Z axis linear bearing rail size 9mm or larger ?

[Docop]

Hi

I'm about to build a cnc of 5ft by 5ft and I plan to use linear rail for the Z axis. The Z will be with a 2.2kW spindle , so let say about 9lbs with the bracket. I will cut alu from time to time too, but mostly mdf.

So is the MGN9c series could do the job, as 2 rail with 4 block OR a MGN12 (12mm) might be better. I'm not quite sure about how to get the proper load..

And for the "threadrod", it could be a 1605 ball-screw, but is there's a smaller one .. or if it's a good idea or just stick with the 1605.. ?
I'm looking to build it light, hoping to get same result as some other z axis with 16mm rod. Don't know if i'm wrong on this..

Thanks

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[ger21]

I'd use at least 15mm rail, as anything smaller uses very small screws. (15mm rails mount with 4mm screws)

And a 2.2Kw spindle by itself weighs more than 10 lbs. You don't want to build light. You want to build strong and rigid.

[Docop]

Thanks for the tips. I've check further and I may had found the load with a speed under 90ipm could give about a 300lbs load around the spindle.
So is the ballscreww sfu1204 could handle that pretty much without any issues ?

and if I go with shaft instead of rail, a 12mm of 300mm long could do the trick and be equal to a 9mm track perhaps ?

[ger21]

A 12mm ballscrew of that length will likely have issues with whipping at that speed.

I would not go with less than 20mm for a round shaft. I use 3/4" shafts on my Z axis, and they are the weakest part of my machine. They are not rigid enough.