It has a 3/8" shank. Grind a little flat on it and stick it in a set screw holder.
What machine are you doing this on.. Maybe its just a floppy POS to begin with..
Here is some of my chatter stopping methods when working with long endmills.
Looks like your endmill is a square corner, try something with a radius or a chamfer.
Feel free to chamfer it yourself. Its not that hard, and if you have some practice,
you can even put your own radiuses on. Sharp square corners on long endmills can
get really grabby.
Make the tool "MORE VARIABLE".. Seriously.. Just grind a big honking chamfer on one
flute. It works, I've done it a bunch of times. I had one machine quite a while ago
that would do it for me.. Shake, rattle, noisy.. Until a corner broke off, and then it
cut clean.. Occasionally taking the cutter with it, so I started just taking new endmills,
Vari flutes, with corner rads and just grinding a big chamfer on one flute. Floppy machine,
FLOPPY!!! The X-Y was pretty stout, the spindle was the problem, and tool interface was
shit. A series 10 Acroloc..
RAMP!!! It puts pressure straight back up through your spindle. Sort of like how a high feed
mill works, and why they can hang out a mile. You're taking that little bit of flex out of the
Z axis.. You're making your machine and tool more rigid than it actually is.
Those are my little tricks. Though one more thing. In my experience using long endmills,
chatter happens when the tool doesn't have much to do. If it doesn't have much to do it
bounces around and makes a lot of noise. Like an employee with a cell phone.. Keep it
busy and it tends to be calmer and less noisy... Almost like a little kid.
I've roughed out a pocket or an outside wall, and left a little more on the last step down.
Keep the bottom of the tool busy on the finish, and the tool calmed right down.. Ran it
faster and ran it harder..
Lots of a little tricks, and essentially none of them have to do with slowing down
the speed, or feeding it lighter. Occasionally that is the only answer, but what you
are doing isn't that crazy.
One last though. Only use the long tools when you need them. Go as deep as you can with
a short one. Go deeper with a little longer one, and then waste the time with the long
expensive endmill, only where you actually need it.