It is not just static deflection you need to worry about. The tool puts an additional load on and so does acceleration and decceleration. Also you can get resonance effects that build up to much larger deflections than you measure with a simple static load. Your theoretical deflection of about 0.014 could easily turn into 0.05, or more, which is in tape measure territory. In addition if you are making parts that fit together even 0.014 turns into a gap of 0.024 which is noticeable. This may be tolerable in wood working but as soon as you try doing plastic you will start finding it a nuisance and if you try any fine engraving it will be intolerable.
Figure out some way to support the rails; one central support will reduce the deflection by a factor of 8, I think deflection goes with the third power of the length.
Alternatively go to a slightly larger tube because the deflection depends on the thrid or fourth power of the beam depth.
Incidentally were you using simple end supports or rigid? That also influences the deflection.
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