This is not the first tool holder I have made. Its the first long reach tool holder I have made. Its turned from 1144, and reamed to size. Its got a set screw. Not optimum for carbide, but it worked for this job. Its got a 9 degree taper. I made it to machine the bottom of an ingot mold with a 10 degree draft angle. 1144 is hardenable, but this is not hardened. I am not setup to harden and grind tool holders.
The tool shank is solid to help increase rigidity, and the end mill bottoms out in the bore for the same reason. Of course this means if (when) I break a tool it will be very difficult to remove. Because the tool holder is not hardened it will probably be ruined if that happens anyway. If I think the tool holder is salvageable at that point I would probably drill the shank on the lathe and punch out the mill.
A 3 inch long mill sticks out about 1 inch. (This is a 3 inch mill, but when measured its actually noticeably longer). At 1 inch of stick out about half of that is beyond the line of the taper of the tool holder. The machine this is used in only turns about 5120RPM so balance is not super critical. A roll test on a couple 246 blocks on the surface plate does not show a significant imbalance. No more than I would expect from a setscrew holder. After drilling a divot on the opposite side I couldn't really find a major settling spot.
The cavities this tool holder was designed for are upto 2.75 inches deep. The tool as designed has a little over 3 inches of reach to the flange including the stickout of the ball nose end mill. The flange is actually unnecessary as I pressed a hardened flange collar onto the shank of the tool holder.
The job this was made for went well.
I also have to do deep-ish hemispherical cavities from time to time. I believe this tool and holder will be very useful for those. My next project on this machine is a mold cavity that includes a large hemisphere for a heavy downrigger weight mold.