Author Topic: Foundry Furnace for the Tiny Shop  (Read 36273 times)

Offline vtsteam

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Foundry Furnace for the Tiny Shop
« on: February 21, 2015, 01:14:49 PM »
I hadn't planned on doing a thread on this little furnace, because it's not much different than many others online. But it's come up a couple times, so I'll just show what I'm trying.

I've run a charcoal briquet aluminum furnace since 2002 using the recipe set out by Dave Gingery in his book (highly recommended) and built my lathe with it back then, a complex milling attachment, many accessories, and a disk turbine (whose blades were actually cast from aluminum melted in a ordinary campfire -- no furnace at all). The furnace was of sand and fireclay construction, but finally deteriorated after ten years when rain leaked through a metal cover over a winter and the shell rusted out.

Then a couple years ago I tired building a sawed off single charge cupola as an experiment. That failed (with wood charcoal, at least -- didn't have coke, which might have worked). That was re-fitted with an oil burner and I was able to successfully cast with iron after much trial and error.

So now with my new tiny shop, I find i need to cast an aluminum pulley to upgrade my Gingery lathe spindle to 1" dia. And I want something less massive and involved than the oil burner -- which is overkill for that purpose. Also located by my distant and unheated big shop, and outside, under snow.

Anyway, those were the reasons for a new furnace. I also hoped to have alternate metal capability -- small capacity iron would have even been possible if I was able to follow Ironman's small furnace design. But I wasn't able to find the very expensive rigidizer he uses, here in the States. So I dropped back in my expectations to maybe being able to melt the red metals.

Instead of fireclay and sand, this time, I wanted to try Plaster of Paris and sand. A recent video post here showed another furnace made from a galvanized bucket using the same refractory, but I'd been thinking about poP for a long time before that. Steve Chastain's books refer to commercil pouring liners of that material, and show data that it has a high insulative ability as well as refractory qualities. Bronze statuary casters have long used it for lost wax molds, flinging it onto the armature to prevent bubbles.

Anyway, wanted to try it and see how refractory, insulative, and long lasting it was. I still want to make Ironman's furnace some day, and will when I can find the right materials. If the plaster doesn't last long, I can always replace it with blanket and rigidizer. So this is a pure experiment, with little to lose.

Materials were sand, a bag of plaster of Paris ($14.95 at the local hardware store), and an expired 30 lb propane tank (a little taller than the 20 lb size) for a  casing.

I had some left over 8" dia. Sonotube (cardboard form used for casting concrete pillars) and I used that for a form inside the furnace. It actually measured 8-5/8" wide so I cut a 1" strip out of it and taped it back together to reach 8".
The seam made it easier to remove when the plaster was cured, too.

The mix was equal parts of sand, plaster, and cold water, and I mixed about 2 quarts at a time. Actually, the first pour was about half that in size, and used a little less water to make a heavier pasty mix which I used to "glue" the outside of the form to the propane cylinder.

This stuff sets off in literally 3 minutes, even with cold water and cold materials (very -- they were stored in a cold shop) so you have to work very fast. DO NOT use warm water!

As soon as that kicked, I ddn't have to hold the cardboard form any more, and could pour the rest of the refractory with no leaks. The proportions I used makes a good pourable mixture that self levels. Within an hour, I peeled out the cardboard form to a nice clean interior. Then I poured the bottom inside the furnace cavity -- about 2" deep.

Finally I found a cardboard mailing tube of exactly 3" diameter and used that to form the lid hole form. in a similar way to the furnace cavity. And then poured the lid.

