoubtedly be aware of this but it is worth mentioning.

Test welds




As can be seen from the photo I was now at the stage where I could produce reliable welds which when "peel" tested pulled the weld nugget away cleanly leaving it on one test piece and a hole in the other. This indicated that the weld was at least as strong as the parent metal. If the weld itself fails this means that the weld time or pressure needs adjusting. I have found that when the pressure and tip geometry is correct the weld time is around 2secs.

The Power Supply 




I decided to use a conventional relay to control the transformer. I used an Omron LY2NJ 8 pin plugin relay Which is rated at 10A 250V and can switch inductive loads up to about 1500 VA It has a 12VDC coil needing about 75mA to energise it. Any relay of this sort of rating or above could be used or even a solid state one of sufficient capacity. A future enhancement of the spot welder will use an SSR but that is still in the planning stage.The relay can be seen in the pictures above in front of the transformer.




A power supply was now needed to drive the relay. Whilst any small 12V PSU could be used I decided (in the spirit of repurposing) to see what I could scavenge to do the job. A friend had given me an old server computer case which still had it's PSU in it along with quite a few switches, LEDs and pushbuttons. This was overkill really but as I had it and it hadn't cost me anything I decided to see If it would work.



As can be seen from the picture It is not a standard size ATX supply and is only rated at 170W so it probably couldn't be used in a conventional P.C. anyway.

To get one of these to "fire up" needs two things. A switch to connect the PSU enable wire (green wire) to 0Volts (any one of the black wires) and a small load on the +5Volt line (red wire) to trick it into thinking it has a computer motherboard attached. This is achieved by connecting two 4.7 Ohm 5Watt power resistors in series between the +5Volt wire (red) and the 0 Volts wire (Black). These can be seen in the picture above mounted on the terminal block. They will get quite warm in use so need to be mounted in free air.

A little bench testing then ensued. I connected the mains input and switched it on. nothing much happened as I expected. But a voltmeter check between the +5 Volt standby rail (purple wire) and 0Volts (black wire) showed  5 volts as this supply stays on even before the main supplies are active.

The load resistors were connected temporarily and a slide switch connected between PS EN (green) and 0 Volts (black, any one) by simply pushing wires into the ATX connector. I  turned the PSU back on and slid the slide switch. Lo and Behold the fans started whirring and a check with a voltmeter showed that it was producing the right voltages on all rails.




Out of interest this is also a great way of making a bench power supply as it gives all the standard voltages used in electronics at quite high currents. The list above shows where they all are

As the PSU still had it's mounting screws a couple of holes were drilled and it was mounted in the casing. I then made a sheetmetal bracket for the connector blocks. and fitted this to the top of the PSU.  Some 10 Amp  "chocolate block" connector blocks were fitted as per photo. I cut off the 20 pin ATX power plug and connected all the wires grouping ones of the same colour together as can be seen. No problem doing this as they all come from the same point internally anyway. the "dummy load" resistors were also fitted. The bundle of wires under the bracket are the other supply connectors from the PSU which I decided not to cut off just in case I needed them.

The relay contacts were then connected as in the circuit diagram above and a slide switch between the Green and Black wires on the terminal block to enable the power supply.

I had decided to put the timer and controls in a separate little control box so for initial testing the relay coil was wired to 0Volts on the terminal block (Black) and  +12Volts (Yellow) via a momentary action pushbutton (Push to make).

It is important to get the polarity right on this the +ve side (yellow wire from PSU via the pushbutton ) and the end of the little Diode with the white band on it should go to pin 8 of the relay and the -ve side (Black wire from PSU) to pin 7. The diode is to suppress back EMF spikes from the relay coil. The pinouts should be written on the relay




To test I plugged in and switched the mains on then slid the switch, the power supply came on so I put some test pieces in and pressed the button for a couple of seconds. The relay clicked, the transformer buzzed, and my weld was complete. Just what I wanted.

This power supply technique may seem a bit big and complex when any 12VDC source that can produce enough current to energize the relay will do but it cost me nothing and I have other voltages "on tap" for other things like cooling fans for example. 

So far the project had cost me about £20 for the welding cable and lugs and the Relay.

Next finishing the casing, and making the control box (time to get the soldering iron out).

Regards Mark


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## Lew Hartswick

Only comment I'd add is to use a solid state "relay" . They are commonly available for
currents well in excess of the primary required here and easier to connect to and silent.
  And even not expensive if you have to depart from the "salvage" line a bit.


