Friday, January 7, 2011

Flux For Soldering Wires

On the 1925 & 1926 WACO NINEs the external brace wires are stranded 1x9 Galvanized cable. From 1927 on WACO used solid streamlined wires, very expensive and slow to order these days. There are four different ways to form an end for attaching cables.

The ends on today's cables typically use a fitting which slips over the end of the cable and is swaged on, very nice and very expensive. The swaged fittings are designed to allow the cable to be loaded to it's full strength without failing before the cable.

Another way is to form a loop on the end of the cable either around a teardrop shaped thimble or around a grooved bushing with a hole in it. The loose end of the cable is the secured to the cable with a Nicopress sleeve which is crimped on with a relatively inexpensive tool.
(Per AC43.13-1B, Nicopress Process is a patented process using copper sleeves may be used up to the full rated strength of the cable when the cable is looped around a thimble.) These Nicopress sleeves and their tools were originally developed to slice telegraph wires instead of the overlapping twisted splice originally used, a very good method for splicing electrical wires, very strong.

On non-flexible 1x19 cable, or flexible cable less than 3/32" in diameter, you can also use a thimble and and secure the end of the wire by wrapping the loose end to the cable with annealed steel wire and soldering it all together. This is how the bracing cables on the NINE were made using 5/32 diameter 1x19 galvanized cable. Unlike the nicopress this method will only hold a load up to 90% of the cable strength and can be replaced with a nicopress sleeve, quicker and cheap but ugly.

On flexible cables such a 7x7 or 7x19 you can use the 5 tuck splice, the way ropes and cables (wire rope) were done before all this high tech stuff. Well I'm sure it seemed high tech 80 years ago. The 5 tuck splice is wrapped with a cotton serving cord to protect and stabilize it. The 5 tuck splice is only rated at 75% of the cable strength. There is also a 7 tuck splice which I believe is rated a little higher. All the control cable ends on the NINE were formed by using the 5 tuck method so we'll get into all that later. John Gaertner at Blue Swallow Aircraft just filmed a video (for sale) on how to do this. John lives down the road from me and does really cool WWI aircraft.

Back to the cables on the NINE. Today we tend to use Stainless steel cables. Galvanized cables don't last as well but cost less for the same rated strength and weight. I'll be using Galvanized because we're restoring this plane not trying to make an improved version. Galvanizing chemically bonds a layer of Zinc to the surface of the steel strands before the cable is wound. As you cut through the zinc to the steel the mix of zinc and steel varies from 100% zinc to 100% steel. This zinc protects the steel from corrosion even if scratched. It's not as good as stainless but this plane will not be spending it's life outside. The other cool thing about this zinc layer is it's easier to solder to then bare steel. You still need to remove surface corrosion for the solder to bind and that's the job of the flux.

So why this long story?

The latest version of AC 43.13-1B drops wrap-solder and 5 tuck splices completely. All the versions before as well as CAM 18 and older books I've found all have very specific requirements for the Wrap-Soldered Splice. We'll only talk about materials here. We'll deal with making the splice in another post.

From CAM 19 August 1, 1949:

The method of making the wrapped and soldered splice is as follows:

(1) The serving or wrapping wire shall be of commercial soft-annealed steel wire or commercial soft iron wire, thoroughly and smoothly tinned or galvanized.

(2) The solder shall be half-and-half tin and lead conforming to Federal Specification QQ-S-571. The melting point of this solder varies from 320 deg. to 390 deg. F., and the tensile strength is approximately 5,700 Pounds per square inch.

(3) Solder flux shall be a compound of stearic acid (there shall be no mineral acid present) and resin. A warming glue pot to keep the flux in fluid state is desirable.

And you thought we'd never get back to the flux. Also this is not a lead-free process so don't eat, drink, or breathe any of this stuff.

First off, NO Mineral Acid. What the heck is that?

To start with mineral acids are CORROSIVE. They'll get trapped in the strands of the cable and slowly dissolve it, not good. The common list includes Hydorchloric, Nitric, Phosphoric, Sulfuric, Boric, Hydrofluoric, Hydrobromic acids. The thing is they are not organic and they release hydrogen ions when dissolved in water because they have hydrogen bonded to some mineral, remember that chart of elements from chemistry class. Almost every commercial flux contains something like hydrochloric acid, zinc chloride, or ammonium chloride.

So what are Stearic Acid and Resin?

Stearic Acid is derived from animal or vegetable fats and oils. It's a fatty acid, you know, like that omega-3 fatty acid stuff that is supposed to be good for you. Fatty acids contain only Hydrogen, Oxygen, and Carbon, No Minerals. Stearic comes from the Greek word for Tallow.

Resin (Rosin) is a hydrocarbon which is secreted from trees, Pine Sap anyone. In solid form Rosin is used on bows for stringed instruments. In powdered form athletes and dancers use it to improve their grip. Did you know Frankincense is a resin?
I bought powdered Rosin at an athletic supply store. I thought it would mix/melt better, I was wrong. If I were doing this again I would use Lump Rosin. Artists use both forms. Daniel Smith has lump Rosin for about $12/lb., If you get 1/4 lb. it will probably be a lifetime supply.
I found Stearic Acid in candle form made by GMP at 3111 Old Lincoln Highway, Trevose, PA 19053-4996. It's their P/N 04320 Candle Stearine Flux. It comes as 6 pieces of broken candles in a bag, probably pieces from making tallow candles for churches or some such.
I used 2 pieces (4 1/2 ounces) mixed with 3 ounces of Rosin, a 60/40 mix. I cut the candle into small chunks so I could see if it would fit in my pot, but you're just melting it. I got the hot plate at the Hardware store for $15 and the iron pot from a gun supply store for $21. You don't need much heat.
Until it got to about 275 deg. F. it was just a milky looking mix with the rosin at the bottom. At that temp the rosin melted and it looked more like coffee, no cream.


It turned back solid at about 150 deg. F. You need it liquid to brush it on/into the cable.

Next step is to start making some practice cable ends.

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