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Post by photoman290 on Apr 20, 2014 9:18:36 GMT -8
as it is raining and work on the RMH is temporarily halted i have been thinking about the best option for the riser. i like the vermiculite idea,but as it is less than ideal being square i thought it maybe worth exploring the idea of making the corners round with castable refractory. don't know if anyone has tried it but it may be a good way of getting a light highly insulated riser without the expense and problems of casting the whole thing in castable refractory. as the corner will only be a inch or so thick pouring it and vibrating it would be quite easy if you did one corner at a time. not sure if the extra work would be worth it though. anyone got any thoughts on whether it would be worth the extra work or not? i was thinking put the 6 inch cardboard tube inside the vermiculite box and tamp down the refractory with a stick then put it on its side and vibrate it. i am getting some castable refractory for the burn tunnel anyway so with have some left over so no extra expense.
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Post by mintcake on Apr 20, 2014 12:37:51 GMT -8
as it is raining and work on the RMH is temporarily halted i have been thinking about the best option for the riser. i like the vermiculite idea,but as it is less than ideal being square i thought it maybe worth exploring the idea of making the corners round with castable refractory. Interesting idea. You should look at the thermal expansion numbers to see if they're compatible, otherwise you're probably going to have your triangles falling off your board into your heat riser. Have you thought about an octagon instead of a square? Much better approximation to a circle, and you probably use less material too. How thick is your vermiculite board? If it's thick then you could do something like I did with my ytong test stove: ________________ \__/__\__/__\__/ etc to make 8 pieces. But make sure you get the lengths and the angles right... |
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Post by photoman290 on Apr 20, 2014 13:02:54 GMT -8
i have thought about them falling off. if you drilled though the corner you could use stainless cable ties or wire to hold them in place. hopefully it shouldnt get too hot that far away from the heat. the board i have in mind comes in 1020mm long by 610mm wide so only just enough for a 150 mm inch riser. from using vermiculite on my woodburner it seems to stay in place with exhaust cement ok so if you glued it as well as tied it it should be ok. only one way to find out...... i like the octagonal idea. not sure if 610 mm with the saw cuts as well will be big enough. i am CAD challenged so tend to use paper and cardboard to design things. lifes too short to learn how to do it on the computer. sorry forgot say the board is 25mm thick.
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Post by photoman290 on Apr 20, 2014 17:29:31 GMT -8
just worked out if i make it octagonal i will only have a CSA of 24 inches with 2 foot wide board. of course i could see if i can get a off cut as well to make it up the required diameter. i calculate i need another strip 4 inch wide to get a 6 inch bore allowing for saw cuts. i did read somewhere that you can calculate the difference between a sq riser and a round one for a given size. still not sure how much difference it makes though in the whole scheme of things. as i don't have the kit to test it i can only go on what others have done. there seems to be lot of variables in the design most of which i will never be able to test. i would think the type of wood and the moisture content would be the biggest variable.
