Author Topic: Square lil bertha  (Read 8114 times)

Offline shipto

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Square lil bertha
« on: April 06, 2016, 03:55:25 PM »
I wasnt going to bother showing this but it seems to me its going to work so here it is.
After my posts about getting my casting sand issue sorted out I decided that my old coal furnace was far too much hassle to do on a regular basis so started looking up electric furnaces and found a few copies of Dave Gingery's lil bertha.

I already had most of the bits needed so decided to give it a go, the fire brick I had was salvaged from a fire proof safe I had to get open when someone at worked locked it without realising the key was lost years ago and it was all in bits which meant I had to cut and shape the 4 sides and cement it all together like 4 crazy paving fire bricks  :lol:.

The elements are from a dismantled convection heater and if I had been more confident I would have brought some slightly thicker nichrome wire but thats still a option.

I got the controller for a good price from ebay along with the SSR and a thermocouple which unfortunatly is only good for 400C so rs supplied me with some that are rated for 1100C, Actually got 2 sets of controllers etc so need a project for the second one now  :coffee:.

so as it stands now I plug it into the supply and the power goes to the 24v transformer to feed the controller and the top switch which when on activates the isolating relay and allows power to go to the bottom element via the SSR the other 2 switches activate the second and third elements. When I first started it all was good so gradually raised the temp until the rcd tripped and at first I couldnt understand why then I felt the second layer of insulation and it was wet. I had let it dry for 3 days before starting it but it obviously wasnt enough so I am currently doing a naughty thing  :wack: I have taken the earth off and running it at 225C to dry it out. The voltage going to "ground is currently at 3.5VAC so it still has a little moisture there.
« Last Edit: April 06, 2016, 04:24:24 PM by shipto »
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Offline shipto

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Re: Square lil bertha
« Reply #1 on: April 10, 2016, 12:09:33 PM »
So had a slight change of plan as I wasnt happy with the salvaged nichrome wire it broke a few times and I ordered some thicker stuff. Had the new stuff come on saturday I probably would have done the element the same as before but it didnt so my mind wandered to the old oven/grill we replaced a while ago. there was a total of 3 elements 2 of them where approximatly 1.5kw but the main grill one is about 2.5kw so decided to give that a go.
After bending it to the desired shape to fit into the furnace I wired it up and melted my first bit of alluminium :ddb: it took around 30 minute to get this melted I reckon somewhere in the region of 50 to 60cc.
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Offline Manxmodder

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Re: Square lil bertha
« Reply #2 on: April 10, 2016, 01:17:51 PM »
Really impressed with your efforts here, shipto  :clap: .

I have also been pondering for some time if domestic oven elements could be suitable to build an electric furnace capable of ally melting.

The other thing that occurs is an electric furnace is more thermally efficient than gas as there is no constant flow of heat escaping through an exhaust vent/chimney.

Even at 6kw for a couple of hours use the cost of running would seem quite economical compared to gas with the added bonus of no noise.

Watching this one with great interest.

OZ.
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Offline shipto

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Re: Square lil bertha
« Reply #3 on: April 10, 2016, 01:35:24 PM »
Thanks manxmodder I think the controller makes all the difference regarding efficiency as I dont know about others but the ones I brought bring the temperature up to 2/3 of set point then allow it to cool for x amount of time and from that it figures out how much is being lost and works out how long it needs to keep element on to keep heat around the level required, pretty clever stuff.
I still have a few things to work through on it but its all looking very good now.
by next weekend I hope to have it completed.
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Offline shipto

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Re: Square lil bertha
« Reply #4 on: April 11, 2016, 04:39:35 PM »
I have got the cradle done for the melting pot now but might have to expand it a little as it got stuck once it was hot, I thought I had left enough room for expansion but maybe not.
The temperature is transferring nicely to the pot the picture shown is about 10 minutes after start up by the time I turned it off the pid was showing 500C and the meter was showing 410C thats when I discovered the pot wouldnt come out. it might be fine once the temperature has stablised I will see once I have made the lid.
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Offline shipto

