Selecting the proper toolholder

The foundation of robust and successful CNC milling and drilling processes consists of numerous elements, including high-quality machine tools, sturdy workholders and high-performance cutting tools.

Toolholders are another of the basic elements. Thumb through trade publications and catalogs and you will see an almost limitless variety of toolholders and options. It can be difficult to sort through the options and opinions.

Collet chucks may well be the most common type because they provide a great deal of value. Collets and collet chucks are relatively inexpensive compared to other types of toolholders. A countless number of sizes are available and each collet in a series will grip a range of tool sizes. Collet chucks provide good runout and gripping force characteristics for all but the most challenging operations. The most common collet styles are ER, TG and DA.

DA collets are the least expensive of the three and provide acceptable runout when performing light-duty milling, but can cause runout problems when performing close-tolerance holemaking operations such as reaming. They can also diminish the ability of high-performance drills to produce tight-tolerance holes, so avoid DA collets for those applications.

TG collets are the most expensive of the group and provide the highest gripping force because of their size and length. TG collets also offer better runout characteristics than DA collets and can be coerced into setups that produce close-tolerance holes. TG collets are an excellent choice for holding tools larger than ⁷/₈” (22.2mm) in diameter and for use in machines with large spindle tapers, such as CAT 50. However, I don’t see TG collets offered in many catalogs. When I do, they are not prominently positioned, which leads me to believe TG collets are losing favor.

Courtesy of C.Tate

The length of the TG collet (left) provides a large contact area with the cutting tool, enhancing gripping force compared to the ER 32 collet (right).

ER collets probably provide the best value and are my first choice when tooling a new machine. They have the gripping force needed to perform many milling operations and have excellent runout characteristics. When paired with high-performance drills, reaming can be eliminated. ER collets can be found in a large range of sizes, from 0.020 ” to 1³/₈” (0.51mm to 34.9mm).

As high-speed machining and aggressive milling become increasingly common, collets sometimes do not provide adequate gripping force. When an operation is too aggressive for the collet chuck, chatter occurs and can cause the cutting tool to pull out of the holder. Mechanical milling chucks present a possible solution to these problems.