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## skyline1

Hi All

Lew, I agree with you about solid state relays and they are more reliable than mechanical ones. There are no contacts to wear or weld and they have the advantage of being able to vary the weld current by phase control. I have used theatrical lighting dimmers in the past and they are quite capable of handling reactive loads like this and have ratings in the order of 5 - 10KW (or KVA in this case) or more sometimes. I am planning to use something like this on the next "iteration" of the spot welder.

But back to the build

Finishing the Casing 




Until now the spot welder components had been mounted in one end of the old microwave casing. I had not cut it down as yet as I didn't know exactly how much space I would need. With all the subassemblies fitted I could now mark it out but everything had to come back out again to cut it. This was not actually too tricky, all I had to do was disconnect a few wires and take the various "chunks" out complete, this also gave me the opportunity to photograph them. Next came the cutting, bashing and bending.

This is where things got a bit "brutal", it was more "Scrapheap Challenge" than "Precision Engineering 101". With nothing to lose the now empty bottom was firmly clamped to a large piece of wood (an old table top) And with the aid of an assistant I set about it with an angle grinder (actually I did the assisting but it's a long story). Good choice of tool as it happens it made short work of it and left a remarkably clean and accurate cut. Where I had cut it there was a pressed depression which needed to be got rid of so I put it on my blacksmith's anvil (another long story) and more "violence" ensued this time with a hammer and a big mallet. The remains of the depression can be seen in one of the photos but it's only on the edge and covered by the lid. Two 1" x 2" wooden battens along the bottom act as feet and strengthen things a little. A coat of black paint later and the bottom of the casing was done.

A start was also made on the cover for it this is still not quite finished as it needs securing down one side and some ventilation grilles fitted (I can spot weld them in now ;D) and it needs painting.

The cover was made again from the old microwave cover which saved me some work as one side  was ready to go (I could reuse the existing screws). It was simply marked , cut to length ( more angle grinder)and bent down to cover the other side of the welder. I do not have a sheet metal folder but I use two lengths of steel angle clamped in the vice then just bend the Mat'l over and dress it down gently with a hammer and block of wood. It is very effective and I have done many bits of sheet metal folding by this method.

Having angle ground, bashed and bent the casing to shape (great stressbuster) it was now time for some gentler stuff but I'll hold that until the next post as I'm still getting the drawings together for the controller. As I'm partially Dyslexic my rough sketches would probably be a bit difficult to read.  It's not that difficult to build and the good news is that most of it is available as a "ready to build" kit intended for educational use so anyone with basic soldering skills should be able to put it together. to whet the appetite of anyone wanting to have a go link to it here http://www.ebay.co.uk/itm/555-Monostable-Timer-Project-Self-Build-Kit-/120916552557?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item1c27302b6d 

The Electronics enthusiasts among you are probably having a chuckle right now but as of 2003 they were making about a Billion of 'em a year (Wikipedia) so it's tried and tested. 

Regards Mark


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## skyline1

Hi All

Sorry for the delay but I've been working on the drawings for this and will post them in the downloads section shortly for anyone interested.

The Controller.






I decided to put the controls in a separate box for a couple of reasons firstly for versatility so I could move them about wherever I wanted and second to keep the electronics away from the high current stuff and shield them somewhat.

I used the somewhat old (and cheap) but well known NE555 timer chip for this but a digital timer would be equally suitable, or even a simple pushbutton although my controller does have a facility for manual control as well. Or a mechanical timer could be pressed into service if you happen to have one. The minimum setting on the original microwave timer was 5 Seconds so I couldn't use that unfortunately.

I make no claims to have designed this exactly, as the circuit is pulled from the datasheet for the chip, I merely adapted it to suit.

I used a premade P.C.B. and Kit of parts for most of this available here http://www.rkonlinestore.co.uk/555-monostable-timer-project-self-build-kit-654-p.asp 

This is designed as an educational project and as such is intended to be easily constructed with basic electronics tools. The complete documentation including quite detailed build instructions is available to go with it. including soldering tips.

This departs a little from my scavenge and adapt theme but at less than £5 for the kit was hardly reckless expenditure.

The controller could also be built on a home made P.C.B. or Stripboard as there are no high voltages or currents involved in this part. 

I use Cadsoft EAGLE for  Schematics and P.C.B.s Freeware Version available Here http://www.cadsoftusa.com/download-eagle/?language=en but there are several others. I shall Include EAGLE .sch files in the download zip.

The Circuit.







The First Stage was to construct the Timer board. The following values for the timing components give a timing range from approx 100mS to about 11 Seconds which in hindsight is probably a little long but can be quite easily changed.