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Re: Square lil bertha
« Reply #5 on: April 12, 2016, 01:58:27 PM »
FAIL! or should I say using a old elements is a fail, if a new only shaped once element was used then maybe it would do better. I allowed it to heat up to various settings and as the pics above show I was able to melt aluminium in fact I was close when it failed the aluminium had started to soften at the bottom of the pot.
Anyway the element had burnt out or something, no obvious signs of the break but its not connected any more.
fortunatly this experiment has cost me nothing but lost time and a bit of electric as I will just proceed now with the origional plan I am however going to remake it more in line with the gingery design where instead of bricks the refractory is set into forms.
Time to go shopping  :coffee:
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Offline shipto

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Re: Square lil bertha
« Reply #6 on: April 14, 2016, 04:02:44 PM »
So as I said I did order some new nichrome wire and I got it coiled, pictured is 12 metres of brand new 1.2mm diameter coiled tight and then stretched to give me a 2.1 metre element which if I worked it out correctly will give me close to 5kw.

Went on the scrounge at work and got a bit of 200mm plastic tube for the inner form which I wound some old gas pipe around and screwed onto the side which will sit inside the outer form which is cut down bit of a space heater again scrounged from work.
I worked out this is going to need 16 litres of refractory mix and am wondering if I could crush up some of the fire bricks I have to add to the mix to save some of the cost of the brought ingredients.
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Offline shipto

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Re: Square lil bertha
« Reply #7 on: April 15, 2016, 05:03:53 PM »
Bit more progress, figured out a way to get the connections from the elements to the outside I am going to use tig gas nozzles they have a m10 thread molded inside.
In the lil bertha book the ceramic is put in after the refractory is cast but I am going to try casting them into it so have used a bit of modelling clay to seal them off.
Also I read on a blog that the person had trouble getting the rubber tube out of his and destroyed the channels so I have moved the screws to the inside so I can remove the inner form and then use a knife to cut the rubber tube in half to get it out.
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Offline Manxmodder

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Re: Square lil bertha
« Reply #8 on: April 15, 2016, 07:55:56 PM »
 I'm still watching with interest here. Just wondering if a smear of Vaseline on the tube would help with releasing it from the lining.....OZ.
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Offline shipto

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Re: Square lil bertha
« Reply #9 on: April 15, 2016, 09:01:03 PM »
I am going to give it a spray with oil on the off chance that it will slide right out but I doubt it will, had to burn the one out my coal burning furnace but that wasnt like this it had the bottom molded in one go.
however if it doesnt come out I will just cut it out with my skilsaw.
Just remembered actually I will have to take a pic of the simple jig I used to coil the element may come in useful for someone in the future, will post that tomorrow.
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Offline shipto

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Re: Square lil bertha
« Reply #10 on: April 16, 2016, 08:54:47 AM »
I guess now I wait  :scratch:
Did a little video of the coiling jig not that its anything special just a bit of angle.
<a href="http://www.youtube.com/watch?v=BCXwJV1ysA4" target="_blank">http://www.youtube.com/watch?v=BCXwJV1ysA4</a>
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Offline shipto

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Re: Square lil bertha
« Reply #11 on: April 17, 2016, 02:11:20 PM »
The mixture is hardening nicely I was able to open up the terminal openings I thought the cement might bind to the ceramic but it hasnt and the tig gas nozzles slide right out so they will be held in with a little of my modellers clay. I unscrewed the inner form and even got it to move a little going to give it  few days before I remove it completely though, Only moved it to give myself a chance of removing it in one piece.
Also to the base underway.
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Offline shipto

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Re: Square lil bertha
« Reply #12 on: April 19, 2016, 05:08:11 PM »
Got the inner form out with a minor break at the top however as said on the blog I read the rubber tubing wouldnt come out without breaking the channels even when it was cut in half not only that but I had failed to secure them tight enough and they tried to float so I have wonky channels. However it not totally ruined they are seperate and run in one channel from top to bottom just need to rethink how I am going to keep the element in place.
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Offline Swarfing

