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How to: Make a Trail Mig (Flux Core) welder

47160 Views 44 Replies 20 Participants Last post by  sherlocktk
Hi all. I just finished making a mig welder that uses 2 car batteries (24v). Here is the how to. Total cost was around 150, but only cost me around 50 as I consider the welder free. This is basically the same system as ready welder but without the cost.

My cost was $35 dollars as I had many of the "parts" lying around.

Parts List:
1. Harbor freight 90 amp Flux core welder. This was my first welder, but it had outlived its usefulness as I got a better machine. Frequently on sale for $90
2. PWM (Pulse Width Modulation) 24v motor speed control ($25) - ebay
3. Jumper Cables ($15) - 8 ga seems to not get hot at all.
4. Plastic Home Depot tool Box ($8)
5. Battery Disconnect Switch ($4)

Step 1. Take out the "guts" from the harbor freight welder. The only parts you want are the ground cable, Torch cable, Wire feed motor assembly and the thing that holds the spool of wire. I don’t have a picture of the gutted welder.

Step 2. Take out the Top plate in the welder that all of the above items and attach into a box of your liking. I used a 8 dollar home depot tool box as seen in Pic 5 below. I cut out the handle part and screwed the metal plate from the mig welder into it to hold all of the items.

Step 3. Install Battery cutoff switch in appropriate position (red handle thing in Picture 5. This allows me to turn off and on the welder without needing to mess with the jumper cables.

Step 4. Wire up the Motor to the PWM speed control and trigger switch on mig gun. Picture 1 and 2 for the Wiring diagram

Step 5. Attach the welding "ground" to the Positive Connection on the battery, Attach the mig gun to the negative battery connection. via the jumper cables

Step 6. Tidy up the wiring (which I have not done yet), attach the PWM Controller somewhere Fit it all in a box and turn the speed control to about 20 % and run a test bead (Picture 4) The picture shows .030 wire, I have since tested .035 and liked the results even better, but either work ok.

Step 7. Put you goggles, Gloves and all the appropriate cables (I put my 4 1/2 grinder) in the tool box for your "welding kit" It all fit in nicely.

Optional: I thought about just putting terminals to hook normal jumper cables to, but I really did not like the idea of having to pull the welder around and have a cable short out somehow. I opted for the dedicated cables, but it would be easy to do some type of high current connectors I suppose. This would take away some of the bulky wiring. I would also consider a better ground clamp. The one from Harbor freight gets very hot, but the harbor freight mig gun does not seem to heat up.

Please post back if you have tried this and liked it.

Your Done, and saved many $$$ over a ready welder. Any Questions or comments?


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That's pretty slick.
I might get around to this. Not because I think I need a welder for trail use, but It would be a cool project. And I like welding.

One of the things I dont like about the HF welder is that it is a "hot tip". Im no electrician but I would guess that I could find a heavy duty solenoid (battery isolation solinoid?) and wire into the trigger switch to make it a traditional cold tip?

Again, Im no electrician, but What is special about a PWM controller for this use? Would a resistance type controller (rheostat) not work? Or is the PWM more predictable by having a 0-100% power instead of 0-X resistance, and requires less math to make sure it will work?
It would be trivial to put in a 24v starter relay inline with the mig gun. You would need to change the green wires to complete the circuit that feed the Pwm controller instead of the motor and wire to make the starter engage off of the pwm power input feed I can make up a diagram next week im out on the trails this weekend

As far as a rheostat that would also work just as well for the speed control. One from a rc car or something would work. At 24 v. I have it set at 20 percent speed or so. That is potentially a lot of heat.
That is a great creative design but I am concerned that it won't put out enough heat for many common trail heavier gauge steel repair jobs.

Using a 90 amp MIG welder for 3/16" is marginal under most conditions, and lots of stuff that can break on the trail is at least 3/16". How well does that work for you with 3/16", especially when that 3/16" is a large piece or part of something bigger that will sink off a lot of its heat?

Cable-wise, I plan to use at least 4 gauge jumpers for mine, 8 gauge is pretty small stuff for jumping batteries and welding.

I like your idea and I am tempted to make one but I am concerned about its 90 amp rating. :confused:
Nothing about it is 90 amp anymore, the 90amp part was the transformer in the welder, which is no longer in use. The only parts from that are the wire feed mechanism, and torch. It can now output hundreds of amps (limited by the cables themselves i would think).

Running off of 120 volts with a 20amp breaker you have 120 amps at 20volts (theroretical), that is why 220v systems with 30-50amp breakers can put out more amps, because there is more power available. But car battery's can output a couple hundred amps, and two batteries puts the voltage in the MIG range. Your limiting factor is the wiring now, and while the HF welder is kinda wimpy in the parts department, for occasional short use it should be fine.
Oh yeah, that makes perfect sense now. Hah, and I normally consider myself fairly astute when it comes to electrical stuff. :rofl:

