The Furnace Shell, part 1

Posted: August 1, 2010 in Furnace Body

The furnace and shell are two parts of the same whole.  A feature in the furnace makes its way into the shell.  However, since I’m using castable insulating refractory and will need to make molds, etc, I’m just going to break this up into individual elements.

I got my initial inspiration from Lionel Oliver’s Backyard Metalcasting site, specifically his “2 bucks” furnace.  However, I was having a heck of a time actually finding 5 gallon metal buckets!  What used to be prevelant, is now rather hard to come by.  I didn’t really want to buy a ton of paint (and what do I do with it?) just to get buckets, and while ordering buckets online is cheap, shipping them is not.

I contemplated buying some cheap stock pots from walmart, but “cheap” is a relative term, and $30 each isn’t especially cheap though it would have been stainless steel and rather cool.  It may have been a little on the small side too, since I wanted to get an inswool coating in there as well as the castable.  And then, I stumbled on a pair of ice buckets from target that seemed to fit the bill… not huge… a good size and relatively inexpensive.

Let me start by saying that at the time I was just happy to find a steel bucket that could do the job.  I wasn’t particularly thinking about the potential outgassing of whatever paint was on here, and it wasn’t until I was virtually done that I’d even heard of Metal Fume Fever.  Galvanized Steel is coated in a thin layer of zinc.  If it vaporizes and is inhaled, you could get a little ill.  Best not to use galvanized steel if you can help it.  I’m not starting over though, so my precautions include trying to strip the paint and galvanized coating via wire brush on my drill, and once complete coating the whole thing with 1200 degree paint to reduce/eliminate fumes by not providing a path to the outside air.   That said, I’m anticipating enough insulation within the shell to make it impossible to get to the zinc melting point (787 degrees F) let alone the zinc boiling point (1665 degrees F) where vapors would be a concern!   Still, best to be prudent.  Keep milk on hand! (old wives tale, or true?  Who knows?)

Ok, with that out of the way, lets move on.  My plan started relatively simple and got progressively more complex as I went on.  Initial plan:  Half on the bottom, half in the top, basically just the 2 bucks furnace, but the castable wouldn’t fill the whole bucket.  An interior form would restrict it to a cylinder, around which I’d wrap 2 inches of inswool.

Then I thought, it’d be good to get a little wool underneath the crucible to keep the heat from migrating downward, both for safety and heat efficiency.  I was thinking 2″, but that was going to shorten up my crucible volume a bit more… hmmm.  Eureka!   Split the top bucket, leaving 3″ welded on the bottom, and the top becomes lighter and more manageable as well!

Next I drew some plans on how I wanted the inner form to go.  The flash makes the picture difficult to see the pencil marks, so my computer-enhanced image is off (and I was using MS Paint, so accuracy is severely compromised!)  The purpose is mainly to demonstrate planning and visualizing how it will all come together.  The larger section at the bottom right is the propane inlet and the smaller hole in the top is a drainage hole in case of leaks.    The flanges on the side are intended to help stabilize the refractory cylinder without the weight and expense of 100% refractory, leaving room to add the wool on the outside. I’m planning on a 1.25″ thick cylinder shell, 6.5″ inner diameter and about 9″ tall in the base.  The lid would have an arch to reduce weight and increase strength.  The interior will be coated in ITC-100.

Then I realized that if additional insulation is good, and reflecting the infrared via ITC is beneficial, it made sense to not put the exhaust hole dead center over the crucible.  By having vents to the outside of the center, more wool can be added  in the cap AND ITC-100 can reflect the radiated heat directly back at the metal instead of shooting off into space.   It sounded good in theory anyway… we’ll see how it turns out.

It’s still important to keep the heat away from the shell at the exhaust, so a signifigant amount of refractory will be needed to surround the exhaust vents.   Initially I was planning on 4 small vents (1″ diameter or so) but quickly came to the realization that it wasn’t going to be enough exhaust surface area and joined the two vents on the sides of the wool strip, making 2 larger exhausts, rather crescent shaped since the vent is conforming to the inner diameter of the lower cylinder.  The cutting had commenced!

All this time I’ve been thinking strictly Foundry use… until I run across a web page describing forging in a vertical forge.  I’ve always been enamored of the idea of creating my own knives and whatnot.  It seemed a simple addition to add in two side ports into the cap, allowing metal to be inserted/heated then withdrawn to work on.  As luck would have it, the blade port could be easily aligned with the additional wool insulation/ITC reflector in the roof.  In theory, it should heat up pretty fast.

(Note that the picture was taken after adding the side ports, but before joining the exhaust ports.)

All of the metal edges were bent inside for additional strength and to eliminate sharp edges on both the top and bottom, then began the tedious work of removing the paint and zinc.

After the top was clean, I bent a steel blank around the lower side edge to serve as both strength, and to function as a handle.  The rod will be inserted into a copper (probably) tube to lift and rotate.  I’ll go into my planned harness later.  If it doesn’t work, it may just be used as a hinge.  The steel ring was secured with sheet metal screws , which also serve to give the refractory a “stop” to keep from sliding out.  The edges on the side ports also serve that purpose, as will a few additional lips that get welded on.  The steel ring will be welded for additional strength and aesthetics.

The bottom has had the propane port cut out of its side, and currently setting up a jig to get welded on to help hold the burner in place.

The next step is welding… stay tuned!

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