I bought another deader

[ddmckee54]

I recently scored my second non-working electric jewelry furnace.  My first was brand new, but had a factory FU.  Imagine that, a cheap Chinese knock-off that doesn't work.  That one was an easy fix, it had a solid state relay that had the 2 input wires to the relay reversed.  Polarity does matter in DC electronics.  This is my second one.


I haven't opened the second one up yet to see what's wrong, but there's not much on the inside of one of these things.  This thing useta work, you can tell it HAS been used.  I'm suspecting a bum power switch, but I'll know for sure when I crack it open. 

I don't plan on using most of the electrical guts of this thing anyway.  The plan is to modify this guy and turn him into a flask burn-out oven for lost wax or lost PLA investment casting.

I like radio control, and I'm especially interested in RC construction type stuff.  I've been building a stash of Bruder construction equipment that I plan on converting to RC.  I'm planning on casting some parts out of ZAMAC to get some extra weight.  I can make flasks that will work with these small electric furnaces, especially if I can get this latest deader working and modified the way I can see it in my head.

Don

[ddmckee54]

I cracked it open.  I'm still not sure what the problem is - but I'm pretty sure I know what caused it.  I believe that the previous owner had a crucible failure.  I've got metal dribbling down into the guts of the furnace. 

I don't see any obvious electrical arcing, so whatever happened probably wasn't a short circuit.  The plastic around one of the spade lugs on the 120V power socket appears to be melted, and the crimp-on wire terminal for that same spade lug seems to be welded to the lug.  There's enough connections into whatever it is that controls the power to the heating coil that I'm pretty sure they used a mechanical relay instead of a solid state relay - but there's no numbers on any of the sides I can see.

[ddmckee54]

I got a little deeper into the guts of the beast last night.  I've got to remember to take some pictures tonight, after all...  Without pictures, it didn't happen.

I did discover that other than "120V AC"  and the terminal numbers, there is NOTHING written anywhere on what I'm assuming is the power contactor for the heating coils.  The power goes from the switch, through the "contactor", then to the heating coil/s.  The PID switches a normally open contact to turn on/off the heating "contactor" coil.  If the "contactor" had screw terminals and/or a heat-sink I might think it's a solid state relay, but this guy has all spade lugs and no heat-sink, so I/m leaning towards it being a mechanical relay.  The PID controller must have quite the dead-band to keep that contactor from banging on and off all the time once the temperature reaches its' set-point.  Or maybe it does bang on and off, until I get this thing turned on I won't know.

That 120V power socket wire terminal that I thought was welded to the socket's spade lug definitely is welded on.  I tried moving it from several different angles with different tools and no-go.  I've got to remember to take my meter back down to the shop tonight so I can determine is it's safe to plug this thing in, or if all the magic smoke has already been let out.  I'm not smelling the typical fried electrical parts smell, so I've still got some hope.

Don   

[ddmckee54]

I forgot to say that the PID connections LOOK like there's a dry contact in the PID that will turn the relay on and off.  The PID says it's 120V, and that matches where those wires go on both ends.  I've got 2 wires that go from the PID to the relay.  If the label on the PID is to be believed they are going to a normally closed contact in the PID.  The other 2 wires from the PID go to the thermocouple.  Those are NOT thermocouple wire, so unless this $10 Chinese PID has built in cold-junction compensation my temperature won't be accurate - there'll be an offset.  But in the hotternhell temperature range required to melt gold and silver, that offset can probably be ignored.

[BillTodd]

I'm surprised it uses a pid controller. A simple thermostat would probably suffice for most casting operations surely?

In fact if only uses a contactor or mechanical relay, it is unlikely to be PID

[John Rudd]

Given the short dv/dt, I’d be surprised it uses a 3 term unit….PI would suffice if really needed….

[ddmckee54]

I's definitely got a controller in it.  It's one of those Chinese $10 specials.  There's not much on the label that I can see and all the controls of this thing will be replaced anyway.  It's only got a single set-point in the controller, and the heat is only wide open throttle all the way.

Anyhewwwww...  I got as many of the dribbles as I could get cleaned up and started checking things out electrically.  Everything checked out to be OK.  At least as OK as I could guesstimate with almost less than no information.  There was nothing left to do but give it the juice and see what happens.  I made sure the switch was off, plugged it in, turned my back, and threw the switch.

This is what happened, just ignore the messy bench:


IT'S ALIVE???...

It was actually rather boring, no boom, no cloud of magic smoke, just the power switch and controller coming to life - just the way electrical startups are supposed to go.  But I DO know that this thing's got a mechanical relay for the power contactor to the heating coil - I can hear it click on and off when I cycle the power to the furnace.

