This step
was considerably less enjoyable than I anticipated it would
be. Cutting IFB makes for VERY fine dust which gets
everywhere. However, this step is by far the most important. The
reason for this is if you are not perfectly accurate the bricks will
not form a circle. It's important to realize that the only cardinal
measurement is the angle of the miters. The length of the hot face of
the brick is not that important, as long as it's close, that's good
enough. The reason for this is that no matter the diameter of your
polygon, the miter angle is constant. If you use 12 bricks to make a
kiln with 1 inch diameter, or with 23, the miter is still 15
degrees. So, set your saw up right before you cut.
Here's a picture of the saw set up for cutting the miters. The first step is to attach a backing plate to the saw for two reasons, one to minimize tearout, second the fences on most miter saws aren't flat, they bend toward the blade gap. I have no idea why, but they do.
The second step is to set the angle on the miter table. Don't trust the protractor that you see under the saw. Use one of these or something else that does the job. If you have a really good miter jig for your table saw then you might be tempted to use that. I'm just not sure I want that amount of dust choking up a good table saw.
When
you think you have the saw set up. Do a test, don't waste a course of
bricks. This little test circle showed me I was a bit out. I had to
readjust the saw more accurately.
Put on a ventilator. Not a little flimsy fabric mask. A real ventilator. You want it to fit snug, you do NOT want to breathe this stuff. Seriously, if you don't own a ventilator, buy one before you do this. And if you don't already have eye protection on (from doing the test) put that on too, I had a small bit of IFB sneak under my glasses into my eye and it was VERY uncomfortable.
Anyways, on to the fun part. You need to some math at this point. Particularly, you need to cut the miters such that the brick is a trapezoid whose short side (the hot face) is the appropriate length such that the circle it will be a part of is the size you wish it to be. This is computed with a little simple trigonometry.
We
know the radius of the inside of the kiln is 11.5 inches, and we know
that the angle from the center to the joint between bricks is 15. (We
also know that the angle at the centre of the brick between the brick
face and the line to the center is 90. That should be obvious.) So,
the hot face length of our trapezoidal bricks is 2*11.5*tan(15). For
interests sake, a kiln with 12 bricks approximating a circle of 11.5
inches radius has bricks with hot face sides 6.162831 inches
long. Obviously we can't measure that very easily. It doesn't really
matter. 0.125 is 1/8th of an inch, and 3/16 is 1.875, so use your tape
measure and measure somewhere between the tick for 1/8 and 3/16 of an
inch. The consequence of being out a little here means that your kiln
will be a tiny bit bigger or a tiny bit smaller. I haven't done any
math on this, but I doubt the difference would be more than a
percentage of your wattage per in3 ratio.
In order to set the saw up to cut the bricks, clamp a stop block to right side of the fence. You need to position the stop block in the correct position. I did this by putting it where I thought it was close, then bringing the saw down on the brick, denting it. Then I used the starret (see link above) protractor to draw the miter that would be cut, duplicated that on the other side and measured. I was out a bit, a little adjustment and good to go. The picture here shows my stop block in position and clamped TIGHT, you don't want it to move. And it's beneficial for the stop block to not be at 90 degrees to the fence, this is to allow some dust buildup and so forth so that you can easily place the bricks accurately. Cut the first side to get a brick that looks like that one. Flip the brick over to cut it into a trapezoid. Note don't cut it into a parrallelogram.
I found
that after almost every single cut I needed to brush dust away. You
want the brick fitting perfectly against the fence and the stop
block. Each brick must be exactly the same size.
You might take notice of the plastic tube running off to the right from the back of the miter saw. Thats for dust collection, it's my shop-vac hose. There are two ports on my saw, preferably three collection hoses would have been better, one for each port, and one right by the blade. It really is a horrible mess to clean up. Take my advice and try to collect as much of the dust while you're cutting. I wouldn't want to do this without dust collection. (Someone on clayart saw these pics and thought I was inside ... I didn't think to mention this ... do it outside.)
So, you've cut one brick. Congratulations. Only 107 more to go!
Here's the final pile of cut bricks with two courses mocked up in my basement. They fit great.
The local suppliers gave me K23 at $3 Canadian a brick. I bought 150 (don't forget the floor and the lid).
Damage so far:
| elements and stuff | $200 |
| bricks | $450 |
| total | $650 |
