The Pole Compass with Articulating Arm (Fig. 3.40)
The easiest way to maintain a precise circle during construction is by using a compass as a guide. For our purposes, a rigid pole compass works best and we use it exclusively for a variety of building designs.
The pole compass can be used for both dome building and the construction of round vertical walls like kivas, hogans, and yurts (Fig. 3.41). The pole compass uses a tall center pole with an arm attached to it that is the length of the desired radius of the building. The attached arm both rotates horizontally and pivots up and down (articulates) from its fixed point on the center pole. Most of the parts and pieces we use to build this compass are from the metal pipes and gate latches manufactured for chain link fence. These parts are available at most hardware stores in the US. Once you understand the function of these parts, however, substitutions can be made when they are not available.
A two- to three-foot (60-90 cm) long pipe is buried two feet (60 cm) into the ground. This is your base for the pole compass. Make sure it is set plumb and level. It remains in the ground and should not wobble or shift. Make sure to compact the dirt around the pipe as it is buried. Adding rocks and gravel along with the infill dirt will give it extra stability. Into this pipe is fitted a slightly smaller diameter pole that is long enough to reach the height of the second floor or loft of a dome, or the finished height of a round, plumb (vertical) wall. The fit should be snug, but loose enough that the center pole can turn in its sleeve. If the center pole binds, a few drops of oil will help it
3.42: Common chain link fittings and parts used to build a compass with articulating arm. Clockwise from upper left: Gate-frame grip; rail-end cap; fork latch.
3.41: A 36-foot-diameter kiva-style earthbag home, going up in Wikieup, Arizona.
turn smoothly. With a four-foot (120 cm) or longer level, check your center pole for plumb. If it is not plumb, you need to reset your base pipe. It is absolutely essential that your center pole is as plumb as possible.
Once you are happy with the plumb of this center pole, you can attach the horizontal arm. Attach the gate frame grip to the center pole. This "clasp fitting" allows us to adjust the height of the horizontal arm by loosening the wing nut and re-tightening it at the desired height. The "pivoting fitting" is a fork latch that is used as a latch for a chain link fence gate. Remove the latch piece of this assembly and replace it with a rail end cap. For our purpose, this part serves as holder for the horizontal arm and allows the arm to pivot up and down. Having the horizontal compass arm pivot makes moving it over the tops of door and window forms easy as it rotates around. Use a chain link top rail for this horizontal arm. You may have to wrap the end of the pipe with some duct tape to get a tight, non-slipping fit into the cap. You can wrap more tape around the outside of the cap and arm to maintain the compression fit (Fig. 3.42 & 3.43).
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3.44a & b: The sliding horizontal compass arm with attached angle bracket.
At the opposite end of your horizontal arm, attach an angle bracket with two hose clamps at the intended radius point of the interior wall. The horizontal arm itself should be longer than the fixed radius point so that it rests on top of the wall and the angle bracket barely touches the inside of the bag wall face after tamping. (Fig. 3.44a & b). We usually set it one inch (2.5 cm) inside of the determined radius to accommodate the bag expanding from tamping. After tamping the first row of bags, the finished thickness will be determined, and the compass arm will be raised on the center pole that amount for the next row. If corbelling the bags for a dome, the angle bracket is moved in toward the center pole the distance the next row of bags is to be stepped in. (See Chapter 12).
Onto the center of the horizontal compass arm, bind a level with duct tape. The level will show you where to tamp down the high spots and make up the difference in the low spots by overfilling the bag(s) above it on the next row. You can mark the low and high spots directly onto the culprit bags for handy reference. In most cases, placing the first row of bags on a level foundation and having all the crew fill the bags consistently with one another, makes keeping the level easy. We like to stay within one-quarter to one-half inch (0.625-1.25 cm) of level, especially if a bond beam is going on top. For domes, it is less critical but still a good idea for level window placement and maintaining the overall symmetry of the dome's shape.
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