Spot-welding cathode grids

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Carl Willis
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Spot-welding cathode grids

Post by Carl Willis » Thu Sep 13, 2001 1:31 am

Hello everyone,

My fusor is all built except for the cathodes (maybe I'll post pictures shortly) so I'm at the point where I need to learn how to spot weld cathode grids. I have tried to weld stainless wire with a 50-amp welder, and have made only one good weld out of about six attempts. So clearly I need your help on this! Our welder is essentially a big transformer, nothing more. Do I need more sophisticated equipment? What is the correct technique? I've seen the pictures on Richard's website of a very nice cathode made entirely by spot-welding, but a "50 joule" welder was used there. Have some hints, Richard? Anyone? Thanks for any help.

P.S. All my sympathy to those experiencing loss and grief over yesterday's tragedy. This is certainly all of us in some measure.
Carl Willis
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Richard Hull
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Re: Spot-welding cathode grids

Post by Richard Hull » Thu Sep 13, 2001 8:00 pm

Carl is not alone in trying to spot weld. I used to use a 60amp 5volt AC filament transformer for spot welding. It worked fairly well. The key is to obtain spot welding electrodes. They are a special non-stick form of phosphorus laden copper alloy. Many full line welding suppliers have these on hand. There are literally thousands of different electrodes. Most are huge and made for giant assembly line welders. I have purchased a number of the smallest larger sizes and machined them to meet my needs.

I wish everyone good luck in cobbling up a spot welder.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Carl Willis
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Re: Spot-welding cathode grids

Post by Carl Willis » Thu Sep 20, 2001 4:17 am

Richard, I'm still a bit frustrated by trying to spot-weld. Good suggestion about the welding electrodes- I got some at McMaster-Carr and they did improve my success ratio. But I may not have the right technique yet. This is what I do when I try to make a closed loop: I clamp the wire so the free ends are overlapping and touching. I turn the welding transformer on, and quickly try to pinch the junction with both electrodes. Usually this approach makes a messy joint with no strength. Should I try to clamp the junction between the electrodes before turning the transformer on? Sorry to hound you on what must surely be a simple and easy process once it's done right...
Carl Willis
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Re: Spot-welding cathode grids

Post by DaveC » Thu Sep 20, 2001 6:43 am

Carl -

I don't know if you have seen the little blade welders on the side of the larger metal cutting band saws..

They are edge welders, and clamp both parts with a bit of spring action.. then apply current for a few secs only.. just enough energy (the joule figure, Richard gave) to melt the adjacent edges for a fusing of the metal. Same technique applies to spot welding the wire stock.

Also regarding spot welders, there are some that use the same size transformer as is in a soldering gun. Replacing the soldering tip with a set of conical copper electrodes and a clamp to squeeze them to the part. Pull the trigger and you have a weld.

Most soldering guns develop about 120 to 150 amps..at the secondary.

Dave Cooper

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Re: Spot-welding cathode grids

Post by Richard Hull » Thu Sep 20, 2001 1:44 pm

Yes! Do firmly clamp the work in place between the electrodes before applying the current. Hitting the fire button should be the last step. Note*** a simple switch is no good! Use a large, very high current relay of at least 50-100 amp contact capacity and close it with a simple push button if using a fixed transformer source.

Ideally, you want an impulse welder (capactive discharge).
When welding wire, you would like about a 1/4 diameter "sink in" or merge by the two pieces. Spot welding is most often referred to as resistance or fusion welding, relying on a small localized area of high resistance at the point of the joins to dissapate the bulk of the weld energy at the point of contact, thereby, fusing the pieces. Make sure you have enough energy or if not using capacitive discharge systems, a weld current of sufficient amplitude. Traditionally this is on the order of 60+ amps for wires in the .025 to .030 range made of stainless steel. The pressure and weld power are critical to success. Even the pros have to play with the system pressure and power levels before a flawless weld of good quality can be achieved. Of course, with proper machinery, it is perfectly repeatable in pressure and energy. Normal use of resistance welding is in a factory setting where tens of thousands of the exact same welds are being made on identical pieces daily and rapidly. A hand spot welder of amateur construction has none of these advantages. The effort becomes an art rather than engineering science.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Spot-welding cathode grids

Post by jim-frank » Fri Dec 28, 2001 8:08 pm

Would carbon rod electrodes work?

guest

Re: Spot-welding cathode grids

Post by guest » Tue Jan 22, 2002 6:51 am

Thanks to everyone who offered support on my spot-welding problems back in September. I got some good insight into correct equipment and technique. But I haven't been getting anywhere -other than frustrated- in the months since. In fact I am pretty embarrassed by my inability to stick together a few pieces of wire, although the entire rest of my fusor down to the deuterium bottle and BF3 counter, stands at the ready. I don't know if y'all can provide any more help -we've already been over my problems in a bunch of detail- but here are even a few more details just in case!

I am using a very large filament transformer from a 5-kW induction heater, running from 240 V through a variac. I have no doubt it does 60+ amps since the secondary appears to be 1/4" rectangular copper bar. Open voltage at the secondary is about 10 V with full 240 at the primary. The primary current is switched with a foot pedal. There was a small shunt of sorts in the middle of the transformer core. I doubt it did much but I knocked it out anyway.

