16mm supported rail how many linear bearings

**progomez** · 12-01-2009

Im thinking of buying some 16mm supported rails from zapp. Would it be advisable to use 2 bearings per length of rail, even on the z axis, which will be 200mm?

Would it twist with only two?

**Lee Roberts** · 12-01-2009

I think given the choice, i would go with x2 bearing blocks per rail. I would think this would give a better distribution of all things considered.

**Gary** · 12-01-2009

Yes it is, but also this type of rail is not the best to use as a Z axis, we suggest either round rail or profiled rail for vertical axis.

Originally Posted by progomez

Im thinking of buying some 16mm supported rails from zapp. Would it be advisable to use 2 bearings per length of rail, even on the z axis, which will be 200mm?

Would it twist with only two?

**irving2008** · 12-01-2009

Gary, whats your reasoning on this? and some links to your recommendations?

**progomez** · 12-01-2009

Originally Posted by Gary

Yes it is, but also this type of rail is not the best to use as a Z axis, we suggest either round rail or profiled rail for vertical axis.

Thanks for the reply.

So supported rail can be used on the x and y? Would 12mm z rails + carriages be sufficient to lift a standard full size wood router?

**Lee Roberts** · 12-01-2009

Originally Posted by progomez

Thanks for the reply.

So supported rail can be used on the x and y? Would 12mm z rails + carriages be sufficient to lift a standard full size wood router?

Thats all im using on my rouckcliff machine and everything seems ok so far.

FOR SALE: x2 12mm Round Rail 1000mm - CHEAP XYZ

or

FOR SALE: x2 16mm Round Rail 1000mm - CHEAP XYZ

I'v also got the SMA units as well, the prices would be the same as Zapp's. :)

**irving2008** · 12-01-2009

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.

**progomez** · 12-01-2009

Originally Posted by irving2008

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** · 12-01-2009

Originally Posted by progomez

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** · 12-01-2009

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.

Originally Posted by irving2008

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...