I do have one big clarification to make. Kurt is using 1mm drillbits to shape his bit slot. I think this is probably too thin to support the bit end when mounted this way - filing the buttons would probably snap the bit right off. I normally use a bit size of around 1.5mm and those hold up much better to the filing action. With the larger bits, you can just mount the stem bit onto the mouthpiece drill held clamped in the tailstock, and rotate the stem during filing to see directly how close you are to the ID of the slot.
Using 1mm bits for the slot nukes this. I used 1mm bits to cut the slot on this stem, just for a change of pace, and ended up having to mount the stem on the lathe via the pointy dead center instead. That served just fine to hold it for filing, but lost the ability to easily check thickness against the ID.
FWIW!
The general idea being to avoid holding the thing while filing it, as that causes wrist tension.
