I'm going to be working on a project where we will be splitting many sonotubes ranging in diameter from 8" to 16" and in length from 12' to 16'. We need to rip them into perfect (as much as possible) half-rounds.
I'm thinking that there must be a jig I can build for my tablesaw that will allow the tubes to be fed through the blade that will make a very straight cut. A V notch would keep the tube centered on the blade, but I can't think of a way to make sure the cut doesn't wander radially from one end to the other. The half rounds are going to be placed on walls and need to be super flat and straight.
One idea was to snap a chalk line along the length of one side of the tube. Then, on the jig, have an arm that extends over the top of the tube that has an indicator on it showing the blade position. As the tube is fed through the blade, keep the chalk line on that indicator. Then flip the tube over, tape the cut just made so the tube doesn't fall apart as the second cut is made. Run the tube through again, this time using the first cut as the guide line on the indicator.
Here's a picture of that idea:
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