Today, after 3 days of drying, I brought the furnace outside, and put a double layer of charcoa briquets in and lit it off. This applies gentle heat, and will help me drive off the water from the plaster -- it will probably take all day, and I'll be adding charcoal as needed. Moisture is obvious as steam this time of year!
I love it when a Plan B comes together!
Steve
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Offline dsquire

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Re: Foundry Furnace for the Tiny Shop
« Reply #1 on: February 21, 2015, 01:25:53 PM »
Good to see you underway with this one Steve. I'll be watching over your shoulder.  :D

Cheers  :beer:

Don
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Offline awemawson

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Re: Foundry Furnace for the Tiny Shop
« Reply #2 on: February 21, 2015, 01:27:44 PM »
Good stuff Steve BUT

 :worthless:

Andrew Mawson
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Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #3 on: February 21, 2015, 02:13:19 PM »
Sorry, I didn't take any pix of the construction. Here it is after running an hour with charcoal. Sorry also -- I couldn't paint the furnace this time of year -- too cold, but plan to make it nicer when things warm -- if they ever do.





« Last Edit: April 30, 2018, 07:49:55 PM by vtsteam »
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Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #4 on: February 21, 2015, 02:55:51 PM »
I added more charcoal to about 3 layers deep after three hours and a piece of old stovepipe on top to increase the draft a little. Still way to early to get out the blower. The brown residue inside the barrel and under the cover will burn off once the furnace is in use at high temps. For now, sorry about the Grapes of Wrath appearance here! You can tell I'm suffering from cabin fever to be doing this at this time of year.....

« Last Edit: April 30, 2018, 07:50:24 PM by vtsteam »
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Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #5 on: February 21, 2015, 03:44:14 PM »
I've debated whether to explain my procedure for cutting up a propane tank. I'll put it down here, but this isn't a recommendation to anyone, just an explanation of what I did, and why I did it that way.

I used to build pulse jet engines, and some of the principles apply, to my way of thinking.

The first principle in avoiding a detonation in a container is to remove all fuel. No fuel, no detonation.

The second is to reduce the volume of potentially combustible mix as much as possible. Reduced volume, reduced energy available.

The third is to provide a relatively large opening in the container. The less containment/restriction the more likely conflagration (rapid combustion) vs detonation (explosion).

Taken all together:

1.) I opened the gas valve on this propane tank and left it outside for 3 years. That still doesn't guarantee no "fuel" since these tanks typically have a tarry residue internally, which if heated will boil and offgas under torch cutting heat and even though it isn't propane itself, it is flammable (and explosive if contained).

2.) Removed the valve entirely with a pipe wrench. This increases the size of the orifice greatly compared to leaving the valve on. This alone reduces the chance of a detonation.

3.) Washed the tank out with hot soap and water. This won't remove most of the tarry residue. So it would be foolish to depend on it. It will remove lighter oils, to some degree.

4.) Filled the tank with water to an inch under the top. This drastically reduces the volume of potential air and fuel mix. The ratio of the current orifice size (about 3/4" diameter) to the contained gas volume (about a pint) has now been increased maybe 1000 times what it was with the open valve attached and the tank empty. There is little volume to burn, and an easy way for hot gasses to exhaust if it does.

5.) With the tank full of water I lit a match and held it over the orifice. No reaction. no pop. Nothing. (Again, I'm not reccomending this to anyone, I'm just repeating what I did and why I did it this way. What I did applies only to this tank, this type of fuel, and this lead-up procedure, not every situation. Make no assumptions about different situations based on this.)

6.) I drained out about a quart of water and repeated. No ignition.

7.) I drained out enough water to lie a little below my cutting line, and repeated. No ignition.

8.) I used a 4-1/2" grinder with a cutting disk in it to score around the tank, still filled with water, and then cut through. The water level was low enough not to wet the disk or grinder, and I wore rubber gloves.

I did NOT use a cutting torch. A cutting torch raises the temperature of the surrounding metal tremendously, and I did not want to vaporize any possible tarry residue in the tank. A torch is in my opinion a bad tool for this kind of thing, and procedures which use inert gasses in the tank are less safe than using water. If the gas isn't right, you can't tell, and the contained volume is way too big for safety. Leaving the valve on the tank really increases the danger.

That's my personal opinion. Many tanks have been cut with torches, and many swear by inert gas fills to do it with. I prefer my way. It makes sense to me, and results are apparent at every step.

