Steel frame cnc router design/build

[CraftyGeek]

As i'm getting further into the design now & tweaking parts sizes to make leadscrew bearing housing line up etc i'm wondering about methods of assembly & tweaking the fit of parts.
This is a 2 pronged question..

Firstly - I see many mentions of shimming parts to get accurate spacings & alignment....what is normally used for shimming?

Secondly - some of the parts that i'll be constructing have a length of steel box with an end cap plate that needs to be welded on.
On the design, i'm drawing this as a neat, flush fit like this:


Given that i'll be completely new to welding (yes i'll be practicing first), the tutorial video that i've watched suggests fitting parts together like the next image so that the actual weld can fit between the box & plate.

That obviously makes getting accurate positioning very tricky & i'm guessing a bit of a nightmare.

So I was wondering if something like this would be a better approach...grind out some notches on the end of the box section - sit the plate flat on the end, then tack weld into the gaps that were created by the grinding...this should help keep alignment more straightforward & accurate?


If i'm missing something here please fill me in...or I could be missing something entirely