Nice, you making me realize it's no longer a 90 amp welder is making me want to do this later when I get some extra $$$. :thumbsup:
coming from a career welder that is pretty slick! :2thumbsup:
I like it,i have an old 90 amp Cambell Hausfeld machine i gave to my family up north,but they never use it,they just wait untill i come to visit if something needs welded"hey Brother,make sure you bring you're welding stuff too"
So i think i'll just get it back and try this out for the heck of it
Now I regret selling my HF 90A Mig for $20...
So the test welds were on 3/16. Got good penetration. I would like to upgrade the ground clamp as it gets pretty hot but the mig gun itself got no hotter than normal. I think the 4 ga battery cables do not matter much as none of the hf wiring is that gauge. Considering you would be welding for a few min at a time I think the smaller gauge cables are adequate
Just a note I would get the largest wire size that the roller could handle for the heavier welding. By adding a portable battery grinder you could some cleanup or a break.
I would like to upgrade the ground clamp as it gets pretty hot but the mig gun itself got no hotter than normal. I think the 4 ga battery cables do not matter much as none of the hf wiring is that gauge.
Yes but as described above, the welder is no longer limited to 90 amps as it was when it came from the HF factory. I'm looking for a used HF 90 amp welder (I made a $40 offer yesterday for a used one that was refused) for this project but I'll be using 4 gauge cables for it. :)
No promises Jerry,but i maybe able to hook you up with one on the cheap,a few farmers i know who bought them and only used once or twice and figured out they were useless to them and sitting collecting dust,i'll see if any of those guys still have one laying around they want to get rid of.

Again no promises,but i'll see what i can do.
That'd be great and much appreciated, thanks!
OK a few extra pictures.

As promised, the diagram on how to make it a "cold tip" welder. Here I utilize a 24v starter relay. It should have enough current capacity. I have not tested this configuration, but I am confident it will work. The most important factor is to get a starter relay that is sized appropriately to handle the current.

I learned to weld with the hot tip, so even on my cold tip welder I really do not miss that feature. My mind is now hard wired on how to use the hot tip.

Also another picture of the welds, this time wirebrushed so you can see a little better. Did not need the welder on the 33 mile Dusy trail this weekend. I was hoping to need to help Someone else out, but I was totally happy not breaking anything.


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I've got one of these in the garage, 125 Amp Single Phase Wire Feed Flux Welder - Homier
After reading this thread, I'm seriously considering building one of these "tool box trail welders".
However, this is currently the only welder that I have at home (I have access to a larger 220v machine, but it's not very convenient). I really don't want to give up my only welder, just to have a trail welder.
Here are a couple idea's/questions that I had, let me know what you think.

What about putting some type of quick connect on the side of the "tool box" for connecting the jumper cables to.
And then, keeping the transformer, and setting it up with some cables that could also attach to the quick connect on the "tool box". In theory, you could use the welder at home, with it plugged into the transformer. And then, when you head for the trail, just unhook the "tool box" from the transformer, and grab the jumper cables, and be ready for trail welding.

What do you think, would it work?
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In regards to the welder listed, based on the price I would suspect the internals are mostly identical to the HF unit, with this unit probably having a larger transformer. But the drive mechanism and motor speed control are basically the same.

If you were to make the welder be "both" you would need a way to disconnect the AC stuff and connect the DC stuff. Inside the welder there are basically 2 seperate circuits that share the same power source.

Circuit one is the one that actually does the welding, basically shorting out the battery.
In your ac welder this is accomplished via transformer. it takes 120v at 20amps and converts to probably 20-30v at 110 amps AC 60hz. a True flux core welder is actually DC, but thoes addtional electronics cost more $$ so we dont get that with the cheapo welder.

Because the transformer is basically a long shorted out wire, hooking up 24v dC directly is probably a bad idea, I expect it would heat up the transformer to a point of making it useless. If it were me I would have a hard disconnect between the two to "switch" from AC operation to DC operation.

Circuit two is the one which makes the motor advance at the proper speed based upon trigger pull.

The motor in this unit is what is called a "universal" meaning it will run on either AC or DC. The speed control in the HF unit I believe is similar to a light dimmer, meaning it only applies AC for a percentage of the full cycle (10-100% of available power) Its power source is the transformer at the 20-30v ac current, and the motor runs on AC. The PWM controller I used is for controlling Motor speed in DC operating range. Agian this woudl require a hard swtich (but much cheaper as it is lower current) or you could get really fancy and hook up a rectifier and small capacator and always use the PWM controller for ac and DC. This is the route I woudl go as you could hard mount the motor speed control in the box easily. It would only require $5 in parts (radio shack has rectifiers and capacitors)
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I just thought about what you were trying to get at with basically having the tool box be the welder and then have a DIFFERENT box that held the transformer. and when you wanted to weld you hook up to the transformer at home and the batteries when out on the trail.

With the addition of a rectifier you can eliminate the stock wire speed controller and use the PWM controller exclusively. (see attached diagram)

The only thing I am not sure about is how large of a capacitor. I am not enough of a EE to help with that. I would just put in a big one and see if it works. (like 1000mf) Many rectifiers are rated for 10 amps or so so this is not an issue. The capicator is there for filtering of the incoming power to generate a more smooth DC current. The rectifier can take either an AC or DC input. The welder will output a AC arc when on AC power and a DC arc when on DC power

When you are at home, hook jumper cables to transformer leads The polarity does not matter. When on the trail hook to the 2 batteries (Polarity matters). It all works the same.


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