Tool holders (toolholders) are the physical interface between tooling and the machine tool. They come in a multiple of different machine mount styles from the older R8 style to newer HSK or VDI mounting. Each tool holder (toolholder) is also designed to carry out a specific application and is tailored to optimize machine performance and operational efficiency. Tool holders have three main parts: the taper, the flange, and the collet pocket. The taper is the conically-shaped area of the tool holder that enters the spindle during tool changing. The flange is the part of the tool holder to which the automatic tool changer is attached when the tool holder is moved from the tool changer to the spindle. The collet pocket is the area into which the collet is inserted before being secured by various types of collet nuts. Some tool holders shrink-fit around the machine tool or cutting tool and remain firmly in place. Others are optimized to the smallest size possible to allow for maximum clearances during machining. Each tool holder is designed for a specific application and tailored to optimize both machine performance and operational efficiency. Driven or “live” tooling is powered. Static tooling is not. 

There are many holder types for tool holders. Machine arbors are motor-driven shafts that turn machine tools. Blank adapters can be customized for specific applications or machining tasks. Boring heads can hold a variety of cutting tools, but are used mainly with boring bars. Collet chucks use collets of various sizes to hold machine tools. End mill holders are designed to hold end mills during milling operations. Milling or drilling chucks are used to hold various cutting tools during milling or drilling applications. Outer diameter (OD) and inner diameter (ID) tool holders can hold a variety of cutting tools. Shell or face mill adaptors are designed to hold shell or face mills, tools used to cut surfaces. Side cutter holders are designed to hold side cutter tools. Saw blade holders are designed to hold saw blades. Tapping chucks are designed to hold tapping tools for threading operations. 

Tool holders differ in terms of mounting style. R8 is used by a majority of Bridgeport machines manufactured after 1965; however, this taper is seldom used with today’s high speed machining (HSM) equipment. Morse taper (MT) is available in four common sizes: #1, #2, #3, and #4. Each size has a tapered fit for fast changes. National Machine Tool Builders (NMTB) defined the NMTB taper for all CNC milling machines. The standard is 3.5 in. per ft. and can require a drawbar. Caterpillar^® developed the CAT^® mounting style, which is sometimes called the V-flange, to standardize the tooling for its machines. Caterpillar and CAT are registered trademarks of Caterpillar, Inc. Designations such as CAT-40, CAT-50 and CAT-60 refer to the NMTB taper size. BT is similar to CAT, but is balanced and symmetrical around the axis of rotation. BT uses the same NMTB body-taper measurement notations, but uses a metric pull-stud thread. Hollow shank tooling (HSK) is designed to increase grip as spindle speed increases. It does not have a pull stud and is often used with HSM. VDI is designed for quick-change tooling and is available in both dovetail and straight-shank versions. Tool holders that use base mount tooling (BMT) and straight shank tooling are also available.