K. Scott Smith

A Technique for Enhancing Machine Tool Accuracy by Transferring the Metrology Reference From the Machine Tool to the Workpiece

High-speed machining (HSM) has had a large impact on the design and fabrication of aerospace parts and HSM techniques have been used to improve the quality of conventionally machined parts as well. Initially, the trend toward HSM of monolithic parts was focused on small parts, where existing machine tools have sufficient precision to machine the required features. But, as the technology continues to progress, the scale of monolithic parts has continued to grow. However, the growth of such parts has become limited by the inability of existing machines to achieve the tolerances required for assembly due to the long-range accuracy and the thermal environment of most machine tools. Increasing part size without decreasing the tolerances using existing technology requires very large and very accurate machines in a tightly controlled thermal environment. As a result, new techniques are needed to precisely and accurately manufacture large scale monolithic components. Previous work has established the fiducial calibration system (FCS), a technique, which, for the first time provides a method that allows for the accuracy of a coordinate measuring machine (CMM) to be transferred to the shop floor. This paper addresses the range of applicability of the FCS, and provides a method to answer two fundamental questions. First, given a set of machines and fiducials, how much improvement in precision of the finished part can be expected? And second, given a desired precision of the finished part, what machines and fiducials are required? The achievable improvement in precision using the FCS depends on a number of factors including, but not limited to: the type of fiducial, the probing system on the machine and CMM, the time required to make a measurement, and the frequency of measurement. In this paper, the sensitivity of the method to such items is evaluated through an uncertainty analysis, and examples are given indicating how this analysis can be used in a variety of cases.

Assessment of the Process Parameters and Their Effect on the Chip Length When Using CNC Toolpaths to Provide Chip Breaking in Turning Operations

Past work at UNC Charlotte has demonstrated that the use of oscillating CNC toolpaths provides a reliable chip breaking alternative to conventional methods such as the use of cutting inserts with special geometries and/or adjusting machining parameters. The specific toolpath geometry and the selection of the oscillating parameters is an important step to reliably and constantly create broken chips using this new method. This paper builds on the past work and discusses the proper selection of oscillation amplitude and its effect on the ability to break chips and to achieve desired chip lengths.Copyright

Continuous Beam Modeling

In Chaps. 1 through 5 we discussed the solution of discrete, lumped-parameter models. For multiple degree of freedom systems, we employed modal analysis to enable us to transform the coupled equations of motion in local (model) coordinates into modal coordinates. In this coordinate frame, the equations of motion were uncoupled and we could apply single degree of freedom solution techniques. In Chap. 6 we shifted our attention to the “backwards problem,” which is representative of a common task for vibration engineers. In this problem, we begin with measurements of an existing structure and use this information to develop a model. We again used discrete models to describe the system behavior.

Two Degree of Freedom Forced Vibration

Let’s extend the two degree of freedom free vibration analysis from Chap. 4 to include externally applied forces so that we can analyze two degree of freedom forced vibration. The general case is that a separate harmonic force is applied at each coordinate; see Fig. 5.1. However, we are considering only linear systems, so we can apply superposition. This means that we can determine the system response due to each force separately and then sum the results to find the combined effect.

Two Degree of Freedom Free Vibration

Let’s extend our free vibration analysis from Chap. 2 to include two degrees of freedom in the model. This would make sense, for example, if we completed a measurement to determine the frequency response function (FRF) for a system and saw that there were obviously two modes of vibration within the frequency range of interest; see Fig. 4.1.

Single Degree of Freedom Forced Vibration

Let’s continue our study of the lumped parameter spring–mass–damper model, but now consider forced vibration. While the oscillation decays over time for a damped system under free vibration, the vibratory motion is maintained at a constant magnitude and frequency when an external energy source (i.e., a forcing function) is present.

Model Development by Modal Analysis

In Chaps. 1–5, we assumed a model and then used that model to determine the system response in the time or frequency domain (or both). More often, however, we have an actual dynamic system and would like to build a model that we can use to represent its vibratory behavior in response to some external excitation. For example, in milling operations, the flexibility of the cutting tool–holder–spindle–machine structure (and sometimes the workpiece) determines the limiting axial depth of cut to avoid chatter, a self-excited vibration (Schmitz and Smith 2009). In this case, the dynamic response at the free end of the tool (and/or at the cutting location on the workpiece) is measured. Using this measured response, a model in the form of modal parameters can be developed for use in a time-domain simulation1 of the milling process. How can we work this “backward problem” of starting with a measurement and developing a model? To begin, we need to determine the modal mass, stiffness, and damping values from the measured frequency response function (FRF).

Single Degree of Freedom Free Vibration

For the discussions in this chapter, we will use what is referred to as a lumped parameter model to describe free vibration. The “lumped” designation means that the mass is concentrated at a single coordinate (degree of freedom) and it is supported by a massless spring and damper. Recall from Sect. 1.2.1 that free vibration means that the mass is disturbed from its equilibrium position and vibration occurs at the natural frequency, but a long-term external force is not present.

Uncertainity Analysis for the Fiducial Calibration System

Previous work has established the Fiducial Calibration System (FCS), a technique, which, for the first time provides a method that allows for the accuracy of a CMM to be transferred to the shop floor. This paper addresses the range of applicability of the FCS, and provides a method to answer two fundamental questions. First, given a set of machines and fiducials, how much improvement in precision of the finished part can be expected? And second, given a desired precision of the finished part, what machines and fiducials are required? The achievable improvement in precision using the FCS depends on a number of factors including, but not limited to: the type of fiducial, the probing system on the machine and CMM, the time required to make a measurement, and the frequency of measurement. In this paper, the sensitivity of the method to such items is evaluated through an uncertainty analysis, and examples are given indicating how this analysis can be used in a variety of cases.Copyright

The Effects of Structural and Servo Modes in Titanium Machining

This paper shows that in titanium machining the metal removal rate may be limited by factors including the dynamic characteristics of the frame of the machine and the servo. Self-excited vibrations related to these components led to poor cutting performance and tool breakage. Measurements of the acceleration were made during a number of cuts. In combination with impact tests, these measurements were used to identify the natural frequencies and mode shapes associated with the structural modes. These measurements ultimately led to adjustment of parameters in the control loop (to modify the servo dynamics), to special tool selection (to disturb the regeneration), and to the choice of stable cutting speeds (to take advantage of the stability lobes). The resulting cutting conditions significantly improved the metal removal rate. The nature of current titanium machining makes the structural modes particularly problematic, and it is important to measure and consider them during process planning.Copyright