In this space, visitors are invited to post any comments, questions, or skeptical observations about Philo T. Farnsworth's contributions to the field of Nuclear Fusion research.
Subject: Re: Home Made Spot Welder
Date: Jun 26, 11:55 am
Poster: Don Harris
On Jun 26, 11:55 am, Don Harris wrote:
>>Let me respectfully suggest that the high voltage pulse techique may be a substantial "over-kill" for the spot welding objective.
>>There are commercial short pulse spot welders.. that use a a cap discharge into a step down transformer... and to make your own is an enjoyable engineering challenge.. but I think there is a much easier and much more accessible apporach to consider.
>>First.. the amount of energy needed to weld is not large, if it is delivered fairly quickly... few msec.. would be ample.
>>This suggests a simple power frequency (manual or triac switched) soldering gun, with the following simple modifications.
>>1. In place of the resistive wire tip used for soldering... you need two movable, conically pointed hefty copper electrodes, between which the pieces to spot welded are clamped. These copper "pole pieces" should be about the same diameter as the main bus bars of the soldering gun and taper to about 1mm diameter, ending with a flat or slightly convex surface.
>>2. One (or both) of the pole pieces must be movable and able to be clamped against the other pole (with the work between) with a few lbs of force. A simple lever like clamp would work.
>>Don't have a recent secondary current measurment to quote, but my recollection is the Weller soldering guns deliver at least 150 amps at 60 hz, to the transformer secondary, even at the low heat setting (70W - 100W). So the 140W or the 250W units should be ample to weld almost any wire grids. The use of a series oil filled capacitor in the primary circuit, will boost the current through the transformer, but one needs to be aware that the voltage across the primary will increase.
>>If you stay below 240VAC, for short bursts, there is no danger whatever, since the magnet wire is typically tested to 1500 v for a few seconds, for UL approved equipment.
>>3. The last point concerns the controls. I would try a simple push button switch for starters. You only need to get enough heat to melt the wires. The electrode pressure will assure good electrical and thermal contact so the wire will fuse.
>>There are sophisticated heating cycles that provide annealing currents to give good mechanical properties to the welded joint. This is what the multi-K$$ units provide. One can no doubt approximate these with a little experimentation.
>>Hope this is helpful.. I think most that communicate here are equal to the task and it is quite safe, since secondary currents while high, are at very low voltages.
>>Hope this is helpful.
Sorry for the above post, which was empty. I hit the submit by accident.
I am still working with the "Soldering iron spot
welder" and thanks to everyones suggestions, am making great headway. This morning I made some beautiful welds on some .035" Iron wire. The wire was from a type J thermocouple. I did exactly what Dave suggested. The gun used was
a 230 watt craftman 115 vac similar to the familiar weller guns. I fabricated some welder probes from some flat copper buss bar and simply clamped them onto the end of the copper posts that were sticking out of the soldering gun. The gun was powered from a 240 vac autotransformer. I squeesed the weld terminal together with my fingers for the initial test. Obviously need to make a way to supply a repeatable pressure. The wire melted and flowed together about half way just as it should.
The gun is rated at 2.5 amps for normal operation. I measured the primary current while welding the iron wire and it was 5.5 amps. I think it will be okay to use the gun in this mode for short periods of time. The was no apparent heating of the primary windings of the gun. I made no attempt to measure the seconday because even before I increased the voltage the secondary current would overange my amp-probe. The probe full scale is 300 amps ac.
As for the pulse discharge method, I quickly found that the price of SCR's that will handle the 1000 Volts discharges are way too expensive, (around $200 for the ones I looked at). I also suspect that the heavy Iron core of the gun is not well suited for pulse discharge. I worked for a short while with the input transformer from a 200 Watt switching power supply. They showed promise as a pulse transformer, but I couldn't figure out how to get the core apart to put on a really low impeadence seconday. Anyone know how to get the Glue or laqure off these things?
I think the soldering gun method is a good way for people to make their own fusor grids. I sure the techniques can be refined as others work with the method. I doesn't get much cheaper than a "garage sale soldering gun".