New build - 1.2M dome

[Gulf]

Here is a quick and easy method to prevent the dreaded droop. Oasiscdm (Colin) prevented this by attaching a guage to the IT, using the floor as a reference.

[jonv]

Thanks for that. I shall be on the lookout for the droop... I have a bit of work to do before I have to decide how to cut the arch bricks. I cut the half soldiers today and hope to get them mortared soon The tops are angled to match the arch bricks for the first proper course.

[JRPizza]

What I did is keep the top and bottom surfaces of my arch bricks parallel, meaning the top and bottom were both "tangent" to my arch form, and I cut all the bricks to sort of match the angle of the top dead center brick (you can sort of see this in the "selfie" I took). The starting point of the angled portion changes as you go around the dome, to account for how much more the arch sticks out at the top vs bottom (see the side view). Since I built the arch as I added courses, I just did custom cuts on the bricks adjacent to the arch to match the angles on the arch bricks. I attached a few pictures to show how I attempted to do this - I got smarter as I went as some of the first cuts were crude. I also had to deal with the dreaded droop, as I didn't pay enough attention to keeping the bricks adjacent to the arch tipped up like the rest of the bricks in the row, an error that adds up and if not corrected results in droop.

[jonv]

Thanks; will definitely give it a go. I'm planning to cut each brick to fit anyway, it's just a case of working out the right angles...

[UtahBeehiver]

If you look at the pic and the inside of the dome area, you can see how one side vs the other side changes as you move down the arch. It does make a smooth transition to the arch and dome interface. But that is up to you. This is a area where it won;t be seen one the oven is done. If you a concerned about cutting each brick then don't do it at all and just tie in the dome to the arch but this is worth the effort.

[jonv]

This picture helps a lot, thanks. I was still wondering if each brick needed to be different! I've seen quite a few that looked as though they were all cut the same, but then perhaps they did not intend them to blend with the dome.

[UtahBeehiver]

I always attach this photo (done by Mr. Chipster) showing how the IT is used to determine the various angles of a tapered arch. Even though this is a partial arch, the concept is the same. But you start the the top dead center of the arch and work your way back down. Each brick is slightly different to you cannot cut identical bricks. Also you need to start with a full brick not a half brick.

[jonv]

I think I'm having the same trouble figuring out the intersection of done and arch, but various posts have given me some clues... I will at least set up the arch in place and see what sense I can make of it with the IT.

[JRPizza]

If you look at two of my "benchmark" builds, UtahBeehiver's and Gulf's, one built his arch first, and one didn't, so it's whatever makes you more comfortable. I had a little trouble visualizing the angles that would intersect with the inner dome so build as you go was the way for me, and having the dome bricks to give extra support seemed like a good idea. When I later built my outer arches I found that the extra support was not something I really needed.

[jonv]

I do share your concerns - I could have made this job easier if I picked flats to work with and hadn't wanted to try and make everything such a tight fit, but that's me I guess - take an already tricky job and make it harder!

I'm aiming for a 500mm width and something around 390 in height. I have a radius of 600mm plus an inch or so for the height of the pivot point above the floor. Should work out in the 61-63% range.

I have some finely ground bauxite for the mortar - it's finer than any sand I've come across. There's a couple of scraps stuck together with it in post #46, with joints as thin as I need - of course, it's easy when just slapping a couple of scraps together, it will be more tricky on the arch/dome itself, but seems doable. Time will tell...

I'm leaning towards building the arch first, unless there's a good reason I should build it as the dome progresses?

[JRPizza]

What is your door to oven height ratio? There is sometimes a little wiggle room there (the FB plans go from 59.5% to 66.6%) and if you made the door a little higher you could gain some joint width. Not trying to redesign your oven, just wondering if you had any leeway in the design.

[jonv]

SableSprings - I tested the mortar I'm using and it seems pretty good for thin joints; the aggregate is nice and fine and it squeezes out fairly well. If I can't keep the arch under control I'm going to have trouble with the dome too as the arch bricks for that are not far from my radius of 600, so it will be a good test. Either I get the joints tight or I get the grinder out!

If I'm honest, I am a little adrift on my numbers (can't hide mistakes from you guys!); I had originally drawn out a 1.1M dome and only later changed it to 1.2M. I made a quick check that the arch bricks were still ok at that radius, but when I rechecked everything a couple of days ago I found it is closer than I thought. Keeping the inner edges close together will dictate pretty thin joints. I will make it work... or adjust the bricks.

The average width of the joints for the arch as laid out is around 1/16" - the black spacers on the outside are 2mm and the green ones on the inside are 1mm. If I have to, I'll trim the keystone to fit.

[JRPizza]

Just curious, how thick are those shims? You might want to try mortaring a few bricks together with your target joint thickness to see if it is going to be doable. Don't know what kind of sand you are planning on using, but for those joints it probably should be pretty fine

[jonv]

You're right, those joints don't leave much room for error. Will definitely mark their positions on the template - would hate to have to cut down the top brick.

The outer arch won't be quite so nice - the stock arch bricks don't match the radius so well but still better than flats and easier than cutting them myself.

[SableSprings]

Jonv, as JR noted you are going to have joint creep... When I laid out my arch as you have done, my joints were tight and I didn't think anything could go "wrong". What I did not realize was that no matter how thin you apply your mortar, the accumulated mortar thickness will affect the dry layout (tight joint) plan. The end result is that your arch will be bigger than you planned and won't fit together the same way as it is laid out "dry". Either put in a little space between the arch bricks (1/16"-1/8") or build the arch up and plan on cutting the keystone to fit at the top of the arch.