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Metal Frame For Sprung Arch Kiln


jrgpots

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I still have to place the arch on my backyard kiln. But I need more help, please.

 

1. Do I need to brace the kiln edges with angle iron before I place the arch?

 

2. When I build the angle iron frame, do I snug it as close to the brick as possible or do I give it "wiggle room?"

 

3. Some designs use Springs to deal with heat expansion. Do I need this. My kiln outer dimensions are 47" wide x 43" deep with a 36" high stem wall. Does this the angle iron bracing?

 

4. All suggestion are appreciated.

 

Jed

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You do not need springs. Put all your angle iron on before setting the arch. The angle iron is the only thing keeping the kiln together when the outward pressure of the arch is on. Put a piece up each corner. Connect them across the bottom all the way around with more angle or channel. Connect them across the sides halfway up the wall. You'll also need angle behind the skew bricks to hold them in place. It's best to connect the top across the front and back with all-thread, so you can tighten it up if needed. Do some Googling of kilns and you'll see how it's done.

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Jed

DO NOT use springs. The arch will just push them out and they will not return (this is from experience) so do not do this.

The arch will expand up when you have the angle iron in place and the kiln heats up.

Your arch needs to be supported with a form (usually wood) and you can not remove this form until the angle iron is in place-hope thats clear enough.Other wise the arch will fall dawn as its not supported as it spreads out. 

You need to have all 4 corners braced as well-the stem wall as you call it. Its best to wrap the arch on all sides-the front  and rear you can support higher up on the corner frames so the arch is not impeded with iron in the way.

Think of this as a box around the arch with the front and back higher up.

You also need this box at the bottom of kiln. You can in the front anchor to a concrete slab to keep iron from the front under your feet. This is an option- Drill and anchor with concrete bolts into the angle with feet wielded on.

I  to build the frame after the walls are up but it can be done 1st as long as your measurements are firm.

Use thick angle on the 4 uprights as they will take the forces of the arch spread and most under size the metal. as well as along the two sides of the arch as thats where it wants to spread out. I like 1/4 angle on my corners.

My salt kiln uses 1/4 x 3 inch wide angle. I also have a little 1/4 x 2 inch and some 1/4 inch 2 inch  channel which has a 2 inch flat side.

Kilns use a lot of steel. I primed mine and spray it with high heat aluminum paint on all kilns.I have  a friend who is a 1st class wielder -I have the cut off saws and wielder but he brings his own DC wielder for bester control.

Do not add wiggle room to the arch make it tight and that will allow it to expand up not out when its hot.Forget the springs idea-You do not need this.

So to be clear put up the side angles going up (veritcal)1st then build a box to floor level and arch level(horizontal) You will find its best to make the front and rear horizontals be higher than the arch to get out of the way so make the angle sides higher to accommodate this in one long piece on each corner.

I wrap the kiln in the middle along the 3 sides as well with channel.

All thread can be used but make sure its big enough. On the salt kiln we just wielded the whole frame and I'm very happy with that . My car kiln has threaded rods but it never repeat never needed to be adjusted in 3.5 decades.

Plan on spending some $$ on iron.

Hope this was in the budget .

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The side walls are up. The temp is in the 70 to 80s already. I was afraid to do the arch without bracing the structure, so it's time to head to the scrap yard for angle iron. But, I'll probably end up getting new stuff.

 

Thanks for the tips.

 

I am planning on placing an outer waterproof layer of Portland cement and fire clay to the kiln. Would you put this over the angle iron or not cover the bracing?

 

Jed

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My choice would be to build a metal roof over kiln and forget the waterproof layer-I have never seen one hold up and they always crack and leak as the kiln will expand and contract more than you think.

Scrap yard is good start-

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Each of two pipes would be welded within an inch of the corner on each side wall of the kiln. These would be welded to the top and bottom as well. The pipe is cheap, about $35.00 for 31 feet of pipe. My kiln is already on the ugly side. The pipe would give it even more character.

 

I'm just thinking out of the box for a while.

 

Jed

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I see no issues as long as long as it does not flex. I think you will need extra wrap around pieces that surround the kiln as Angle Iron is much less apt to flex. Flex is not good. You still need the support of flat stock or angle at arch buttress.

My experience is pipe can flex but there are so many kinds and wall thicknesses of pipe. I'm just not familiar with drillers pipe.

You need to see how much the pipe flexes in your longest piece (6 feet???)

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Guest JBaymore

Mark brings up an EXCELLENT point that I make all the time in my kiln building workshops and classes......... that a lot of potters tend to kind of "miss".  Steel flexes.  You have to understand this concept to make a good kiln buttress frame (or a frame that deals with the racking forces of a hinged door).

 

I've seen many a "saddleback" sagged sprung arch because someone did not get this idea when they designed the actual buttress member that sits behind the arch skews.  Near the corner uprights the steel does not move .....but in the center of the long span.... it bows outward.  This lets the center of the arch sag.  Over time... it bows permanently.  Kiln slowly starts to look like an old tired work horse.

 

Another important point is that steel flexes MORE when it is getting hot.  So you have to make sure not to enclose the steel within refractories.  Allow heat energy to dissipate off the surface to the surrounding air.  Otherwise it might be inflexible enough when it is cold.... but when hot... it bends.  

 

Since  about 1980 I have used channel steel for resisting flexing for arch buttress spans (also for kiln car frames) .  Flat face against the back of the skew bricks.  And flat face up toward the car kiln's floor refractories.  If you use angle for the arch buttress ... do NOT put the one side of the L inside a groove in the refractories.  Unless the kiln is a "short term one".  (This also accelerates rusting in the hotter steel area.)

 

best,

 

...................john

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Q

Mark brings up an EXCELLENT point...

 

I've seen many a "saddleback" sagged sprung arch because someone did not get this idea when they designed the actual buttress member that sits behind the arch skews.  Near the corner uprights the steel does not move .....but in the center of the long span.... it bows outward.... 

 

I have used channel steel for resisting flexing for arch buttress spans (also for kiln car frames) .  Flat face against the back of the skew bricks.  And flat face up toward the car kiln's floor refractories.

 

 

best,

 

 

.........john

 

On a small kiln like this, (48" span), what size channel works?

 

And BTW, thank you Mark and John.

 

Jed

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Pipe would structurally be stronger than any flat metal. Interesting idea.

I have built sprung arches using used car valve springs. I also tightened them up on occasion. My kilns lasted a couple of decades. They were fired several times per week during the semesters.

SO I go with the springs.

One was a 60 cu. ft car kiln. The arch was a beauty and I always maintained it. The car valve springs were something scrounged at car dumps.

for the corners and front to rear I used 2.5"-3"heavy  angle iron. 

 

The angle iron and threaded rod is above the arch and not visible. The tightening of the nuts on the threaded rod lifted the arch up ever so slightly and lined it up nicely.

post-1954-0-47975400-1490989030_thumb.jpg

 

 

 

Marcia

post-1954-0-47975400-1490989030_thumb.jpg

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Guest JBaymore

Jed,

 

That's a pretty short span.  You did not mention if the arch is hards or soft.... so the weight and resultant thrust will differ.  No time to do a deflection calc, but 3" with 1/4" meniscus should be fine.  For overkill and absolute "certainty" ... use 4" with 3/8" meniscus.  Cost difference between the two for such small lengths would be minimal.

 

Note in the picture below how the channel is notched out at the corner upright ends (and welded there) to fit into the corner vertical angles so that the flat faces of all of the steel are sitting flush against the brickwork.  Not butt welded and not laid on top of the corner uprights.

 

gallery_1543_1143_134209.jpg

 

best,

 

......................john

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