Jules wrote: How accurately can these things be printed?
Printing the pin-and-hole test piece I used will give you an idea:
http://softsolder.com/2013/04/21/makerg ... t-objects/
In round numbers, a feature will be within ±0.2 mm of its nominal XY dimension. For large features, that's no big deal, but for a 2 mm pin that should fit in a 2 mm hole, it's a problem.
You can print with slightly better tolerances by paying vastly more attention to detail. However, the process boils down to extruding a string of molten goo, so it's not particularly precise.
Are pegs just generally not a good idea?
A vertical peg has very little strength in the XY plane, because there's not much surface area for bonding. Particularly if it has a hole down the middle, the poor thing will just shear off. Better to use a filament snippet or a metal pin; if you need more strength than that, maybe 3D printed plastic is the wrong material for other reasons.
How high can you make a walled print?
About 5 mm less than you want... [grin]
Very tall, very thin walls require careful attention, because there's not much mechanical strength holding them against the force of the plastic shrinking as it cools: the edges tend to warp upward and snag the nozzle. While you can make it work, minimizing the problem lets you worry about other issues.
How strong is the bond using glue?
Ideally, it'll be as strong as the base plastic. I usually sand the mating parts to flatten out minor imperfections and scuff the surfaces, then glue 'em together and it works fine.
Do you think the fins will print okay as they are
They seem chunky enough to work well, but it's hard to say without a few experiments... which is what this is all about, anyway.
your expertise!
Let's just say that I've made far more mistakes than you, but you'll catch up quickly![grin]
