Hello Jim,
I've caught up with this rather late, but thought I'd throw my 2d's worth in.
En24 _is_ tough -T is tempered, but still tough compared to average steel. If it's been heat treated as very probably it has, then it will certainly be hard work for HSS. For reference, you can get a wide range of material specs (including BS970, ISO and many ASTM specs) and heat treatments from the West Yorks Steel site eg:
http://www.westyorkssteel.com/EN24.htmlI have reduced holders for tipped tools and they are generally pretty tough and certainly carbide is better, but you might still get away with HSS. You need to have the cutter speed quite slow to prevent over heating the cutter tips, a good flood of suds helps. A steady squirt from a washing-up liquid bottle if you haven't got a suds pump. If 300 RPM was shagging your HSS cutter, slow it down some more, carbide doesn't really suffer overheating problems teh same way as HSS, but you may get chatter or other problems with over speed.
Different materials react differently to different machining operations. eg stainless saws OK with effort, files fine, but is a notorious sod to drill. Allowing the work to work-harden may also be a problem in your case and one way to help minimise that is to keep the feed quite high, even with a relatively slow cutter speed. Once hardened it can be very difficult to start to cut again.
I have a Speeds & Feeds list (attached below) which I have found generally acceptable. It looks like the data probably came from Alan Marshall's site, or possibly we both plagarised it from the same place, and I have added somewhat to it.
The work-piece material and cutter size results in a selected RPM, the type of cutter then implies the feed in thou/turn, multiply the two together and you get the feed in In. (or thou)/min. eg 2 thou/tooth, two teeth => 4 thou per turn, 300 RPM => 1200 thou or 1.2" per minute. The depth of cut is mostly down to how rigid the machine is and how brave you are, but 0.1" should be no bother for your machine.
Dovetail cutters are damned expensive to buy, but for a simple one, using a trianglar (TCMT) tip, it's not too difficult to make one. Google 'home made dovetail cutter' and there are a fair few examples.
One problem with milling cutters, unlike lathe tools, is that once they're toast, you can't go to the bench grinder and tickle them up again and have another go. It's try another cutter time and that can get expensive and disheartening. One reason I recently got a T&C grinder, but that's another story....
Cheers
Richard