I have just discovered this problem also, having purchased all Metric sized hardware. Yesterday I printed my Tool Holder and then the Nut Traps and thought I was going mad when I took an M4 screw and Nylock nut and test fitted them together – the M4 Nylock nut spins in the nut trap!
While the Parts Page lists both Imperial and the Metric equivalent, in this case I don’t think it works. M4 nuts are 7mm wide (across flats) but SAE #6 nuts are 5/16″ (~8mm, across flats) which is what the nut traps are designed for. For parts like the Rollers, Corners etc, it isn’t a big deal as you can access the nut to hold it or jam something in to stop it spinning while you tighten it. With the Nut Trap, you can’t access it while tightening. If you jam something in there and it falls out, how do you ever undo a spinning Nylock inside a tube? Drilling it out the screw head would be the only option.
I am thinking I will remix the Nut Trap and Tool Holder to fit M4 hardware to solve my problem. I may have to do that anyway once I get my router and design a mount for it (the mounting holes on the Tool Holder are all the same size, suit #6 hardware).
Can we get a note put beside those parts in the parts list please (both the printed parts and hardware)? This may help others in Metric countries understand the pros and cons of substituting Metric for Imperial. Maybe even a sticky in this sub-forum?
I’m not trying to have a go at Ryan, the design or anyone, just wanting to highlight a problem.