Welding

[Little John]

Had a bid in on an AC welder on Ebay. Lost it as I wasn't prepared to go ver a hundred.

However, necessity being the mother of invention, I think I've found a way round the uneven carbon consunption problem on a twin carbon arc torch a DC welder. I just need to reverse the circuit every three minutes as it takes about three and a half minutes for one carbon to wear down sufficiently more than the other to break an arc. All I need for this are 4 inline junction boxes and 4 inline switches. These can be part of the wiring that comes out of the back of the torch and so don't have to be a part of the main welder leads.

It means I have to stop the job every three minutes for about 10 seconds or so to press 4 switches to reverse the circuit. But that's no major hassle.

Anyone see a problem I might have missed? That includes you JSD....

[Little John]

Had a bid in on an AC welder on Ebay. Lost it as I wasn't prepared to go ver a hundred.

However, necessity being the mother of invention, I think I've found a way round the uneven carbon consunption problem on a twin carbon arc torch a DC welder. I just need to reverse the circuit every three minutes as it takes about three and a half minutes for one carbon to wear down sufficiently more than the other to break an arc. All I need for this are 4 inline junction boxes and 4 inline switches. These can be part of the wiring that comes out of the back of the torch and so don't have to be a part of the main welder leads.

It means I have to stop the job every three minutes for about 10 seconds or so to press 4 switches to reverse the circuit. But that's no major hassle.

Anyone see a problem I might have missed? That includes you JSD....

It works.

I've also figured out the optimum combination of amperage and carbon width to get 3 minutes of arc before needing to reverse current. So, for example, 4mm carbons will operate at 25 amps for 3 minutes before needing to reverse the current, 6mm at about 35 to 40 amps and so on. I'm guessing 8mm will give me 50 odd amps and 10mm will take me nearer to 80. The little bit of brazing I tested out yesterday suggests that 50 amps is about right for brazing thin steel.

What all of the above amounts to is a "torch-like" tool that can be used to weld and/or braze very thin steel. It also allows for brazing of some unlike metals as long as both their melting points are above the melting point of the brazing filler metal. Finally, it allows for the heating and forging of steel and other forgable metals.

Essentially, it can be seen as an electric "oxy-fuel" torch, albeit a rather crude one. But, it does the job, so that's good enough for me.

Including the twin carbon arc torch itself plus all of the other switches, wiring and junction boxes, the lot comes to about a 100 quid.Carbons can be had for about 15 quid for a hundred.

[adam2]

Had a bid in on an AC welder on Ebay. Lost it as I wasn't prepared to go ver a hundred.

However, necessity being the mother of invention, I think I've found a way round the uneven carbon consunption problem on a twin carbon arc torch a DC welder. I just need to reverse the circuit every three minutes as it takes about three and a half minutes for one carbon to wear down sufficiently more than the other to break an arc. All I need for this are 4 inline junction boxes and 4 inline switches. These can be part of the wiring that comes out of the back of the torch and so don't have to be a part of the main welder leads.

It means I have to stop the job every three minutes for about 10 seconds or so to press 4 switches to reverse the circuit. But that's no major hassle.

Anyone see a problem I might have missed? That includes you JSD....

The only problem that I can forsee is cost and complication, suitable switches for that much current are far from cheap.
If you can find a suitable switch, you can do it with a single switch.
You need a four terminal reversing switch, these are also called intermediate switches for switching lamps from 3 or more locations.
Readily available in low currents for lighting control, but not readily available in a large enough current rating for your application.

It might be easier to use contactors controlled by a single switch, two contactors each 2 or more pole would do it.

Or buy a cheap AC welder for brazing, possibly better than than the DC unit for this purpose.
Having both AC and DC welders is a good precaution against failure of either, since either could in a pinch substitute for the other one, though not the optimum tool for the job.

[adam2]

Another option would to be to use carbon rods of different sizes for positive and negative, such that they burn at roughly the same rate.

If have not heard of this being used for brazing, but it was certainly common practice for arc lights worked on a DC supply.
Cinemas used carbon arcs until recently, a handful probably still do !

For arc lighting it was important that as the carbons were consumed, and automaticly or manually adjusted, that the arc remained in the same place relative to lenses and reflectors.
This was more readily achieved if both carbons burnt at the same rate.

[Little John]

Had a bid in on an AC welder on Ebay. Lost it as I wasn't prepared to go ver a hundred.

