When selecting a machine spindle, the following are some general rules of thumb:
1. Consider the gage length of the tool. Doubling the gage length of a tool can increase the deflection at the end by a factor of 8. A way to compensate for this would be to go from a 40 taper to a 50 taper spindle and tool holder.
2. Choose a spindle that can transmit the required amount of power. See the data charts for MILWAUKEE motorized spindles for further detail
3. When boring, select a spindle that has a nose bearing ID larger than the bore being machined
4. Select the nose bearing arrangement suited for the application
a. Duplex set: suited for mostly radial loading
b. Triplex set: suited for radial and axial loading
c. Quadruplex set: extra high capacity
When in doubt between two spindle sizes, a general rule of thumb is to select the larger spindle, assuming it has the speed capacity required. Although the initial capital investment will be higher for the larger spindle, the increase in productivity, tool life and workpiece quality over the life of the machine usually more than makes up the difference.