end mill size rule of thumb?

last night, I had to mill a 1/2" notch in a chunk of steel. Normally, being
of a "Tim the Tool Man" mentality, I'd just grab my 3/4" end mill; the
biggest my little China 3 in 1 is spec'd for. But, I've been thinking (for
lack of actual experience) and it occurred to me that a smaller end mill
would have less distance between the flutes and thus, with a higher RPM,
more cutting edges per second for the same surface speed. So I used a 1/2"
end mill and it worked significantly better than I'd have gotten with my
trusty 3/4". It just cut plain faster.
So, my questions to the knowledgeable are:
1) was this just a fluke of using a marginal machine?
2) or, is there some rule of thumb for how large an end mill to use when
it's size is not determined by some internal cut? You know, like: "minimum
3 teeth in contact with the work" for a saw blade.
Is bigger really better?
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Bigger is better if you have the rigidity and the horsepower to run them correctly. The chipload/tooth on a 3/4" endmill is significantly higher than on a 1/2". As the "web" (is that what it's called on an EM) doubles in diameter, it's shear strength is squared.
For finishing, larger tools aren't really very useful unless they have lots of flutes (not usually very economical for the HSM), however the last cut (finishing cut) can usually be done by feeding the roughing tool slowly.
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Robin S.
"Robin S." wrote in message news:a2gqb.6322$ snipped-for-privacy@news20.bellglobal.com...
rigidity and the horsepower to run them
Well, that settles it... my little machine is optimistically spec'd when they say 3/4" so I'm getting poor results when I try it. The 1/2" end mill isn't working better, it's just working properly. The larger one is probably flexing the machine too much and I'm backing off to compensate, thus getting slower results. My logic might have been wrong but I'm glad I tried going smaller - I learned something new in the process.
So, I guess I'll stick with the smaller sizes... thanks for the info, it's good to know.
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