CNC Router, no preload - what are the consequences

Hi and thank you for your help.
I am building a CNC Router based off of the CNC Router Parts design using 80/20 extrusion and a rack and pinion drive. The cutting surface will be 4ft by 4ft.

I got some Bosch Rexroth linear rails and bearings (25mm wide rails) but just now figured out that the carriages are the version with no preload.

My question is, what are the consequences of running bearings with no preload? Realistically, I am interested in routing wood and maybe machining a little bit of aluminum but rarely. What kind of problems might I run into compared to medium or high preload bearings?

Thank you for your help

Hi Nick - Cutting wood I doubt you will see a difference unless you are trying for very tight tolerances. The issue is that if the bearing is used where the load changes from one side to the other (say you are cutting a circle) then at the cross over point the bearing has clearance so it flops across. I say it has clearance as a zero clearance bearing will become a clearance bearing as it settles in. I used light preload bearings on my router and about 1.5 years in it started to visibly wobble around corners. Taking it apart I could feel the clearance in the cars. When new you could not feel any clearance. I replaced them with heavy fit bearings. So depending on what you do and how often, just wait until you see some wobble. Keep the grease up to it it helps. Peter