On the Machinability of Powder Metallurgy Austenitic Stainless Steels

Powder Metallurgy (P/M) materials, especially those made of high strength steels, are often reported in the technical literature to have poor machinability when compared to their wrought or cast counterparts. In order to characterize the machinability of single phase P/M materials and to identify the influence of porosity on that behavior, the machinability of P/M 304L austenitic stainless steel was evaluated as a function of porosity, in the range of 64 to 90 percent of theoretical density. Machinability was defined in terms of the average drill point temperature. It was found that the drill temperature increased with porosity to a point. Further increases in porosity produced decreasing levels of average drill point temperature. The nonlinear machinability response was attributed to the offsetting contributions of the thermal conductivity, the work-hardening, and the bulk properties of the P/M material.

Sequential Estimation of Machinability Parameters for Adaptive Optimization of Machinability Data Base Systems

Mathematical model type machinability data base systems require suitable model building procedures to estimate the model parameters. The estimation procedure should be capable of using subjective prior information about the models and must also be capable of adapting the model parameters to the particular machining environment for which the data are needed. In this paper, the sequential Maximum A Posteriori (MAP) estimation procedure is proposed as the mathematical tool for performing these functions. Mathematical details of this estimation procedure are presented. The advantages of this method over conventional regression analysis are discussed based on the analysis of an experimental tool life data set. Details regarding the selection of the various initial values needed for starting the sequential procedure are presented. The use of prior information about the models in order to improve the parameter estimates is investigated. The adaptive capability of the procedure is analyzed using simulated tool life data. The results of this analysis indicate that the proposed sequential estimation procedure is a valuable tool for estimating machinability parameters and for the adaptive optimization of machinability data base systems.

A Stochastic Approach to the Measurement and Analysis of Leadscrew Drive Kinematic Errors

A practical way to achieve machine tool drive system accuracy is through compensation in the position control loop. Various strategies have been developed in which the compensation data are stored either in a computer or by alternative techniques. The approach described in this paper demonstrates how the dominant causes of the inaccuracies in machine tool drive systems can be determined in a simple and straightforward manner and how the compensation data can be generated using a microprocessor-based calibration technique. The proposed method was demonstrated on a Giddings & Lewis Model 10V Numericenter and consists of three major steps: 1) accuracy measurement, 2) error-map representation, and 3) error-map analysis and error cause diagnosis. Accuracy measurement was accomplished by reading both the resolver counts and the output of a laser interferometer which served as a reference (master) scale. The procedure is based on a strategy in which small 0.5 mm (0.020 in.), highly accurate ±2.5 μm ( ± 0.0001 in.) linear displacements of the machine tool table motion are taken as the measurement reference. An ARMA (Autoregressive Moving Average) model of order (16, 15) is the statistically adequate model that fit the data of the angular displacement of the resolver. The Dynamic Data System technique was used for this purpose and it can also be used to represent the error-map function. A spectral analysis of the ARMA model revealed the sources of the kinematic errors.

An Investigation of Computer Control of Machining Chatter

Based on stochastic process modeling, a scheme has been developed for the detection and control of machining chatter. The range of the tool vibration signal is computed by a hybrid computer and compared with permissible limits to exercise automatic change of the speed and feed rate. The control scheme was evaluated for its adaptability and effectiveness by forcing a chatter condition and subjecting the process to computer control. The scheme is sensitive to process variables.

Analysis and Design of a Drill Grinder and Evaluation of Grinding Parameters

A prototype drill grinder was designed and built based on a computer aided drill point geometry analysis. The new grinder controls all essential drill point grinding parameters. The new grinder was evaluated by grinding drills, measuring their point geometry parameters, and comparing these measurements with their expected values. The effects of five parameters, consisting of three grinding and two cutting condition parameters, on the drill thrust and torque are determined by an experiment using a two-level factorial design.

Analysis of the Chisel Edge and the Effect of the d-Theta Relationship on Drill Point Geometry

Following the investigation of drill point geometry previously reported [1, 2], the chisel edge profile and the chisel edge angle are analyzed mathematically. The relationship between the cone semiangle, θ, and the x-coordinate of the cone vertex, −d, is derived for a given chisel edge angle and nominal relief angle at the outer corner. This relationship is used to study the effects of grinding cone parameters on drill point geometry. The analysis indicates the existence of an optimum θ value for best drill performance.

Drill Temperature Distributions by Numerical Solutions

The backward finite-difference method is used to determine three-dimensional drill temperature distributions. The geometry of the drill was described by (1) approximating the drill as a one-quarter cone and (2) sectioning a true drill point and measuring its profiles. The three-dimensional temperature distributions provided both drill cutting edge and drill flank temperature profiles which were close to prior experimental data and showed improvement over the previous analytical solutions.

An Analysis of Drill Geometry for Optimum Drill Design by Computer. Part I—Drill Geometry Analysis

A comprehensive analysis of the twist drill point geometry is made in order that the high-speed digital computer can be used as an aid in the design of a drill. This subject is treated in two parts. In Part I, the drill geometry is analyzed with respect to the drill flute and flank contours by considering cross sections of the drill cut by planes perpendicular to its axis. Since several important drill angles are defined in planes inclined to the drill axis, the analysis is extended to cover the general case where the drill is cut by any plane inclined to its axis.

Evaluation of the Effects of Design Variables on Drill Temperature Responses

The effect of five design and three operating variables on three different drill temperature responses is investigated. A total of 512 observations was made using a replicated two-level factorial design. Problems associated with the selection of the temperature responses are studied including the transient nature of the response, the location of the thermocouple, and the interrelationship of the workpiece volume and drill wear. The five drill design variables are web thickness at point, margin width, relative lip height, helix angle, and surface condition, of which the helix angle and web thickness at point had the most significance within the experimental range. The effect of two-factor interactions was also discussed.

An Analysis of Drill Geometry for Optimum Drill Design by Computer. Part II—Computer-Aided Design

In Part II, a computer program that aids in the design of a twist drill is described, based on the analysis presented in Part I. A true drill and a “computer-designed” drill having identical design and grinding parameters are compared in orthogonal cutting planes. The effects of the design parameters on drill geometry are investigated utilizing the computer program.

Temperature Distributions in Drilling

An analytical temperature distribution along the cutting edge and on the flank face of a drill is obtained using Tsueda’s and Loewen and Shaw’s equations after modifications. Experimental temperature distributions are also investigated and the effects of a pilot hole and the heat sink are evaluated. Agreement between the analytical and experimental results is fairly close except near the chisel edge and near the drill periphery. The use of workpieces containing a pilot hole provides a method to account for the discrepancy between the analytical solution and the experimental results near the chisel edge while some explanation is offered to describe the discrepancy near the drill periphery.