John C. Ziegert

Fundamental Comparison of the Use of Serial and Parallel Kinematics for Machines Tools

Abstract Classical cartesian kinematics Machining Center (MC) structures are compared with parallel kinematics hexapods (HX) structures from the point of view of workspace, stiffness, accuracy, acceleration ability, and motion dynamics for use as high speed milling machines. Concrete stiffness values are used as achievable. It is concluded that variable strut length HX are fundamentally inferior to the MC and cannot practically be used as high speed milling machines. The constant strut length HX offer larger workspace and higher strut stiffness and may produce characteristics comparable to MC in particular designs.

The difficulty of measuring low friction : Uncertainty analysis for friction coefficient measurements

The experimental evaluation of friction coefficient is a common laboratory procedure; however, the corresponding measurement uncertainty is not widely discussed. This manuscript examines the experimental uncertainty associated with friction measurements by following the guidelines prescribed in international standards. The uncertainty contributors identified in this analysis include load cell calibration, load cell voltage measurement, and instrument geometry. A series of 20 tests, carried out under nominally identical conditions, was performed using a reciprocating pin-on-disk tribometer. A comparison between the experimental standard deviation and uncertainty analysis results is provided.

The laser ball bar: a new instrument for machine tool metrology

Abstract Current techniques for mapping the volumetric positioning errors of machine tools are time and labor intensive. To address these shortcomings, a linear displacement measuring device is introduced to rapidly and easily determine tool positions via trilateration. The laser ball bar (LBB) consists of a laser interferometer aligned within a telescoping ball bar. The design of the device is discussed and an error budget is developed to estimate its predicted accuracy. Results of repeatability and linear accuracy test of the assembled prototype LBB are given. The LBB is used to map the volumetric errors of a two-axis turning center. Comparison of the LBB error map and the error map obtained through parametric error measurements shows the LBB can accurately map the machine errors in a timely manner.

Wear-Rate Uncertainty Analysis

Wear due to relative motion between component surfaces is one of the primary modes of failure for many engineered systems. Unfortunately, it is difficult to accurately predict component life due to wear as reported wear rates generally exhibit large scatter. This paper analyzes a reciprocating tribometer in an attempt to understand the instrument-related sources of the scatter in measured wear rates. To accomplish this, an uncertainty analysis is completed for wear-rate testing of a commercially available virgin polytetrafluoroethylene pin on 347 stainless steel counterface. It is found that, for the conditions selected in this study, the variance in the experimental data can be traced primarily to the experimental apparatus and procedure. Namely, the principal uncertainty sources were found to be associated with the sample mass measurement and volume determination.

Examination of surface location error due to phasing of cutter vibrations

The purpose of this research is to investigate the relative importance of spindle speed, system dynamics, and cutting conditions on the accuracy of surface location in computer numerical-control (CNC) finish machining operations. The relationship between the spindle speed, the most flexible modes of the machine/cutting tool system and the final part dimensions is rather complex. The underlying theory, based on the situation of forced vibrations, is outlined. It is shown that the critical factor is the ratio of the tooth passing frequency to the system most flexible mode and corresponding natural frequency. Simple analytical calculations are carried out to illustrate the overcut/undercut surface error phenomenon. A simple simulation for end milling operations is also described which calculates the force on the cutter, the resulting cutter deflection, and the final error of surface. A comparison between the simulated and experimental results is presented. From experimental data, it shown that a change in surface location (and part dimension) of up to 50 μm is seen for a set of given conditions (i.e., cutter, material, chip load) simply by changing spindle speeds. Furthermore, it is seen that certain spindle speeds produce surfaces with no error introduced by the machining process.

Neural network thermal error compensation of a machining center

Abstract A neural network based on Artificial Resonance Theory (ART-map) was used to predict and compensate the tool point errors of a 3-axis machining center using discrete temperature readings from the machine’s structure as inputs. A combination of kinematic error modeling, curve fitting, and the neural network were used to maintain the machine’s three-dimensional (3-D) accuracy within ±7.4 μm, regardless of the thermal state. The network model was evaluated with diagonal measurements and part machining tests. A laser ball bar was used to take the necessary measurements for training the neural network.

