I found some ABEC-7 (best accuracy) bearings that are originally for skateboards. They are about $1/each. See the web link below. I plan to use these bearings to build my own bearings & sliding rails. It has a 3.25 KGF dynamic load. My question is what is the load ratings in LBS (US pounds)? Can these bearings handle a 5' x 5' metal mill used to cut steel and aluminum? How do they compare with bearings normally used for metal CNC mills?

http://www.vxb.com/page/bearings/PROD/SB/1ASB/session_id/6824d46ee68307f6b757e70856323141

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One kgf is equal to 2.2 pounds, so 3.25kgf is a little over 7 lbs. I would say that something is wrong with those specs. Using those specs, a skateboard with 4 wheels and 2 bearings per wheel would be rated for less than 60 lbs, not enough to support a large child even standing still.

That is a good price for those sealed bearings. Regardless of those numbers, I would go ahead and use them. I wonder if they meant those as an axial load. I bet if you look up a standard 608 bearing that you could get an actual rating. Besides most of your loads will be of a radial nature anyway.

How do they compare with bearings normally used for metal CNC mills?

THK type bearings use on metal mills are rated at several thousand lbs

coolman,
try these bearing same price and can take a lot more pressure.
1080 lbs static 2148 lbs dynamic
it has 17 mm shaft size.
http://www.vxb.com/page/bearings/PROD/17mm/6203Z/session_id/3735590ff0816236d08e55c4d91caa4c

I am using those abec-7's on my plasma table. I have used 4 bearings to support the downward weight of my gantry, which at the moment weighs 45kg, or 100lbs. From memory these bearings can handle about 90kg each i think. Hey, I was sitting on my gantry the other day because I was so excited about actually making the gantry and how good it slides along. I weigh 100kg the gantry weighs 45, so say 150kg which is probably around 330lbs and I tell you what, unbeleviable! I could easily push myself along while I was sitting on the gantry. COOL!

Apples

I ordered 100 of those bearings from vbx.com for only $34 this week.