DIY spot welding machine

[kagliostro]

I want to build a spot welding machine

in the web you can see many video about the use of a microwave oven for this use

https://www.youtube.com/watch?feature=player_embedded&v=RnjF5Hj2Udg

I've a pair of this transformers ( ~2000v @ 600mA), but I've find one that I think is better, the transformer is recovered from a broken Telwin arc welding machine 48v @ 140A and I can regoulate the current via a knob who acts on the core of the transformer

My intention is to roll out the secondary winding (that is physically broken at one extremity) and to use a 24mm² cable to wind the new secondary

usually the people that build this DIY version of spot welding machine has a small space and obtain 1.6v-4v

but usually a spot welding machine has 5v-22v on the secondary

I've more space on this transformer and so I can think to have a pair of secondary in parallel to have more current (that is important for resistance weld) or to have only one secondary with the double of volts disposable

Usually a DIY spot welding machine is used as to spot on iron or steel, as to spot aluminium is necessary a machine with a much larger current (very large) because of the low electric resistance it has and because the heat dissipation is very high on aluminium with respect to the iron

However with a trick is possible to use a common spot welding machine to spot on aluminium

Metal Shaping with Lazze: Spot Weld Aluminum with a 220V Welder

Spot welding 99.9% Aluminum with DeltaSpot

I would like to adopt the best solution for the use of this trick, so, which is your opinion about the secondary ? is better to have double current and a lower voltage (5-8v I think) or 1/2 current and double voltage (10-16v) ??

This will be a question for SILVERGUN I think

K

[PRR]

You really do NOT want Volts for spot-welding.

A 1/8" (3mm) diameter copper wire is 0.7 Ohms per 1,000 feet.

Aluminum is about twice, say 1.5 Ohm per 1000'.

Assume 1/16" total thickness. 0.000,01 Ohms in the spot.

I would assume a "120V 20A" wall-outlet could supply 110V 40A for a split-second. Call it 5,000 Watts.

(I do not know if 5,000 watts is enough for this job.)

0.2 Volts in 0.000,01 Ohms is 4,000 Watts. And 20,000 Amps.

So you only need like a quarter of a Volt at the spot.

That assumes you can magically transport the power to the spot without loss. In fact the transformer is some distance from the spot. You minimize this. You don't want your wires or arms to get hot enough to melt, also you want low loss, so the arms are much wider than the spot.

You probably also want the winding to be bigger than the spot, though we do have to compromise to fit it in the space.

Say arm is 4 times the diameter of the spot, 1/16th the resistance per foot. 1.5/16= 0.1 Ohms per 1,000 feet. Say two 1-foot arms and the equivalent of 2 feet in the winding. 4 feet at 0.1 Ohm/ 1000' is 0.000,4 Ohms.

This is not great. We have 0.000,4 Ohms dead loss in series with 0.000,01 ohms of useful load, the spot. Only about 1/40 of the total power ends up in the spot. With 4,000W input we get 100 Watts in the spot.

4,000 Watts heat input to 0.000,41 Ohms is 1.3V 3,000 Amps.

So "needing Volts" on a spot-welder means you have more resistance in arms and winding than in the spot you are trying to weld.

> usually a spot welding machine has 5v-22v on the secondary

I have to think on that.

Additionally: most Aluminum welding needs flux. Very high temperature will cut the Oxide without flux. However the oxide is still in the weld layer. You need large pressure to upset the molten metal and break through the hot oxide. (You also need large pressure for low contact resistance.) The one time I used a spot-welder, HIGH pressure seemed to work better than adjusting the current or time.

[kagliostro]

Many Thanks PRR & Drgonzonm

So "needing Volts" on a spot-welder means you have more resistance in arms and winding than in the spot you are trying to weld.

and isn't it so ?

also with large arms and cable for the winding (24mm² for me, I wasn't able to find something larger)

the thin space to be spotted hasn't a lower resistance anyway ?

---

About pressure I agree that it is very important and I'll try to build a system with good pressure abilities

---

The info about the range of 5v-22v  for spot welding comes from here (pag.491)

http://books.google.com/books?id=sERj55e85bEC&pg=PA491

K

[SILVERGUN]

Ciao K,
PRR is right on with his thoughts about current and pressure.....that is ALL you need...BUT, lots of both
The more pressure you can put on the work piece, the more current you will be able to conduct through it,,,,that's why they are BOTH equally important

It is much more important that you have enough current, to "back up" whatever voltage you wind up with on your secondary
The actual voltage is not significant.....the current IS

It is the resistance of the work piece that is responsible for the flow of current....and that is why aluminum is a difficult subject
The heat is more likely to "fan out" away from the desisred weld point, due to it's excellent thermal conductivity
It is more likely to dissipate the heat than focus it into a nice spot weld

Part of the reason I have no help for you is because my documentation doesn't even touch on it,,,and the schematics for the units I see are very simple
Just a big transformer with a set of high pressure tonges (tips)
The size of the tip is also important to the outcome.....
The 208/230 volt models that I'm familiar with use a primary xfmr to initially step down to 115 volts, which doubles the available current before it gets to the secondary xfmr

The closest that I can get on a spec for you is:
Material thickness: 0.016" magnesium alloy
Electrode force: 300lbs.
Welding Current: 17,000 amps
 
Al should have similar requirements
I guess it might help to have some type of "capacitor discharge" system for this.....