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Post by mintcake on Apr 21, 2014 13:22:53 GMT -8
just worked out if i make it octagonal i will only have a CSA of 24 inches with 2 foot wide board. of course i could see if i can get a off cut as well to make it up the required diameter. i calculate i need another strip 4 inch wide to get a 6 inch bore allowing for saw cuts. i did read somewhere that you can calculate the difference between a sq riser and a round one for a given size. Peter Berg has said that, in terms of fluid dynamics (for the riser), the equivalent of a square side X is a circle diameter X, never mind about the corners, they don't matter. I assume the same holds true for octagons. So a 6 inch diameter internal diameter circle would be equivalent to a 6inch (internal) square, only with less wall. As for how much material... I'm not sure how you worked it out, but I don't come up with the same answers you did. A simple square with each piece overlapping all the way to the back of one neighbour would need 4*(150+25)+3 cuts=709mm, so you can't do a square unless you skimp on the overlap, at which point you have problems holding it together, I think. If you've only got 610mm, then with 3mm saw cuts you've only got 1mm left for 4 overlaps... Ouch! I hope you've got a thinner saw than I have! For the octagon, I worked it out like this: Each bit the bottom 25mm of an isosceles triangle, 45 degrees at the top, where there are 75mm from the inner surface to the centre of the octagon. WARNING: NO ONE USE THIS: it should be tan, not sin, see recalculation below. Each Inner face: 75mm*2*sin(22.5degrees) = 57.4mm, each Outer face: (4in)*2*sin(22.5degrees)=76.5mm. the difference between inner and outer faces is 19.1mm, and you lose half that much to get the first and last edges angled. By cutting it like I tried to draw, with the "top" of the board being waste,inner,outer,inner,outer,inner,outer,inner,outer, then you need 4 inner surfaces, 4 outer surfaces, and one waste bit. I think it's 10 cuts. Therefore for a up/down octagon, 150mm internal diameter with 25mm board, you need: 4*57.4+4*76.5 = 535.75mm for the riser 10 cuts at say 3mm, = 30mm. plus wasted material of 9.55mm. Thus I end up with you needing a total of 575mm of board, so unless my maths is buggy, you should have space to cut them oversize so you can fine-tune the angles. Hope this makes sense! David |
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Post by photoman290 on Apr 22, 2014 0:24:47 GMT -8
bit early for me so i may come back on that,but i used the 180(n-2)/n formula to give me an angle of 135 degrees. will work out the rest later after i go over other my numbers again.
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Post by photoman290 on Apr 22, 2014 9:39:10 GMT -8
assuming the maths is right this little program seems to say that the total width is going to be 87mm*8=696m plus 8 saw cuts.i shall use the empirical method and saw up a 610mm piece of 25mm wood and fit it together and get back to you on the result.
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Post by mintcake on Apr 23, 2014 4:08:50 GMT -8
I'm not sure what your 135 degrees is measured from or to. Angle to bend the edge by? 3
I'm using angle at the centre.
60/8=45 degrees, but for
the trig you need to half that.
oops.. My maths was buggy: it should be tan, not sin.
So: Each Inner face: 75mm*2*tan(22.5degrees) = 62.1mm, each Outer face: 100mm*2*tan(22.5degrees)=82.8mm
Total length is thus 579.6mm plus cuts, or 609.6mm if you've got a 3mm cut.
Totally agree with using empirical method. I did it on paper myself.
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Post by photoman290 on Apr 23, 2014 16:25:37 GMT -8
i am sure you sure right. i gave up on the maths and just cut up bits of paper to arrive at the same point.inside 77mm outside 87mm angle 22.5 degrees.. that comes to 620mm overall with a hole in the centre of just over 150mm with a CSA of 28.5 sq mm so allowing for saw cuts it should be fine. if i just use the flat sections and join them together on the corners that will only give me a 100 mm hole with the same amount of material.
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Post by mintcake on Apr 24, 2014 10:07:38 GMT -8
A warning, having been there myself... if you're going to try to do it with a hand saw, you'll need some kind of a jig, you'll probably still get wavy lines and you're risking a repetitive strain injury (I gave myself tennis elbow).
If you're going to use a hand-held circular saw (my second attempt. It's very dusty, not nice for the blade, but it seemed to work on ytong no idea about vermiculite board), you need a different jig, plus lots of scrap wood because it's a right pain getting the angle right. Even a few degrees means you can't join the pieces properly. Measure about 5 times, don't be satisfied with a protractor, like I was. Instead, cut 8 bits of wood and make sure they really do make a nice octagon. Wish I had...
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Post by photoman290 on Apr 25, 2014 3:08:38 GMT -8
thanks for the tips. i was going to use a bandsaw as the blade is a lot thinner than a circular saw. i will makea jig at the correct angle so the blade runs in slot. i was planning to use exhaust cement to seal up any gaps and wrap the whole thing in ally foil.
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