P. V. M. Rao

Experimental investigations for improving part strength in selective laser sintering

Selective laser sintering (SLS) is a powder-based rapid prototyping process in which parts are built by sintering of selected areas of layers of powder using laser. Nowadays, SLS is emerging as a rapid manufacturing technique, which produces functional parts in small batches, particularly in aerospace application and rapid tooling. Therefore, SLS prototypes should have sufficient strength to satisfy functional requirements. Apart from the energy density which is the combination of laser power, beam speed and hatch spacing, various other parameters like refresh rate, layer thickness and hatch pattern influence part strength. In the present work, relationship between strength and the various process parameters namely layer thickness, refresh rate, part bed temperature and hatch pattern have been investigated. Experiments are conducted based on Taguchi method using L16 modified orthogonal array. Tensile specimens of polyamide (PA2200) material as per the standard ‘ASTM D638’ are fabricated on SLS machine with constant energy density and tested on a universal testing machine for tensile strength. Optimum strength conditions are obtained by maximising signal to noise (S/N) ratio and analysis of variance (ANOVA) is used to understand the significance of process variables affecting part strength. A regression model to predict part strength has been developed. Confirmation test conducted subsequently has revealed that the results are within the confidence interval.

Data and knowledge modeling for design–process planning integration of sheet metal components

Realizing design–process planning integration is vital to the competitiveness of manufacturing organization and its ability to respond rapidly to market changes. Many attempts have been made in the past proposing the integration of the two activities based on product data models. However, both design and process planning activities are knowledge intensive. An effective integration is possible only if both data and knowledge models form a basis for integration. This paper presents key issues related to data and knowledge modeling for integration of design (CAD) and process planning (CAPP) activities for sheet metal components. Previous attempts to model data and knowledge have concentrated only on either design or process planning and not from an integration point of view. Moreover, in these attempts data and knowledge models have been proposed without attempting to relate the two. The same has been overcome in the present work. An integration framework based on data and knowledge is proposed at the end and discussed for domain of design–process planning integration of sheet metal components.

Automated manufacturability evaluation system for sheet metal components in mass production

Automated manufacturability evaluation of a given design is a key requirement in realizing complete integration of design and process planning. It would still be better to control the designing process itself with manufacturability information. The purpose of such an evaluation is to assist designers in their effort to come up with manufacturable parts economizing in terms of cost and time, without compromising on quality and functional requirements. The present work deals with one such system developed for manufacturability evaluation of sheet metal components. Unlike most of the work done in the sheet metal area in the past, which concentrates on specific domain or phase of manufacturability evaluation, the present work is more comprehensive combining characteristics of all the existing methods and phases of manufacturability evaluation. The prime components of the present system are design evaluation and process plan generation. Design evaluator and process planner use different types of data and knowledge to identify design and process planning violations which are overcome by suggesting design changes. A process planner also uses manufacturing resource and process information to arrive at manufacturable parts by generating feasible process plans. Results of manufacturability evaluation are presented for typical sheet metal parts to be produced by bending and shearing (blanking and piercing) processes.

New model for shrinkage compensation in selective laser sintering

This paper presents a new model for shrinkage and a new approach for shrinkage compensation to enhance the accuracy of parts produced by selective laser sintering (SLS)–a solid freeform fabrication process. The present prevailing approach as proposed by machine manufacturers is simple but not accurate. A new shrinkage model which accounts for part geometry as well as beam offset is proposed in this work. A new compensation scheme which accounts for nonlinear shrinkage is proposed, implemented and validated. The proposed compensation scheme compensates for shrinkage at every layer and at every hatch length, unlike a uniform compensation scheme applied to entire part. A new algorithm which accounts for this is developed and implemented. Experiments carried out with the new shrinkage model as well as with the new compensation scheme have shown significant improvement in the accuracy of the parts produced which establishes the effectiveness of the proposed methodology.

System for early cost estimation of die-cast parts

Early cost estimate of a part is important information and forms a basis for preparing quotations, which are competitive from a market point of view. It is seen that a commonly adopted approach of variant cost estimation based only on geometric information of the component is not always accurate. This is also true in the case of die-cast parts. The geometric complexity of the part, tooling complexity, part and tool material, processing cost, and manufacturing resources for producing the part all need to be considered for accurate cost estimation. This paper deals with a comprehensive system developed to estimate and analyze the manufacturing cost of die-cast parts. A computer-aided cost-estimation system has been developed that applies manufacturing process as well as manufacturing resource considerations in addition to part feature complexity. Use of the proposed system is demonstrated in selecting the optimum number of cavities and the appropriate manufacturing resources under machine-related constraints. Further, the cost-estimation system developed herein is used for carrying out feature sensitivity analysis to identify design features that add significant cost to the part. The use of this system for optimal machine loading in multiple parts situation is also demonstrated.