[SILVERGUN]

K,
I have one of these here:

I attached the tech manual

[kagliostro]

Hi SILVERGUN

Many thanks for the info and the manual

---

In pic 9-2 is the #5 (that flat surface that remember a U) the secondary of the transformer ?

that is really a large winding, many mm² of copper

Franco

[PRR]

> cable for the winding

Figure 9-2 in the manual that Silvergun posted.

The secondary is a SOLID BAR maybe 2" 50mm wide and 20" 500mm long, bent in a U. I would guess 1/4" 6mm thick, but I don't trust that part-drawing to scale.

So it is "one turn", but the core is very extended. If the core were more cube-shape, you would use 2 or 3 turns to get the same voltage. Say 15mmX6mm copper bar. You would have to get it DEAD-soft to make those bends.

The manual gives this small data: "Resistance spot welds are made with very low voltage (1-3 volts) and with very high current (4000-5000 amperes) applied under pressure for a brief time period (1-3 seconds) which causes intense localized  heating and a weld."
____________________________

> The 208/230 volt models that I'm familiar with use a primary xfmr to initially step down to 115 volts, which doubles the available current before it gets to the secondary xfmr

The primary could be wound for either voltage, and do the same work. Use twice as many turns of wire of 0.7 times the diameter, you get the same job done.

The limit here is that "120V" outlets are mostly 20A, so you can only suck 2400 VA from the wall. The 120V model is rated 1500 VA output, which suggests large internal loss. (Also 50% duty cycle, while a core that big should handle 1500VA continuously; there are many changes and compromises when you really want CURRENT!!)

(However they seem to want a 30A fuse for 110V operation?)

Given 230V power they can supply more power at the same "safe" cord voltage, 110V. They also give you 8 pounds more core. Now the limit is how big a tool you can handle comfortably. (If working on cars and such. If all work comes to the bench, the core could be as big as you can afford.)

I don't know how many Amps is in Kagliostro's outlets. I would use the clothes-dryer or kitchen stove circuit, not the bedroom lamp circuit; something wired for BIG load. Even the microwave oven cores *could* pull a LOT of power (more than the microwave oven) if re-wound.
___________________________

I have a *heavy* book that I wish I could ship to you. It has 38 pages on spot-welding processes. Mostly we got things close-enough. However it says (without evidence) that Aluminum "requires" ramp-up/down control. That it should rise from 5,000A to 20,000A, hold, then fall back to 5,000A before cut-off. Silvergun's manual clearly does not do this. It also says "Not recommended for aluminum or copper alloys." It is for cars, ducts, brackets, grates, in Steel. Surely you "can" weld aluminum with it, but the capacity may be thin and the weld strength may be poor. (And Miller is probably happy to sell you a different model they say is good for Aluminum.)

[kagliostro]

Thanks Again PRR

The line in my house is ~220V @ 3KW (also if now here the standard is 230V)

As to act the circuit I've a SS relay (220V 25A) that I'll drive with an industrial timer

that can give me times from 0.05s to 1s continuous

and intermittent (the timer acts sequentially two equal times of ON and OFF)

from 0.2s till 4s - from 0.5s till 20s

and other longer times

Today I've find a supplier also for 35mm² and 50mm² wire, much better than the 24mm² I've found previously

I'll build the spot welding machine in the better way I can, I think that it will be able to spot on iron and steel and give acceptable results, about aluminium I'll try hoping to be able to obtain some results, if not I'll use the machine for steel

K

[SILVERGUN]

I took a look at the schematic and realized that I was wrong about the primary transformer....
It only acts as a control xfmr for the internals of the control box,,,,contactor, timer

The spot unit taps directly off of the primary

The unit I have here is the LMSW-52T from 2004....never used
230 VAC model with timer/contactor box......no tongs
If you are interested, I can sell it for $400 plus shipping (not sure what that would be to Italy)
Thanks for bringing this up, because it prompted me to look around and realize that this is just sitting here collecting dust.
I know you would like to build one, but I just figured I would let you know that this is here if you need it.
 

[kagliostro]

Many thanks for the offer but I must decline

(you must consider also that  shipping charges and customs fees would be ridiculous)

Maybe someone in the forum could have interest in the machine

Franco

[kagliostro]

I've a question

to joint the terminal lug to the cables (35mm² - 50mm²) of the secondary of the transformer

I haven't a so big crimping tool, can I soldering with tin the joint or the current is too high and I must find an adequate measure crimper ?

Thanks

K

[kagliostro]

I can find both (crimp lugs and screwed lugs) I must decide what to do

Thanks

K

[SILVERGUN]

You might want to end up with a mechanical (screwed) crimp rather than crimp. 

I agree.....in most welding machines there is a mechanical (nut and bolt) connection between the heavy xfmr secondary windings and the rectifier / stabilizer assy.
In some cases they just smash the end of the wire a little flat so that it's wide enough to drill a decent size hole through

[kagliostro]

Grazie Dave

Franco

[SILVERGUN]

Here's an example:

[kagliostro]

WOW, that is really a Solid Core wire 

Franco