Bob Oswald replies: That’s a really tough subject. Working at Rockler for several years, I’ve dealt with this problem almost weekly. Sadly there are no standards in dimensioning of products for dust collection. From the simple shop vac that has somewhere around a 2 inch fitting to the four inch systems, everything varies a bit. My own system is a combination of parts from Rockler and Home Depot. To make work using a “four inch” fitting from Home Depot that might be irrigation pipe to mate with a “four inch” elbow or gate from Rockler or other suppliers, is to test fit things from both places. It often turns out that the inside diameter of one component might fit the outside diameter of another. Unfortunately it is a mix and match process. One thing I’d recommend is don’t be trying to buy precisely what you need and running back to one of the stores to return things that don’t work. Have a collection of parts on hand, save yourself the grief, and build as you go. I built my fist system about four years ago and I’m still mixing pieces for some new equipment additions.