I use 6" PVC drain pipe for my dust collection system. One of the problems I faced was making a clean and perpendicular cut of the 6" pipe. I tried making a miter box to accommodate the 6" pipe, but it didn't work as I hoped. I then tried this vise/holder design and when used with an 18" PVC saw, I'm able to get good right angle cuts.
I used two 3/4" plywood boards and screwed them together. One of the dimensions was 2" larger than the outside diameter of the 6" pipe -- this is to minimize the space the facing takes against the blade, the less facing, the move distance of a cut I can make. Then I place a section of pipe on the board and drew a circle. I drilled a pilot hole for my scroll saw and did a rough cut removing most of the interior of the circle. I then cut into the circle with my band saw (the band saw kerf is thinner than the jig saw) and carefully cut up to, but not including, the pencil line. The fit is very tight, you cannot just slide it down the pipe, the vise has to be jiggled down.
Then, to make a cut, I simply line the blade up against the housing and carefully start a cut to get a good perpendicular cut started. The remainder of the cut goes well and keeping the 18" Lenox blade against the housing works fine.
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Hope this proves helpful to other building a PVC duct system.