Principles and Challenges in Workholding

The positioning of a workpiece during the machining process critically influences the quality and output of the machine tool. Despite the diverse methods available to secure a workpiece, there are fundamental principles to ensure proper positioning.

The primary goals of workholding include providing maximum access of the cutting tool to all surfaces and features of the workpiece, securing the workpiece firmly enough to prevent it from dislodging during machining, and minimizing warping and deformation due to clamping forces. This task becomes particularly challenging with varying workpiece blanks, such as castings and forgings, which require a flexible workholding device.

Traditionally, parts are clamped based on the contour of the part blank. However, due to variations in the blanks, this method can lead to inaccurate positioning and increased setup times.

ROEMHELD’s Part in Revolutionizing Workpiece Clamping

ROEMHELD North America, located in Fenton, Missouri, has developed a new solution to address these challenges: the concentric clamping system. This system focuses on bisecting the workpiece blank using an imaginary centerline rather than locating off the contour of the part.

The modular clamping elements of this system include passive and active components, with the active modules being hydraulically actuated. These can be operated using the machine's hydraulics or an independent hydraulic pump. Two active designs are available: a linear clamping element for holding the outer diameter (OD) of the workpiece with sliding clamps, similar to a vise, and a round concentric clamping element for gripping the inner diameter (ID) of a hole or cutout on the workpiece blank. The ID clamping elements have a standard range from 1 to 10 inches in diameter, with two or three-way elements for precise location or centering. The repeatability of the round elements is +0.0008 inch, while the linear units offer +0.0002 inch.

Both clamp types use a “sliding dog” mechanism, where the moving clamp elements slide on precision machined inclined planes. Hydraulic pressure causes these elements to extend equally, providing a clamping force range from 200 to 15,000 pounds. This mechanism ensures high repeatability and sufficient stroke to accommodate variations in workpiece blank surfaces.

Flexibility with Modular Concentric Clamping

Designed for medium to high production volumes, this system allows fixtures to be built with routed manifolds or conventional hydraulic tubing. Palletized systems can utilize a de-coupling unit for portability within production cells, and the modules are reusable across different fixtures.

For shops aiming to improve the positioning accuracy of castings, forgings, or sintered blanks, the concentric clamping system offers a reliable solution. Its capability for automation reduces human error, ensuring better accuracy and efficiency in the manufacturing process.