Hi OLD
I used pvc 100mm drain pipe in my workshop. It is relatively cheap and easy to install plus the fittings don't cost an arm and a leg either, and there is a huge variety of them!
I discovered, as you will see from a previous thread, that it is possible to use a rubber reducer to attach alloy blast gates to the pvc pipe without great expense as well. Normally, you would have to pay around £30 for a plastic blast gate that fits directly to the pvc pipe. A nasty plastic affair that is inferior to the alloy ones that only cost a tenner.
Go and have a look in your nearest plumbers merchant. I did just that, told them what I wanted to do and they could not have been more helpfull.
We practically designed the entire system over the counter!
You will hear a lot about grounding the pvc. I read everything I could find on this subject and there are conflicting views but the most sensible article I found proved that it is actually impossible to ground plastic because it is not a conducter. Also, as I was a firefighter for thirty years and a fire safety officer for many of them I can tell you that the chances of a dust explosion in a small, essentially domestic extraction system are next to nil. Even if it did happen you would probably not even notice!
Cheers.
SF