Workholding

For turning centers, three-jaw chucks are the most common workholding device, gripping the outside of a bar or other type of material as the main spindle spins and turning operations are performed.

Expanding mandrels are available to expand inside a tube or machined bore to grip the part during turning. These provide the cutting tool full access to the entire outside diameter of a part.

Source: Expanding The Expanding Mandrel's Usefulness

Collets are generally used when the diameter of the part or bar to be gripped ranges from 1/16 inch to 2.5 inches. Compared with standard triple-jaw chucks with soft jaws, standard collets tend to be inexpensive and more accurate for gripping in the small-to-medium range.

When a collet-type workholding device is used, the part being gripped and machined will be closer to the headstock bearings on the machine tool or indexing fixture than is the case when using a power chuck.

Source: Collets Vs. Chucks

According to Mr. Wolfe, "any vise has a bending moment associated with the clamping force applied. Take for example a 6-inch vise held down by its four corners. When the vise jaws close to clamp on a workpiece, the unsupported middle will tend to bend up from the machine table. It can move 0.002 inch or more. On precise parts that may be all the tolerance there is. My suggestion is always to secure the vise base as close as possible to the center distance between the movable and fixed jaws."

Another problem for many shops is consistency from operator to operator. "Rarely do two operators use the same clamping force on a vise," says Mr. Wolfe. "Deflections of the workpiece blank can cause inaccuracies. A more expensive but more accurate pull-type vise will help reduce some clamping variability. A simple torque handle will help achieve consistent clamping on both the pull-type and the more common push-type vises."

Make it a rule of thumb," says Mr. Wolfe, "to always try to keep the workpiece as close to the vise base as possible. Usually parallels work better on this type of setup than step jaws because the operator can check the seat of the part by trying to move the parallel. If it's snug, it's good."

Source: Flexibility Is A Vise