Overview of Workholding System Components - Part 1

You don’t need to guess what your workholding system needs to accomplish. It must hold a workpiece in place, but also keep it aligned. If it doesn’t complete any of its main functions, you may end up with scrapped parts or worse, a damaged machine.

Workholding systems are typically composed of a number of discrete components. Each of these helps to fulfill one or more of the functions that must be accomplished for the system to perform as intended. The 5 functions that all workholding systems must perform are:

1. Hold the workpiece(s) securely against the machining forces.

2. Provide access to the surfaces to be machined.

3. Clamp and release the workpiece(s).

4. Align the workpiece(s) to the machine’s coordinate axes.

5. Locate the workpiece(s) origin(s) relative to the machine’s origin.

Examining the types of components available to accomplish these functions, you may recognize that many workholding system components contribute to more than one function. We can use the following criteria to evaluate and compare various components:

1. Productivity improvements. Increasing productivity and reducing cycle time is one of the most important aspects of workholding. This can come from:

   a. more efficient use of the machine’s work volume to allow multiple parts to be machined in the same setup

   b. the ability to more rapidly load and unload parts

These advantages must be weighed against the time and effort required to set up the workholding system in the machine. 

2. Ability to accommodate workpieces of different sizes and shapes. Many shop owners and managers want to invest in workholding systems that are capable of handling a wide variety of workpiece shapes and sizes.

3. Requirements for special preparation of workpieces. Ideally, the workholding system will accommodate workpieces directly as supplied without the need for preliminary operations to machine locating or clamping surfaces. However, special preparation may be required where there is wide variation in the dimensions of the raw workpiece, like castings or forgings. It may also be justified if necessary to better resist high cutting forces or to provide machining access to otherwise blocked surfaces, eliminating the requirement for refixturing. 

4. Amount of clamping force generated. The majority of clamping systems use friction to restrain the workpiece in at least some directions. The clamping force must be sufficient to create frictional forces large enough to resist the machining forces; yet low enough to avoid distorting or damaging the workpiece.

5. Static and dynamic stiffness. All real mechanical systems deflect when forces are applied. In previous articles in this series we looked at the machining forces which the workholding system must withstand. The static and dynamic stiffness of the system is of paramount importance in reducing deflections and minimizing vibrations.

6. Workpiece access. Many workpieces have machined features on multiple sides. It is highly desirable to avoid having to refixture the part to access these features. Therefore, the workholding system should maximize cutting tool access to multiple surfaces of the workpiece. 

7. Accuracy and repeatability. Workholding systems are commonly used to align and locate the workpiece(s) within the machine’s workspace, reducing or eliminating the need to “indicate” the parts into alignment, or to “touch off” surfaces on the part to tell the machine controller where in the workspace the part is located. The accuracy and repeatability with which the workpiece(s) are aligned and located is critical to achieving the desired productivity gains.

8. Modularity and reconfigurability. Not all shops have the luxury of production volumes high enough that machines and workholding systems can be dedicated to making a single part for their entire useful life. Therefore, the components of the workholding system should ideally be able to be reconfigured to allow for different jobs. Modularity is often a key characteristic to achieve this.

9. Cost. Workholding systems ideally improve productivity. However, to also improve profitability, the system cost must provide an adequate return on investment (ROI).

Understanding these criteria is vitally important to comparing workholding components. In future posts, we’ll break down how and when to use each class of component, and why each is useful.

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