PR is polyester resin, and alumilite is urethane resin. Both products are awesome, but they have different properties that make each one better at certain tasks. Although technically Alumilite is harder than PR, PR is harder in texture and performance, Alumilite has elastic properties which allow it to have some stretch and give to it. This is why Alumilite doesn't quite reach that super shine like PR does. The new Crystal Clear Alumilite is far better than the original, as they have removed a bit of the elastic property, but by no means removed all of it. PR doesn't have stretch, therefore it is somewhat brittle. You take a piece of PR and a Piece of Alumilite and whack them with a hammer 1 time only and the PR will be 50+ pieces, the alumilite might break into 2 or 3 pieces. There are other similar products that have some stretch to them, like ebonite as just one example, but most of the other products are not feasable to pour your own. I'm not saying you can't tap PR either, I'm just saying that you will have far easier and better success rates with Alumilite in this situation.
Thin walls and then tapping..yes, that is where it gets tricky. This is why I said to tape the blank real good. If you wrap some electrical tape around the end of the blank a half dozen times or more..now the walls are not so thin anymore.
A drill press is not much different than a lathe and will work just as good. You mount the drill bit in the drill press. Maybe you already drilled it before you spun it, maybe not, it doesn't matter, you should still mount that same drill bit in the press. Put the blank in a vise, no need to tighten the vise very much either, just enough to hold the pen. Pull down the press and seat the bit into the hole, then clamp the vise down. Now you have the center of the hole lined up with the drill chuck. Remove the drill bit, install your tap. Pull down on the drill press with the power off of course! Enter the pen and turn the chuck by hand as you pull down, back off to clear hole, re-enter etc etc. Be sure and use a lube of course, cooking spray works good...actually I prefer you use a cooking spray and be sure the spray contains no silicone. WD 40 has silicone in it. Silicone contamintes wood. You get it on your tools, eventually you get it on your hands and on your wood, it's guaranteed, so just dont have any silicone in the shop...junk it all, buy cooking spray for threading, or use a non silicone lubricant, like the Bostick products for lubing your table saw and other tools. That's my advice..no silicone ever !!! <no wire hangers either:wink:>