Z-axis - mounting rails on spindle side, rather than tower side

[guru_florida]

Hi guys,

If I mount the rails on the carriage side (vertical moving part) of the z-axis and mount the trucks on the tower part. I can get better clearance for higher work pieces. I will have about 6-8" travel on the z-axis.

The concern is then the ball screw supports must also be on the carriage side, and the ball nut is mounted to the tower. This means I have significantly more weight on the carriage, the carriage is extended farther out which also creates a larger twisting moment about the linear rails.

Anyone have some insights about whether its worth it or not?

Thanks,
Colin

[harryn]

Maybe you can fix the ball screw and rotate the ball nut. That keeps most of the weight, such as motors, and belts, etc on the fixed part of the gantry.

[Zumba]

Originally Posted by guru_florida If I mount the rails on the carriage side (vertical moving part) of the z-axis and mount the trucks on the tower part. I can get better clearance for higher work pieces.

I'm picturing your design in my head and can see what you're getting at. This is for a gantry router?

[ger21]

The new design I've been working on has about 8" of travel with the trucks moving with the spindle. The screw is supported at the top only.

http://www.cnczone.com/forums/showthread.php?t=45844

[guru_florida]

Zumba: It's a moving table style router actually. The tower part of the mill will be completely fixed and rigid, made out of solid aluminum, or 90mm^2 8020 extrusion (the big stuff!).

ger21: Your picture illustrates what I am talking about. Your router bit can go above the clearance of the rail backboard (for lack or a better word.). So if you intend to cut anything that is this high of your table, the rails and the metal or wood they are mounted too wont clear the workpiece. This is why I though of going the other way, put the trucks on the tower and the rails on the spindle assembly.

When I think about it. I dont think it is worth it to have more weight on the spindle assembly just for the few parts I *might* think up that are tall. Come to think of it, I have this HUGE MASSIVE linear rail lying around that I could probably use in the tower assembly and make it's height manually adjustable. I would use this in place of the 90mm 8020. Right now I am using the rail as a barbell for my lunch workouts!

harryn: It's an interesting idea making the ballnut move, thus at least moving the motor back to the tower assembly. How could I really do this? Mount some kind of housing over the ballnut with a belt groove? Some kind of bearing on the top and bottom of this housing would have to hold the ballnut rigidly relative to the tower assembly or lost motion would occur.

Colin

[guru_florida]

btw: That large linear rail I have is a THK HSR55 !!!

[ger21]

Originally Posted by guru_florida ger21: Your picture illustrates what I am talking about. Your router bit can go above the clearance of the rail backboard (for lack or a better word.). So if you intend to cut anything that is this high of your table, the rails and the metal or wood they are mounted too wont clear the workpiece.

I've designed about 8" of clearance under the rails, and about 8" of travel. So in theory I 'll be able to make parts 8" thick. Keep in mind that you need the router bit to actually clear the workpiece, and you also need it to reach the table. And you want to do it with the least amount of travel possible, because the longer the Z axis travel, the more flex it's gonna have.

If you're only going to have 6-8" of travel, you should be able to do it similar to the way I am without having to move the rails.

[harryn]

guru - I did a quick thread search with the words "rotating ballnut". Quite a bit comes up in the engineering area, but here is one thread I looked at in more detail http://www.cnczone.com/forums/showth...tating+ballnut

You might try searching through poster svenakela 's posts as well.

I don't think it is rocket science - you are dead on in that it will take an assembly with strong thrust bearings and some kind of housing mounted to the Z.