This is the mystery metal that I removed from the beast. 



The piece in the front was chipped out from the bottom of the furnace itself.  The rest were removed either from the base of the furnace or from between the bottom of the furnace and the base of the furnace.  (There's an air gap there.)

I did let it start to heat, and the temperature DID start to rise, so I'm pretty sure the heating coil is still functional.  There appears to be a factory defect in the ceramic heating chamber liner, probably good enough?  Time will tell.  It was late when I got to this point  so I only let the temperature come up a couple of degrees before I shut it off.

The controller's set-point was 1150 degrees C, which is right around the melting point of silver, so I'm thinking the mystery metal MIGHT be silver?

Tonight I'm going to figure out how to change the set-point of the controller, drop that to about 300 degrees, and let the beast heat to temperature - see how long it takes.  It seemed like it took FOREVER for the temperature to change those three degrees, actually it was only about 10 minutes.  Ten minutes probably wasn't that bad considering that there's the ceramic heating chamber liner and a steel tube sheath between the heating element and the thermocouple.  I let it heat 3 degrees just to be sure I wasn't just seeing the temperature flicker from the 25 to 26 degrees it was claiming as the ambient temperature.  My basement is NOT that warm so I know it was lying about that, probably the cold-junction compensation I was talking about.  Doesn't matter, for this purpose it's close enough.

Don

[tom osselton]

If it stamped “Hi Yo “ it’s silver.

[ddmckee54]

There was no "Hi Yo", but the the pieces did say "neigh" to being easily rounded up and herded away.

Over the weekend I figured out how to change the change the controller's set-point.  I dropped the set point to 300 degrees, checked the time, then went away for a while so I wouldn't be tempted to stand there watching the process variable display change.  I wanted to see how much the temperature would change in 30 minutes.  The answer to that was Nadda, Zip, Zilch and Bupkiss.  Alas, there was no joy in Muddville.

I tore the thing apart - again, and did a little trouble-shooting.  I could hear the relay pulling in after the controller booted up, so I knew that it was at least trying to work.  I checked the voltage across the heating coil, that was 120V so I knew the relay contacts were at least sorta good.  When I tried to measure the resistance of the heating coil I got NUTHIN' this time, it was an open circuit.  That 3 degree temperature rise that I saw last week was probably the last gasp of the heating coil.

I've got a new coil on order.  It's going to to be taking a Slow Boat from China, supposedly it will be here about 2-4 weeks from now.

[tom osselton]

Just wondering what the thickness of the wire is I’ve got some nicrome wire here that I’ve been wondering if I could try making one although I’ll probably just buy one.

[ddmckee54]

Kelly (Al203 - over on thehomefoundry.org) has built a couple of electric furnaces.  I believe that he has posted a link to an on-line calculator for winding your own heating coils.  Nichrome may not be the best choice, don't think its' max temperature is high enough.

If you just want to do small parts, these electric jewelry furnaces are pretty cheap, you can pick them up for USD $200-$300.  If you are electrically inclined, you can pick up non-working furnaces at about 1/3 of that cost.  My first unit cost me less than $100.  It was a non-working factory return, all I had to do was flip 2 wires around and I had a working furnace.  They had the polarity of the 12VDC signal to the solid state relay backwards.  This unit I got for less than $70.  It had 2 issues, they had a crucible failure and a mystery metal dribbled down into the control guts of the furnace, and the heating coil burned out.  I got the mystery metal removed and it didn't cause any serious damage.  I've got a replacement heating coil on the way, it was less than $50.

When I get that coil replaced I'll have 2 working furnaces for a little more that $200.  I plan on turning the 2nd unit into a burnout oven for lost-wax and/or lost-PLA casting.

Edit: Corrected some spelling errors.  Spell-chequer isn't all ways rite.

[tom osselton]

Ok thanks what are they like for brass do they Handle it well another thing I see is them being either 120 or 220 volts I’m wondering if it is for the North America verses European power supplies or if it affects the melt time.
( in a pinch I could fire up the kiln if push came to shove during the winter. )

[ddmckee54]

I haven't done any casting with mine yet, still getting things set up to do that.

Myfordboy has made a number of castings using an electric furnace that is very similar to what I've got.  I think he's done brass, aluminum(Or aluminium depending on your side of the pond), and ZA12 - but it might have been a different zinc alloy.  The size of some of the castings that he has been able to pull off using that little jewelry furnace surprised me - as in bigger than I expected.

Because they are temperature controlled I would think that you'd get less zinc burn-off when doing brass by keeping the set-point as low as possible.

[tom osselton]

Yes I watch Myfordboy and have seen the things he makes I think it’s the smaller gates and risers that help out or I have a spring loaded centrifical casting machine I bought off of Kijiji for $50. ( Can ) out here that’s itching to be used.

