On 17 Oct, 06:36, Matty F wrote:
I want to make some brass boxes, about 65x45mm, with hinged lids.
I thought I'd use 2mm brass plate, and solder the bottom on with
silver solder. They need to be oil-tight!

First of all, soft or silver? You'll have a pig of a job soft-
soldering lubricators like this, because of the thickness of the brass
and the amount of heat you'd need to apply.

If you do go for soft, forget irons, forget torches, and go for
sweating them together.

This is basically heavy tinning in the joining areas, assembling cold,
dribbling liquid flux in (Baker's fluid) and then heating slowly in a
furnace (or best approximation with a few firebricks and a broad torch
flame), until the solder runs together. You're reliant on capillary
action here, and no amount of faffing about with an iron or individual
jointing with a torch will give you a reliable leak-proof joint in
thick brass.

Mechanical fit is important. It's too thick to fold and yet get a
square edge, so you're building up from flat sheets. I'd leave the
front, back and base over-long, then file them down into your cuboid
afterwards. I'd also use a tiny chisel to raise a few tiny burrs on
the inside of the base as a backstop for the sides, then wire the box
up with soft iron wire before heating. You can even use iron
toolmaker's clamps inside (whitewashed to stop them sticking), or even
(easiest for bulk runs) make up a welded or ground steel inner
scaffold block (with big chamfers) so that you can clamp the brass
plates easily into place onto it.

Personally I'd silver solder it. Viscosity is lower and capillary
attraction greater, so it just loves to flow nicely and give a well-
sealed joint. Again, pre-build the load and furnace it, rather than
taking a torch around inch by inch. Silver solder is expensive (but so
is time), so make the parts fit well. Again, I'd make the edges
straight, stack the box up with overlaps, solder and then trim
afterwards.

Brazing is possible, looks good (colour match), but hard work. You
have a tiny (if any!) difference in melting point between a hard sheet
brass and a soft spelter.

Cleanliness beforehand is important. If you have any lead solder
around, stick with using lead, as silver soldering over old lead
residue is a right old pain.

Silver solder used to (and bought via eBay still does) contain
cadmium, which makes it easier to work with and the fumes toxic.
Silver soldering flux (Easyflow) is fluorides, so that's pretty nasty
when heated too. Ventilate!

You need a firebrick hearth and plenty of bricks around it before you
do any of this stuff. Firebricks are cheaper than bigger torches and
encourage more even heating, with less distortion.