A plasma table has reappeared on my potential project list, and I do have a couple thoughts about mitigating dust.
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I wonder if mounting the rails on the underside of the side frame would help dust from settling onto the rails, then have the screw mounted below the rails/gantry/carriage? On the inside edge you can then have a bit plate hanging straight down to stop dust coming of the cutting area, and arrange a couple brush strips on the outside to minimise dust blowing around and in.