[Gadgt]

I'm new to this forum and am planning to make a mini desktop CNC mill (12"x10"x4" or so) in the near future. While searching ebay for some cheap linear bearings I kept coming across these Chinese made bearings. The model number is LME12UU and cost as little as $5 for two. They have a 12mm inner diameter which I think will be fine for all axis on my build. I searched the forum a little and haven't seen too many people mention them before.

Has anyone had any experience using these? I spent awhile modeling my design in SolidWorks, the first picture shows how I plan on mounting the bearings (using pipe straps). The second shows the semi-finished machine. What are your opinions on this setup?

Please remember that:
a. This is my first build and I'm trying to keep it cheap ($300 range)
b. I'm not expecting insane tolerances here, just enough to help me build my small projects out of wood or plastic.

Thank you!

[ger21]

Even on a machine that small, you will get a lot of flex with 12mm shafts. I'd recommend using 20mm for X and Y, and 16mm for the Z axis.
20mm Linear Motion Systems:Ceramic,Miniature Ball Bearings

[Christian Knüll]

The bearings are also relatively short. Even if they are free of backlash it would be e.g. easy to flex the Z-axis forward and backward.
Even more critical is the axis moving the base plate: it will tilt forward and backward easily unless it is supported by something else.

If the bearings aren't free of play its gets worse: you wouldn't even need force to flex the machine considerably. So besides a larger diameter using 4 instead of 2 bearings per axis (so that there are always 2 bearings on one shaft with some distance between them) would also be reasonable.

However - if you just like to mill some soft wood with small diameter tools it should be OK to have some fun with it - but use at least 4 bearings for the base plate - its just $5 more.

[Gadgt]

Thanks for the quick replies. I'm anxious to start building but am open to all suggestions.

I anticipated there being some flex in the 12mm shaft, especially where the spindle meets the x axis, so I added a couple sliders underneath the spindle. You can just barely make it out in the second picture, but I've attached another picture with the base plate removed. The sliders would be fixed to the frame and give the base plate that extra support in the place where it needed it most. I was hoping that this would help to remove any flex in the x axis. It also helps maximize the area I can cut without having to add any additional bulk.

I'll look into increasing the diameter as well though.

[ger21]

The best place to eliminate flex is in the shafts themselves. Using "sliders" will likely cause binding.

[Gadgt]

The best place to eliminate flex is in the shafts themselves. Using "sliders" will likely cause binding.

Hmm what about using a roller bearing then that the base plate rolls on? So the main support would come from the rails, plus a little extra from the bearings. I feel like having support directly under the spindle at all times would help a lot with the flexing.

[ger21]

Again, the best place to eliminate flex is in the shafts themselves. If you can spend a little more, you'd be much better off just using supported shafts. Just my opinion.

[Gadgt]

Okay I hear ya. I'm going to give the roller bearing idea a shot with a thicker rod design. If it doesn't work out I should be able to change the design pretty easily since it's mostly going to be made from 8020.