I am building a CNC machine based on Chinese ballscrews and rails, with aluminium frame. The ballscrews are supported by BK12/BF12 blocks, and I include part of the datasheet in the picture bellow. I have a pretty basic question: what screws do those parts expect? I can see that the four vertical mounting holes are 6.6mm in diameter. My first reaction is that a metric M6.5 or imperial 1/4 inch hex socket screws would fit, but M6.5 is a bit exotic and imperial screws for metric part... sounds weird. Or is this all not important? I worry that the part will not be supported well if I use "wrong" diameter screws.
Also, there are four smaller holes perpendicular to the 6.6mm ones (they are marked as fi d2 and are 5.5mm) - are they just for alternative way of mounting?
Those holes would normally be for M6 bolts, but 1/4" would fit, too. 1/64" or .5mm are common clearances, and allow a tad of adjustment if the threads aren't quite in the right spot. The actual position is held by the clamping force, not the bolt locations.
The 4 smaller holes could be for alternative mounting, but I find those are ideal for the motor mounts--easy to get the motor axis colinear with the ballscrew axis that way.