I stumbled upon a great idea to make through mortises using a hand held router so I thought I'd share it with the group! The problem with making through mortises with a hand held router is that you have to align the router with one face of the workpiece make a mortise about halfway through it using a straight cutting bit and then flip it over and repeat the process. I have found that a little bit of error creeps in when I flip the piece over and sometimes the mortises dont align exactly. To solve this, I make the first mortise as before- about halfway through the piece. Then I drill a hole in the centre of the mortise, smaller than the width of the mortise and all the way through. I make sure that the diameter of this hole is just bigger than my flush trimming bit( this is important). I then flip the piece over, remove the fence from my router and change the bit from a straight cutting bit to a flush trimming bit. I then insert the flush bit into the through hole I've made and using the mortise as a guide, I trim the bottom half of the mortise! Gives perfectly crisp through mortises every time.
It is hard on the bit and I sometimes take the majority of the material out with a drill bit before routing. I've been thinking of investing in one of these upcut spiral bits, though.