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Need Help With A Mini Spot Welder Answered

I've been trying to gather information to build a welder for a special application. The use is for welding the commutator tabs and magnet wire that passes over them for small DC motors. While there are specialized and often automated welders made for this purpose, they are both large and very expensive and really intended for industrial users. I have been able to get small bits of information as to how others have done this, but it's been difficult to get the whole project laid out (there is apparently some "secrecy" involved here!?). The materials being welded (soldering isn't a preferable way to make this connection...these motors run hot and very fast) are the brass commutator tabs (approximately .030" thick material about .060" wide formed as a "U") and the copper magnet wire used to wind the poles that then passes over each brass tab...from #23 awg to #28awg. From what I've been able to gather, people have done this on a "homebrew" basis using 6V automotive battery chargers/starters that can deliver around 30 amps or more. I have very little knowledge of electronics and none at all about welding and circuit design, but am looking for information on how I might proceed after sourcing the charger/starter. I'm guessing that the "negative" cable would be adapted to use as a clamp at the commutator and would also double as a heat sink to prevent damaging the commutator and that the "positive" cable would somehow be adapted to hold some sort of fine rod that would touch each com tab to complete the welds. -Maybe a footswitch could or should be used to start/stop the weld? -Is the positive cable simply used to hold a rod for the "spot weld"?, or are there other pieces that need to be added? To my mind, it would seem that simply touching the positive to the commutator tab to make the weld and complete the circuit would simply trip a breaker at the panel or any protection device on the charger(dead short!?). -Any other information or thoughts? Thank you in advance -john


Updating the post below; The 1-1/2 to 3 turns of the transformer below are the secondary turns. Also, yes, a short circuit is what you have when spot welding. The duration is a short time and the secondary winding is beefy Your Primary may have to be over fused slightly, should be OK as this is not a continuous load.

Hi Yeasayer and THANKS for the information. As a way of getting a handle on all this, I did some little test today with a variable DC power supply I have (0-12V @ up to 15 amps). I clamped the negative to a small piece of .030" brass and held various width and length pencil lead in the + clamp. When touching the workpiece near the neg clamp, the lead/graphite would come to a nice bright yellow within a second or so...but then would trip/reset the internal protection of the supply. So I can definitely see what you're talking about! So from your information and what I've gotten from another guy: 1)I would fabricate a circular clamp/heatsink from some aluminum and cut fins into it to clamp to the commutator. This would only have to be clamped once for all three welds (these are three-pole armatures) 2)I would use some lead from carpenter's pencils sharpened to a point in the positive clamp (unless you or anyone else know a better material?). 3)Rather than start off hacking-up a transformer as you suggest...I'm more inclined to try the automotive charger/starter as suggested. I see one from Grainger that'll do 6V at up to 55 amps for relatively little (around $60). If it turns out that 6V is too much...I'll have to dig-in as you suggest. I'm only concerned that the charger I'm looking at will also have some internal protection circuit. 4)The timer relay and footswitch were also suggested by the person who gave me the 6V @30 amps information. I can see how it would be easy to install a momentary footswitch on the mains cord if using the above unit...the timer seems a bit more involved. I think I may just try this in baby steps to see if the concept works by manually making/breaking the connection. I really appreciate the help and any other thoughts you or others may have on this. -john

Sounds like you have a plan. I would suggest using the Carbon rods found in dry cell batteries because they are not soft like the pencil lead and because carbon is not a good conductor of electricity, the larger diameter will carry your current with less voltage drop. Grind the end of the rod at a 72 degree angle, similar to the angle you would sharpen a drill bit . Leave a small flat surface at the point for contact. I see no problem with using 6 volts there may be excessive sparks when contacting the work piece. would like to know how this works out.

Thanks again Yeasayer. I will take your advice on the carbon rods from dry cell batteries as the pencil graphite is rather fragile and will post back here when I get the battery charger/starter. When I first wanted to try this, I had thought this was more of a "stick welding" process...but I see now that it will be more like a spot weld or maybe even a kind of brazing since I have also heard that some people will feed some sort of filler metal into the joint while applying the heat. I will do some experimenting with all this, but step one is to have the actual "heat source"/welder and I think I'm on the right track. Thanks again for your help. -john

Yes; The Carbon rod can be used for locating and executing the Weld, I am not an expert but do have some knowledge about electrical systems. Spot welders use the principal of electrical resistance in a conductor to generate the necessary heat for the metal to flow. it is necessary to have a large area contacting your negative or ground Lead and clamped tight. This prevents damage to your work. Your carbon Lead should be ground to a very sharp point, this will concentrate the amperage flow to a very small area, similar to a small wire diameter, You know what happens when a high current passes on a little wire . The Carbon can withstand extreme temperature but the metal will be molten. you really should have a timer relay connected to the primary of the transformer. and a foot switch. It only takes about 2 seconds or less. Another thing, Spot welders use a voltage of 3 or less, the amperage is the high value. I have seen the transformer of many spot welders, only 1-1/2 turns #000 AWG, Some Had 3 turns. I suggest you try to fish 3 turns of #6 onto the core of an old battery charger You will need a diode bridge or combination of, that will pass 80 to 100 amps. Hope this helps.

Hi Linux, Thanks! That sounds very much closer to what I'm looking at here. So using the carbon battery rods, you simply have them hooked up to the power output leads of the transformer and make contact where you want the connection? -john

I use an 85 watt train transformer with carbon rods from batteries to weld VERY thin sheet.

I have very little knowledge of electronics and none at all about welding and circuit design

That in itself is scary. Playing with high amperages with little knowledge of electricity or welding is downright dangerous.

A spot weld is normally created by the contact point of the two metals being welded, with the amperage being applied to another point of contact (albeit very near the weld).

One thing is sure, you can't weld anything heavy gauge with only 30 amps. With my 100 amp arch welder it is hard to strike an arch at less then 60 amps (without a LOT of practice).

Hi Goodhart,

I didn't say I had little knowledge of electricity...only electronics and circuit design. I have worked with residential wiring and dangerous tools for many years and good safety practice is part of my MO. Surely this isn't more dangerous than building an arc welder from discarded microwave oven transformers or automotive batteries!

A spot weld is normally created by the contact point of the two metals being welded, with the amperage being applied to another point of contact (albeit very near the weld).

I understand the basic priciple, it's getting from here to there that's the rub :-) I also could very well be using ther wrong terminology, I've seen similar processes described as "resistance or resistive welding??? Basically, the wire is crimped in the tab and some sort of weld is done right at the tab that fuses the wire and tab together forming a sound electrical and mechanical connection (as well as burning off the wire's insulation right at the contact point as a handy byproduct of the process)


One thing is sure, you can't weld anything heavy gauge with only 30 amps. With my 100 amp arch welder it is hard to strike an arch at less then 60 amps (without a LOT of practice).

Well, per my first post, this involves welding/fusing very small brass parts...approx .030" thick by .060" wide with copper magnet wire that has been crimped behind the tabs ranging from #23awg down to #28awg. The weld is preferable to soldering because of the stress (heat and mechanical)the spinning armature undergoes.