Ok so what do those two cups do that a pair of large washers, or a block of wood doesn’t?
Well, I suppose that small lip on the outside edge means you’re pressing on the bearing housing rather than the bearing itself. Whether you can really damage a bearing that way I don’t know…
As for the instructions, some may say it’s a ploy to get people to buy yet another tool they don’t really need
You’ll see from the instructions that Shimano actually suggest using a vice – no mention of a special tool, although they do make one, so I’d give’em the benefit of the doubt its not just a cynical ploy to sell us a new tool.
I didn’t even have the frame faced
As far as I’m aware, you don’t need to face the shell for the press-fit cups, that’s just for the external ones that seat on the outside edge. Probably about the only advantage of press-fit BBs!
Anyway, not saying you can’t do a perfectly good job with home made tools, just that at 50 quid I’m willing to invest in the right tools, as with two bikes and an average life of 2,000kms per BB, it looks like I’ll be doing this job relatively often!