On milling of thin-walled tubular geometries

Abstract Machining of thin-walled tubular geometries poses interesting problems from a process planning perspective. In machining of such geometries by milling, cutting force-induced tool and workpiece deflections have to be overcome in order to realize part accuracies without compromising productivity. This calls for a systematic study of surface errors on machined parts due to both tool and workpiece deflections. The present paper investigates the effect of cutter and workpiece flexibilities on surface error during peripheral milling of thin-walled tubular geometries. Unlike previous attempts to study milling of thin-walled straight geometries, the present work focuses mainly on machining of tubular geometries. Tubular geometries need to be treated differently from a process planning perspective by exploiting the workpiece rigidity they offer. The process planner has an option of synclastic and anticlastic machining possibilities during machining of tubular geometries which needs to be explored and understood. More importantly, the complex and varying profile of surface error with cutting conditions has to be accounted for. From the outcomes of the present work it can be summarized that surface errors in machining of closed tubular geometries are due to multiple factors which include workpiece rigidity, tool overhang, curvature effects, thinning effects, and nature of tool and workpiece engagements. All of these parameters are affected by process parameters chosen during machining. The present study also demonstrates that the understanding of surface error profiles due to cutter and workpiece deflections not only helps in realizing dimensional accuracy, but geometric tolerances as well.

Die-Casting Feature Recognition for Automated Parting Direction and Parting Line Determination

Determining parting direction and parting line for die-cast parts is a nontrivial task that not only depends upon shape and topology of the part, but also on many process related factors. Normally, a die-casting expert decides parting direction and parting line, intuitively taking into account a large number of factors, and this process can be time consuming and cumbersome in many cases. This study addresses automated determination of parting direction and parting line for a die-cast part from part CAD model. The proposed methodology takes STEP file of the part as input for extracting die-casting features, which consists of protrusion or depression regions of the part. These features are classified into those with single, double, or multiple withdrawal directions. Geometric reasoning is used for feature recognition, which includes nested and interacting features. Global visibility instead of local visibility is used for planning withdrawal direction, which makes the decision arrived by present system closer to industrial practice. Parting line is determined based on selected candidate parting direction considering process constraints and priorities. The contribution of this paper is in terms of development of an automated parting direction and parting line determination system, which is more comprehensive and overcomes limitations of the previous work. Results of this system have been validated with those arrived at by experts from the die-casting industry.

A flexible surface tooling for sheet-forming processes: conceptual studies and numerical simulation

All sheet-forming processes suffer from a limitation that tooling in the form of molds/dies is a precursor for realizing any simple or complex shape. In order to overcome this limitation, discrete surface tooling has been proposed in the past. This consists of a large number of closely spaced surface tool elements arranged in a matrix form whose heights can be adjusted to approximate the contour of desired surface shapes. Such a tooling has been successfully demonstrated experimentally. In this paper, a variation of discrete surface tooling, called as flexible surface tooling, has been proposed. The proposed tooling consists of discrete surface tool elements draped with flexible sheet of rubber-like material to provide a continuous surface required for tooling applications. Computational simulation of the proposed tooling is carried out for feasibility studies. Computational simulation involves the deformation analysis of rubber-like membranes in multiple contact by FEM. The results of the analysis indicate that the flexible surface tooling is an option for sheet-forming processes in general and for processes like composite layup in particular.

Statistical modeling and minimization of form error in SLS prototyping

Purpose – The purpose of this paper is to report experimental investigations performed to analyze the effect of process parameters on the shape accuracy of selective laser sintered (SLS) parts.Design/methodology/approach – The effect of process parameters, namely build orientation, laser power, scan speed, cylinder diameter and build chamber temperature has been studied on shape accuracy by using geometric tolerances such as cylindricity and flatness. Central composite design (CCD) is used to plan the experiments and a second order regression model has been developed to predict flatness and cylindricity. The significance of process variables on flatness and cylindricity has been evaluated using analysis of variance technique.Findings – It is observed that interaction effects are more dominant than individual effects. In case of cylindricity, it is found that the interaction between the scan speed and orientation is the dominant factor next to the orientation and quadratic effect of the geometry. In case o...

Computer Aided Manufacturability Analysis of Die-cast Parts

AbstractAutomated manufacturability analysis is an important tool with the designer which is meant to incorporate manufacturability aspects at initial stages of design. This paper deals with a system developed for automated manufacturability analysis for die-cast parts. Purpose of this system is to assist designers in their effort to come up with manufacturable die-cast parts economizing in terms of cost and time without compromising with quality and functional requirements. Unlike most of the work done in the past, which concentrates on manufacturability assessment of a part to be machined, the present work deals with die-casting process. This system uses geometric reasoning to extract manufacturing features and other information from a part CAD model. It uses a knowledge base consisting of die-casting process knowledge and rules, to present a manufacturability advice to the designer. Use of the proposed system is demonstrated for the manufacturability assessment of typical die-cast parts.