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:
  1. 100 lbs silica sand
  2. 10 lbs sourthern bentonite
  3. 15 lbs porcelain clay
  4. 7 lbs water
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.

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 friction.

  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 magnification.

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.