If you are cutting plywood you need to pump air down with the beam so the tarry gungy horrible smoke cannot get back on to your lens rendering it opaque and destroying the surface coating.
The 6040 I got had a particularly naff fume extraction, a vague suck somewhere at the back, sudden steps in the ducting, no particular air inlet to guide or encourage the flow.
The driver board and ghastly dongled Chinese software, open frame high voltage PSU scarcely justified the enormous amount of case set aside for them. Every expense had been spared.
What it really needed was a water flow/temperature measure to stop me firing up the tube should the flow stop for any reason.
If making your own, remember that everything below the beam wants to scatter light, not reflect