Matt Helliwell wrote:

I wonder if I used 3" and then a smaller diameter into the cyclone
whether that'd give the appropriate speed. Only one way to find out.

There's a balance to be struck between the volume of air you can
move, its velocity, the pressure it will suck against and the size
of the particles it needs to carry.

The vacuum cleaner (henry/vax) type of arrangement will move modest
amounts of air at high velocities and pressures, but won't work
too well with large bore pipes.

Workshop extracters move lots more air, but don't work well against
large pressures and so are less satisfactory will smaller pipework.

I developed mine four or five years ago initially using an Earlex,
but moved on to a bigger dust extractor.

http://www.geocities.com/nicknelsonn/cyclone.html

This system is based on 4" ducting and flexible pipework.

Whats really good about this whole issue is that it's so
interesting and easy to simple experiment with different
approaches, starting from what you happen to have already
in terms of materials and blowers, and develope it into
a system which works for you.

Nick