Sorry to go on at length here, but I haven't ever seen this explained this way before and thought it might be useful. Nothing in this guarantees safety, and every person must assume personal responsibility if they try to use a propane tank. Other substances and tanks can behave completely differently.

I love it when a Plan B comes together!
Steve
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Offline RobWilson

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Re: Foundry Furnace for the Tiny Shop
« Reply #6 on: February 21, 2015, 03:48:27 PM »
 :clap: :clap: Nice one Steve , I do like a good foundry/furnace build  :thumbup:

Rob

PS ,That white stuff looks cold  :(

Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #7 on: February 21, 2015, 05:14:54 PM »
Thanks Rob, Andrew, Don!  :beer:

Maybe also a word on cutting the exhaust hole in the tank top:

I took a piece of hardwood and whittled it into a tapered plug, then hammered it into the threaded valve hole. Then sawed it off flush. Marked the center, punched it. Then used that as a guide for the pilot of a 3" bi-metal hole saw in an electric hand drill. Low speed and light pressure cut a perfectly clean hole, centered on the top. I did this before filling with the plaster mix.

The tuyere hole was also cut with a hole saw sized to fit U.S. 1-1/4" steel water pipe (1-38" ID actual, and about 1-5/8" OD) started square radial but after cutting through the outer metal and into the plaster angled to be more or less tangential to the furnace chamber. The bottom of the tuyere is about 1" above the furnace chamber bottom.

The tank and top were wire-brushed to remove the tarry residue in the tank before pouring with plaster, as well.
« Last Edit: February 21, 2015, 06:05:53 PM by vtsteam »
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #8 on: February 21, 2015, 06:01:41 PM »
It's started to snow, and get dark out and the furnace has been cooking for 6 hours now, so I added more charcoal and about a minute of soft blast to get the coals all lit. They're carrying on well, now and the heat is definitely up a notch:



« Last Edit: April 30, 2018, 07:51:03 PM by vtsteam »
I love it when a Plan B comes together!
Steve
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Offline mattinker

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Re: Foundry Furnace for the Tiny Shop
« Reply #9 on: February 21, 2015, 06:57:13 PM »
Steve,

I see where you're going, plaster does have good refractory characteristics. My only fear is that it may not stand up to repeated heat cycles. It is relatively soft. It is used to make moulds for lost way casting which are only used once and are then broken open. I've never seen any references to it in kiln or foundry construction. I would think that something like ITC100 painted on as a surface protection might be useful to increase usable life. This is an interesting experiment!


All the best, Matthew

Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #10 on: February 21, 2015, 07:03:05 PM »
Matthew, I'm hoping the sand will give it toughness.

If I got ITC100, I'd be thinking kaowool blanket/iron melting -- it may come to that if this doesn't work.
I love it when a Plan B comes together!
Steve
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Offline mattinker

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Re: Foundry Furnace for the Tiny Shop
« Reply #11 on: February 21, 2015, 08:13:14 PM »
It's only as tough as the binder. ITC gives a surface protection and reflects heat. It's uses are not limited to Ceramic fibres.

Regards, Matthew

Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #12 on: February 21, 2015, 09:49:50 PM »
I don't agree about the toughness of a binder, Matt, but let's see what actually happens first before pre-judging the results of an experiment just starting out.

And Matt, I know what the ITC product is used for and what it is. My point is, if I'm going to spend more than five times what the plaster costs on a coating to protect it, that's kind of silly!

As I said if I wanted to plunk down serious money for a coating, I might as well go whole hog with a fiber blanket refractory under it -- a refractory itself which costs only about half of the price of that expensive coating. And hopefully gain the capability to melt iron.

And actually, if I were going to spend $100+ a pint for a coating specifically to melt iron, I'd prefer to use a zircon based coating, that ironman uses.

Which brings us back to the very reason for starting this thread....I can't source that zircon coating for the present here in the US. So I'm giving up on iron, and high tech solutions in this furnace and focusing on melting lower temp metals using low tech refractory. And plaster of Paris is what I 've wanted to try for a long time.

Now, preliminary impressions -- the sand and plaster are much tougher than plaster alone. It was obvious even molding it and cleaning off drips.