[ddmckee54]

I got the replacement coil about a week ago, I think I got this damned cold about that time too.  I'm going to try and get the coil replaced this weekend and see if I can get some heat out of the furnace.

Then I need to figure out how I can turn the furnace into a burn-out oven.  On HMEM, Foketry is building a scale model of an Italian 18 cylinder aircraft engine.  He casts some of his parts and in his build thread provided a link to an Arduino based controller for a burnout oven, I'll have to take a look at that again.

Don

[tom osselton]

There are some articles on this site about converting a kiln into a programable oven.

[tom osselton]

Well Black Friday got me and I picked up one of the furnaces along with a Resin Printer, Resin and Wash and Cure Station! Wish me luck! lol

[ddmckee54]

Good luck Tom, let us know how it goes.

I finally got around to cracking the top half of this thing open last weekend, and the coil is definitely dead.  I don't know if I'm going to be able to save the ceramic furnace liner / coil winding guide or not.  I'd like to save it as the ceramic liner seems to be in good condition.  After they wound the heating element on it they smeared some type of goop over the coil on the outside of the liner.  It MIGHT have been a refractory compound of some type, but as cheap as these things are, I doubt it was a proper refractory.  It's cracked pretty badly so I doubt it was rated for the temperatures it actually saw.  It's probably just plaster and sand, but since it's been smeared over the heating element the coil is acting like rebar.  I'm going to try chipping that crap off the liner, but I'll probably just have to get a new liner - they are less than $50.

Or I may just get some decent castable refractory and make my own liner, the size I actually want it.  I've got to make a new top and bottom for the oven anyway.  This is going to be a burnout oven, not a foundry furnace so it won't be seeing metal melting temps.  I've got 3D printers, I can easily print the molds to make a liner.

Don

[ddmckee54]

At first glance, this ceramic liner doesn't look too bad.


As a jewelry furnace this one is definitely dead.

That is the remainder of the heating element that I can't get off of the ceramic liner, all the rest of it came off with a little persuasion.

This stuff refused to let go, it's like it's fused to the liner.

There seems to be some of the mystery metal mixed in with the heating element, and I've got no clue what the purple stuff is.  All I know is that it DOES NOT want to let go of the liner.

Here's a look at the hole from the inside of the liner.

It looks like whatever happened to that liner started from the inside where the crucible would be, and went out towards the heating element.  Anybody seen a failure like this before?

Since I've already got a new heating element, I think we'll be going to Plan B and casting my own liner.

Don

[hermetic]

Problem is some of that stuff fused to the former will be conductive. This is what mine was like, albeit to a lesser extent, when I stripped it. The problem on mine was that the ceramic wool insulation appeared to be in small peices, and had been forced into the gap between the element former and the outer case so that some areas were under more pressure than others and some more insulated than others. When I tried to order a replacement I was told that there was no UK stock  of spares, and eventually got a kiln element maker to wind me a new element. He queried the dimensions and diameter of wire,saying that for this wire and loading, the coils were too close together, and would cause itto overheat and fail. The element he supplied was a comprimise, having a slightly lower loading and wire coils further apart, and after winding it onto the former I wrapped a completely new ceramic blanket around the whole former and element until it was just over the diameter of the outer case and slid it into the casing. So far so good. I think the chinese element wire, and quality of assembly may leave something to be desired, although it was used at the upper setting to melt copper, and that is at the very limit of what Kanthal wire will stand. Interesting project, good luck with it!
Phil

[ddmckee54]

Plan A was to reuse the existing liner when building the burn-out oven.  That would have limited me to the ID of the existing liner, which I think is around 78-80mm.  That's not much bigger than the 3" exhaust tubing that I plan to make my flasks out of.  So this liner being unusable is probably a good thing.  This furnace originally had the liner/heater assembly with 2 layers of about 1/2" thick insulating blanket wrapped around it, and all of that was stuffed into the metal shell.  The shell wound up around 7-8" in diameter.

Plan B will increase the liner ID to around 4" (100mm) or more, and the heating element will be on the inside of the liner instead of the outside.  I can understand why they put the heater on the outside though, a crucible failure probably wouldn't damage the heating element like that.  But that also meant that the heat had to be driven through the ceramic liner before it got in where it could do any good.  Since this will be a burnout oven it won't be exposed to those types of temperatures or possible crucible failures.  So the heating element is going inside the liner.  I also plan on increasing the insulation layer, going from about 1" thick to about 2" thick.

Now I need to start doing some 3D modeling - so I can get the plastic-pooping-robots making some molds for the various parts I'll need to cast.