Machinist

[Lawrence London <lfljvenaura@gmail.com>]

http://www.practicalmachinist.com/vb/general/ot-want-reline-muffle-furnace-heat-treatment-any-tips-276190/index2.html

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2. 12-03-2013, 12:48 PM #22

   adama

   Diamond

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   Dec 2004

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   uk

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   Rather than just groves you can get hard inserts with a trough for the wire to sit in these then in turn simply sit in groves in the soft bricks. Its not a cheap option but realy extends the kiln linnings life if used in a process thats not overly hard on the actual bricks themselves.

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3. 12-03-2013, 01:26 PM #23

   Marty Feldman

   Stainless

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   I would suggest using soft refractory brick, at about $4-5/brick, from a commercial supplier, not the hard "firebrick" which dulls tools as noted in an above post. The soft bricks are easily cut with ordinary tools such as a hand saw. With a router and a hemispherical bit it is easy to carve out a groove for the Kanthal coil. Angle the groove down about 45º so that the coil nestles down into it and stays there by gravity. You might want to look into coating all inside surfaces with ITC-100, which is commonly used in kilns and forges to reflect radiating heat back into the interior of the chamber. There is sound information to be had by talking on the phone to competant commercial suppliers, such as Paul at Budget Casting Supply, or to the guys at Bailey Ceramic. I know that things are sometimes dictated by a tight budget, but my feeling is to steer clear of homebrew cheapie methods when constructing a heat treat oven.

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4. 12-03-2013, 06:06 PM #24

   

   SAG 180

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   Cairns, Qld, Australia

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   Thankyou

   I just wanted to say thanks for all the thoughtful replies. This unit has no name visible, with the entire 10"x10"x6" chamber sawn from a single block of very soft, lightweight zirconia insulation. The 3AG fuse is rated at 6.4 A so at 240V it'll probably be a 1000W unit, the heater wire is fairly fragile so it can't be straightened out for measuring. I've bought a coiled length of Ebay Kanthal (fence wire) that is long enough for three 1000W elements so I'll make the unit either 1000 - 1500W if the gauge of wire can handle the current. It has a simple 1970's vintage thermocouple based temp control that is showing signs of bad corrosion so I'm fitting a cheap ramp controller and a 40A TRIAC I have lying around.
   
   There's plenty of room to scrape out the soft damaged heater grooves from the chamber: scrape 3/4" inch all round and fit grooved brick walls to retain the heater coils in position without the original grouting. Lab supply is a funny business and it wouldn't surprise me if the design of the furnaces with grout is for no real reason except selling custom heater blocks that regularly blow.
   
   Once again, thanks for the thoughtful replies with the excellent advice. I'll have a bit more of a play with my aluminium oxide clay mix with a view towards casting grooved tiles to line the furnace walls (Like Adama mentioned) and failing that, buy the proper bricks and cut and groove them.

   Last edited by SAG 180; 12-04-2013 at 04:38 AM. Reason: spelling

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5. 12-04-2013, 02:07 AM #25

   Fulmen

   Aluminum

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   Oct 2010

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   Glad it's working out for you, do feel free to post any progress on your grouting mix if you continue down that path. Unless someone can come up with a different solution I will need something similar for my furnace. Perhaps I could place ceramic rings on a heavy wire between the coils to keep them from shorting? Either way, since it's on the outside of the tube it will be embedded one way or the other. I don't know if the powdery insulator can cause unwanted reactions with the coil, perhaps unlikely since it shows no signs of being affected by the temperature? I'm also uncertain of how much distortion one can expect in the coils over time, with exposed coils you at least have the opportunity to adjust the wires if it becomes a problem. Embedding might ensure longer life, after all this unit has probably been running continuously for decades (lab furnace). In fact it failed due to brittle wires, over time they turned crystalline making them as brittle as glass unless red hot.