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The Artful Bodger
12-14-2010, 05:43 PM
I am looking for ideas for a carbon arc muffle furnace. I am thinking of an oven for heat treating of metal and oven brazing items. I foot cube or maybe a little bigger.

It seems it would be easy to make a carbon arc heat source using the techniques of the early street lamps where a solenoid drew the rods apart as the current increased (I think that was how they worked? :rolleyes: ).

Power source would be a 240V 140A arc welder, moving choke type.

Any comments? I am looking for comments on this concept in particular rather than a discussion of the myriad of alternatives.

Thanks.

Tait
12-14-2010, 06:28 PM
Sounds like a cool idea.

I have no comment on the idea - I AM curious though - what would you be wanting this for?

Tait

macona
12-14-2010, 06:37 PM
Sounds pretty hot to me. ;)

But why not just nichrome heating elements? Or do you want to get past the 1000c mark?

rohart
12-14-2010, 06:43 PM
My immediate reaction is that you won't get any more heat out of the electrical energy than if you just ran the current through a resistor/heater element.

The carbon arc was designed to get the energy into radiation. When welding, the arc goes from the work to the rod, so both get heated, which is what you want.

If all you want is to make heat, which then has time to get transferred to a large box, there's no point in going to the noisy dirty unstable extreme of using a carbon arc.

The only problem is making sure that the heater wire, and its support material, can take the temperature. If nichrome won't take it, you'll just have to look up to see what will.

Lew Hartswick
12-14-2010, 06:47 PM
The thought that occured to me was: 35 mm theather projectors.
That is a lot of light and UV in adition to the heat. Distribution of the
heat for uniform treatment may be a bugger. As it is a "more or less"
point source.
Don't think I like it very well BUT if you do try it I'd sure like to hear
about and see it. :-)

oldtiffie
12-14-2010, 06:52 PM
http://www.google.com.au/search?hl=en&source=hp&q=carbon+arc+furnace&meta=&aq=f&aqi=g1g-v5&aql=&oq=&gs_rfai=

The Artful Bodger
12-14-2010, 07:04 PM
Thanks for the comment everyone.

As suggested, one of the objectives is to avoid the maximum tempertures of using resistive heaters such as nichrome wire.

One of the other motivations for this potential project is that I have the welder on hand and I dont think it has felt the surge of hot electrons for about five years or more, but an oven that could braze and heat treat that would be useful.

macona
12-14-2010, 07:21 PM
So what you want then is a control that brings the carbons together and then pulls them apart and maintains the set arc. When I did the welding gig we actually had such a device. It was used with a gullco track burner that held a fed carbon instead of a torch. You set the voltage on the control box and it does it all. The arc gap voltage is proportional to the distance between the electrodes.

This would be pretty simple to do with a small microcontroller and stepper motors to feed the rods. Even an arduino. Though you will want some sort of PID loop to stabilize the rods.

You would use the analog input to monitor the voltage between the electrodes and a cheap hall current switch on one of the lines connected to one of the digital inputs. On start the uC would feed the rods together until the hall sensor sensed current. At that time it would separate the electrodes until the arc voltage matched the preset voltage level. As the carbons burn the voltage will rise and the uC will trigger the stepper to move the carbon in.

Something like an arduino would be perfect. There are stepper libraries all ready for something like this.

The Artful Bodger
12-14-2010, 07:35 PM
Macona, I think it is even easier than that. The old style arc lamps had a solenoid in series with the arc. When the gap was closed current would flow and the solenoid drew the rods apart, when the gap got too big the current reduced and as the solenoid also reduced its force gravity moved the rods back closer together.

lazlo
12-14-2010, 07:42 PM
Sounds like a cool project -- take pictures! :)

The Artful Bodger
12-14-2010, 07:52 PM
Sounds like a cool project -- take pictures! :)

.........through my welding helmet!:)

macona
12-14-2010, 08:09 PM
.........through my welding helmet!:)

No kidding, otherwise you will end up like my arm that got too close to a super high pressure mercury arc lamp I was playing with.

Not one of the brightest things I have done...

http://farm5.static.flickr.com/4086/5186654166_46a882f0df_b.jpg (http://www.flickr.com/photos/67292116@N00/5186654166/)
Ushio UHP Mercury Vapor Lamp (http://www.flickr.com/photos/67292116@N00/5186654166/) by macona (http://www.flickr.com/people/67292116@N00/), on Flickr

jugs
12-14-2010, 08:24 PM
I am looking for ideas for a carbon arc muffle furnace. I am thinking of an oven for heat treating of metal and oven brazing items. I foot cube or maybe a little bigger.