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Re: Square lil bertha
« Reply #13 on: April 19, 2016, 06:59:46 PM »
A few years ago I helped a friend build  new pottery kiln. It was made in sections from a single mould. To get the grooves for the element we used foam gap filler from the builders merchant which you can get in 16, 20 and 25mm. After we pulled the sections from the mould we just torched the foam out and cleaned up the gap so the element could be pushed in. As far as I know the kiln is still in use today
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Offline Swarfing

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Re: Square lil bertha
« Reply #14 on: April 19, 2016, 07:03:49 PM »
Also to answer your question, just use some element pins like these http://www.paragonweb.com/KilnAccessory.cfm?KID=178
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Offline Will_D

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Re: Square lil bertha
« Reply #15 on: April 20, 2016, 05:39:28 AM »
 I am also think of a similar project.

Love the idea of the Tig shrouds!

One thing to think about is electrical safety. I will be using SSRs to switch the elements via a PId controler. As the furnace will have a lid, it will be simple enough to put a micro-switch or 2 in series with the 12v switching circuit for the SSRs, so that when the lid is removed the elements are automatically switched off.

Why 2 in series? - so that if one failed closed the other will still break the circuit!
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Offline shipto

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Re: Square lil bertha
« Reply #16 on: April 20, 2016, 12:49:11 PM »
I agree foam would have been a lot better it would need securing well though and I was thinking about pinning it in will see how this works out once it has dried properly which I think is going to take a while.
Good luck Will the tig shrouds where a godsend and I may not have thought of them except I recently brought a arc,tig and cutting unit so had them in the shop.
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Offline shipto

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Re: Square lil bertha
« Reply #17 on: April 20, 2016, 04:37:02 PM »
I have left it with about 2 kilo's of silica gel inside it in hope that it will dry it out a bit quicker. Also got the electrical connections sorted out forgot to take a pic of them in place but will do that tomorrow. The modellers clay is supposed to be baked at 130C for 15 minutes so I need to borrow my mates paint stripper to try and do this.
Regarding electrical safety I too am planning to add a safety switch on the hinged lid to shut off the power when its open.
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Offline Manxmodder

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Re: Square lil bertha
« Reply #18 on: April 20, 2016, 05:03:48 PM »
I agree foam would have been a lot better it would need securing well though and I was thinking about pinning it in will see how this works out once it has dried properly which I think is going to take a while.
Good luck Will the tig shrouds where a godsend and I may not have thought of them except I recently brought a arc,tig and cutting unit so had them in the shop.

Agree,the foam and burn out method sounds really good. Small dabs of adhesive from a hot melt glue gun would be a good way to fix the foam to the inner lining tube as the glue wouldn't be too difficult to separate from the  liner. A liner made from thick cardboard and waxed on the outer face would be easy enough to remove as you could cut it lengthwise and collapse it inwards to remove

OZ.
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Offline Swarfing

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Re: Square lil bertha
« Reply #19 on: April 21, 2016, 03:58:10 PM »
To stick the foam we used a bit of copydex adhesive using it like contact adhesive because it was to hand. the good thing about it was we just peeled it off before torching the foam out.
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Offline shipto

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Re: Square lil bertha
« Reply #20 on: April 21, 2016, 04:11:30 PM »
Well here is the terminals in place and as I said yesterday just need to get the clay to 130C for 15 minutes to set it.
I installed the element tonight too the pins go into the refractory quite easily but it doesnt seem to want to let go of this water I have put the multimeter on it at far ends for 3 days now and am getting the same reading as I did on day one 1.6m ohm the reading across the terminals (without the element)was 68m ohm.
I have wired it up to my variable power supply at the moment which by my calculations should be supplying about 60 watts been on a few hours now and I am just going to nip out and see what temp i have as long as its not stupidly high I will leave it until tomorrow hopefully it will drive a bit of the moisture out.
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Offline shipto

