The two most common linear rail systems for CNC routers are known as simply supported and fully supported.  I’m going to use an equation to estimate the maximum deflection in a simply supported system.

The equation is Deflection = WL3/48EI where W is the load, L is the Length, E is Young’s Modulus, and I is the inertia. You can find calculators on the internet. I found this one at The round support is the third one down.  If you for example the X axis is 30" long and the Y axis 24" long (keeping in mind there are 2 rails for the X and 2 rails for the Y). The actual travel distance for the X axis may be less maybe 24" and the Y axis less than 18".

If the rails are 3/4" in diameter and there is 20 lbs. of force as the router plunges into the work, then the maximum X rail deflection is 0.19" and the Y maximum deflection would be 0.09". This means the deflection could be as much as 0.29". This drops to 0.09" if the rail diameters are 1".

The closer the spindle is to a support the less deflection there is. This means the router will have different deflection depending on where it is and the amount of load there is on the tool bit. This deflection is not just in the Z, but it also affects the X and Y axis when the bit is being pushed through the material.

What can be done about deflection?

  1. Make sure the router has fully supported beams.
  2. Use large diameter linear rods.
  3. Slow down the feed rate and reduce the load on the bit. If the loads are small the deflection is small.

All BobsCNC Routers use fully supported beams.

June 07, 2015 — Bob Wood