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Post by Karl L on Oct 20, 2023 13:19:05 GMT -8
Thanks, Trev.
Maybe I missed you saying, but why were you putting vermiculite board on the firebox sides?
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Post by Karl L on Oct 5, 2023 6:39:31 GMT -8
... so I had to quickly grab the rotary hammer and use that. I vibrated it much longer than last time, so I'll see what effect this had. I have to say the casting seems to have turned out very well. Only a few very small air bubbles on the bottom surface.
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Post by Karl L on Oct 4, 2023 11:00:55 GMT -8
The board is made out of 18mm plywood, about the same size as the base of my mould. The edges of the board sit on some bits of 20mm thick packing foam, which bear onto some wooden rails. The board is not fixed down at all.
What is the width of your U-bolt - either between the centres or across the outside of the legs? And the length?
Maybe using a U-bolt with more weight will do it...
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Post by Karl L on Oct 4, 2023 6:47:27 GMT -8
I built a vibration table using an old electric drill and tried doing another casting.
Short story: it didn't work well - I had to go back to using my rotary hammer.
I made the table using a very old Black and Decker drill with a metal body - dating from the 1960s. The U-bolt was a piece of M8 threaded rod, which I bent into a U with about 30 mm between the two centres. I also added a couple of flanged M8 nuts to the offset leg of the U-Bolt, to give it a bit more weight. The drill was in high gear - stated as 2400 rpm on the gear control.
I tried it out before doing the actual casting and it felt like it wasn't going to have enough vibration to fluidize the mix, but I thought I should try it anyway.
I was right - it barely had any effect on the mix, so I had to quickly grab the rotary hammer and use that. I vibrated it much longer than last time, so I'll see what effect this had.
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Post by Karl L on Sept 25, 2023 13:15:09 GMT -8
Here's a pic of my Heath Robinson / Rube Goldberg vibration table I threw it together out of junk about 10 years ago and it worked so well I'm still using it. It sits on top of a plastic fish box, so the drill is under the board inside the box. The molds sit on top of the board to be vibrated. I took out the screws that held the 2 halves of the plastic drill body together and just put some wood screws right through into the board to hold it on there. Trev Thanks - it's very useful to see what it looks like.
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Post by Karl L on Sept 25, 2023 13:14:03 GMT -8
You dont really want any air voids, I made a short video just recently that might help you I did watch your video before doing the casting - thanks very much. Is there a big problem if there are air voids - like explosions on heating - or are there other kinds of problem?
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Post by Karl L on Sept 25, 2023 8:51:48 GMT -8
Refractory casting mix is very dry compared with standard mortar mixes. I am using large amounts so I use an electric cement mixer but mixing in a barrow with a trowel is fine. What we want from the vibrating, is a solid dense mass with zero trapped air. You will just have to do your best with whatever method you can, basically you vibrate until no bubbles rise. On a hight frequency vibrating table (very expensive machine) it would take about 5-10 seconds with a diy table maybe 60-120 seconds and with a hammer drill, as soon as the bubbles stop coming to the surface? Its all good fun…… Thanks for all the tips on casting Martyn and Trev. I made a mould and tried casting one of the firebox sides. Everything went fine apart from the vibration using a hammer drill, which I didn't work as well as I hoped. I loaded the drill with a chisel and used hammer action only (no rotation). The easiest places to hammer onto were vertcally down on the mould edges (70mm x 45mm timber), all around the perimeter of the mould. It looked like the hammer action did quite a good job close to where I applied it, but that decreased with distance from the edge. I now have a casting with a visible void near the middle of the bottom face, and I guess there may be a few more voids or bubbles inside. Is this casting usable? I.e. is a casting with a few voids OK to use? I do have a very old corded drill, so I might be able to try Trev's method, spinning a U-bolt. What size U-bolt did you use? Thanks, Karl
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Post by Karl L on Jul 3, 2023 8:42:23 GMT -8
Thanks Martyn, that's useful.
I was thinking to use a paint stirer to do the mixing, as that's what I use for making up mortar.
The datasheet says to use 11-14% water - I presume this is by weight...
For the vibration I was thinking to sit the mould on some white foam packing material, then put a coach bolt in a hammer drill and press this on the edges of the mould.
How can I tell when it's vibrated enough?
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Post by Karl L on Jul 3, 2023 1:43:34 GMT -8
I've got some 1400C dense castable and will make the wooden moulds soon.
I'm planning on using plywood for the base of the moulds, and ordinary softwood construction timber for the sides.
Also, I'll coat the insides with water-based mould release, as used for normal concrete.
Is it OK to use unpainted wood for the moulds, like concrete shuttering? Is there any problem with water in the castable mix getting drawn out by the dry wood?
I worry that this might reduce the water ratio in the mix, or cause the wood to expand and so move the castable mix as it cures.
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Post by Karl L on Jun 22, 2023 4:02:32 GMT -8
Thanks very much, Trev.
I understand the implications of fully closing a damper, and not allowing people to use it who don't understand that.
I did quite a lot of searching for dampers a few months back, but didn't see what I wanted for a 5" flue. I may end up having to make something myself.
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Post by Karl L on Jun 21, 2023 5:48:18 GMT -8
Hi Trev, Where did you get hold of your airtight chimney damper? Thanks, Karl
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Post by Karl L on Jun 15, 2023 22:02:41 GMT -8
Hi Karl, The top of the primary air slot is 14mm above the top of the threshold. The front of the threshold is 70mm back from the inside of the primary air slot. The mesh is just attached at the top by a few twists of copper wire so it might appear angled back in the pictures but it's meant to be vertical. The threshold and mesh were all designed to deal with the issues that the primary air blowing on the bottom of the fire sometimes caused. The 275mm wide primary air slot right across the front directing the incoming air down towards the floor, probably reduces the importance of the size and position of the mesh and threshold, but that's just a guess at the moment. This is how much it needs to be open for 20% csa primary (I use the tip of my index finger as a guide), so you can see how it directs the air downwards allowing it to then flow evenly up across the whole fire. I can move this forward again now that I don't need the old stove to keep warm... Trev, do you think it would work if I set the primary air slot a bit lower, so that the *lower edge* of the slot is just above the insulating sub-floor of the firebox - that is, 23mm below the bottom of the V-shaped floor in my 115mm design? (The primary air slot would go through the steel shell of the stove, rather than through the door).
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Post by Karl L on Mar 22, 2023 12:03:06 GMT -8
I've used the same bits of vermiculite board for each new design of firebox for several years. They have been in contact with red hot ashes daily and they've never cracked.
I think it might be because I cut them small enough so the thermal stresses aren't so big. (I cut them to the same size as a standard fire brick split, and mount them vertically.)
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Post by Karl L on Mar 16, 2023 2:42:30 GMT -8
How important is it that the bypass is capable of opening to full system CSA?
I've remember someone suggesting that 50% system CSA is sufficient, and maybe safer if it's accidentally left open.
One thing suggesting that 50% system size might be OK is that the port is 50% system CSA -- but it would be good to know what works in practise.
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Post by Karl L on Mar 15, 2023 3:36:51 GMT -8
I tried a piece of my 1000*C rated 1" calcium silicate board as a firebox roof last night, it buckled and cracked up within the first 5 minutes. Hi Trev, I bought some calcium silicate board to try to get a feel for it. I plan to use it behind the cast refractory, as you do. But I was also wondering if it's possible to use it in contact with hot gasses at all? Do you have a sense of why it failed in your firebox? Do you think it would fail if situated just after the top chamber exit, in direct contact with the hot gasses at that point? Thanks, Karl
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