Friday, October 24, 2014

Blade Switch Modification - Part 2

My last post laid out how I planned to modify a high current DPDT blade switch, making it easier to connect eye terminals from a solar charger, starter motor, and battery.  The goal is to add threaded posts to the terminals by soldering some screws through them.    This post is just an update of my efforts.

I started by drilling a hole in the end of each terminal plate and tapping an M5 thread in it.  It may seem strange to tap a thread for something I'm going to solder in place, but I thought it would be an easy way to hold everything together while I soldered the screw to the plate.  I originally intended to use 20mm long brass M5 hex bolts, but they were over a dollar each, while M5 cheese head screws the same length were 20 cents.  The head didn't really matter so I went with those.

To solder the parts together I used a 2% silver solder and a MAPP torch.  The image below shows the results from soldering the brass screws to the plates.  Those of you with a keen eye may notice the first four look like crap.  After I did the first two, I took some time to reassess things and thought my surface preparation could be improved.  So I spent some more time thoroughly filing all the surfaces to be mated.  You can see from parts three and four, things didn't get much better.

It was at this point I realised I'd been using the wrong flux.  I'd been applying the stuff for just basic plumbing work using lead free tin solder.  I needed something a little bit more powerful for silver solder brazing rod.  Luckily I had some flux that contained boric and phosphoric acid.  The poison warning on the bottle was bigger than the brand name so it had to be good, right?  Indeed it was. My last two joints were a lot better.  Joint five is almost perfect, but I think joint six got too hot.  It was going well and then the solder started spitting (always wear goggles).  I was tempted to rework the first four parts, but even though they were they ugly, they were electrically and mechanically connected.  I didn't have spare parts, so if I made a mistake it would be a month before new parts arrive from China.  I played it safe and left them alone.  If I had to do 100 of these I think I'd know what I was doing after a few more attempts.

Soldered Brass
Brass strips with screws soldered in place
It was almost easy sailing from then on.  Joint 6 caused me some problems.  The solder wicked its way through the thread and about 5 mm down the bolt.  This was easily fixed by running an M5 die down the thread to re-cut it.  The flux was then removed and the entire switch was reassembled.

Switch Parts
Screw Terminal
All I need to do now is make a mounting bracket and finish the wiring.

Switch Parts
Reassembled Blade Switch
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