Current status of the furnace -- I shut it down after 9 hours of baking the lining, and put it away for the night. We'll do a post mortem in the morning.
I love it when a Plan B comes together!
Steve
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Offline Manxmodder

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Re: Foundry Furnace for the Tiny Shop
« Reply #13 on: February 22, 2015, 12:04:10 AM »
Steve,very interesting thread I'll be following this experimental build closely as I have an old scrap propane cylinder that I took the top off a couple of months ago(pillar valve removed and filled with water and surgery performed with small angle grinder, just as you did :thumbup:)

I agree with you on the water vs inert gas option as a more certain safety measure when cutting the top off though I have seen a couple of other guys cut them open after filling with dry sand,which does the job of reducing the free internal volume.

Watching with interest,I hope this works out well.....OZ.
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Offline SwarfnStuff

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Re: Foundry Furnace for the Tiny Shop
« Reply #14 on: February 22, 2015, 01:33:36 AM »
Thanks for posting Steve.
   For what it's worth, if you need to extend the set time of plaster of Paris a trick I have used often when patching up the dings on the plaster walls is to add a little repeat little, milk, you know, the white stuff from cows?
   Don't know the chemistry but it does work. Too much milk and you will wait hours. I got this tip from a plasterer friend many years back.
  Now I will watch along with the others to see how you furnace performs.
John B
Converting good metal into swarf sometimes ending up with something useful. ;-)

Offline bertie_bassett

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Re: Foundry Furnace for the Tiny Shop
« Reply #15 on: February 22, 2015, 05:40:40 AM »
looking good  :thumbup:

will be watching as this progresses and stealing all the good ideas when I get round to making one myself.

have started saving up ally heatsinks, but that's as far as iv got.
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Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #16 on: February 22, 2015, 12:22:36 PM »
Thanks Oz, John, Bertie!  :beer:

I emptied the ash out of the furnace this morning, and the lining is in fine shape. No deterioration. In fact the impressions of the cardboard tube seams are preserved as is the "shiny" areas where I'd used plastic tape on the form. The lining rings when I tap it, so it seems reasonably dry. The lid however still seems slightly soft when scratched -- most of the lid surface is blocked from the heat by the shell, so that will take some more time to fully dry out. There's also probably still bound water in all of the lining --  this bake isn't really a test yet -- that will come with full melting heat. But I should be able to leave the furnace in the shop now -- freezing temps shouldn't hurt it. I did bring the lid back indoors to dry further.

I love it when a Plan B comes together!
Steve
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Offline mattinker

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Re: Foundry Furnace for the Tiny Shop
« Reply #17 on: February 22, 2015, 04:24:15 PM »
I hope it survives the first firing! The sand should allow the plaster to bind at it's maximum a minimum of plaster between the grains of sand, but it is still that plaster that holds the sand together! The results will be interesting!Something cropped up on the Hobycast forum today, ITC 100 in 1/4pint bottles for $25! I know it's not in the budget, just thought I'd let you know!

Regards, Matthew

Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #18 on: February 22, 2015, 09:06:12 PM »
John B, thanks for the milk tip. It's interesting, I think the sand acted in the opposite way, as a catalyst for the Plaster of Paris. The reason I think that is, my wife decided she wanted to cast about a cup of Plaster of Paris (without sand) today for a project she had. We mixed it up and I expected the near instant cure I was getting with the refractory mix. But it ended up taking a LOT longer -- at least 20 minutes. So I think the sand must really accelerate that stuff.

Thanks for the tip about milk. If I ever have to do this again, I'll probably try it to slow things down a little.
I love it when a Plan B comes together!
Steve
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Offline Eugene

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Re: Foundry Furnace for the Tiny Shop
« Reply #19 on: February 23, 2015, 04:15:19 AM »
Meanwhile even further off the main topic ...... I'm a bit leery of using a steel crucible for melting ally. Is there a coating or liner that could be applied to the steel interior to prevent the iron dissolving into the charge? I ask this from the safety point of view as much as anything else.

