16mm supported rail how many linear bearings

[progomez]

The issue isnt the weight... its the amount of flex and that depends on the accuracy you want to achieve.

Remember that the cutter, router and mounting at the lowest part of the Z motion is acting as a lever trying to bend the z-rails. Assuming you are cutting wood with a typical router and a 6mm cutter you will be pushing something like 100N sideways force at a distance of say 200mm which is a moment of 20Nm (torque). That will flex 12mm unsupported rail by about .06mm and will result in a deviation of the cutter point about 0.6mm. If you are OK with that worst case inaccuracy then fine but even for wood thats high. 16mm rail will give a worst case deviation of 0.15mm, which is better.

Thanks for the reply.

That is why I was going to buy supported rails but zapp advised against. Maybe because they are opened backed? Just need a simple and quick way of making an x axis so that I can used the machine to cut a joe cnc.

[irving2008]

Thanks for the reply.

That is why I was going to buy supported rails but zapp advised against. Maybe because they are opened backed? Just need a simple and quick way of making an x axis so that I can used the machine to cut a joe cnc.

Supported for X and Y is preferable because the weight of the load (router + vertical forces) causes significant negative deflection in the vertical plane which the support effectively resists. However supported rail is strong to resist loads into the support but poor when being pulled away from the support which is why Gary recommends something stronger such solid profiled rails where the glides run on the edge and better resist forces equally into and out of the plane of the rail as well as turning moments.

Thats doesnt mean you cant use unsupported rail in thr Z direction. In reality the deflection will be much smaller than what I suggested and for a small machine 12 or 16mm rail may well be adequate if the depth of cut is kept small enough. As Lee says, his works fine...

[Gary]

Yes this is why.
I have had customers use supported (TBR) rail for Z, and it worked, but i dont suggest it.
If the Z is a short stroke, standard 16mm should be sufficient, for light wood and plastic, or 20mm if you can stretch to it.
I dont suggest using 12, 13 or 16 mm rail for X and Y, unless doing very light work or engraving.

Supported for X and Y is preferable because the weight of the load (router + vertical forces) causes significant negative deflection in the vertical plane which the support effectively resists. However supported rail is strong to resist loads into the support but poor when being pulled away from the support which is why Gary recommends something stronger such solid profiled rails where the glides run on the edge and better resist forces equally into and out of the plane of the rail as well as turning moments.

Thats doesnt mean you cant use unsupported rail in thr Z direction. In reality the deflection will be much smaller than what I suggested and for a small machine 12 or 16mm rail may well be adequate if the depth of cut is kept small enough. As Lee says, his works fine...