I am using spot-welding electrodes (from McMaster-Carr).

I am trying to make grid rings out of 0.025" stainless (316 alloy) wire, also purchased from McMaster-Carr.

For practice, I'm just trying to stick two pieces of wire overlapping at right angles together. It doesn't work well on any settings of the variac. At the highest, the junction gets yellowish hot with moderate pressure and the steel is ruined by the heat, by oxidation, or both. At lower settings, the wire only gets reddish hot and sticks some, but the joint can always be broken by repeated tugging on the wire.

I get best (but still poor) results from using a moderately high power to heat the junction to an orange color, application of rather weak pressure on the joint with the electrodes, for slightly less than one second. As Richard told me before, I always pinch the joint with the electrodes before applying power and let up on the power before removing the electrodes (otherwise the joint is just arc-vaporized). The joint is by far the weakest point on the finished product.

Heavy pressure on the joint conducts heat out of it and into the copper electrodes, and fails to melt the metal together albeit the metal gets orange hot.

I have gone to extreme measures to try to make this easier, short of shelling out big bucks for a proper impulse spot welder. I have tried washing the wire in stainless steel acid flux, for example. Does not the slightest bit of good. I went to a local welder with the "proper equipment", and he laughed at the idea of joining the wire in the manner I have been trying. He did join one ring for me, by first knotting or splicing the ends of the wire together in an ugly ball and then fusing the ball together with the welder. I was not impressed.

I've often wondered if the wire is somehow to blame, but I cannot think of why.

So once again, I am begging for any further assistance on this problem. Otherwise, I may have to give up this project until I can afford an impulse welder- I'd hate to let this be my biggest obstacle.

Thanks,
Carl

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Re: Spot-welding cathode grids

Post by teslapark » Tue Jan 22, 2002 8:31 am

Carl,

Assembling grids by no means has to be the biggest obstacle. I'm sorry to hear that you've had so much frustration over the spot welding, and know exactly how you feel as far as how it often takes some false starts before the welding works right.

First off, I'm not exactly for sure what is wrong with your setup. It sounds like you've analyzed it all pretty deeply, and now you're wracking your brain trying to think of some lurking variable that's causing you trouble.

You may want to try placing a clamp-on ammeter on the output of the transformer, to see how much current really goes through when you short it out.

My crude homemade welder only puts out a two or three volts at max, but the current can go as high as a few hundred amps.

I don't know exactly how much current is there when I'm welding, but I'm usually pretty aggressive, finding the point on the variac where the .030" wire explodes into a shower of sparks and then backing off from that point only slightly for my weld setting.

I hold the work with a pair of pliers, place pressure on the joint, and tap the foot switch for about a quarter of a second. The joint turns bright yellow for a moment and the wire does discolor some around the joint.

How do you turn on the transformer? A relay would probably be better than just a regular switch.

My welder uses "common" copper pipe and tubing for its electrodes.

I often like the wire to overlap a enough so I can make more than one weld. These welds don't have to be super tough, they just need to be strong enough to withstand construction and installation. As has been stated before, the grid will become very brittle and "set" once the fusor is operated for awhile, and very little stress is ever put on the joints during operation.

You may want to check the songs archives for the "classic" microwave oven transformer welder recipe; the process is a little messy, but can can also double as a stress reliever (hacking away the secondary coil of an old transformer).

With our method of welding, achieving a good weld is more technique than technology, so keep experimenting.

I hope this post has been helpful, and that some more experienced list members can help more.

Great to hear that you are very close to having your fusor ready to go. Best of luck with the welding, and wish you a speedy and stress free completion.

Adam Parker

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Richard Hull
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Re: Spot-welding cathode grids

Post by Richard Hull » Tue Jan 22, 2002 3:26 pm

Carl,

Your transformer can't have any magnetic shunt in it!!

You need hundreds of amps instantly. A shunt will not allow the iron to transfer burst current! It is doing its job.

Again, I first used a 60amp rated 2.5 volt filament transformer and just relay closed it to a variac to control current. I tended to use rather severe pressure, never light and adjusted the variac accordingly. Just like Adam, I measured about 600 amps weld current for the optimum weld. The wire would sink into its mate about 1/4 depth. With the transformer system, the weld could be broken with a good deal of effort, which is OK. The weld time for me was a fraction of a second.

With the impulse system it is absolutely impossible to separate the weld as thousands of amps are used over a controlled number of milliseconds.

The bottom line..............If you get the simple crossed wires to come apart with some several forceful efforts, you are probably fine as the fusor puts zero strain on them.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Tom Dressel
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Re: Spot-welding cathode grids

Post by Tom Dressel » Wed Jan 23, 2002 12:23 am

In a pinch you can try silver soldering the grid rings. The grid that I made in May 2001 has had about 100 hours on it and still not fallen apart. You must use the purer jewler's silver solder. Though I dont know how the grid will stand up if I start "burning" deuterium.

Tom Dressel

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