Timing Capacitor C3 10µF

Timing Resistor R5 10K&#937;

Timing Control Pot VR1 1M&#937;

I used flying leads for VR1 and L.E.D.s 1 and 2 as these are mounted off board (on the front panel) as can be seen in the pictures. The board was tested using a 9Volt PP3 battery and a spare relay and base. The kit actually includes a battery clip and 9Volts is just about the "Must operate" voltage of the relay used so the controller could be built as a standalone battery powered unit if required (useful for other jobs)

There are some extra components Like the pulse shaper, the manual override and the power switch but more about that next time along with connecting it all it all up.

Regards Mark


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## starnovice

This is really interesting.  Thank you for taking the time to post it.

A couple of questions, probably obvious but I have never used a spot welder so don't know:

1. What do you expect to be the maximum thickness that you can weld?

2. How do you bring the jaws together, just push down on the bolt with the spring?

3. Would a foot pedal to bring the jaws together be useful?

4. Can the length of the electrodes be made shorter, say if you were spot welding a tube and needed for the electrode to fit inside?

Again thank you very much for presenting such detailed information!

th_wav

Pat


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## skyline1

Hi Pat

Until I started building this one I had only used a spot welder very occasionally myself and it was many years ago so I was on a learning curve myself.

This is only designed for thin sheet the maximum I have used it on is 0.5mm (each piece) which is around 20 thou'  but I think it would be able to do a little more say around 1.6mm overall (63 thou') It would probably struggle at much more than that.

You don't need to bring the jaws together simply lift the top jaw against the spring and slide the components in the gap. You then just gently let it drop against the bottom one and you're ready to go, use gloves though as it may be quite hot. Check the components are lined up as you want and readjust if needed. Then as the song goes "push the button" .

You could use a spring loaded pedal if you wish, this is simply how I did it. Experimentation is the name of the game with this sort of thing. A foot switch to "fire" the welder would be a good idea though so you could use both hands on the job. You could wire it across the weld pushbutton on the controller and put a socket in the back. A momentary action guitar footswitch would be ideal and they come with a suitable plug fitted (1/4" Jack). Very good idea to turn the welder off unless you are actually welding with it lest you accidentally step on the pedal.

Yes you could make the electrodes shorter for things like tubes as you describe. I think the best way to do it would be to make one longer than the other thus bringing one of the weld arms closer to the circumference of the tube and enabling it to fit inside easier. Have a look on Ebay for spot welder arms for inspiration on this, prices are pretty scary though that's why I had a bash at building one.

Regards Mark


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## skyline1

Hi All

I've had a little trouble uploading the drawings but Austin is looking into it for me meanwhile should anyone want them PM me and I'll send them to you.

Back to the build












Having built the timer board it was tested and worked fine so I hooked it up to the welder (only 4 wires, +12V to PSU yellow, 0V to PSU black, + and - to the relay). The board will operate from quite a high voltage range but 12Volt relays are easily obtainable (and SSRs). 

"Live" testing then commenced and this is where I hit a slight snag, the timer worked great for thicker material but on very thin stuff was a bit inconsistent. (Like my experiments making wacky bits of "modern art" out of old tin cans, which was great fun, but back to the plot).

The problem was that the pulse from the trigger pushbutton needs to be shorter than the timer delay or it "holds on" until you release the button. On really thin stuff like tin cans. (which is only a few thou' thick) the weld time is so short that it's less than the time it takes to press and release the button.

A little research was needed, luckily a search for "NE555 timer circuits" yielded hundred of sites devoted to the "little blighters" including this one http://www.doctronics.co.uk/555.htm which I found very interesting. It also has breadboard layouts and calculators for working out the timing component values for anyone who fancies having a play with them.

It turns out that what was needed was a "Pulse Differentiator" (What a mouthful) which despite it's grand name actually consists of only 4 components 2 resistors a diode and a small capacitor. As one of the components is already on the board (R4) the other 3 were mounted on a small piece of stripboard which can be seen in the photos next to the main board.

I decided that I also needed a manual override and this is where S1 on the diagram comes in. This is a single pole double throw toggle switch with a centre off position. It is spring loaded back to off on one side (the manual side) and latches on the other. (the auto side) I thought that this might be a difficult and expensive component to obtain but not so I obtained mine for Maplin in the U.K.http://www.maplin.co.uk/sub-miniature-toggle-switches-2341 Part No                                              FH02C R.S. and Farnell also do them. In other parts of the world they should be stocked by most Electronic Component suppliers. Failing this try your local motor spares shop they are widely used in motor cars for auxilliary switches.

