Subject: [baidarka] Working with Aluminum
From: Charles Hall (chall@rtpnet.org)
Date: Wed Sep 10 2003 - 12:05:47 EDT
I know we've pretty much exhausted this thread, but I just cames across an
article in the current issue of "Home Machinist" magazine that sounds very
authoritative. The author makes those electronic equipment cabinets that
you see in laboratories. Most of the article is about making pretty brass
boxes, but this section talks about aluminum...
Charles Hall
>From "The Home Shop Machinist", Sep-Oct 2003, page 17-18.
----------------------------------------------------------
SOLDERING ALUMINUM
In order to solder aluminum, a very aggressive flux together with
special-alloy solders must be used. Only those fluxes that can penetrate
the oxide layer on aluminum are satisfactory. These are often included
inside the solder as a flux-core as in WELCO COR-AL or separately as a
paste with ALSOLDER 500, both products of the J. W. Harris Company and
sold through dealers. An aluminum solder in rod form is available as
Bernz-O-Matic Model AL-3 4043 Aluminum Soldering Rod available from
outlets selling this brand. And for readers who really want a challenge,
try aluminum brazing using AL-BRAZE 1070. It's my opinion, however, that
brazing aluminum should not be attempted unless you are very skilled
indeed ... otherwise you'll end up chucking your work in the garbage can.
Typical tin-lead solders for brass could be used as solders for aluminum
were it not for the fact that aluminum lies high on the electromotive
scale for the metals. With tin-lead, a potential of about 1.53 volts
exists between aluminum and the solder, an intolerable situation because
it results in galvanic action that soon destroys the joint. Most alloy
solders for aluminum consist of zinc, magnesium and aluminum and exhibit
an electromotive potential close to that for aluminum.
By now, you must have concluded that soldering aluminum is difficult.
Indeed it is for, in addition to solder-flux problems, there is also the
matter of soldering temperature. All solders for aluminum melt at
temperatures considerably higher than those for brass. Coupled with the
extremely high heat transfer for aluminum, maintaining the proper
soldering temperature becomes very tricky. Not enough heat and the solder
crumbles and scatters about only to melt eventually into little puddles.
Too much heat and the molten solder oxidizes. Oxidation arrests solder
flow and prevents solder from traveling along a solder line. At best, the
solder line is lumpy, spotty, often weak and more often than not, looks
just awful.
Appearance of the solder joint may not be a concern if all you want to do
is solder in aluminum screws in order to prevent them from loosening when
constructing a beam antenna from aluminum tubing, for example. Nor is it a
problem when you want to replace stripped threads in an aluminum part. You
achieve the latter by heating the part to the melting point of the solder
and filling the hole with melted material, then redrilling and rethreading
the hole. But if you desire to make small metal boxes, like those
described above in brass, forget it. Aluminum cannot be soldered with the
ease and perfection that brass can and making assemblies like those shown
is simply out of reach.
OTHER PROBLEMS WITH ALUMINUM
There is another property of aluminum, a non-solder property that impedes
certain machining operations and easily sidestepped. Aluminum is called a
long chip metal. This means, simply, that when the metal is drilled for
instance, the cuttings (or chips if you prefer) tend to be long and
stringy. And because aluminum is also a soft metal, the chips have a great
tendency to adhere to the drill, filling up the flutes resulting in
friction and build up of heat. The cuttings rub against the wall of the
hole resulting in a rough surface. There is an easy fix to all of this.
In order to prevent chip buildup or loading, use a cutting lubricant, but
not just any lubricant. Use one of the modern environmentally friendly,
biodegradable kinds. These modern liquid products are of low-odor,
non-objectionable in smell and non-toxic. My favorite is Tapmatic Natural
Cutting Fluid, a light green liquid prepared from purified natural fatty
acids and available locally. Fill a squirt-type oilcan with the fluid and
squirt a few drops into the hole, from time to time, as it is being
drilled and you'll obtain a hole with smooth, polished walls.
<snip>
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