Sender Fontwit used to put the (very important) v-groove in the backs of
his tongues with a router, but, frankly the whole idea scares the bejeezus
out of me and I do it by hand. I can't say I *like* doing it, but neither
does it really take all that long.
 
I have a little wooden holder, which is a block of beech with a foot piece
so's it can be clamped into the bench vise in front of me. A rabbet of
sorts holds the tongue, which is restrained by a wooden cam on my right. I
make two or three passes on each side with a very, very sharp 1/4" chisel
and, bob's your uncle. Sandy used to clean his up with a carver's v-gouge
or parting tool, but I've never had much luck with that.
 
owen
 
 
>A V-groove bit in a router, clamped upside down to the bench top, and a little
>sled inclined at about 10 degrees from the horizontal, running between two
>tracks installed on the router base, with a stop at the high end for the top
>of the tongue and side-guides.  You hold the work down with the index finger
>and move the sled with thumb and ring finger.  Once you get used to moving
>your finger directly over the router bit--usually not a very good idea, but
>here not a safety issue, as long as the ungrooved portion of the tongue is
>perfectly supported--it goes really fast.
>
>But I am sure there are less nerve-wracking approaches.
>
>Tyre,
>Dunedin