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Innovations in Machine Tool Design Through Determinism

  
  
  

Jason Walter
Plant Manager, Star Cutter Company

Machine tool designs and their engineers have evolved over the decades, constantly pushing for an enhanced design that provides an increase in value over the current designs available to manufacturers. It is widely accepted in the machine tool industry that all sources of error can be modeled, researched, and isolated. Manufacturers today are using engineering principles to explore though deterministic processes sources of machine tool errors and designing and integrating solutions to minimize the effects.

There are many sources of error when dealing with the accuracy and repeatability of a machine tool. In addition, the level to which these errors are isolated and designed around depends heavily on the application of the machine in manufacturing. For example, the principles applied to designing an ultra-precision diamond turning lathe for the manufacturing of optics would have a totally different approach than an engineer designing a conventional turning lathe. Machine geometry, control systems, and environmental factors are just a few of the many different topics to consider when creating an error budget.

Machine geometry is a basic but important element to machine design. If grinding a surface to .0001” of an inch flat and parallel is desired, the machine slide ways must be designed and verified to meet this specification below .0001”. Straightness, parallelism, pitch, yaw, roll, squareness, rotational run out, are all important variables that need to be measured and understood to optimize machine performance. As stated before, the level of precision depends on the application. In the some cases, orders of magnitude of these values could be millionths of an inch and arc-seconds, where in other conventional machining centers, these values could be 10, 100, or even 1,000 times larger. As manufacturers, it is easy to see that as these numbers approach zero, the cost to manufacture the product increases at an exponential rate.

Control systems are directly tied to the mechanical components they are moving. In order to maximize the benefits of today’s high-end control systems, all mechanical accuracies are driving to be as accurate as possible. This type of mentality has led to a wider integration of linear motor and torque motors instead of conventional ball screw and belt driven servo motor solutions. Ball screws and belt driven servo motors inherently introduce an element of backlash and other lost motion characteristics, which over time get worse with wear. Linear motors and torque motors coupled with the high resolution encoders available today have led machine tools to become more robust, and a focus has moved to enhancements within control systems. With the processing power of today’s computers, the ability exists to address things such as real-time digital systems, adaptive control and compensation methods.

Two of the more interesting areas for analysis for design consideration center on environmental characteristics, particularly vibration and thermal considerations. These external factors may not be as important as a pure mechanical interaction, but again, the level of interaction centers around the application. The sensitivity to room temperature, the season outside, floor vibration, or proximity to a mining operation is very critical to an optics manufacturer, whereas the local job shop making parts to print may not see any effect on production based on these variables.

Taking environmental considerations into account as a design engineer can be as straightforward or complex as the application demands. When it comes to vibration, simple rubber leveling feet on a base could provide a solution, where on a more demanding application a self-leveling set of air isolators may be the only solution. Thermal effects on a machine tool can be much harder to model, isolate, and design engineering solutions for in order to minimize. Innovations in base materials and integrated liquid cooling circuits can isolate and remove heat from sources, but still remains as one of the major challenges for precision machine tool design engineers.

Determinism is defined as the doctrine that all events, including human action, are ultimately determined by causes external to the will. As engineers and designers, it is critical to isolate those external causes and design engineering solutions. This simple deterministic principle will make tomorrow’s manufacturing better than today’s.

Redistribution courtesy of AMT News - www.amtonline.org

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