DIY Build of Aluminium CNC Wood Router without Workshop & Tools

[Jonathan]

I guarantee that an M5 or M6 hole will not strip in 5mm of aluminium for the loads that are imposed upon it on your gantry.
When calculating the length of thread engagement required the standard method is to calculate the required thread for it to require a higher force to strip the thread than for the bolt to snap when placed under tension.

For M5 the calculation (reference here) results in minimum 4.0mm engagement and 5.0mm engagement for M6. However this assumes the thread is steel, so you have to compensate by multiplying by the ratio of the material's tensile strength. This clearly results in slightly greater values.
This is still not a problem since the force on each bolt is small in your application, particularly if there are a lot of them. You are not going to get anywhere near the force required to snap a bolt. The concern is more ensuring they do not work loose, so I would thread-lock them to be safe. Since you are using SBR rails I reccomend thread-locking the bolts that fix the rail to the aluminium mount since they can also work loose.