3D printing offers the opportunity to design and make replacement parts to exacting specifications when needed. This is particularly helpful for space applications where stand-alone replacement mechanisms are required. Samples of 3D-printed polylactic acid (PLA) were subjected with up to 200 kGy of gamma radiation from a Cobalt-60 irradiator. The mechanical responses to destructive testing were successfully modeled with a combination of linear and exponential functions and may be understood given the underlying chemical changes due to said radiation exposures. We find that for doses up to 50 kGy, the performance of 3D-printed PLA is largely unaffected, which is beneficial for applications in space and in medicine. At larger doses, it appears that decomposition processes win out over cross-linking, which may aid in the degradation of PLA in waste streams.

This paper presents a new method for stability analysis of the variable spindle speed face milling process whose dynamics are described by a set of differential-difference equations with periodic coefficients and time varying time delay. Fourier analysis and Floquet theory applied to the system equations result in a characteristic equation of infinite order with constant coefficients. Its truncated version is used to determine the limit of stability by employing standard techniques of control theory. Analytically predicted stability boundaries are compared with lobes generated by time domain simulations. Experimental results are also presented that validate the proposed analytical method for chatter stability analysis. Finally, an example is presented that demonstrates the advantage of using spindle speed variation when machining a workpiece having multiple modes of vibration.

Plunge-turning processes of round-edged cutters is analyzed and its cutting force modeled in the light of a general theory of non-free cutting developed by the authors. In this study the whole cutting tool is treated as a combination of a series of Elementary Cutting Tools (ECTs). Due to the non-linearity of chip-ejection interference among all the ECTs the total cutting force of the whole cutter, however, cannot be calculated by simply superposing the incremental cutting forces generated by all the ECTs. A Non-Linear Synthesis Method (NLSM) is therefore suggested for modeling this non-free cutting force. The main feature of the method is that the chip-ejection interference is under consideration and modeled on the basis of the Principle of Minimum Energy (PME). Good agreement between the predicted and measured main cutting forces is identified. Furthermore, a Coefficient of Non-Free Cutting (CNFC) is applied to quantitatively indicate the strength of chip-ejection interference among the ECTs and the degree of freedom confinement of the cutting process.

This paper considers chatter in diamond turning of aluminum. A nonlinear chip area model is developed for use with the classic single degree of freedom cutting model. The chip area model is formulated as a function of the change in depth of cut between consecutive passes. This function is shown to be highly dependent on the quadratic term when turning the diameter of cylindrical workpieces with round-nosed diamond tools. The cutting model is further extended to include an impact disturbance to the cutting process. The recent research in ultra-precision machining is incorporated to properly account for the rise in specific cutting energy at extremely light depths of cut. Simulations of the resulting tool displacement show close agreement with experimental measurements.

This paper presents the basis for optimizing the design of a three degrees-of-freedom (DOF) variable reluctance (VR) spherical motor which offers some attractive features by combining pitch, roll, and yaw motion in a single joint. The spherical wrist motor offers a major performance advantage in trajectory planning and control as compared to the popular three-consecutive-rotational joint wrist. Since an improved performance estimate is required, a method for optimizing the VR spherical motor’s magnetics was developed. This paper begins with a presentation of the geometrical independent and dependent variables which fully described the design of a VR spherical motor. These variables are derived from examination of the torque prediction model. Next, a complete set of constraint equations governing geometry, thermal limitations, amplifier specifications, iron saturation, and leakage flux are derived. Finally, an example problem is presented where the motor’s geometry is determined by maximizing the output torque at one rotor position. The concept of developing a spherical motor with uniform torque characteristics is discussed with respect to the optimization methodology. It is expected that the resulting analysis will improve the analytical torque prediction model by the inclusion of constraint equations, aid in developing future VR spherical motor designs, improve estimates of performance, and therefore will offer better insight into potential applications.

The calibration scheme of robot forward kinematics presented in this paper has a number of features. Firstly, robot kinematic errors are modeled in a recursive format and as such, the number of measurements that need to be taken for calibration can be determined by studying the rate of convergence of estimation error covariance. Secondly, a simplified adaptive filtering algorithm is used to deal with unknown measurement noise statistics and unknown robot motion repeatability characteristics in estimating the kinematic errors. Thirdly, a laser interferometry system is used to measure positions of a robot end-effector in world coordinates. The measurement system was implemented in experiments involving a three degree-of-freedom gantry robot. The adaptive filtering of the experimental data identified 0.5 to 1.5 percent errors in representative kinematic parameters of the given robot by taking into account measurement noise and robot repeatability.

This paper presents a new technique for digitizing an unknown sculptured surface using a new multi-Laser Displacement Sensor (LDS) and an articulated Coordinate Measuring Machine (CMM). The new multi-LDS measures the tangents, curvatures, and displacement of the surface. This local information is utilized in the technique to extrapolate the surface position and normal vector at the next location to be digitized. In addition, this paper derives the relationship between the digitizing error parameters and the ratio of sampling arc length to the radius of maximum curvature. This technique is illustrated experimentally using a six degree-of-freedom PUMA robot and a Keyence LDS.

