which rail/ bearing type?

[bearwen]

I have posted this question in the general metal working forum but thought it should be here as well so dont shoot me for double post.

I am looking at building my own cnc mill. I am looking at building the design that is in the plans section here at the zone. I plan to mill standard steel and aluminum for the most part. I need it to be accurate at least to the thousands. I am looking at different bearings and need your guys opinion. Do I use the flat rails or supported round rails or does it matter. Round rails I am looking at 20 mm for xy and 16mm for z. As for the flat rails what ever i can get a hold of for a good price I have found a set of flat rails the are hiwin egh20ca for the xand y and find something else for the z. What do you guys recommend for this supported round rails and open block bearings or the hiwin/thk type flat rail bearings? I am attaching pics of the rail types so everyone knows what i am talking about.

[ger21]

HiWins or THK's are far superior to round rails.

[bearwen]

Ger21,
I have come to know you in the forums over the years and rescpect your opinion. Please if you could explain why the THK types are bettter.

[ger21]

If you look at load ratings, the THK type are rated much higher. Also, they tend to be more rigid, as they are usually preloaded. You need to check the specs to make sure, but I know the HiWins are available with 4 or 5 different preload amounts.
If you look at commercial machines that use linear rails, you won't find any that use round rails, except for those aimed at hobbiests.
The $100K routers that we have at work use 25mm THK's to carry thousands of pounds.