Linear Bearings for small CNC

I was just looking at the Zen 12×12″ CNC which is made of Polyethylene, just like my big red colored mill.

This machine uses 16mm linear bearings on the X and Y axis and 12mm bearings on the Z axis.

So that’s:

4-16mm guide rails
8-16mm linear bearings
2-12mm guide rails
4-12mm linear barings

The linear bearing cost is easy. 4-12mm is $19.95. 8-16mm is $39.90. So the bearings are $59.85. I’m a little confused why they list two kinds of 16mm bearings and one is twice as expensive as the other.

The guide rails are a bit trickier to choose since they need to be longer than the travel. The 16mm x 60″ rod is $39.95. I suppose this could be sliced into 4-15″ segments.

The 12mm x 13″ rod is $4.95. That can probably be sliced to create the Z axis rails.

So if the above is right, that comes to $104.75. My only concern is that the 15″ rails for the X and Y axis could be short. This only allows 3″ for the the space taken by the linear bearings. That’s assuming the guide rails can be mounted flush which would probably require drilling into their ends. I don’t have tools for that (unless I can figure out how to do that accurately with my drill press).

I just noticed that the Zen is 18x18x14″. That means there is an extra 6″ over and above the 12″ travel. So if I take enough off th 60″ guide rail for one 12″ axis, that leaves only 24″ left for the other axis. So if I split th remaining, that just leaves 12″ guide rails. If we still need 6″ of extra, then there’s only enough left for a 6″ cutting area. So we’d only get a 12×6″ cutting area.

If we want the machine to be 12×9″ or 12×8″ we’d need more guide rail. If we add a 14mm x 30″ at $24.95, that boosts th cost up to $129.70.  But with the 30″ rod we can get a 12×9″ machine. Plus we have 24″ of 14mm rod left over.

But this begs the question of why we are using the 12mm hardware on the Z axis. We could just convert the 12mm bearings to 14mm (same price) and use the left over 14mm rod on the Z axis. We can then drop the 12mm rod (which was only $4.95). So that’s $124.75 if everything is 14mm.