The quality of parts produced by a composite tape-laying process is sensitive to the contact force exerted and the abutment of adjacent tape courses kept when the tapes are laid. Thus, a prerequisite for a high performance laying process automated by means of industrial manipulators is the ability to regulate laying force and position precisely in the presence of robot motion inaccuracies and laying contour variations. An on-line adaptive force/position controller for robot-automated laying processes was presented. The contact force error normal to a laying surface is measured by a wrist sensor in real time, while the position error tangent to the surface is measured by sensors found in manipulator joints. Both are modeled as autoregressive (AR) processes, on the basis of which future errors in Cartesian coordinates are forecasted. Via a compliance selection matrix, both predicted errors are fed to joint actuators in a similar manner for taking compensatory actions. In view of the similarity, the force control portion of the scheme was implemented in an experiment using a three degree-of-freedom manipulator and the results indicated a 40 percent average reduction in contact force variations.

The stability of periodic cutting operations, the dynamics of which are described by linear differential-difference equations with periodic coefficients, is studied. A new stability theory that uses parametric transfer functions and Fourier analysis to obtain the characteristic equation of such systems is experimentally verified. The theory is applied to single-point turning of a compliant work piece with two degrees-of-freedom. The theoretical predictions of both the critical depth of cut for chatter-free turning and the corresponding chatter frequency were found to be in good agreement with the measurements obtained from actual chatter tests under various surface speeds.

The implications of a fundamental asymmetry in the lathe turning process are considered. The asymmetry is the directional dependence of system stiffness that exists at the cutting tool edge. Depending on tool edge displacement, the system stiffness is determined by the workpiece stiffness or the relatively much stiffer tool support structure. A one degree-of-freedom, lumped parameter model with nonlinear stiffness is constructed and subjected to a periodic forcing. Time and frequency domain analyses show that extremely complex dynamic behavior can occur in this simple system.

The dynamics of a cutting process are very complex in nature. It involves not only the changes of plastic state in the intensive deformation zone but also the elastic behavior of work material surrounding the deformation zone, especially in the vicinity of the tool nose region. These changes are induced by the inner and outer modulations of the uncut chip thickness during the process and at the same time govern the variation of the cutting force. Based on these causal relationships, the transfer function between the vibration variables and the dynamic force components for a single degree-of-freedom machining system has been developed. The characterization of the mechanics of the cutting process by the new model provides more insight into the physics of the cutting dynamics. The model has been tested through computer simulation for both orthogonal wave-generating and wave-removing processes. By reference to existing experimental evidence, the theoretical predictions show a very good agreement with the test results.

Two types of sheet forming processes are analyzed by rigid-viscoplastic FEM (Finite Element Method): axisymmetric punch stretching and hydrostatic bulge forming. The present formulations, based on the membrane theory and the Hill’s anisotropic flow rule, include the rate sensitivity which is a key factor in controlling the forming of superplastic materials. Normal anisotropy is taken into account and Coulomb friction is assumed at the interface between punch and sheet. Nonsteady-state deformation processes, investigated in this study, were quasi-statically and incrementally analyzed. An FEM code was developed, using two-node linear elements with two degrees of freedom at each node, and applied to solve four categories of problems: (1) A.K. steel punch stretching, (2) hydrostatic bulging of a rate-insensitive material, (3) hydrostatic bulging of rate-sensitive materials, and (4) hydrostatic bulging of a superplastic material (Ti-6-4). Strain distributions and shape changes predicted in the first two problems were compared with experiments and results of other analyses. The results of the third problem could not be compared with experiments; however, the results showed that the rate sensitivity affects the deformation as expected. The fourth problem is the main theme of this paper. To maintain the superplasticity in forming processes and to produce sound products, the control of the strain-rate is a key factor. A hydrostatic bulge forming process, which is often used for manufacturing structural aerospace parts, was analyzed and discussed. Further, an optimum pressure curve (pressure versus time), which maintains the desired strain-rate in the deformed material, was obtained and compared with the results of an analytical prediction, available in the literature.

Optimization procedures to install a viscoelastic dynamic damper into a single degree of freedom primary system is briefly reviewed. Excitation methods are shown to identify elastic modulus and loss factor of a viscoelastic material at given prestrain, which are needed in the optimum design of the damper. An optimum-designed damper is attached on the toolpost of a lathe and its excellent chatter-suppressing effects are observed under six cutting conditions in terms of integrated power of the accelerations around the chatter frequency. Because one of the resonance frequencies responsible for the chatter varies depending upon the location of the carriage on the sliding surface, the prestrain of the viscoelastic element, which is initially optimum-tuned and damped at a location of the carriage, is readjusted for optimum tuning at the other locations. The effects of the readjustment are discussed in terms of the reduction of structural compliances and magnitudes of chatter vibrations.