However, necessity being the mother of invention, I think I've found a way round the uneven carbon consunption problem on a twin carbon arc torch a DC welder. I just need to reverse the circuit every three minutes as it takes about three and a half minutes for one carbon to wear down sufficiently more than the other to break an arc. All I need for this are 4 inline junction boxes and 4 inline switches. These can be part of the wiring that comes out of the back of the torch and so don't have to be a part of the main welder leads.

It means I have to stop the job every three minutes for about 10 seconds or so to press 4 switches to reverse the circuit. But that's no major hassle.

Anyone see a problem I might have missed? That includes you JSD....

The only problem that I can forsee is cost and complication, suitable switches for that much current are far from cheap.
If you can find a suitable switch, you can do it with a single switch.
You need a four terminal reversing switch, these are also called intermediate switches for switching lamps from 3 or more locations.
Readily available in low currents for lighting control, but not readily available in a large enough current rating for your application.

It might be easier to use contactors controlled by a single switch, two contactors each 2 or more pole would do it.

Or buy a cheap AC welder for brazing, possibly better than than the DC unit for this purpose.
Having both AC and DC welders is a good precaution against failure of either, since either could in a pinch substitute for the other one, though not the optimum tool for the job.

I did it with two double-pole 100 amp circuit breaker switches from BnQ at 7 quid each

[adam2]

No reason that it wont work.
These switches are only designed for AC but will be fine on DC provided that they are not operated on load.

Ideally they should be mounted in an insulated enclosure, but remembering the amount of exposed live metal involved in welding, I would not worry.

[Little John]

Another option would to be to use carbon rods of different sizes for positive and negative, such that they burn at roughly the same rate.

If have not heard of this being used for brazing, but it was certainly common practice for arc lights worked on a DC supply.
Cinemas used carbon arcs until recently, a handful probably still do !

For arc lighting it was important that as the carbons were consumed, and automaticly or manually adjusted, that the arc remained in the same place relative to lenses and reflectors.
This was more readily achieved if both carbons burnt at the same rate.

Problem with carbons of different sizes is that because of the way the dc circuit works, one of the carbons burns twice as fast as the other. The largest carbons that could be put in the torch are 10mm. this means that that size that i would have to put in the other side of the torch would be a 5mm. The torch could only be run at the optimum temperature/amperage of the smallest carbon. In this case, 5mm. This is too small to do anything other than very light brazing and so would be too limiting. from my limited experiments over the last day or two, it seems 8mm is about right for most brazing/welding of reasonably thin steel, 6mm for very light brazing of very thin steel and 8 to 10 mm for heating and forging.

It's cos of the above limitations that I went for the circuit reversal option. Having said that, some more experimentation this afternoon following the installation of the circuit reverser, have revealed a very good combination is, say, 6mm and 8mm (or 8mm and 10mm). One way, it lasts about 5 or 6 mins before needing to reverse. The other way, it only last about a min. But this seems less inconvenient than 3 mins each way for some reason.

[Little John]

No reason that it wont work.
These switches are only designed for AC but will be fine on DC provided that they are not operated on load.

Ideally they should be mounted in an insulated enclosure, but remembering the amount of exposed live metal involved in welding, I would not worry.

Yep.

It's only about 20 to 50 odd volts. (It's a floating voltage on it, I think, depending on the amperage demands. It's a bit flash and I bought it when I was teaching and had more money than sense... ). I've had the torch demand up to 80 amps and the switches haven't tripped. The carbon arc torch never needs more than 80 amps (I easily cut a 4mm steel bar in half with 80amps on 10mm carbons) so I'm assuming the switches will be OK long term.

I've currently got it mounted as a single big exterior junction box with the two double pole trips switches mounted at either end of it. All three items are mounted on a big square of waterproof ply. I'm going to turn that into a box with two studs sicking out of one side and two studs sticking out of the other side. That way, I can connect the two welder leads to one side and the two terminals from the torch on the other. When it is in use, the lid will be off so I can access the two switches. When not in use, the torch can be wrapped up and put in the box. I should say, I priced up some heavy duty singe core cable for the wiring and it worked out at about 7 quid per meter! However, I got round that by buying a cheap jumper lead set for a tenner and butchering the cables for the job.

