Hi

Hoping you may be able to offer me some advice.

I am currently restoring an old lorry cab and I am using a MIG welder on 0.9mm sheet metal and am experiencing distortion when trying to perform plug (pool) welds.

I am considering purchasing a Spot Welder (can be seen in the URL below) but would like to know what distortion I am likely to get with it before spending too much money. I am welding 20 guage in approx 5 feet lengths.

I have been informed by a friend (who doesn't have much experience) that there would be zero distortion with a Spot Welder due to the short time interval in which the weld takes place. I would be disappointed however to buy the machine and find that it distorts like the MIG welder.

http://www.machinemart.co.uk/product.asp?p=010110013&r=2036&g=105

Many Thanks for your help and experience

Arthur Ford

-- Arthur Ford (bethford50@hotmail.com), March 10, 2003

Answers

Hello Arthur,

Resistance spotwelding is used for all sheet metal in automotive assembly plants. The only places I see distortion (and this is rare) is where the spot is too close to the radius of a flange, and puts a slight bend in the panel nearby. The heat is confined to the 4 to 6 mm area between the tips, and since the tips are water cooled (very important) they limit the transfer of heat very effectively.

You should be very pleased with a resistance spotwelder for this, but be aware that the electrical service can be quite large. Usually it would require 250 amps at 480VAC, and other users on the same power grid may experience computer-crashing noise spikes on the line.

The welder you show as an example uses 230VAC, and probably puts out about 7 to 8 VAC secondary. This means the transformer would have a winding ratio of about 30 to 1. Normal secondary current will be about 11,000 amps for 10 cycles, (cycles of the AC line) so normal primary current would be approaching 400 amps.

The unit you referenced seems to be setup for a different operating range than I see in automotive plants.

The weld force in automotive applications is usually 600 pounds to 1100 pounds. (272Kg to 500Kg) This unit produces between 40Kg and 120Kg. It is possible that the weld is done at low current to accomodate this low weld force. I would guess that they would use 6000 amps for 60 cycles, at these low forces. This would be a 200 amp primary current, which could be supplied by a 100 amp service, and may not disturb others on the power grid very much.

The 13KW figure they state would only be 57 amps on the primary, but it would only be 4000 amps on the secondary at 3 volts. These numbers seem low, I would like to know what the max current is into a 2mm + 2mm stackup. I would be uncomfortable if it is below 6000 amps.

As far as the automatic adjustment feature, I have never seen this sucessfully demonstrated in automotive plants, and I am skeptical. I prefer to manually set the time as the smaller unit does, but you need the larger unit, as you will often have to weld material larger than the stated capacity of 1mm + 1mm.

The lack of water cooling on the tips means you should make your welds no faster than 3 or 4 in any 5 minute period. If the copper heats above 390 degrees F, it will go "dead soft" and they will mushroom (flatten) easily. You could immerse them in water for 15 seconds after every few welds and avoid this.

Good luck with this, please tell me how it turned out. You can also write me for assistance on setting up the pressure, time, & heat.

David Bacon dbacon@updatetechnology.com

-- David Bacon (dbacon@updatetechnology.com), March 17, 2003.

Hi David

Many thanks for taking the time to reply comprehensively to my query.

I am currently working on a different area of the lorry cab, so the Spot Welder isn't quite as urgent as it was. This will give me time to research the machine further.

I was contemplating purchasing the optional extension arms (50 cm) for difficult to reach areas, but these would presumably reduce the 40-120 kg pressure quite significantly. I wonder what effect this would have?

The lack of water cooling shouldn't be a problem for me considering the limited use that I have for it.

I have a 30 amp fuse (and plugs & sockets) in the garage. I will need to find out if this is sufficient.

Again, many thanks for your help, and I'll let you know more if/when I buy the machine.

Thanks Arthur Ford

-- Arthur Ford (bethford50@hotmail.com), March 21, 2003.

I just noticed your reply, and I am concerned about the water and power. Most resistance welding requires an instantaineous draw of about 60KVA to 500KVA. Thirty amps at 110 volts is only 3.3KVA, so there will be a major problem. If you have a 200 amp service, and there are no computers on the line you might be able to weld from that, with some of these specialized repair welders for small shops. The water cooling is very necessary for the cooling of the tips, as if they reach 600 degrees or so (Depends a LOT on the copper alloy) they will go dead soft, and flatten easily. If you have no water available, I would restrict my welding to allow the tips to cool below 100 degrees between welds. Plunging them in a bucket of water is acceptable, even though it sounds a bit crude.

-- David Bacon (dbacon@updatetechnology.com), May 12, 2003.