Mill Tool Holders 1- The R8 Tool Holder
This video shows how to install an end mill and an R8 collet into the milling machine spindle. First insert the end mill in the collet. Use a rag to hold the end mill and insert the end mill and collet into the mill spindle. Thread in and tighten the draw bar until snug. Hold the spindle brake and use a closed end wrench to tighten the collet. Now release the brake.

Except for a few tools that mount directly in the machine's spindle taper , tool holders are used as an intermediate between the spindle and the cutting tool. Most of the milling machines you will work with in a machine shop will be of a standard female taper size.
The most typical tool holder for a vertical knee mill is the R8 type. They can be collets , drill chuck, tapping head, boring bar , or any of dozens of other tools with the R8 taper. The R8 has a key slot which fits the milling machine's spindle pin. In all cases the R8 tool holder has a threaded female end into which a draw bar is threaded and pulled snug into the taper of the milling machine spindle.

Like most tool holders, the R8 has a taper that grips radially when linear force is applied by the draw bar. It does not take much linear force to produce a great deal of radial grip. This is why over tightening is a problem... especially with R8 collets. It takes only a moderate force on the draw bar to keep the tool safe in the spindle but it takes a lot of time to remove it if it has been over tightened. To remove an R8 tool loosen the draw bar a few turns. If the R8 does not come loose then tap downward on the draw bar while it still has a lot of thread engagement. Once the taper breaks loose, remove the draw bar. Be careful not to damage the threads of the draw bar or the R8. R.S.