ISSUE #06

Clamping Cylinders

Securing the Part
The previous chapter discussed positioning cylinders, which accomplish the first of the three steps in workholding. In this chapter we look at clamping cylinders, used to implement the second step.

Holding a workpiece in place while working on it is so fundamental that generations of metalworkers have devised countless variations of mechanical clamps such as the one illustrated (Refer to Chapter 3 for more examples).
These devices are based on principles such as the lever, inclined plane, and screw thread. They are simple and comparatively inexpensive to purchase. But, like virtually every other choice in life, there are tradeoffs. In return for the lowest upfront cost, the purchaser of a mechanical clamping system must accept two key limitations:
  • Modest production runs;
  • No critical tolerances.
Bar Clamp with Handle
Production runs can be limited by the labor-intensive nature Production runs can be limited by the labor-intensive nature of mechanical clamping. For many parts, tolerances are limited by the need to allow for distortion of the part and inconsistent clamping forces from part-to-part due to the variability of manually tightened clamps.

Hydraulic workholding overcomes those limitations. The tradeoff is that the purchaser must pay more upfront for a hydraulic clamping system. However, the numbers readily show that for any significant production volume, the cost of using hydraulic workholding is lower than for manual clamping. The lower total cost of hydraulic workholding combines with an increased production rate, improved accuracy and repeatability, less congested fixtures, increased safety, and automation compatibility.

An over-located workpiece
Whether the workholding method is mechanical or hydraulic, the same three fundamental steps must be accomplished: positioning, clamping, supporting. Regardless of how it is accomplished, the positioning step is based on the 3-2-1 locating principle explained in Chapter 2. It’s important to avoid the mistake of over-location. This occurs when there is more than one locating plane or point for any given degree of freedom. Over-location can cause in distortion or mis-orientation of the part, resulting in degraded accuracy and consistency.

“Clamping” is the secure fastening of a positioned workpiece in a fixture. Clamping always requires force transmission through the part. Each clamping force vector should, as nearly as possible, describe a line that extends from the application point of the clamping force through the workpiece to the bearing points and is parallel to the axis of the clamping cylinder and perpendicular to the plane of the bearing points.