[jwest]

Im desiging a cnc router (75cm x 50cm in size) with a moving table and stationary gantry to cut wood. I've decided to use linear bearings on round metal rods. but. How can I work out what diameter rod to use (and material). given that the table will weigh a couple kilos. with a couple more kg pushing down on it from the router.

Many thanks.

[jsheerin]

To answer the basic question of what size rods to use, you can start with a basic beam deflection calculation:
http://www.engineersedge.com/beam-deflection-menu.htm

In particular, you might want:
http://www.engineersedge.com/beam_be...m_bending2.htm
This will be a conservative estimate as the ends of your rods would not really be simply supported. This will let you see how much your rods will deflect for a given load. Loads will include the weight of components and cutting forces. Your goal would be to pick a rod that gives a deflection that is small enough to meet your goals for the machine.

You'll need to know the area moment of inertia to use the above formulas:
http://www.engineeringtoolbox.com/ar...ia-d_1328.html

You'll also need to know the modulus of elasticity of the material you want to use. This can also help you pick a material - different materials will have different values of E and will give different deflections. Try www.matweb.com for lots of material properties. Some conversions might be necessary to make all the units work out.