had my first go at this today, I stopped pretty quick as it was more of a trial run than anything.
I started off by cutting a chunk of birch, about 4"dia and 5"long which I split about a third off for the hearth. I cut a notch at the edge of this and a notch right in the middle of the other part for the bearing block.
Then, I cut a drill from birch too, about as thick as my thumb (a bit under an inch), 7" long and whittled both ends to a bluntish point and de-barked the middle 3" or so leaving it rough for the string to grip.
The bow was a birch sappling about 10mm dia and about 2/3 the length of my arm.
After messing about with the tension I eventually got the drill spinning well and after about 10 strokes I had smoke and brown dust falling out of the notch, a little while later I had lots of smoke and darker dust. The way it was going, I'm sure I could've got an ember but I'd learnt enough for today.
so, although they say the drill needs to be softer, it seems to work with all birch parts. Hopefully I'll prove the point next time.
Anyone see anything I did wrong (apart from materials)?