Deleted all that - off topic.
So an end mill like this is extremely long and I think it would tend to deflect or light cuts would be needed. So I think I’m missing something. I was wondering if this would be perhaps used in a horizontal mill and the bottom end could be supported by the other side with the arbour support.
I've never seen an endmill that was meant for a secondary support.. Not to say it doesn't exist but I'd think it likely isn't meant for that or there wouldn't be flutes to the end. Horizontal machines use arbors because you can make up essentially any profile you want in a near endless size (obviously there are limits to the overarm and table and arbor), and cutters can be reground for them a lot easier than endmills. I'd guess it's just a run of the mill extended endmill, it wouldn't be meant for roughin, just cleaning up sidewalls all the way down.
I've also seen very long endmills used in timber processing machines, a place I worked when I was 16 ended up buying a humdinger k2 that could handle 24"x18"x30'+ beams, it would use endmills like that for pocketing for the tenons. They will use carbide in wood processing as well, but it's not unheard of to use hss for that.
Ps, autocorrect can suck my balls.. If there's mistakes in that posts spelling or entire words you will have to read between the lines.
I agree with @kylemp The photo is of a long flute end mill for surfacing the sides of a project. They are meant for taking lighter cuts than standard lengths. I strongly prefer the plunge method of mulling the sides, and taking multiple finish cuts in depth increments so I've never needed the long flute version.
They are useful as long as the flutes don't get chipped. That happens when they get dull (or so my toolmaker friend says)...
I pretty much agree that they are for light duty finishing; in my experience, primarily to get a sharp 90 degree corner in a deep recess. Another application is straightening a hole that has wandered when drilled and for a deep plunge cut of a slot end.
I have HSS cobalt in .375/.500/.630", from 4" to 6" progressively. Mine are center cut.
Purchased through Thomas Skinner, I as well have one .500" x 6" 2 flute aluminum end mill. My use is primarily automotive applications where I want to keep a given radius in a corner with a long reach ( 4" ) and an initial 90* angle at the surface. This looks like intake manifolds and intake runners in cylinder heads.