Everyone in the machining business has heard these two sayings. The first one urges us to optimize productivity and profitability by reducing total cycle time for machining operations. The second reminds us that underutilized machines are a waste of capital resources. It also highlights the fact that cycle time for any component being produced is composed of the time when the machine is actively removing material from the workpiece, i.e. making chips, and, inevitably, the time spent on all of the other activities that must also be performed. Above all, It urges us to minimize the non-chipmaking component of cycle time to improve profitability.
The choice of fixtures and devices to hold the workpiece during machining can have a large effect on cycle time and profitability of machining operations. In this series, we will explore how the choice of workholding systems impacts both the economics and the physics of the machining process.
Economics is about time and money, and it prompts us to minimize the time spent in all phases of the machining business in order to increase profitability. Essentially, it tells us what we want to do. The economics of the machining business pressures us to constantly improve the productivity of our operations. To do this, we must make optimal use of our shop floor personnel and of the machine tools in our inventory. This can take the form of effective scheduling to minimize idle time, e.g. excess capacity; as well as minimizing cycle time for making individual pieces.
Unfortunately, machine tools and machining processes, including workholding and fixturing, are governed by the laws of physics, and those laws place constraints on what we are actually able to do. The physical properties of the machine tool and workholding system include such things as spindle torque and power, number and arrangement of axes, axis velocity and acceleration, workspace size and shape, and stiffness and damping. The selection of tooling, machining parameters, and tool paths combine with the physical properties of the machine tool and workholding system to determine the resulting cutting forces, deflections and vibrations, and thermal distortions of the system; and ultimately the accuracy and surface finish of the resulting product.
The economics of machining and the physics of machining strongly interact, and are often at odds. Finding the sweet spot for operations can require a good understanding of both. We will see in this series of articles that the selection of workholding systems plays an important role in both the economics and physics of machining.
The goals of this series of articles are to provide the reader with:
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An understanding of basic machining physics and how this constrains what is possible in machining operations.
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An understanding of the range of options available for fixturing and workholding, including tombstones, fixturing plates, clamping systems, etc.
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An understanding of how selection of workholding system components, including materials, affects both the physics and economics of machining operations.
In a series of articles we will present a comprehensive guidebook on stationary workholding and fixturing that can act as a resource to aid the machining community in optimizing their operations. “The Complete Guide to Stationary Workholding and Machining Fixtures” will cover the following topics.
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Understanding the economics of production machining and how proper selection of workholding can increase productivity, reduce cycle time, and increase profits.
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Understanding the various functions that workholding systems must fulfill, and the technical requirements they must meet.
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Basic physics of machining and chipmaking. Cutting forces. Stiffness and deflection. The importance of stiffness in machining systems and workholding.
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Static vs. dynamic stiffness. Vibrations. The importance of damping.
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Overview of the types of elements available to make up a complete workholding system.
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Tombstones and fixture plates
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Trunnion systems and rotary tables
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Modular workholding systems
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Edge clamping systems
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Face fixturing systems
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