I'm a little bit confused as far as the differences between the styles of THK rails.
Here is the summary from the 5bears website which I found helpful:
SHS (Best, 4-way loading, caged balls)
HSR (Good, 4-way loading, uncaged balls)
SSR (OK, need to be used horizontally, caged balls)
SR (OK, need to be used horizontally, uncaged balls)
RSR (Avoid; balls fall out)
For the Z axis on my mill, presumably I should be using the SHS or HSR rails/blocks. These have proven difficult to purchase on ebay for a reasonable price.
What I do not understand, is what about the SR/SSR blocks makes them unsuitable for vertical use. Will they pop off the rail or bind up if a pulling force is exerted on them, or do they have considerably more play?
Also, could I one each of the SR rails on the sides of the vertical column rather than two on the front? It seems like this changes the weight of the mill head from a pulling force to a sideways force, which the SR/SSR blocks are designed for.
This might be a more optimal design no matter which rails I choose. Wider column = more stability. It also would move the milling head further in since the rails wouldn't be hanging off the column in the front.
That said if you want accurate information about the application of those rails you should go to the vendor site...
There shouldn't be any problem using any of these rails in a vertical configuration. Pretty much all linear rails can take force and torques in all directions. The exact rail you need depends on your load (usually not an issue) and your stiffness requirements (depend on what you want to cut, what depth cuts, etc.), follow these links: