28 replies to this topic

[havlicek]

Mr. John,

 

Did you ever get your "com welding rods"....arc gouging rods?

 

 

REDD

...a long time ago!  For doing slot car armatures, I have a lifetime supply.

 

-john

[Coal Train]

Ide love to be pointed in the direction of how to build a com welder.

[havlicek]

Ide love to be pointed in the direction of how to build a com welder.

 

Hi Robert, the information is in one of the "Arm Winding" threads...but only the lord knows where  Building an actual com welder, such as is used for production is way beyond the scope of what most anyone could do, but there's no need to build one.  All you need is a power source and a few doo-dads.

1)The power source should be able to put out high current and low voltage DC.  50amps should be about right, but the lower the voltage the better.  Lots of volts increases the possibility of arcing...instant "poof" and your com tab is vaporized.  I use an inexpensive "car starter/battery charger".  The "car starter" part is important because it means the thing can put out big current immediately, whereas a regular battery charger ("trickle charger") will only do low current.  If a device can start a car with a dead battery, it needs to put out high current...so look for 50amps or so.  This one is something like the one I use...so I suppose it would work, but mine will do 50 amps at 6VDC, and I'm not sure if this one will...but it still should work.

 

    Basically, it's all very simple.  You're going to feed high current through the com tab to heat it up QUICK to a high enough temperature to melt either the copper tab itself or the brazing metal (silver).

 

The positive from the power source will go to a footswitch (momentary single pole single throw type that is normally open).  From the footswitch, the current will go to an alligator clip or some other holding device for the brazing rod.  That is what will get touched to the com tab.  At the top of the com tab, you will have placed another alligator clip (I modified one to fit around the com and make good contact with the pole you're welding/brazing) that gets connected back to the power source's negative.  With the power source set to "50 amps", you touch the com tab with the brazing rod tip and, while keeping good contact, trip the footswitch to send the current through the tab which will heat it up FAST.  My footswitch is made from a 50 amp marine engine starter switch I got for like $2 from a surplus electronics place online and is housed in a plastic project box that has two brass screws sticking up to use as terminals to make the connections to the starter and the brazing rod clip.

 

***There's a LOT of feel involved here, and I've found (from having ruined many commutators) that short "pulsing" of the current works better than simply keeping the current on long enough to soften the metal.  If you pay attention to how things look and feel you get to know when the tab is "done".

 

***You don't want to press too hard with the brazing rod tip.  When the metal softens, too much pressure can send the tip too far and screw things up.

 

***You don't want to press too lightly either.  As things heat up, this can cause you to lose contact with the tab and that much current at 6 or 12 volts will easily strike an arc, and the com tab will be either ruined or gone altogether...OR the magnet wire at the com tab will be.

 

***The shape of the brazing rod tip is VERY important.  I cut a two or three inch piece and sharpen it in an old fashioned crank-type pencil sharpener, then I slightly "blunt the tip" and then form a small slot across that blunted end.  That slot will be the "female" to the com tab's "male".  The same shape tip will also work fine on "hook type" tabs by just holding it on them...but the tip will need to be reshaped fairly regularly as you work, since it will lose it's shape.

 

***You can make a heatsink that slips over the commutator out of aluminum and connect your negative to that instead of to the com directly, but it's not absolutely necessary and is more of a bother.

 

***Practice practice practice.  Plan on ruining a lot of junk coms until you feel comfortable enough to try this on good arms.  You will still ruin some, but that becomes less of a problem with time.

 

***Always practice good safety, you're working with a lot of current, and even at only 6 or 12 volts things can get dicey, but it doesn't have to be dangerous at all.  Even with the power source "on", no current will pass through the com with your footswitch in the normally "open" position.  Of course, both cables from the power source are "hot" when the power source is on so keep them apart.

 

***For brazing metal, I use jeweler's silver in the "easy" alloy that comes in a fine wire.  This melts at somewhere around 1200F, well below the approximately 2000F or so of the copper.  I cut a short piece ...VERY short piece of the silver wire (like 1/32" or so) with an Xacto knife and place it on the tab and wire with a small blob of silver powder flux that will hold it there.  You touch the com tab, hit the footswitch in short bursts until you see the silver melt and flow then STOP.  As I move from tab to tab, I also clean off each tab with a very small wire brush to inspect all is well.

 

***As with regular low temperature soldering, the parts to be welded/brazed should be clean.  I clean the tabs when winding with some fine sandpaper, always make sure the arm wire is completely clean/stripped when wrapping it on the com tabs and then also use a little wire brush on the wire/tab just before welding/brazing.  

 

***Making sure things are clean is even more important here because, you will also be making contact between the tab and brazing rod tip.  A little dirt/oxidation on the tip and/or the tab can prevent current flow.  It's also important to be sure the com segments are clean for the negative terminal contact.  Current will always take the easiest path...the path of least resistance.  If you have a clip on the com for the negative that touches more than one com segment (as I do) and that segment is dirty, the current may "want" to flow through another cleaner one.  That will force the current to flow through the whole arm and the cleaner segment.  You will feel and see the actual laminations heat up and within a few seconds you'll burn up a pole...goodbye arm.

***Both the silver wire and the syringe with silver powder and flux are available online from jeweler's supply places.

 

Like I said, take your time, practice, make sure things are clean and you're working at a comfortable height and position with good lighting.  This is sort of a low-tech, but very effective method of welding coms.  What you lack in "tech"nology, you can make up for with "tech"nique.  There are welding tools available also at jewelry supply places.  I have no idea how effective they would be for this, but it seems like a good bet since what we're doing here is basically very similar.  However, they cost anywhere from $1,500 to 2 or 3 thousand bucks and would most likely have to be modified a bit to work well with armatures.

 

-john

Here are some helpful links:

For welding/brazing, silver wire...the "extra easy" alloy will work best:
http://www.ottofrei....ire-Solder.html

For the powdered silver and flux that you put on the tab and use it to hold the little piece of wire (again, extra easy ):
http://www.ottofrei....ste-Solder.html

 

 

[Coal Train]

Awesome John , thanks