Most work in a muffle furnace is below 1000°CegHardening 650-900°C Tempering 150-220°CBrazing 400-800°C NiCrC (nichrome wire)Chemical Composition: 61% Ni, 15% Cr, bal. Fe Approx. Melting Point: 1350 deg C I don't see a problem, it's- Easy to control (some processes take 24/36 hrs), Clean & quiet .:)

Carbon arc is good for Hi-temps for melting, but is dirty, noisy, needs constant adjustment & replacement of rods + lots of nasty UV. :(

john
:)

The Artful Bodger
12-14-2010, 08:28 PM
Good points Jugs.

I thought I would be keeping the temperature down when required by pulsing the arc.

Dirty, I thought the idea of a muffle furnace was the work was not exposed directly to the heat source?

John

paulj84003
12-14-2010, 08:49 PM
FYI RE: theater 35 mm projectors with carbon arc light houses. Back a few years ago 1954 to be exact I was chief projectionest in a military theater at Ft Bragg. The carbon rods as I recall lasted about 30 plus minutes with an adjustable feed. movie reels lasted 20 minutes to change over. The arc feed moved the rods togeather as they burned. PaulJ

The Artful Bodger
12-14-2010, 09:10 PM
The arc feed moved the rods togeather as they burned. PaulJ That would be right Paul as they would need to keep the arc near the required point in the optics. In the case of this oven I think moving just one would be enough.

randyc
12-14-2010, 09:13 PM
...I am looking for comments on this concept in particular rather than a discussion of the myriad of alternatives...

Apparently most folks didn't note that particular part of your request, not unusual.

An interesting idea and reminds me of one of my earliest "experiments" which entailed smashing "D" cells open to obtain the carbon rods that I attached to metal plates nailed to blocks of wood.

The plates were connected to my Lionel train transformer, of course. Probably not much more needs to be added. Don't recall doing anything useful with the rig although I DID stick various materials into the arc to see what would happen.

Cheers,
Randy C

lazlo
12-15-2010, 12:32 AM
How about this 1933 Scientific American article:

http://blog.makezine.com/MAKE_426.jpg

Arc furnace that melts anything
(http://blog.makezine.com/archive/2007/03/arc_furnace_that_melts_an.html)

HEAT so terrific that no known substance is able to withstand it for long can be developed in your home laboratory with nothing more than a pair of electric light carbons, a small crucible, and some means of controlling the flow of the electric current from the house mains through the arc.
Most electrical experimenters attempt to use an old toaster or electric grill in series with the arc. This works all right, but the current flow is limited to three or four amperes and is greatest when the carbons are in contact and the arc is producing the least amount of heat. Adding another toaster or grill in parallel with the first one doubles the current through the arc, doubles the cost of operation, and still is open to the objection that the current flow is greatest when the arc is least effective.

The difficulty is that a carbon arc, operating on the ordinary 60-cycle, 110-volt current, actually requires only about 35 volts. The difference is wasted in useless heat from the grill or toaster.


Some good ideas from one of the posters:

I did this using a cheap $59 arc welder from homier. I used carbon gouging rods (used for cutting metal thats too thick for a plasma torches or gas torches) for the electrodes. Instead of a crucible I used two bricks that I hollowed out with an air hammer to hold the molten metal. The bottom has large area hollowed out to contain the molten metal. The other one is a lid with two channels for the electrodes. Its got to be about the cheapest way to melt metal for small castings. Without the lid I wasn't able to get the metal hot enough to pour into a mold thou. This is very dangerous and life threatening by the way. It has the potential to cause blindness, electrocution, burns,... to many ways to kill or permanently mame yourself doing this to list. Some things to note about this are the cheapo arc welder listed above came with a 220 volt euro plug that I had to cut off and rewire. I run it at its lowest (45 amp) setting for the greatest duty cycle(I wired it up to 110 once and it ran cranked all the way up to the 100 amp setting without the thermal protection tripping). The smallest carbon rods I found work best (1/4 inch). It also seem to help if I sharpen them and cut the copper coating off from the ends that are arcing. The carbons are kind of hard to find. I found them at the local farm and fleet store. My normal welding supplier did not carry them. The bricks do crack open after a while and drop the molten metal out so don't do this on a table while wearing flip-flops. My metal of choice so far has been rusty nails for small casts to turn in my lathe. All the rust seems to get pushed out of the molten metal and the castings seem to be good all the way through as far as I can tell. This is sort of a micro scale version of the way scrap steel is melted back down.

macona
12-15-2010, 12:54 AM
Good points Jugs.

