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-   -   Keystone fabrication (http://www.fornobravo.com/forum/f28/keystone-fabrication-2918.html)

gjbingham 11-11-2007 08:44 AM

Keystone fabrication
 
Hi all,
I checked around a bit and didn't see any posts specific to this topic, so I thought I'd just throw it out there for anybody fighting this issue or getting ready to build.

Each row of brick in the dome is supposed to have a keystone to lock that row together, or at least that was my interpretation of the plans. I'm not sure how important this is in the first 3 or 4 rows in the dome, as gravity provides significant resistance to movement.

As the angles of the rows get steeper, it becomes quickly apparent that a good keystone is required.

My first few attempts at fabricating a keystone failed miserably. I tried measuring the top and bottom widths of the gaps and marking them on a brick. My cuts were always way off at the time of try-in.

Even using a piece of paper on the inside of the dome against the final gap, tracing the shape, cutting it out and transfering it to the brick and cutting it didn't work. I was kicking myself, thinking how can this not work?????

It turned out that I was making one procedural error. You must take the template traced from the gap, and after cutting it out, flip it over before placing it on the brick to trace. If you forget to do this, the angles are reversed on the final cut brick.

Suddenly the keystones were very snug, sometimes too much so.

My last technique you may want to suffer through: I wanted my final keystone to be as tight as possible and intimate contact with all adjacent bricks. I went through a process of cutting the template and brick as I had been, but rather than just trimming it with the saw till it fit, I instead put a half cup of flour in a baggy and coated the keystone with flour. When inserted into the hole, any spots that are "high" or stopping the keystone from seating will leave a rubbed spot on the brick, where the flour has been removed.

I recommend using a sharpie pen or other marked and marking the spots before using a grinder or saw to remove them. The second you put the keystone down, other floured areas will be removed and you quickly forget which ones were the ones you need to remove.

This is kind of a pain, and took an hour or so of going back and forth between the flour, dome, and grinder before the stone finally fit tight. Also, a good scrub brush will help remove the pasty flour build-up on the brick and hole, which if not removed occasionally, will itself impede the seating of the keystone.

Hope this helps.
George

dusty 11-27-2007 06:23 PM

Re: Keystone fabrication
 
Quote:

Originally Posted by gjbingham (Post 19134)
I coated the keystone with flour. When inserted into the hole, any spots that are "high" or stopping the keystone from seating will leave a rubbed spot on the brick, where the flour has been removed

Ahh... The dentistry tecniques applied to oven building! Very clever. I'm going to use that one.

Thanks Doc,

dusty


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