21.5.13

Welder Rehab & Foot Switch

In which I buy an old school welder off the friendly neighbourhood chopper freak, fix it up, make an illuminated foot switch for it (which works although I have no idea how), and start gluing things together with hot metal, badly. Like warty Oscar here.



Old school welder


It's a good day when you score a TIG welder for less than $100, even if you do have to drive the length of New Jersey to get it. The Duke was kind enough to throw in all the leads, a helmet, some stick electrodes, a welding history of the neighbourhood, pep talk, and brief tutorial. I left North Bergen with a smile on my face.

The Miller Econo Twin HF is big, heavy, and built to last. At its heart is a giant step down transformer for pumping out high current low voltage AC. A jumbo bridge rectifier and inductor allow DC operation. The only trace of sophistication is a spark gap oscillator that generates high frequency (HF) bursts of high voltage, which helps initiate the arc and greatly facilitates TIG welding. Although primitive and bulky compared to modern inverting welders, there is very little that can go wrong with these machines because the circuitry is so simple and the components are extremely durable. Occasionally you may hear an anecdote about an old welder firing up first time having been found submerged by flood water or in the basement of a building that has burned down.



Welder rehab


As far as I was aware no such disasters had befallen this machine. But it had seen plenty of service — the Duke had been using it to build motorcycles in his garage — and I reckoned it to be 40 years old if it was a day, and due a little TLC.

First I took off the hood and vacuumed out a few decades worth of crud and dust. Next I needed to replace the red nut for the work (negative polarity) jack socket. Not wishing to shell out for the complete receptacle, I cheaped out with a ½"-20 nut wrapped with electrical tape. It did the job. For a modicum of visual appeal I brushed surface rust from the hood with a wire wheel and sprayed with primer and gloss. The Rustoleum brilliant blue was a good colour match for the original Miller paint job. An easy but important task was to repair the insulation in the work and stinger (electrode) cables, which was cracked in many places. I wrapped the exposed wires with many turns of electrical tape. Finally I needed to wire the garage. (This was the expensive bit.) My electrician Bud put in a 100A service to a subpanel and a 40A 240V breaker and NEMA 6-50 receptacle for the welder. Sorted!


HF foot switch


One thing that did not come with the welder was a foot switch for the HF (high frequency) arc starter. This is not strictly necessary for the stick welding that I was doing, but does make it easier to initate the arc. I made a robust foot switch in a matter of minutes.

HF secondary output primary coil operates at 36V AC according to manual (see schematic below). All that is required to turn it on is a SPST switch connected to a twist lock receptacle on the front panel of the welder. I had a DPDT momentary stomp switch handy so I used a pair of NO contacts for the HF switch. I mounted the switch in a robust die cast enclosure which I attached to a board for when conditions underfoot are not ideal.


This seemed to work fine, so I decided to gild the lily by putting in an LED for a visual signal of when the HF was activated. (This was hardly necessary as the HF generator made an audible hiss.) Without really thinking it through, I put a rectifying diode parallel to the LED with opposite polarity and a 1 KΩ resistor in series. When I connected up this sub-circuit to the same set of NO contacts, the LED illuminated when the HF was off, and turned off when the foot switch was pressed — exactly the opposite of what I had intended. Still without thinking, I fiddled about by disconnecting the LED from the NO contact and connecting it to an NC contact.


The schematic above shows the circuit with the pedal switched off. The SOP coil, connected via the plug through the NO contact, is part of an open circuit so no current flows and the HF is off. The anode and cathode of the LED, connected via the NC contact, are at the same potential so the LED remains unlit. When the pedal is depressed, 36 V AC flows through the SOP coil and the HF turns on. The LED connected to the NC contact is then part of an open circuit, but nevertheless it illuminates (albiet dimly). Quite how it does that is a mystery to me. My best guess is that the cable between the pedal and the welder acting as an aerial and picking up electromagnetic noise from the high frequency oscillations. Any suggestions?



Materials



Test pieces


For a portable welding table, I got a shop cart from the scrap dealer, turned it upside down, and sanded the paint off the bottom.


I was finally ready for a bit of practice. I hit up the big box store for 5lb of 1/8" E6013 electrodes — the classic "farmer rod" that can be burned using an AC buzz box with minimal skill and still give a nice looking bead — and laid down lines of ugly stringers on some scrap pipe. My first attempts were pretty dire, so I made a virtue out of my splattery welds. Because frogs are supposed to be warty.

After a while I managed to dial in a reasonable current setting and started to get the hang of controlling the rod angle, arc length,and travel speed. I found scratch starting the arc pretty tricky. Happily, using the HF pedal made this unnecessary.

The Duke's helmet served well for my first few attempts but the strap was worn out and the wobbly pivot was driving me crazy. After a few hours of it falling off my face I had arc eye and a stiff neck. A Wel-Bilt autodarkening helmet was $50 well spent.


Next I made a rectangular flap for the chicken cage using ½‎" EMT conduit and E308 rod. I used a flap wheel to remove the galvanized coating to avoid porosity and zinc fumes. (Better still would have been to invest in a respirator and particulate filters.) The stainless weld will stay shiny, but the bare steel is already starting to rust.

The conduit was thin so I blew plenty of holes along the way. Eventually I realized that the metal was too thin to lay a bead on and I needed to make repeated tack welds until they joined up along the mitre. The angle grinder got plenty of action removing the various blobs and slag inclusions. Still, the finished structure should help keep the fox out.

6 comments:

Anonymous said...

Does the pedal turn the welder on and off? I have the same welder and I don't understand what the remote actually does. I'm hoping it and on\off switch so I don't have to scratch ark

Paul Mann said...

No, pressing the switch does not turn the power to the welder on/off; it only controls the Hi Frequency (HF) arc stabilizer circuit. The HF arc stabilizer performs two functions when enabled: (1) it generates a high voltage / high frequency signal to ionize the shielding gas so you don't need to strike-start the arc (to avoid risking damage to the TIG needle or disrupting the work surface, for example), and (2) to keep the shield gas ionized by superimposing a HF/HV signal over the weld current to prevent the arc from wandering or extinguishing during low amperage weld operations (i.e., to stabilized the arc). I found a good summary here if you like reading technical literature: http://www.google.com/patents/US4767912

Tom said...

^ what Paul said!

Anonymous said...

I am looking at this machine with everything except a bottle for 450. Is this a good price? I am looking for a good machine for around my farm. I am not a production weld or anything. My current machine is a very old century ac unit. I am hoping this machine will be quite an upgrade. So anyway in your opinion will this be a good machine for the farm, non production fab work, and maybe build a maple syrup pan or 2?? Any help will be great thank you.

Ashan said...
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