The paper addresses the problem of end effector trajectory planning and control of seven degrees of freedom (DOF) kinematically redundant robots. An off-line optimal continuous path planning method is developed for on-line control at joint level. The specified end effector path is approximated by a set of location nodes selected on the desired path. The motion control of the robot is provided by cubic polynomial interpolation at joint level. The proposed approach to trajectory planning of kinematically redundant robots consists of obtaining an optimal set of nodes which guarantees minimum deviation from the desired Cartesian path. The redundant DOF of the robot is used as a constrained variable in the optimization search. The method is illustrated in an example.

Control of a kinematically redundant robot arm requires an optimization procedure to determine the motion of the end effector. The criterion for optimization can be minimum motion time, minimum joint displacement increments or a combined merit function specified according to the requirements of the user. Three different methods may be used to perform the computations and obtain the joint coordinate increments for the point-to-point motion control of the robot. The methods are the “direct,” the “pseudoinverse” and the “generalized inverse” methods. These methods are described in detail in this paper, and results obtained with the three methods are compared on the basis of performing simulated tasks. It is concluded that the generalized inverse method is the most suitable, general method for point-to-point control of robots with more than six degrees-of-freedom.

A robotic wrist unit has been constructed and is functioning on a large industrial robot used to load, assemble, and finish parts in an automated metal working cell. The wrist uses force feedback and remote-center-compliance techniques similar to those that have been developed for assembly robots. The wrist employs four spherical hydraulic actuators that behave as adjustable springs with a stiffness that varies between 5.8 × 10 3 and 7.9 × 10 3 N/m. This allows the center of compliance to be projected over a range from 25 to 157 mm out from the upper platform of the wrist. The wrist has 5 1/2 degrees-of-freedom, being compliant in each direction except axial extension. Deflections of ± 4.6 mm in the radial plane and 5.1 mm in compression are possible. The accuracy of the sensors over this working range is within 0.025 mm for translations and within 0.001 rad for rotations.

A unique methodology is proposed to identify the joint structural parameters of the machine tool by combining the Dynamic Data System (DDS) methodology with the Finite Element Method (FEM). The structural parameter identification of a simple system with complete modal matrix is introduced together with the modal analysis by the DDS method. The simulation study of a two degree-of-freedom system shows the high accuracy of the identification procedure. The identification of joint parameters of a general system with an incomplete mode shape is analyzed next. The condensation and digital filtering techniques are developed for this purpose. In a shaft model experiment the joint parameters are identified when the response data at boundary points are difficult to be measured. Furthermore, a column model experiment revealed the ability of the proposed identification technique to deal with a large number degree-of-freedom system.

An analytical model of a locomotive due to S. Levy is modified to accept random track input and used as the basis of a locomotive study with 63 degrees of freedom. This model differs from precursors in that it is faster, it accepts nonlinear suspension components and wheel/rail interactions, it includes routines so that track irregulatiries may be incorporated by means of vertical and lateral PSD curves, it accounts for adhesion limits, checks derailment conditions, and provides time histories of generalized variables. With this code, a specific locomotive model, similar to the General Electric Company’s U33-C locomotive, is evaluated to ascertain what changes in either the model or the track parameters will permit higher safe speeds. This parametric study addresses speeds in the range of 70 to 280 ft/sec. (21.34 to 85.34 m/sec), three classes of track, 4, 5, and 6, primary suspension stiffnesses from 18 to 300 klbs/ft. (.263 to 4.38 MN/m), wheel flange clearances from .05 to .75 inches (1.27 to 19.1 mm) and the effect of vertical and lateral rail stiffnesses in the ranges of .5 to 8 and .25 to 4 Mlbs/ft. (7.3 to 117 and 3.65 to 58.4 MN/m), respectively. The results generally agree well with current standards governing safe running speeds on various class tracks. They indicate that at speeds equal to accepted safe operating speeds on class 4, 5, and 6 tracks, the rms values of the vertical forces at the contact of the first wheel and rail do not increase beyond 25% of the nominal vertical load at this point. Under these same conditions, maximum wheel-rail separations, if they occur, are for each case less than the wheel flange height assumed one inch (25.4 mm); for class 6 track, no separations larger than the flange height occur even at 280 ft/sec. (85.34 m/sec.).

The hunting stability of the 3-axle locomotive truck is evaluated, using a linear eleven degree of freedom model. Predictions from the model are validated with test data, and the effects of wheel profile and secondary suspension damping on hunting stability are shown.

This work deals with the efficiency of energy transfer in impact forming machines due to the interaction of the process with machine structure as represented by a two degree-of-freedom (dof) system. The forming process is simulated by a rectangular force pulse as a function of time [1]. The effects of various parameters of the structure and also of the pulse on the energy transfer have been explored. It is found that the shock absorbers, commonly used for isolating the vibrations originating from such machines, restrict the attainment of 100 percent forming efficiency. Also, there exists a pulse-insensitive zone in the efficiency variation in which efficiency is independent of the forming operation and that the design of the machine should aim to achieve it.