MK1 Steel router build log 1500x800x200

[routercnc]

8.8 means tensile strength of ~800 MPa and yield strength of ~80% of this

12.9 means tensile strength of ~12 MPa and yield strength of ~90% of this

So 12.9 will be much stronger than 8.8 but you are usually not interested in that. You are unlikely to create forces high enough to break the bolts on a diy machine. 8.8 MPa is quite a lot !

What you are interested in is the stiffness of the bolts as this tells you how much deflection you will get when machining parts. But this will be similar in both bolt types so 8.8 will be fine. No harm in using 12.9 but they won't give you any benefit.

If you are interested in why it is because the stress/strain curves have the same slope, it is just that the 12.9 bolts will go further along the curve as they are stronger.

If you buy stainless there is a different naming system as stainless does not really yield before it breaks (by comparison) but again there is no diy CNC advantage other than looks nice and won't rust