This item , was cheaper on vevor than other places I had looked, the rails arrived very quickly and were well packaged, the bearings came pre adjusted and fit perfectly, as I,m using these for a cnc laser engraver build ,accuracy is quite important, I must note at this point that because the mounting rails are pre drilled , the aluminium is under stress and causes the rails to bow slightly, however a slight hammering on the inside of the curve on the ALUMINIUM ONLY , tends to straighten these , its something and nothing really, don,t let it put you off buying them its an easy fix, the rails seem to be induction hardened , you won,t cut these to length with a hacksaw, a great experience thanks VEVEOR I will be back .
Hower 
