Design/Construction Log for a Hobbiest Foundry
This is a brief overview behind design and construction decisions associated
with the construction of hobbiest foundry.
15 Nov 2001
Received green sand recipe (and a lot of other advice and material
sources) from Robert Grauman:
It takes time for the clay to absorb water and therefore the mixture should
be left overnight in order to get a good feel of the actual moisture content.
Too much moisture, apparently, is the number 1 cause of problems for newbies.
Too much moisture is dangerous as the moisture can be explosively released
when molten material makes contact with it. Too little moisture,
however, does not allow the sand to retain its shape.
100 lbs silica sand
10 lbs sourthern bentonite
15 lbs porcelain clay
7 lbs water
pre 24 Nov 2001
Selected Rupert Wenig's
mini-mongo since it has sufficient BTUs for casting iron as well as
aluminum. Parts are readily available.
Selected the standard Gingery gas-fired furnace. It has sufficient
size and appears to be (reasonably) safe to operate.
Selected a castable refractory (Dukast 2800) instead of a home-made
mix (suggested by Robert Grauman).
25 Nov 2001
Chose to use an old discarded oil barrel (14" inner diameter) as the
outer form for the furnace. This is slightly larger than Gingery's
specifications. Inner forms were:
Bottom section: beer bottle for drain hole, plastic bucket, styrofoam
for taper of furnace bottom, wood dowel split in two (4 pieces) to
form drain "gullies". Assembled together using hot-clue, tabe, and
Middle section: two plastic bucket bottoms for the ends, aluminum
sheeting for the side. Aluminum pop-riveted into place on the pail
bottoms, and along the seam.
Top section: plastic container (with a slight taper). Casting
reinforced with wire.
This seems to have worked satisfactorily. Burner port was created
using the burner itself wrapped 1 1/2 times with sheet aluminum.
Overall wall thickness (approximately) is 2 11/16". This (small)
increase has increased the calculated refractory volume from approx 1600
in^3 to 2100 in^3. More refractory is required.
26 Nov 2001
Received Dukast 2800 Coarse data:
Mixing water: 3.3L (5.9 pints) per 55 lb bag.
Recommended service limit: 1540 deg C (2800 deg F).
Minimum time before firing: 24 hours.
Weight to place: 2211 kg/m^3 (138 lb/ft^3).
Manufacter states: "Data are averaged results of laboratory tests
conducted under applicable and current ASTM standards, except where noted.
Data should not be used for exact specification purposes." And I
may have typed it wrong.
06 Dec 2001
Went to Andre's and saw his setup for casting aluminum and brass.
We will likely modify the furnace lifting mechanism to reflect his improvements.
The lever-arm is increased, making the lifting action much easier.
08 Dec 2001
When firing the furnace refractory lining, we had trouble getting the
burner output sufficient. The propane was burning in the tube.
A piece of teflon tape had worked its way into the burner jet, restricting
propane flow. Upon removal of the tape and modifying the aperture
on one of the fittings, the burner began to output properly. Nonetheless,
it is expected that the burner output can be increased by reselecting the
connection method to the propane tank. Currently, we are using a
setup which uses a hose that adapts a 20lb tank to that of a disposable
propane cylinder intended for small barbeques and other appliances.
This hose has a #60 size hole which is likely reducing gas pressure.
We played with the jet size and found little difference, likely due to
this restriction. It has been decided that this hose will be replaced,
and at the same time a regulator will be installed for fine-tuning of propane
output. More will follow on this.
Andre suggested a source for the regulator: G D Liquidators.
He uses a dial guage from Princess auto. The guage is selected to
give fine readings between 15 and 20 psi.
A photo of the furnace thus far.
04 January 2002
Designed the furnace frame and lifting mechanism. The design is
based upon Andre's modification to the Gingery lifting mechanism, but sized
for our furnace. Drawings to follow.
A new regulator, hose, and fittings improved propane flow compared
to the last firing. We were able to complete curing of the
furnace lining. The inside of the furnace becomes almost too bright
to look at with the naked eye.
05 January 2002
Began constructing the furnace frame and lifting mechanism. Good
progress was made this weekend.
18 February 2002
A lot has happened since the last entry:
The furnace frame has been completed.
The inaugral melt (of piston heads from Robert Grauman).
A subsequent melt of pistons (from Marc), a lawnchair (courtesy of some
neighbour - who had thrown it out!), and the rest of the piston heads.
Gordon Dick (a colleague from NAIT) has agreed to construct a large flask
for us to start with.
Arrangements have been made to get some petro-bond from a fellow in the
metal enthusiasts club.
Some used petro-bond was delivered to Marc's by Robert.
We took slices from 4 different ingots and I used my good old "Intel
Play" Microscope for some work: zooming in on the aluminum.
We noticed that our second melting session may have resulted in ingots
that were poured too hot. These thoughts were brought about by the
fact that (relatively dirty) piston stock ingots ended up being quite rough
on the outside. Here are the captures. They were taken from
cross-section cuts out of the ingots that Marc faced off in his lathe.
First are shots of the unpolished samples, followed by polished versions
at two magnifications.
The layout of the following captures is consistent:
|Piston head (from the first melt).
||Piston head (from the second melt).
|Piston (from the second melt).
||Lawnchair (from the second melt).
Unpolished samples, all 200 times magnification:
Polished samples (using 1500 grit paper) all at 60 times magnification:
Polished samples, 200 times magnification:
Here is what has lead us to believe that there may be trouble with temperature.
Top of a piston ingot (from the second melt) at 200 times magnification.
Top of a piston ingot (from the second melt) at 60 times magnification.
A void in the piston aluminum cross-section (polished) shown at 200 times
We'll need to do a little experimentation to figure out if the problems
are caused by pouring temperature or just muck in the materials, or maybe
even just crappy aluminum.