MACHINING Copyright Goodheart-Willcox Co., Inc. Chapter 26 Milling Machines 467 26.8.1 Toolholders for Vertical Milling Machines The spindle taper and mount style play a role in determining the toolholder that can be used on verti- cal milling machines. The taper is the conical shape on the shank of the toolholder that enters the spindle when the tool is changed, Figure 26-66. It needs to be the same as the taper in the milling machine spindle or the toolholder will not fit properly. For example, a Brown and Sharp taper is a long, low-angle taper as opposed to the Association of Manufacturing Technology (formerly the National Machine Tool Builders (NMTB)) taper, which is a steep taper com- monly used on vertical milling machine spindles and toolholders. Designations such as 30, 40, 50, and 60 refer to the shank taper size of the tooling (e.g. CAT®30, CAT®40, CAT®50). Draw-in bars are commonly used to hold the tool- holder firmly in the milling machine spindle. On older vertical milling machines, a draw-in bar is inserted down the spindle from the top and screwed into the adapter sleeve, spring collet, or adapter, Figure 26-67.7 7 This is a slower method of changing tools compared to the use of power draw-in bars. CNC vertical milling machines with automatic tool changers use various types of power draw-in bars. A clamp system in the milling machine spindle is used to grasp the pull stud bolt and draw the toolholder shank into the spindle. Drive keys (often called dogs) on the end of the spindle fit into corresponding slots in the toolholder flange and are used to provide positive (nonslip) drive and torque while the machining takes place. Tools are quickly changed with an automatic tool changer, Figure 26-68, by mating the standard- ized taper and the drive key. Quick change tooling is beneficial when manually changing cutting tools, Figure 26-69. End mills may be mounted in adapters, collet chucks, or shell end mill holders. The toolholder is typically fastened into the milling machine spindle by pressing a button or by turning the collar tight around the flange. Milling shank styles such as BT, CAT®, and HSK are widely accepted and commonly used in modern vertical milling machines. The design of the toolholder will vary based on the shank style. For example, the design of a HSK (hollow shank taper) toolholder is dif- ferent than a BT or CAT® toolholder with an NMTB taper. HSK toolholders are used in applications where the milling machine spindle will be rotating at higher speeds because the tooling increases its grip as the spindle speed increases. Republic-Lagun Machine Tool Co. Figure 26-65. Angular head adjustments are possible on most vertical spindle milling machines. monstArr/Shutterstock.com Figure 26-66. Common parts on a vertical milling machine toolholder are a taper shank, flange, and pull stud. Taper shank Flange n slot Drive key s Pull stud