[scott0999]

Is it better to have the Z axis rails on the side vs facing the spindle. It seems most machines have the later, but is that actually better?

I was searching around and came across this design, the idea was to reduce binding.

thoughts?

[elfrench]

Which part do you think will bend or bind less?

[widgitmaster]

Because you have open bearings on the Z axis, the forward & reverse forces will tend to open them more while the side force is the strongest area!
Imagine the force directions while interpolating a circular pocket!
U should use square rails & trucks on the Z axis, they will be more compact and absorb forces from all directions!

[scott0999]

Because you have open bearings on the Z axis, the forward & reverse forces will tend to open them more while the side force is the strongest area!
Imagine the force directions while interpolating a circular pocket!
U should use square rails & trucks on the Z axis, they will be more compact and absorb forces from all directions!

I agree but I already bought the round rails unfortunately. on my next build I will get square rails&trucks, or maybe even a THK actuator

so with the round ones, on the side would be better?

[inventor83]

I would put them on the front of the Z, this might be stronger and put less stress on the axis when you are plunging or drilling

[JerryBurks]

That is exactly how I use the round rails on my machine for better/similar rigidity in all directions. I have even a double set of blocks in the lower position (total of 6 for the z-axis) because the horizontal forces are higher there than in the top position due to the spindle and bit leverage. The machine is pretty good but still, I regret not using profile rails like THK and may rebuild the machine at some point. These open bearing blocks just flex too much and the machine is not as good as it could be.

[scott0999]

That is exactly how I use the round rails on my machine for better/similar rigidity in all directions. I have even a double set of blocks in the lower position (total of 6 for the z-axis) because the forces are higher there than in the top position due to the spindle and bit leverage. The machine is pretty good but still, I regret not using profile rails like THK and may rebuild the machine at some point. These open bearing blocks just flex too much and the machine is not as good as it could be.

ok sounds good.

with the round rails, say for instance you mount just the Z-axis plate to them (with the rails facing the plate). the plate itself can still pivot right? it seems the only thing keeping it aligned would be the drive screw?

[JerryBurks]

...with the round rails, say for instance you mount just the Z-axis plate to them (with the rails facing the plate). the plate itself can still pivot right? it seems the only thing keeping it aligned would be the drive screw?

Not sure how that is meant. But with at least 4 bearing blocks spaced apart reasonably (mine are in a 10" square for Z) there is nothing to pivot. If the linear bearings were ideal and not flexing, the only degree of freedom for the z-plate is a linear motion along the z-axis and that is obviously guided by the lead screw.

[dmalicky]

Good comments from widgitmaster and JerryBurks. SBR20 bearings have decent stiffness in compression, but are pretty flexy in tension (http://www.cnczone.com/forums/linear...ml#post1412778).
Lateral stiffness is probably in-between tension and compression. The highest priority is to avoid tension.

Here is a breakdown on how the cutter load direction affects the SBR bearings for the 3 axes and orientation choices, for a moving gantry design. I'll use the convention:
Y axis: lateral, along the gantry tube (4' in a 4x8 machine)
X axis: longitudinal (8' in a 4x8 machine)


Z Axis
Longitudinal orientation (traditional, mounted to the front of the Z plate):
1. X cutter loads put either the top or bottom pair in tension. This results in a lot of deflection at the cutter, and different chatter tendencies for +X vs -X forces.
2. Y cutter loads put all 4 in lateral loading.
3. Z cutter loads put either the top or bottom pair in tension. Same problem.
So, 2 of the 3 cutter load directions result in tension on a pair. This orientation is easiest to build, but it has the worst stiffness.

Lateral orientation (opposing, like JerryBurks and the OP's pic):
1. X cutter loads put all 4 in lateral loading.
2. Y cutter loads put 2 diagonally opposite bearings in compression, and the other 2 in tension. The 2 in tension will do little but that's acceptable since the 2 in compression will be stiff. Since they are diagonally opposite, if they have sufficient vertical spacing, they can effectively resist the torque from the cutter.
3. Z cutter loads put all 4 in lateral loading.
So none of the 3 cutter load directions result in tension on a pair.

Y Axis (along the gantry tube, lateral)
Longitudinal orientation: same effects as with the Z axis longitudinal orientation, so tool will deflect a lot for X and Z loads.
Up and down orientation: same effects as with the Z axis lateral/opposing orientation, so preferred for stiffness but again, more work.

X Axis (longitudinal)
The bearing loading for this axis is fundamentally different than the other two.
- For the above two axes, the cutter load is cantilevered off the bearings, so cutter forces get amplified at the bearings and bearing deflection gets amplified back to the cutter. Double trouble. This is why these axes benefit the most from profile rail. With limited $, profile rail on the Z axis has the best benefit/cost ratio.
- For the X axis, if the feet bearings have good longitudinal separation (at least 10") and the cutter sits in between them, all cutter forces get distributed between the bearings, and bearing deflection does not amplify at the cutter.
- If the gantry is heavy, Z drilling forces are light, and the cutter sits between the X bearings, it would be ok to orient the X bearings with their opening face down -- it's unlikely they would see tension.
- If not, face them laterally/opposing like JerryBurks machine.

Also, for the Y and Z axes, the vertical separation between the pairs of SBR bearings needs to be at least as much as the Z clearance.


On THK linear actuators, keep in mind that they tend to be heavy, and many of them are designed to have slop in the ballscrew and/or bearings. The no-slop ones can be found by decoding the part number. Last time I looked on ebay, almost all of them were the slop type.

[scott0999]

thanks for the excellent replies, I'm learning