if you are using very small (2mm?) cutters the cutting forces are tiny - a 2mm, 2flute cutter at 20000rpm and 800mm/min feed with a 0.1mm DOC endmilling on aluminium ineeds about 10w of cutting power and will exert about 1.5N cutting force (if the cutter doesnt break first!). At those loads unsupported rail is possible - the deflection at the centre of a 200mm length of 20mm rail is <0.001mm at 25N working load