You may not notice much of a difference, but the 4x15mm blocks are preferable.
Most hobbyists install their linear motion components incorrectly due to lack of resources. The proper way to install a rail is to mill a very accurate slot with a 90 degree shoulder on one side and a 60 degree shoulder on the other... not easy if you don't already have a decent sized VMC to play with. Tapered wedges are installed every few inches to push the rails against the 90 degree shoulder before they're bolted down from the top. The blocks are installed in a similar fashion. That is why the sides of the rails and blocks are ground.
With a single HRW-45 block, the area of the mounting pad is something like 3.5" x 3.5", whereas four HSR-15 blocks can be spaced out to whatever you want. Without a mounting shoulders, a larger mounting area is preferable.
But again, for these very small mills, you probably won't notice any difference. |