Spot Welder homebrew? Soldering gun?

I'm pretty certain a solder gun, even a 250 watt one, won't put out enough current to butt spot weld anything as heavy as a bandsaw blade. Steel foils maybe, but not a blade.

Here's one I hadn't seen before. It claims to weld bandsaw blades using a 12 volt car batttery for its high current energy source:

Do you have a proper fixture to hold the blade ends lined up and butted together while soldering them? It's a real PIA to do that job right without one. Usable commercial ones are pretty cheap, and it's a pretty easy thing to DIY.

Like this one:

Just my .02,

Jeff

-- Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to place the blame on."

I tried building a bandsaw blade welder. I got close, but still had to deal with the whole mechanical aspect, where one jaw of the fixture needs to move closer as the ends fuse. I eventually snagged one on eBay when eBay's computers gagged at a fortuitous time.

Anyway, you need LOTS of amps. Enough to make steel particles fly through the air like sparks from a grinder. I'm guessing that you need something like 1000 amps to do it right. These welders take close to 15 A from the wall socket during the weld cycle. (Some take even more, or need 220 V) So, a slodering gun is hopeless, they get around 40 A output.

I got a 1 KVA isolation or control transformer and cut the secondary windings off it, and made a 1-turn secondary of #4 stranded wire, that I connected to the ends of the blade. I was then able to make adjustments to the voltage by adding 1/2 turn on the transformer core as needed. I think I found 1.5 turns worked best on this particular unit.

One feature of all the commercial units is a tricky linkage that applies inward pressure to the blade ends, and automatically turns off the current when the ends have moved together a certain amount, maybe .050". Too much upset and you have too much of a blob to grind off. Not enough upset and you have a weak weld that is not fused all across the blade width.

Jon

I have a welder on my little vertical saw and it does a good job after practice, especially drawing back the weld. However, I have found that it is cheaper to get the blades welded from my supplier at $2.50 per weld and they stock the blade stock. I get charged the same per foot if I buy 100' or have them weld-up finished blades. So, for $2.50 per blade, is it worth your time? Or, do you do some internal cuts?

I have the blade welder made by Advance Car MOver, and its called a Badger Bandsaw blade welder, and have one real thing to say abaout it. IT KICKS butt when it comes to welding up a bandsaw blade. I have had mine over a year now, and its used quite a bit, and it works mighty fine and does a superb job of sticking a blade together. It works on Bimetal as well as carbon steel blades, but you do need to make sure you have a good fully charged battery and good connections from the welder to the battery. Other than that, the instructions are straight forward and its a iece of cake welding up blades. I have done 3/16 thru 5/8" with it so far, mostly 3/8 and 1/2 in both carbon and bimetal. Properly annealed (follow instructions once again) and I have yet to have a blade break at the weld. Sure beats silver solder or brazing them by far.

I have on one occassion run up on some carbon blade stock made by F E Morse (IIRC) that I could just not weld with it. Since I bought the roll of blade stock, I figured I would just carry it down to the local place that I used to have weld my blades up at, as they have a sure nuff high dollar setup for this job. They also had nothing but problems welding up the blade, so the material got sent back, so it sure was not the welder that made problems. Another coil of saw blade stock I received in the previous coils place worked like a champ. Go figure.........maybe quality control was on vacation that day it was made.

Fot the price the BAdger Blade welder is hard to beat. Until you clean up and bevel a blade clamp it in a fixture and silver solder it, you can cut a piece and clamp it in the BAdger and even if you have to carry it out to your vehicle and open the hood to use it, you can weld up a blade better and faster. Its pretty well idiot proof if you follow the instructions.

Visit my website:

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I make a lot of internal cuts from time to time., For me however they charge $5.00 a weld, have about a 40+ mile round trip to and from the shop to get them welded up, and then its only one person that makes them and he also delivers the welding sup0ply stores gasses etc, so it usually involves a two trip deal to get blades made up or rewelded. I buy a lot of coils of bandsaw stock on Ebay. Got some great deals on

100 and 250 foot rolls, much cheaper than I can get it for through MSC etc. This way with a welder I can have the saw stock in the 1/2" most commonly used width and make up blades out of it for both my H/V and Homebrew vertical saw when I need them. LAst coil I bought was new old stock made by DoAll, 3/8" width wavy set in 16 TPI for under $20.00 each plus shipping for 2 100' rolls.

Just when I ama down to my last blade I usually always tear it up, and its usually on a Saturday afternoon when everyone is closed and I have to wait until Monday and hope the fellow that welds them is in. Now I don't have that problem! Visit my website:

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Often on sale for $130.

Joel. phx

I (probably among others) suggested discharging a capacitor into the primary of a soldering gun to get a high current pulse out the secondary as a spot welder, but I've never tried it myself. However, for blade welding you need continuous current to do resistance welding, not a pulse of current for a spot weld. Maybe this is what to do with all those old low duty cycle ac stick welders now that everyone has moved up to MIG? :-)

-- Regards, Carl Ijames snipped-for-privacy@verizon.net

40 Amps sounded a bit low to me. I couldn't see that amount of current heating up something roughly the size of a piece of number 12 copper wire to soldering temperatures in a few seconds.

It only took a moment to stick the jaws of my Amprobe through the soldering tip on my ancient 250 watt Weller gun.....the one with the brown Bakelite housing.

I pulled the trigger and (as Claude Rains put it) I was shocked, just shocked, to see it displaying 460 amps.

I think that was a valid reading, but my electric field theory is quite rusty, so somebody ping me if I used an incorrect measuring technique.

But, I still doubt that a soldering gun transformer would be ballsy enough to resistance weld blades.

Jeff

-- Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to place the blame on."

While obviously welding a bandsaw blade is a better join Silver Soldering can produce a satisfactory join, provided the preparation and soldering is done correctly. When I first started Silver soldering I had several failures, But now my joins are good enough to hold so that a break has been in the blade steel not at the joint. Made sure that the blade ends were chamfered appropriately I don't have a jig (yet) but grind both ends together so that the angle is similar. I Tin the chamfered ends first and then place the blade into a jig that holds the blade in correct alignment, then I just heat the ends until the tinned ends remelt and touch. When the blade is cool I clean up the joint to make sure the joint thickness is no more than normal blade thickness and that there is no step at the back of the blade. The job is done! The hardest thing seems to be to get the chamfer ground, but a jig will go a long way to making that easier. Cheers, Peter.

******************************************** ,-._|\ L. Peter Stacey / Oz \ Melbourne Australia \_,--.x/ snipped-for-privacy@melbpc.org.au v

my crappy Silver soldering has produced to date. Before I rush out and buy a cheap soldering gun, has anyone done this

I have some doubts about that, as the transformers of these guns radiate HUGE fields, and may have affected the clamp-on pickup coil. You might hold the clamp-on next to the soldering gun, but without the probe clamped around the tip wire, and see if you still get a reading. My guess is you will get a substantial reading from it.

I'd be willing to believe currents up to maybe 100 A for a 250 W gun, but not much above that. The problem with the blade welder is the steel is not such a great electrical conductor, especially when heated red hot. So, the blade welder needs to deliver a lot more voltage across the blade to work. The soldering gun is heating a very short length of copper bar to a much lower temperature, so the voltage drop must be quite a bit less, a fraction of a volt. The blade welder probably needs several volts open circuit, and at least a Volt or two when doing the weld. Two volts at 460 A would require almost a KW.

460 A at half a volt would only require 250 W, so maybe your numbers are OK, if the complete circuit of the gun, including the one turn coil in the transformer secondary only drops half a volt. But, then it would need a resistance of 1 milli-ohm, which is AWFULLY low. I can't believe the resistance of the tip, with two pressure joints, is that low.

Jon

That won't work. The transformers have a HUGE leakage inductance, and run right at the edge of saturation as it is. You really can't get much more out of them than they already produce. Also, the primary wire is much too thin for high current pulses. If you made a somewhat larger transformer, with just a few turns of primary, and a one-turn secondary, and interwound the pri and sec windings to get the leakage inductance down as far as possible, and then dumped energy storage caps into the primary, that could very well work, sort of. But, you'd get a VERY narrow weld zone, which might lead to a weak joint. The commercial welders take about 1/4 second or so to make the joint, providing time for the metal to fuse and upset (push together) a bit. I don't know if this is necessary, but I suspect it really is for this type of weld, a butt weld. The strength comes from the merging of metals on both sides of the joint, and a millisecond pulse weld probably is not the correct way to butt weld thin strips. (It does work fine in spot welding thin sheets.)

Jon

You know, I thought the same thing when I was taking the current reading, so I moved the closed amprobe jaws around the outside of the secondary "U" and the tip and read zippo on the 1000 amp range I was on. Then I ran off work and wrote my post about the

460 amp measurement.

I thought about it during the day, and when I got back home, the first thing I did was measure the voltage on the tip clamp bolts securing the tip to the "U". It was

0.25 vac as soon as I pulled the trigger, rising and stabilizing at 0.32 vac when the tip heated up. I measured it with a digital voltmeter and double checked it with my scope, which confirmed the expected sinusoidal waveform

I took a look at the Weller's nameplate and realized my memory was off a bit. It is rated as a 200 watt iron, not 250 watts as I said in my post.

So, 460 amps and a third of a volt makes 150 watts or so, close enough for me. I suppose I could have measured the power draw from the 120 vac line, but I think it's time to quit playing mad scientist. I'm sure the "remaining" 50 watts is heating the primary winding, not to mention all the power being gobbled up by that little prefocused light bulb.

I'm open to a wager (for our favorite charities) on that statement....Wanna play?

I certainly will buy that!

I think it could well be...The resistance (cold) of No.12 wire is 1.6 ohms/1000 feet, so the four inches or so of copper tip on my Weller would have a resistance of only about half a milliohm. (And I bet if I actually measured and calculated the tip would be closer to a piece of No.10 wire, so even lower.) The pressure contacts are brass on copper, socked down damn hard. Consider that...'eh?

Jeff

And those guns won't work if the nuts aren't 'squeaky tight.' My former boss actually calculated what was optimal - and it turned out to be 5.25 inches of no. 14 wire for the tip, bend into a hairpin shape.

Jim

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Now you've got me wondering what approach he took when "calculating" that length. If I had a "need to know" that fact I'd probably just do empirically by trying a few different lengths of wire and measuring their dissipations with the same kind of current/voltage measurements I barfed off about earlier in this thread. Then, I could plot power vs. wire length and find the max.

Or maybe one should shoot for maximum tip temperature, or...or...or...

Jeff (Who abviously has *way* too much time on his hands this week...)

-- Jeff Wisnia (W1BSV + Brass Rat '57 EE)

"If you can keep smiling when things go wrong, you've thought of someone to place the blame on."

I suspect he probably actually measured a new weller tip with a bridge setup, and then trimmed a piece of no.

14 wire to match. But that guy was a sharpie, so there may have been more than that going on there.

Jim

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