I had thought of just using a loose-ish liner of potters clay; if some means could be found of retaining it during the pour, that would seem on the face of it to be workable. Is it? Or what?

Eug


Offline awemawson

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Re: Foundry Furnace for the Tiny Shop
« Reply #20 on: February 23, 2015, 05:02:11 AM »
I used to use a Zircon wash on ladles I used for aluminium. Warm the ladle over the furnace and paint it on with an old broad paint brush - it flash dries. Same for stirring pokers etc.

This was to prevent iron dissolving into the melt. I've never been happy with the multitude of ferrous 'pipe crucibles' that you see on the web from a metallurgical point of view. I've always used clay / graphite conventional crucibles.
Andrew Mawson
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Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #21 on: February 23, 2015, 07:44:24 AM »
This is one of those perennial user purpose and style disagreements. I've used a cast iron plumber's pot for all of my aluminum melting career, other than some bigger pours which used a 4" pipe nipple and welded plate bottom.

Here's the deal -- I mainly melt pistons for stock metal. Those pistons were cast with steel clips right in them. So no way are you going to avoid "iron contamination" by using a ceramic crucible or a ladle wash with these. Nor do most of the world's engine makers for what is obviously an extremely demanding application. Melted pistons, despite the steel clips make great castings -- naturally. That's what they are! My castings for my lathe, accessories turbine, etc were fine castings, the lathe hasn't fallen apart after a dozen years, and I expect it to last a lifetime.

Now there is nothng wrong in my opinion with using virgin alloys, a non-ferrous crucible or wash to control a melt and get single named alloy results. If that's what you enjoy doing, and/or you have a demanding application, I don't say no. It suits the application and the style of the caster. And vise versa. If you're melting scrap, and casting heavy sections for non-NASA parts, as most of us do a ferrous crucible isn't a problem and shouldn't be disdained.

What IS a problem is too thin a crucible if steel. It should be at least 1/8" thick and preferably double that to last any time at all. Thin stainless steel canisters burn through in only a couple pours, too. Using a steel soup can is asking for trouble -- not to mention the coatings inhalation problem. The recent video elsewhere on the site using a fire extinguisher bottom, seems both hard to obtain and too thin. Why bother? A pipe nipple is much thicker and readily available. To me the best iron crucible is a cast plumber's pot. It doesn't waste in the flame as steel does -- I've used one for 12 years.

 If you are worried about iron, you can line any ferrous crucible on the inside with fireclay and sand if you don't have the bucks for an expensive ladle wash. It will then be a ceramic crucible to all intents and purposes.
« Last Edit: February 23, 2015, 08:32:23 AM by vtsteam »
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Steve
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Offline vtsteam

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Arctic Pour
« Reply #22 on: February 24, 2015, 05:38:07 PM »
I shoveled snow and chipped away ice until I saw frozen ground then covered with some coarse sand and set-up the furnace. As I mentioned in the Tiny Shop thread temps last night were the lowest of the winter -26C and today it only warmed to 11C and it was windy too. I still wanted to try the furnace out and I used some of my poor grade aluminum and an old lost foam pattern which I'd made last summer for an engine casing. It was only 6mm thick and I'd had some difficulties back then with this gated version of it. But it was something to try with the furnace -- any excuse!

« Last Edit: April 30, 2018, 07:51:42 PM by vtsteam »
I love it when a Plan B comes together!
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Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #23 on: February 24, 2015, 05:42:10 PM »
Gear set out, charcoal lit ready to go. No blast yet Lost foam mold setup in the small orange child's bucket lower left :

« Last Edit: April 30, 2018, 07:52:05 PM by vtsteam »
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Steve
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Offline vtsteam

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Re: Foundry Furnace for the Tiny Shop
« Reply #24 on: February 24, 2015, 05:50:53 PM »
Blast on, coals coming up to heat....4:10 PM :

« Last Edit: April 30, 2018, 07:52:34 PM by vtsteam »
I love it when a Plan B comes together!
Steve
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