Select Spindles and Toolholders… from the Manufacturing menu.
To modify a spindle, select the desired spindle as the Current Spindle and click the Properties 
button. The spindle properties dialog box comes up.
To create a new tool holder, select an existing spindle as the Current Spindle (the dimensions of this spindle will be used as the initial dimensions for your new holder) and click the Create new spindle 
button. The toolholder properties dialog box comes up.