What is the best way to mount the axles to the chassis of my robot.
If decide to use ball bearings how would i attach the bearing to my metal frame because i know bearings are not meant to be welded.
Use a Ball Bearing Holder for each.
With the right tools you can make them yourself out of a block of aluminum.
Drill out for a the bearing (tight, but not overly so), drill the piece for a tightening bolt at the top and cut through the aluminum radially, so the bolt can secure it.
Another way, when you have the hole for the bearing, is to make two thinner plates with 1..2..3 mm smaller diameter hole to go on each side and bolt them to the center piece (for retaining the bearing.
Cheap and easy, but not nearly as resilient, would be to cut a notch into the frame, cut a hose clamp in two and bolt one half to each side - then you can tighten the clamp to retain the bearing.