Spindle thermal drift measurement using the laser ball bar

Thermally induced errors are major contributors to the overall accuracy of machine tools. An important component of thermally induced errors is the error associated with spindle thermal drifts. In this paper, a novel method is developed to measure spindle thermal drifts in machine tools using a laser ball bar (LBB) as the calibration instrument. The method is implemented on a two-axis CNC turning center. The LBB is used to measure the coordinates of the spindle center and the direction cosines of the spindle axis at various thermal states. The axial, radial, and tilt thermal drifts of the spindle are then computed from the changes in these coordinates. The new method is verified by comparing the spindle drifts measured with the LBB to those measured by capacitance gauges. The results obtained by the new method show good agreement with the capacitance gauge technique. The primary advantage of the new method is the ability to measure the spatial coordinates of the spindle center and direction cosines of the spindle axis with the same instrument used for measurement of the geometric errors of the machine axes.

Automated measurement and compensation of thermally induced error maps in machine tools

In this paper, a direct method of machine tool calibration is adopted to model and predict thermally induced errors in machine tools. This method uses a laser ball bar (LBB) as the calibration instrument and is implemented on a two-axis computerized numerical control turning center (CNC). Rather than individually measuring the parametric errors to build the error model of the machine, the total positioning errors at the cutting tool and spindle thermal drifts are rapidly measured using the LBB within the same experimental setup. Unlike conventional approaches, the spindle thermal drifts are derived from the true spindle position and orientation measured by the LBB. A neural network is used to build a machine model in an incremental fashion by correlating the measured errors with temperature gradients of the various heat sources during a regular thermal duty cycle. The machine model developed by the neural network is further tested using random thermal duty cycles. The performance of the system is also evaluated through cutting tests under various thermal conditions. A substantial improvement in the overall accuracy was obtained.

In Situ Lubrication with Boric Acid: Powder Delivery of an Environmentally Benign Solid Lubricant

In situ deposition of boric acid in dry powder form is investigated as a potential environmentally benign solid lubricant for sliding metal contacts. Boric acid is widely used in industrial processes and agriculture, is not classified as a pollutant by EPA, and produces no serious illnesses or carcinogenic effects from exposure to solutions or aerosols. In this study, boric acid powder is aerosolized and entrained in a low-velocity jet of nitrogen gas, which is directed at a self-mated 302 SS sliding contact in a rotating pin-on-disc tribometer. The effects of powder flow rate, sliding speed, normal load, and track diameter on coefficient of friction and wear rate are investigated. Friction coefficients below μ = 0.1 can be consistently reached and maintained as long as the powder flow continues. Wear rates are reduced over 2 orders of magnitude. Review led by Paul Bessette

Design of Honeycomb Meta-Materials for High Shear Flexure

A numerical study for a functional design of honeycomb meta-materials targeting flexible shear properties (about 6.5MPa effective shear modulus and 15% maximum effective shear strain) is conducted with two material selections — polycarbonate (PC) and mild-steel (MS), and five honeycomb configurations. Cell wall thicknesses are found for each material to reach the target shear modulus for available cell heights with five honeycomb configurations. PC honeycomb structures can be tailored with 0.4 to 1.3mm cell wall thicknesses to attain the 6.5MPa shear modulus. MS honeycombs can be built with 0.2mm or lower wall thicknesses to reach the target shear modulus. Sensitivity of wall thickness on effective properties may be a hurdle to overcome when designing metallic honeycombs. The sensitivity appears to be more significant with an increased number of unit cells in the vertical direction. PC auxetic honeycombs having 0.4 to 1.9 mm cell wall thicknesses show 15% maximum effective shear strain without local cell damage. Auxetic honeycombs having negative Poisson’s ratio show lower effective shear moduli and higher maximum effective shear strains than the regular counterparts, implying that auxetic honeycombs are candidate geometries for a shear flexure design.Copyright