Machine vise, that is, machine flat-nose pliers, is a machine tool accessory used to clamp workpieces in machine tool processing, so it can also be called machine tool vise.

The design structure of the machine vise is simple and compact, with strong clamping force and easy operation and use.The inner nut of the machine vise generally adopts a relatively strong metal material to maintain a large clamping force.Generally, the inner nut is equipped with a chassis with a 180° scale, which can be rotated in a 360° plane.When machine vise is used, most of them are assembled on machine tools and equipment such as medium-sized milling machines, drilling machines, and surface grinders.

When the workpiece is clamped with a machine vise, the fixed movable jaws of the machine vise should find a position perpendicular to or parallel to the direction of movement of the workbench to ensure the verticality or parallelism of the workpiece to be processed.The commonly used correction methods for machine vise are：

　　(1) Angle ruler alignment: The base of the angle ruler is pressed against the vertical rail surface of the machine tool.The wing of the angle ruler is used to check the fixed jaw and rotate the flat jaw so that the fixed jaw is aligned with the angle ruler in the same straight line.

　　(2) Tool shaft alignment: Lift the workbench so that the fixed jaw iron is along one side of the tool shaft, and turn the vise for the machine so that the fixed jaw and the tool shaft are aligned in the same straight line.When the vise is used for the machine, try to be at the root of the knife shaft as much as possible.

　　(3) Use the return needle to find the correction: clamp the needle slightly on the knife shaft washer.Move the workbench back and forth, calibrate with the tip of the needle, pass through the fixed jaws and rotate the vise for the machine, and keep aligning until it is parallel to the direction of movement of the workbench.

　　(4) Calibration of the dial indicator: Fix the dial indicator (or lever meter) on the tool shaft or machine tool, and make the contacts of the meter close to the fixed jaws.Move the workbench back and forth and rotate the machine vise until it is aligned with the required requirements.

　　When the verticality of the workpiece is high, it is also necessary to move the workbench to correct the verticality of the fixed jaws with a vise.