I thought I would be keeping the temperature down when required by pulsing the arc.

Dirty, I thought the idea of a muffle furnace was the work was not exposed directly to the heat source?

John


Hmm, I dont think you can pulse the arc. I think it will just extinguish. I had not even thought of temp control. And the temps needed for heat treating are pretty critical.

A muffle furnace is just one kind of furnace. The whole chamber is referred as "the muffle". Mine is direct heated:

http://farm5.static.flickr.com/4128/5153700372_800e3f361b_z.jpg (http://www.flickr.com/photos/67292116@N00/5153700372/)
Vulcan Box Furnace (http://www.flickr.com/photos/67292116@N00/5153700372/) by macona (http://www.flickr.com/people/67292116@N00/), on Flickr

The Artful Bodger
12-15-2010, 01:13 AM
Hmm, I dont think you can pulse the arc. I think it will just extinguish. I had not even thought of temp control. And the temps needed for heat treating are pretty critical.

A muffle furnace is just one kind of furnace. The whole chamber is referred as "the muffle". Mine is direct heated:

http://farm5.static.flickr.com/4128/5153700372_800e3f361b_z.jpg (http://www.flickr.com/photos/67292116@N00/5153700372/)
Vulcan Box Furnace (http://www.flickr.com/photos/67292116@N00/5153700372/) by macona (http://www.flickr.com/people/67292116@N00/), on Flickr

Nice furnace!

Having read some of the response I am now not so sure that this really is a good idea but having come this far:-

I thought the 'muffle' was a panel of ceramic material that was heated on one side by gas or oil flame etc and the material being heated over was on the other side of the panel isolated from the combustion gases etc. Obviously this isolation is not a consideration in a resistance electrice furnace.

As for the arc, I was thinking of a system like the old street lights where the electrodes are brought together by gravity and when the current flows a solenoid draws them apart. Using something like 'simmerstat' from a cooker to control the primary of my welder would give me longer or shorter periods of arcing. But that is just a notion, it is not an argument that this is the best or even a possible way of controlling the heat in home made arc furnace.

John

macona
12-15-2010, 01:52 AM
I am just going by the parts manual for mine. They refer to the whole ceramic assemble as the muffle. I really done have a clue where the name came from.

You might look for a used one like mine. Mine was in really rough shape coming out of a lab and was in close proximity to HCl. I pulled out the single set point control and installed the control you see in the pic. It gives me up to 30 step ramp/soak. And they are only about $70US new.

I think your simmer stat is what we call infinite switches. A bimetallic finger with spring pressure from the know changes the rate it kicks in/out when current flows through it.

I think you would have a better chance using a phase controlled device like a heavy duty light dimmer. You wouldnt want DC as the positive electrode would get eaten a whole lot faster. Or use a much more massive carbon for the positive side.

Weston Bye
12-15-2010, 05:42 AM
...One of the other motivations for this potential project is that I have the welder on hand and I dont think it has felt the surge of hot electrons for about five years or more, but an oven that could braze and heat treat that would be useful.

You will need to be sure that your arc welder has the duty cycle for extended continuous operation. Most don't. Proper heat treating and soaking can be hours long, depending on the thickness of the part. You may get by if you arc furnace is of sufficiently low curent.

oldtiffie
12-15-2010, 06:08 AM
The welder duty cycle is the $-age of time the welder runs at a particular current in any 10 minute period.

There will or may be a current setting on a particular machine where the duty cycle will be 100%.

The Duty Cycle reduces with increase in current.

This is my MIG welder which has a Duty Cycle of 100% for a current of 120A @20 volts, 60% @150A and 35% @180A. It is rated at 35A @ 230V single-phase and is on a 50A circuit.

https://www.machineryhouse.com.au/Products?stockCode=W185

Similar principles apply to "chokey" welders as well. My chokey is a 220A machine.

I would think that if AP kept his machine current just below the machine's 100% Duty Cycle current he should be able to run it continuously.

But I'd be sure of the ventilation and if it does not have fan-forced automatic cooling, I'd have a big shop fan blowing on it.

The house electricity meter may need water-cooling too!!