[adam2]

If anyone wants to try carbons of different sizes, one carbon would probably need to be about twice the crossectional area of the other, not twice the diameter.
If for example 3mm and 4mm carbons were used, I would expect them to burn at nearly equal rates. The squares of the diameters are in the ratio of 9 to 16, or nearly one twice the other.
I would expect that the 4mm carbon would burn quicker than the 3mm one, but not by much.

[JavaScriptDonkey]

I'd go with Adam2 on this as with pretty much anything electrical.
Resistivity is proportional to CSA so that should be the factor to consider rather than diameter.

I say 'should' as I've never had to do this so let us know how it plays out Steve.

[Little John]

If anyone wants to try carbons of different sizes, one carbon would probably need to be about twice the crossectional area of the other, not twice the diameter.
If for example 3mm and 4mm carbons were used, I would expect them to burn at nearly equal rates. The squares of the diameters are in the ratio of 9 to 16, or nearly one twice the other.
I would expect that the 4mm carbon would burn quicker than the 3mm one, but not by much.

I absolutely don't doubt the veracity of your maths or your knowledge on the resistivity of substances Adam. Indeed, on a welding forum on this very topic, the advice from one or two of the guys was also that you should only need to move up the next size, which is about 2mm. All I can say is that over the last couple of days, I have found that unless one carbon is twice the diameter of the other, it will burn down faster. 4mm and 8mm burn down more or less at exactly the same rate. With 6mm and 10mm, the 10mm burns down slightly faster. With, say, 6mm and 8mm, the 8mm definitely burns down faster but not as fast as if they were both 6mm.

[adam2]

Experience beats theory, I would suggest

[Little John]

Experience beats theory, I would suggest

Nope, you were right in the first instance. Theory, in this case, beats numb-nuts experience on this side of the screen.....

It turns out I was mixing up some of my brazing carbons that I got with the torch and some other gouging carbons I got from a local welding outlet. For a start, the gouging carbons have a much more diffuse and less controllable arc. Secondly, they seem to burn down at a faster rate to the brazing carbons. I wonder if they are, maybe, less dense?

Anyway, I put in a 6mm and an 8mm brazing carbon about an hour ago and, hey presto, they very nearly burned down at the same rate. Not quite, but very close. I got about six minutes out of them before needing to reverse the current for about a couple of minutes to bring them back into line with one another. Which is OK, since I would have thought that's not too far off the duty cycle anyway. Also, the brazing carbons burn much cleaner and the arc is much more controllable. The brazing carbons cost more than the standard gouging carbons, though. About five times as much at £1 each. However, they seem to burn down half as fast for some reason. This means that, in terms of actual usage, they are about 50p each when compared to gouging carbons, which makes their cost only two and a half times as much, Again, I wonder if this is because they are more dense?

Given that each pair of carbons should last at least 20 minutes, an hours continuous brazing will cost about 3 quid in carbons. I'm not sure how this would compare to the cost of 1 hour's continuous oxy-fuel brazing, but I would have thought it would be favourable. Plus, there are no additional costs in terms of hire charges on the gas tanks and other peripheral expenses. Finally, there is the convenience and safety of storage of carbons over long periods as compared to oxy-fuel, I think what I will do is get a stock of brazing carbons in for brazing and fine welding and a separate stock of gouging carbons in for basic heating and forging. Standard and heavy welding can be done by the normal arc welder. Between the three, pretty much everything I'll ever need or want to do is covered.

[Little John]

Ok, just had a read on the web. Some people have built an ac welder for next to nothing from a couple of microwave transformers. I might have a look into that. I'd only need it to put out about 60 amps for carbon arc brazing.

[adam2]

Ok, just had a read on the web. Some people have built an ac welder for next to nothing from a couple of microwave transformers. I might have a look into that. I'd only need it to put out about 60 amps for carbon arc brazing.

I have my doubts about the viability of this.
Relatively large transformers can indeed be found inside scrap microwave ovens. But microwave ovens use very high voltages internaly, these being obtained from a step up up transformer.
The output voltage varies but is typicly a few thousand volts at relatively low current.

Welding or brazing requires high current at relatively low voltages and microwave transformers therefore seem unsuitable.

With a bit of ingenuity it may be possible to remove the high voltage winding and add your own secondary winding of a suitably large current and low voltage.
This is a non trivial task and I cant recomend it for the beginer !
If determined to have a go, please take great care, both mains voltages and the output voltage of microwave transformers are definatly fatal.