Exploring Spot Welding - A Failed Experiment

RE: Exploring Spot Welding - A Failed Experiment

three-phase — Fri, 03 May 2019 20:51:30 GMT

A very interesting project.

I have never seen a capacitive discharge spot welder, I have only seen it carried out with either a lead acid battery as the source, or a transformer and rectifier. A lot of home made spot welders for lithium battery tabs utilise a rewound transformer from a microwave, in a similar system to that suggested by [mention:b0bc65b9ecdc4307bd967592f00e340a:e9ed411860ed4f2ba0265705b8793d05]. The duration of the current flow is also controlled to produce consistent welds. For the welds you are doing there may even be enough energy in a lithium battery pack, but current limiting and battery temperature precautions will need to be considered.

There are some videos from a company on Youtube that make resistance welding machines, they do provide a little detail the setup they are using, so this may give you some ideas to look at.

https://www.youtube.com/watch?v=HHChJi0PJfk

The particular one linked above shows two copper wires being cross welded, but there are plenty more videos from them showing other processes.

The alternative to resistance welding is ultrasonic welding parts together, this for me would be much more of a challenge, but I do not know if your background gives you access to knowledge and apparatus that could be utilised for such a design?

It also makes me think of exothermic welding process that we use on large copper tapes. The heat generated is a chemical reaction but we clamp the tapes together in a former that hold them in place, this retains the heat and stops the molten metal escaping. I am thinking that perhaps making the bottom plate with a small channel that you could lay the wires in to, so that they are supported and then the top electrode lowered on to them to keep them pushed together. Not sure how such a small intricate former could be made without machining?

I have come across a technique of forming small coils in the wire end to be joined that the component wire is fed into and then soldered on, a bit like wire wrapping. This may provide more support and prevent the joints from falling apart when soldered into a board.

Reply number 9 in the linked blog from the vintage radio forum, shows the above technique.

https://www.vintage-radio.net/forum/showthread.php?t=106515

Adding heatshrink around a joint would also help to support it, but that is also increasing the cost and time of making the joints.

Kind regards.

RE: Exploring Spot Welding - A Failed Experiment

DAB — Thu, 02 May 2019 18:49:05 GMT

Hi John,

No experiment is a failure if you learn something new.

To do proper spot welding, you need to understand the charge flow rate from the materials involved. I suspect you are dumping too much current too quickly. You might want to consider installing a current limiter circuit so that you can control the maximum current being transferred. You might find that you will limit the destruction from the initial discharge.

You can also look at a variable pulse control circuit so that you limit the time the current flows. You can greatly reduce thermal cycling of the material by limiting the time by which the charge is applied. You can also keep your capacitor from being damaged.

Keep up the experimenting, knowledge is power.

DAB

RE: Exploring Spot Welding - A Failed Experiment

ralphjy — Thu, 02 May 2019 15:15:30 GMT

Hi John,

Interesting experiment. I do agree with Shabaz that safety is a big concern using high discharge rates and big electrolytic caps. My wife would freak out if she saw me doing something like that. I guess I've had my share of smoke and explosions.

Have you thought about using crimp splices to extend the component leads? I know that crimping has its drawbacks - cost and the length of the splice, but AMP does have splices for solid wire down to AWG 26. https://www.newark.com/amp-te-connectivity/321198/terminal-butt-splice-26-22awg/dp/57AC7859?st=solistrand%2026%20awg

Ralph

RE: Exploring Spot Welding - A Failed Experiment

shabaz — Thu, 02 May 2019 14:08:13 GMT

Hi John,

There's a PDF doc here

https://www.millerwelds.com/-/media/miller-electric/files/pdf/resources/resistance.pdf

which suggests very low voltages (1.5-3.5V), but also has a diagram showing a transformer for a (presumably) non-capacitive discharge spot welder.

I'm wondering if some high power transformer could be applied to the output, to generate a reduced voltage pulse. But not sure where to source such a transformer!

Many of the (badly documented : ( projects online seem to use 12V or higher directly though, so it's all conflicting information : )

RE: Exploring Spot Welding - A Failed Experiment

dubbie — Thu, 02 May 2019 09:39:07 GMT

John,

I'm no expert on welding or metallurgy but I do remember that when I first started working that on one of my factory placements one of the technicians made a mains powered spot welder. It was massive, I cannot remember how many hundreds or thousands of amps it was. But when he used it is just hummed a little bit. There was no sparking or flash or anything. If fact he said that when you see spot welders flashing and sparking (as they do on TV adverts) that was because they were not set up properly. He explained that the idea was to apply pressure to the two metals being joined using the tips which were about 5 mm diameter and flat, and then passing enough current to heat the two bits of metal being joined until they just started to melt so that they could be pressed together to fuse them. Maybe you could make some sort of foot operated press that would allow you to position the two wires so that they cross and apply pressure to them and then gradually start increasing the current until they start to fuse. You might be able to get enough current from a low voltage power supply, provided it can give a few amps current when shorted (this type of power supply might not be that common - I have used a Chinese one that was pretty chunky and would give several amps when shorted. After-all, a resistor can get hot pretty quickly when even a fairly low current is passed through it.

Dubbie

RE: Exploring Spot Welding - A Failed Experiment

fmilburn — Thu, 02 May 2019 05:27:07 GMT

Hi John,

Up to your usual interesting experiments I see... Among my credentials is a secondary degree in metallurgy which has never been applied and to which I normally don't admit to for fear of being questioned. [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-6e60740f-891c-406c-8f81-a6c2dd93d63e/contentimage_5F00_1.png:16:16]. I always thought of spot welding as a process predominately used on sheet material. Your theory about the weld material being "blown out" seems possible when applied to wire - limited surface area in contact. It may also imply temperature is too high - maybe back off energy a bit? Are the leads clean / oxidation free? Try using snips off of other leads to see if similar material welds better if your conventional tinned extension wire is different. I doubt any of these suggestions are good leads (pun intended) but couldn't help commenting.

Frank

RE: Exploring Spot Welding - A Failed Experiment

shabaz — Thu, 02 May 2019 05:08:30 GMT

Hi John,

This is a really great experiment! I'm hoping to learn how to do this one day too, so I'm going to follow your work with interest.

It sounds like you're doing the right stuff, using copper plate and short distance for low resistance and inductance, and copper rod to get the current to the right place. Nice idea with the braid to get the flexible connection!

Maybe the energy isn't released quickly enough through bouncing on the switch. The debounce time of a switch can be in milliseconds or maybe 10msec or so, so perhaps that (in conjunction with the inductance) is spreading the energy over this period.

Or it could be related to the internal resistance of the capacitor perhaps.

This is just speculation, I've never succeeded in making a good strong weld either, although I've only tried crude stuff like directly shorting the wires of a capacitor!

Also, (I'm sure you're aware and are doing it safely anyway) it could be worth putting a plastic screen between you and the experiments (e.g. a perspex cover over the cap), just in case it blasts out : ( I've had an old electrolytic capacitor do that to me, perhaps it was the wrong way around, or it was just old, anyway charging it caused it to burst out gray stuff all over the room : (