In operation when the timer energises it turns on TR1 and grounds the negative pole of the relay thus energising it so to pull it in manually simply bypass the transistor to 0Volts, this is how S1 works. It has the added advantage that the timer also runs although it doesn't actually do anything which can be handy to set the weld time. If this is not needed the controller will work perfectly well without it.

With everything now working I had quite a "birdsnest" of wires and bits of electronics next to the welder, (and several tin can "sculptures") so it was time to mount it up properly. 

I had, in my vast collection of electro-mechanical junk (yes I am a hoarder, I don't see rubbish, I see projects). Some little project boxes. R.S. part No 508 - 605 if still available. These were not new as they had no front or back panels and had had some holes drilled in the bottom. To cut a long story short front and back panels were made from thin galvanised steel. (something else I had quite a bit of) and in went the various switches and LEDs  Mostly either from the timer Kit or salvaged from the old server case. A gland in the back for the cable and the weld time pot. I will make a label for the front panel shortly to tidy things up a bit. (Scarcity of round tuits). 

The two circuit boards were mounted into the case along with the chocolate block connector and the timer was wired up with 16/0.2 wire, the connection diagram above should be fairly easy to follow. I used twisted pairs mostly for neatness (well sort of, it's still a bit messy). Quick way to do it BTW is put one end in the vice or put a weight on it then wind 'em together with a pistol drill. 

The controller circuit can be adapted any way you want and could be built in but it should be in a metal box for shielding (555s are pretty tolerant though).

Well there you are spot welder pretty much complete. just one more post about the cable connections and the "round tuits" (When I get one). And hopefully a video of it in action I haven't forgotten, Gus.

Regards Mark


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## skyline1

Hi all

I hate Stainless Steel, It fights you !:rant:

Sorry to start with a bit of a rant but I've been breaking up the inside shell of the old microwave to recover the stainless steel and guess what, it's spot welded together.

Now I am breaking spot welds rather than creating them I am beginning to realise what a monster I've created. Spot welds even tiny ones are very strong indeed especially in Stainless. It is taking quite a lot of drilling and grinding to " Resolve" this into some flat sheets I can use.

Final bits of the build.

View attachment Spot welder cable termination.pdf


In a previous "life" I ran a small P.A. Company doing concert sound and the like. As a result I had a quantiy of 3 pair screened audio Cable which proved ideal for the welder. The Connection diagram is above. Its the same old story slight overkill but it was lying around doing nothing.

It is probably not strictly necessary to go to to these lengths and only 5 wires are needed as the 2 0Volt connections go to the same point on the PSU but I had a 6 Core cable so I used all of them. 

It is probably possible to use plain unshielded cable although I haven't tried it and as we are talking about low voltages and currents here a piece of computer network cable would probably be suitable. Or indeed anything flexible and rated at 12Volts or above that can carry 75mA.

Further "Round Tuits"

During the course of this post many good ideas have been suggested which I shall take up in due course

I need to finish and paint the casing and make a front panel (not sure what of yet plywood possibly.)

I shall reroute the cables from the transformer to the welding heads (shortest possible route).

I will do a label for the controller with all the names of the various controls and a scale for the weld timer.

I will probably be reworking the controller to put it all on one P.C.B. as I can see uses for it elsewhere.

Following Lew's comments about SSRs I am designing an SCR (Triac) based
power controller to vary the welding current with an Enable/Disable Input from the controller and an overheat protection function.

As well as the advantages previously discussed it has 2 other I think, quite important ones.

It will energise the transformer in a much more controlled manner and can be to an extent "soft started"  reducing switch on surge.

The Transformer as it stands has a magnetic shunt fitted which I mentioned right at the start of this build (it is the metal piece between the two windings). The purpose of this is to limit the output of of the transformer. Which is why Microwave transformers ere quite well suited to this sort of job. In the microwave it also has another function but it is a bit technical and needn't concern us is this application.

The effect of removing it  would normally be that the transformer would draw as much current as it wanted (too much) resulting is blown fuses, tripped circuit breakers, and very likely a burnt out transformer, (It wouldn't do the relay or the wiring much good either).

But with a means of effectively reducing the input voltage by phase control (the same way a dimmer or a speed controller works) I think It would be possible to extract more current from the transformer than it's original design allows In short bursts of course and the supply wiring would need to be beefed up a bit (you can't get out what you don't put in)

There are problems to be overcome with this idea and this is taking me more into the realms of commercial units and away from my scavenge and repurpose theme but is worth future experimentation.

Well that just about wraps up the spot welder from scrap project I hope it has proved interesting and given some food for thought.

Regards Mark


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