Hi Joe,
You're right, my machine used THK's HRW series guides. There's a single HRW50 block and rail for the Y, 2 HRW35 blocks on a single rail for the X and a single HRW35 block and rail for the Z. For this design they work beautifully, and will never even come close to their rated capacities on this mill, being way oversized for the application. It runs really smooth and quiet.
The machine is designed for complex 3D parts requiring a good accuracy and good surface finish, and where minimizing the cycle time per part is less critical. I use a high speed spindle, taking light cuts at higher feedrates, not hogging off serious metal in a single pass... It's capable of a nice high resolution (0.00025 mm / 0.00001") and limiting the ballscrews to a fairly safe 2500 rpm gives feedrates up to 5,000 mm/min or 200 ipm. I run brushless servos and a smoothstepper board (Mach3 won't come close to driving it at that speed directly, even with the kernel at 100 Khz).
If you can't get the HRW blocks or similar (NSK do a wide, low profile series too, others may as well), I'd suggest you go with the 4 blocks and 2 rails - the 45 mm rails are just too high. The height alone will contribute to a greater moment load due to inertia and cutting forces, and that series isn't designed to be used in that configuration as the HRW blocks are.
If you want to go nuts with big caged roller linear bearings, check out THK's SRW series
http://www.thk.com/documents/us_pdf/...e/en_B_219.pdf
Best regards,
Jason