PVC is what I used. 6" flex hose and my machines have 4" ports. I know about the advice to enlarge machine ports but I just couldn't see how to do that especially on the Jointer/Planer. a 4 X 6 PVC adapter happened to fit over 6" flex hose, I used some silicone and small bolts/fender washers/nuts to secure the hose. I used short lengths of 4" PVC over the machine ports and the other end slotted and bent in a little so the 4" PVC hose adapter would fit over them easily. My thinking with the 6" hose and 4" adapters is that there's a lot of turbulence close to the interior wall of the hose so functionally a 6" corrugated hose would move about as much air as a 4" smooth tube. I could certainly be off on this - no engineer - but it seems to work pretty well.
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