I dropped by the dealer today to pick up a 90 degree 4" ell for a temporary dust collection setup. I've never purchased in PVC dust fittings before. This was a Jet part number, but I think its the same universal Taiwanese-made fitting that everyone sells (other brand Woodstock?). It looked like reasonable quality, then I looked inside of it. Expecting a nice smooth sweeping curve, I was shocked. The inside of the fitting appears to be two pieces of PVC that have been cut on a miter and bent enough to come together. The outside of the ell is simply a nice looking shell. Its not smooth and it actually swages down to restrict the flow - not exactly what you want for a nicely designed smooth flow network.
Bob