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I am building a fire pit out of Square driveway type stones. I am trying to figure out at what degree I need to cut them to make them fit together in a circle tight.
The squares are 9.75" Sq.
How do I figure out at what degree they need cutting, or is there a standard degree say 45?
Decide what SIZE circle you want.
Do the math. 360 degrees in a circle divided by the angle of cut = number of stones
If you leave the wide side full-size, 9.75 x number of stones = circumference in inches
inches / 12 circumference in feet
circumference / Pi (3.1415...) = diameter.
A 1 degree cut will get you a circle (technically a polygon) of 360 stones about 93 feet across.
5 . 72 . 18.6
10 . 36 . 9.3
If you make a cut on BOTH sides to make the stones symmetrical, the effective angle is doubled (10 degrees on both sides = 20 degrees = 18 stones = outside diameter is 4.65 feet)
ie /_\ .vs. |__\ for stone shape.
45 degree cuts on one side will get you an octagon (roughly) pinwheel with no place to put a fire in the middle, while both sides would get you a 9.75" square, still with no place to put a fire in the middle.
A hint from other media - try it with paper or cardboard before you commit to cutting stone. The circle formula gets increasingly incorrect as you use fewer and fewer stones which approximate a circle less well. The effect of one side or two side cutting also increases with fewer stones/greater angles.
Something to mention that has possibly not been considered... not all manufactured stones are intended for fire pits. Many of the concrete-based pavers used in driveways applications will "pop" and/or explode under prolonged exposure to heat (as within a firepit). This is in part because they are relatively porous and will "absorb" moisture and water. they will retain this water content, which will eventually turn to steam and contents under pressure. Review this SE answer: https://outdoors.stackexchange.com/questions/801/how-to-avoid-exploding-rocks
An important thing to consider is to make certain you have a fire ring insert to protect the bare stone from the direct heat source. Many of these manufactured metal inserts come in a variety of set sizes and this will help you to determine the exact measurement for the cut as needed.
In fact, to save you some time, and possibly money (unless the stones are leftovers from another job, or were gifted to you) There are fire pit kits available at many box stores. ie: http://www.homedepot.com/b/Outdoors-Outdoor-Heating-Fire-Pits/N-5yc1vZc6na
These include the metal insert, as well as the pre-cut cast stones for ease of installation. You can put one together for as little as a few hundred dollars and an hour of time on a prepped flat surface. maybe a half day if you start from scratch in a lawn area.
If you don't want that route, and want to go with a natural stone look, try granite or basalt or any type of igneous rock...as opposed to a sedimentary (sandstone, limestone, slate, etc)
A sedimentary style rock settles during creation into layers (as opposed to the process of cooling the "liquid hot Mag-ma"). The settling of these layers can end up allowing water/moisture to seep in and create fissures which allow moisture to turn to steam and create pressure then a face full of stone as opposed to s'mores.