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Re: Square lil bertha
« Reply #21 on: April 23, 2016, 05:46:47 AM »
Well 28volts didnt do much (not that I expected much from 60 watt) but it did make it sweat a little by keeping the temperature in the high 20's. So I have wired up the spare pid with the ssr and a 110V transformer and have the temperature currently sitting at 130C I have put a lid on with a small opening and the bottom open too so the heat will cause the air to flow freely through the body and this seems to be dying it out nicely.
while its drying I am going to try to sort out the proper lid for it.
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Offline shipto

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Re: Square lil bertha
« Reply #22 on: April 23, 2016, 05:01:17 PM »
Today while the refractory was drying out I got the clay on the terminals baked hard, drilled out the 30X30 box for the feet and made a start on the top for it but I have run out of cat litter for the refractory mix so need to get some more of that tomorrow. the top looks like a poor attempt at dream catcher right now.
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Offline Will_D

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Re: Square lil bertha
« Reply #23 on: April 23, 2016, 05:05:19 PM »
... but I have run out of cat litter for the refractory mix so need to get some more of that tomorrow.
So what is your refractory mix? Or have I just missed it in the posts?
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Offline shipto

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Re: Square lil bertha
« Reply #24 on: April 23, 2016, 05:24:01 PM »
... but I have run out of cat litter for the refractory mix so need to get some more of that tomorrow.
So what is your refractory mix? Or have I just missed it in the posts?
No I havent mentioned it but I am using 3 parts cement, 3 parts perlite, 4 parts sand and 4 part betonite clay (cat litter) mixed with as much water required to get desired thickness.
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Offline shipto

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Re: Square lil bertha
« Reply #25 on: April 24, 2016, 03:54:17 PM »
Lid is poured and base is attached to body. Had the temperature set at 200C most of the day and took it to 250C for the last 30 minutes so I am ready to get it wired up properly and take it up further.
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Offline shipto

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Re: Square lil bertha
« Reply #26 on: April 28, 2016, 03:45:14 PM »
Did most of the wiring up just need to add the safety switch and sort out power for the fan which I am putting in to keep the heat out of the controls but its enough for me to start increasing the temperature to get this moisture out which can be seen collecting on the oven door glass.

Also started on the hinge for the lid still need to sort out the handle and stopping the lid just over upright.
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Offline shipto

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Re: Square lil bertha
« Reply #27 on: April 29, 2016, 04:46:10 PM »
Its standing on its own 3 feet but I did realise what I hadn't given thought to while building, it needs a few handles for moving it around.
I made a change to the mix for the lid to make it a bit lighter basically took a little of everything out and added more perlite. It has indeed resulted in a much lighter mix but it already has cracks but this may be me knocking it about a bit but time will tell.
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Offline shipto

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Re: Square lil bertha
« Reply #28 on: April 30, 2016, 03:04:26 PM »
First melt  :ddb: got a couple of cracks though will fill them with cement. Also the new thermocouple I brought does not seem to be reading properly, have it at maximum input shift and its still reading a little low might take it out and try one of the ones I brought from RS.
EDIT: Forgot to mention from almost cold to this melt took me 45 minutes and that includes adding more to the pot. However the heavens opened up just as I was about to pour it so had to hold it for a further half hour till the rain stopped as I didnt want to pour in the shop.
« Last Edit: April 30, 2016, 06:06:05 PM by shipto »
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Offline tom osselton

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Re: Square lil bertha
« Reply #29 on: April 30, 2016, 04:04:50 PM »
I wouldn't think a little cracking would make any difference on it, some cracking seems to be a normal part of making furnaces.

Offline Toolshed

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Re: Square lil bertha
« Reply #30 on: May 04, 2016, 09:46:46 PM »
Sweet furnace build there!  I'm running a propane burner at this time....Whole lot less to futz with....
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Offline shipto

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Re: Square lil bertha
« Reply #31 on: May 05, 2016, 04:46:20 PM »
Hi and welcome Toolshed I did consider propane for a little while but the cost for me would have been quite high. Had the origional one worked better I would have made do with it but I am very happy with the second attempt (actually I should change the title since its not square) and if I ever feel the need I could in theory fit a pot to melt about 8 litres in it, although it might take a fair while to get it to temperature.
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