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 that any successful design must accomplish, along with a list of requirements and constraints that define the performance objectives for achieving each function, and any inherent limitations on the hardware used to achieve them.
It’s always very tempting to just jump right in and start grabbing components and assembling them, but experience shows that better results are achieved if we take a few moments to step back from the hardware and first consider the design problem a little more abstractly. Doing this helps to make sure we fully understand what we want to achieve. In this article we will apply this methodology to the design of workholding systems.
Functions performed by the workholding system
All workholding systems must perform the following functions, regardless of the specific hardware used:
Hold the workpiece securely during machining -- Once loaded, the workpiece must be prevented from unwanted motion due to the machining forces. Rigid bodies have 6 degrees-of-freedom of motion in space, i.e. translations along the three coordinate directions, X, Y, and Z, and rotations about those same directions, Rot_X, Rot_Y, and Rot_Z. For each of these degrees of freedom, motion can occur in both the positive and negative directions. The workholding system must restrict all of these potential motions.
Allow access to surfaces to be machined -- The workholding system must hold the workpiece in such a way that all of the surfaces to be machined are accessible to the cutting tools and within the range of axis motions of the machine tool.
Clamp and unclamp workpieces -- A method must be proved that allows new workpieces to be secured into the workholding system, and finished workpieces to be removed.
Align the workpiece to the machine tool’s coordinate axes -- A fundamental problem of CNC machining is that the motion commands embedded in the NC part program are expressed relative to a coordinate system embedded in the part. However, the machine tool’s controller can only execute motion commands relative to the machine tool’s coordinate system, which are defined by the physical layout of the motion axes. Therefore, the workholding system must ensure that the axes of the workpiece coordinate system are precisely parallel to the machine tool’s motion axes.
Locate the origin of the workpiece coordinate system relative to the machine coordinate system -- Even when the workpiece axes are aligned parallel to the machine axes, the machine still needs to know where the workpiece is located within its workspace. Ideally, the workholding system will position the workpiece at a known and constant location within the workspace. If it does not, then the machine operator will be forced to “touch off” surfaces of the workpiece to establish its location, adding to the cycle time for the part.
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 degrees-of-freedom of motion of the workpiece. For example, in the figure below we see a workpiece held in two common ways, a conventional vise and via toe clamps.
For the vise, physical interference with surfaces of the vise prevent motion in the +Y, -Y, -Z, and Rot_Z directions. All of the other possible directions of motion must be restrained by friction. In contrast, for the toe clamp setup, physical interference of surfaces prevents motion only in the +Z, -Z, +/- Rot_X, and +/- Rot_Y directions. All other possible directions of motion must be restrained by friction. Of course, the toe clamp(s) also restrict access to portions of the top surface, precluding machining in those regions.
The maximum frictional force that is generated is generally a small fraction of the clamping force. To be safe, assume it is about 20% or less. Therefore, the clamping forces must be high enough to generate frictional forces that are significantly larger than the anticipated cutting forces. If particularly heavy cuts are planned in one direction, it may be wise to design the workholding system so that physical interference counteracts these forces instead of friction.
Avoid excessive deflections of the workpiece due to the clamping forces -- Workpieces are, by definition, not perfect in geometry; and this 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 tends to “straighten” the bend, that bend will reoccur after finish machining and unclamping of the workpiece, likely leading to a finished part that does not meet dimensional tolerances.
In some cases, workpieces have sections or features that are thin and flexible. For these types of parts, great 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 be a significant contributor to cycle time. The economic benefits of more complex systems that clamp all parts with a single input motion, or automatically clamp and unclamp hydraulically or pneumatically, must be weighed against the added cost.
Orient and locate the part relative to the machine axes with enough precision so that required tolerances can be achieved -- Nearly all raw workpieces lack precision surfaces to locate them, especially castings and forgings. For these parts, the requirements for orientation and location accuracy only need to be good enough to be comfortably accommodated by the machining allowance on the workpiece. However, it is often necessary to refixture a part to finish the machining of previously inaccessible surfaces. In these cases, the requirements for orienting and locating the part are much more stringent, and different strategies may be required to ensure meeting the specified tolerances.
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. Of course, these are generally very stiff springs; but the allowable deflections are also very small. Collectively, the system of interconnected springs must be stiff enough so that the deflections resulting from the cutting forces are negligibly small compared to the tolerance requirements.
The stiffness of the elements, combined with their masses and damping, will also determine the system’s vibrational response to the time varying cutting forces that occur in virtually all milling operations. Now the situation becomes more complex since these dynamic systems have natural frequencies or resonances, and when the rotational speed of the cutter causes the tooth passing frequency to be near one of these resonances, the amplitude of vibrational deflections can increase dramatically.
Avoid degradation of the machine’s positioning performance -- If the fixturing system, fully loaded with workpieces, is too heavy, it may degrade the dynamic 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. However, for machines with axes driven by ballscrews, this effect will likely be very minimal even for very heavy loads, since the ballscrew drive provides such a low transmission ratio.
Provide modularity and flexibility -- Very few shops have the luxury of production volumes and runs that are high enough that a machine tool and workholding system can be dedicated to a single part for their entire useful life. That means that workholding system components should be designed to provide flexibility and modularity so that they can quickly and easily adapt to new and different jobs, thus providing more benefit.
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 able to be loaded through the access doors.
Weight -- All machine tools have maximum allowable 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.
This article is intended to provide a high-level view of workholding system design, including the discrete functions they must perform, the requirements they must meet, and constraints imposed by the machine tool. Keeping these in mind will allow us to more effectively evaluate different components and systems.
In the next few articles we will look at the physics of the machining process so that we can begin to put numbers on these requirements and answer questions such as “how large are the machining forces that the workholding system must counteract” and “how stiff do the workholding elements need to be”?
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