Designing Workholding Systems

Part 3 of The Complete Guide to Stationary Workholding and Machining Fixtures

The creation of a workholding system is an exercise in engineering design, even if the system is assembled from catalog components. The first step in design is to identify all of the functions to be accomplished and constraints for achieving each function. 

It’s tempting to just jump right in and start assembling components, but better results can be achieved if we take a few moments to step back from the hardware and first consider the design problem.

Functions performed by the workholding system

All workholding systems must perform the following functions, regardless of any specific hardware used:

• Hold the workpiece securely during machining to avoid unwanted motion on all axes. 

• Allow access to surfaces to be machined.

• Clamp and unclamp workpieces.
  

• Align and locate the workpiece.

Requirements of workholding systems

The workholding hardware selected to perform the functions listed above must satisfy the following requirements:

• Provide enough friction to withstand the cutting forces. Virtually all workholding systems rely on friction to restrain at least some of the workpiece’s degrees-of-freedom of motion. Clamping forces must be high enough to generate frictional forces larger than the anticipated cutting forces.
  

• Avoid excessive deflections of the workpiece due to the clamping forces. Workpieces are not perfect in geometry, which can cause problems during clamping. For example, consider a workpiece that is cut from raw bar stock, and contains a slight bend. If it is clamped in a vise in a manner that “straightens” the bend, that bend will reoccur after machining, likely leading to a finished part that doesn’t meet dimensional tolerances.

  

  In some cases, workpieces have sections or features that are thin and flexible. For these parts, care must be taken to design a workholding system that can clamp the part securely enough to resist the cutting forces without distorting it.

• Enable rapid clamping and unclamping of the workpiece(s). For workholding systems designed to hold multiple workpieces, the time required to clamp and unclamp them can heavily impact cycle time. The economic benefits of more complex systems that clamp all parts with a single input motion (or automatically), must be weighed against the added cost.

• Orient and locate the part relative to the machine axes with precision. Nearly all raw workpieces lack precision surfaces to locate them. For these parts, orientation and location accuracy only need to be good enough to be comfortably accommodated by the machining allowance.

• Avoid excessive deflections or vibrations under the cutting forces. All machine tool components, including workholding systems, deflect when forces are applied to them. The amount depends on how stiff they are.

  It is useful to think of the machine tool and workholding system as a network of interconnected springs. The system of springs must be stiff enough that the cutting force deflections are small compared to the tolerance requirements. The stiffness of the elements determines the system’s vibrational response.

• Avoid degradation of the machine’s positioning performance. If the fixturing system is too heavy, it may degrade the performance of the axis motions. The axis drive motors have a finite amount of torque output, and as the load to be moved gets larger the maximum achievable acceleration will drop.

• Provide modularity and flexibility. In cases where the expected part volume is very high, it can make sense to design a customized workholding system that is dedicated to that single part or family of parts. However, when the expected volume is smaller, it usually makes more sense to select workholding system components that are designed to provide the flexibility and modularity needed to adapt to new and different jobs.   

Constraints on workholding systems

Workholding systems also face constraints that are normally machine-dependent, including: 

• Size: The workholding system must be able to physically fit within the machine’s work volume and be loadable through the access doors.

• Weight: All machine tools have maximum weight limits the machine can safely accommodate. The fully loaded workholding system must not exceed this limit.

• Compatibility: The workholding system will need to be tightly attached to the worktable of the machine tool. Therefore, its mating surface must be compatible with the attachment methods the worktable is designed to use.

Summary

Identifying the functions, expectations, and constraints of a potential workholding system is the first step toward improving. Keeping these in mind will allow us to more effectively evaluate different components and systems. By considering the design challenges involved, we can develop an effective and customized workholding system.

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