Esko Niemi

Determination of material model parameters from orthogonal cutting experiments

Flow stress models in finite element analysis of metal cutting require material parameters that are essential considering the accuracy of the simulations. This article presents a method to acquire material parameters from cutting experiments using the extended Oxley’s shear zone theory. The novelty in this approach is to use measured chip geometry and temperature instead of determining them analytically to calculate strain and strain rate. These values are used to calculate the resultant cutting forces with the extended Oxley’s model and Johnson–Cook flow stress model. Flow stress model parameters are optimized to fit the calculated forces to those measured from cutting experiments. The Johnson–Cook parameters acquired with this method perform better than those found in the literature.

Optimal workforce allocation for assembly lines for highly customised low-volume products

Traditionally, the assembly of large make-to-order products has been organised as assembly cells, where the mainly manual assembly is performed by a group of workers. This is a flexible arrangement and thus suitable for variable products. An assembly line facilitates worker specialisation, work standardisation, low investment in equipment, and other benefits. However, line balancing in variable production is difficult. The use of a flexible work force balances the line, but lowers line efficiency. The objective of the present study is to develop a simple and efficient workforce allocation policy in an environment where variable products are assembled by teams of workers on a relatively short line. Since the work is mainly manual, the efficiency of worker utilisation is critical. The research method is empirical experimentation using discrete event simulation. The results show that the proposed use of an expert team of moving workers can be used to recover some of the efficiency lost as a result of processing time variability.

Part allocation to nesting layouts under variable demand

The problem formulation presented here deals with the choice of parts for nesting layouts when cutting arbitrary sheet metal parts under variable demand. The parts are chosen based on their demand characteristics and size, but their actual positioning on a larger sheet is not discussed. Guidelines are created for a nesting strategy that minimizes the needs for part stocking and renesting while still meeting the production requirements. A new, practical and robust method of determining the approximate numbers of parts of the different types to the nesting layouts is proposed. The performance measure used to compare the nesting strategies is the work in process, expressed as the average percentage of the required production. According to this method, at least the same number of nesting layouts is created as that of the different part types. The use of the method ensures that the work in process stays low, even if the demand differs considerably from the forecast on which the nesting was originally based. Numerical experiments were made with several random and deterministic parts, nesting layouts and production programmes. In the experiments, the nesting layout selection problem was solved for several successive planning periods and the effect of various nesting strategies, and the characteristics of the demand and the part types were studied.

Investigation of the effect of different cutting parameters on chip formation of low-lead brass with experiments and simulations

Poor chip breakage causes problems in machining of low-lead brass. To improve chip breakage, finite element model simulations were implemented in cutting tool design. Finite element model simulations enable high number of experiments that would be expensive and slow to perform by conventional cutting tests. Compression tests and cutting experiments under different temperatures and strain rates were performed for lead-free brass, to acquire material parameters for the finite element model. It was observed that the coupled effect of thermal softening and rate sensitivity of the material was difficult to take into account with the existing material model. Furthermore, it was found that there are no reported material models that can take rate sensitivity–temperature coupling into account. This was counteracted by fitting the model with least square method to the stress–strain data at the cutting temperature, although this causes error in simulations with temperatures higher or lower than the supposed cutting temperature. Nevertheless, the simulated results proved accurate enough to model the chip breakage. Based on the simulations and experiments, the use of a positive rake angle, high cutting speed and low cutting feed rate improve chip breakage from continuous chip to a chip of average length of 4 mm.

Modified Johnson-Cook flow stress model with thermal softening damping for finite element modeling of cutting

Results of materials testing for lead-free brass show that the effect of thermal softening decreases significantly when the strain rate is high. This behavior is referred to as thermal softening damping. In this article, a flow stress model with thermal softening damping based on the Johnson–Cook flow stress model was developed. Finite element simulations with the proposed model are compared to cutting experiments to estimate the effect of damping in metal cutting.

Lean manufacturing methods in simulation literature: review and association analysis

The lean manufacturing philosophy includes several methods that aim to remove waste from production. This paper studies lean manufacturing methods and how simulation is used to consider them. In order to do this, it reviews papers that study simulation together with lean methods. The papers that are reviewed are categorized according to the lean methods used and result types obtained. Analysis is performed in order to gain knowledge about the volumes of occurrence of different methods and result types. Typical methods in the papers are different types of value stream mapping and work-in-process models. An exploratory analysis is performed to reveal the relationships between the methods and result types. This is done using association analysis. It reveals the methods that are commonly studied together in the literature. The paper also lists research areas that are not considered in the literature. These areas are often related to the analysis of variation.

Worker coordination policies in parallel station systems: performance models for a set of jobs and for continuous arrival of jobs

Varying workloads and uncertain processing times in parallel assembly cause idle times for skilled, high-cost workers. This idleness can be avoided and the utilisation of the workers improved by allowing workers to move between the stations to help each other. Worker movement between assembly stations needs efficient and feasible coordination, and therefore, this paper compares four different worker coordination policies: no helping, floater, pairs and complete helping. The dynamics of the policies are modelled by studying the parallel assembly as a continuous-time Markov process. The system is studied with two different job release cases for non-identical jobs (customised products). In the first case, a given number of jobs have to be completed by the entire system. In the second case, new jobs arrive with a Poisson-distributed rate. The models assume that when one worker helps another, their collaborative inefficiency reduces the productivity. The models are used in numerical experiments to compare the performances of worker coordination policies as average job cycle times. The main conclusions from the results suggest the use of the complete helping policy in minor collaborative inefficiency conditions, especially with a given set of jobs. The pairs policy is a reasonable alternative in major inefficiency conditions with the continuous arrival of jobs.

A comparison of rescheduling policies for online flow shops to minimize tardiness

In practical situations, flow shops usually have some policies on rescheduling previously scheduled jobs. This article compares three of these rescheduling policies: an unrestricted one where previously scheduled jobs can be moved freely, one where jobs can only be moved forward in the schedule, and one where jobs that have already been scheduled cannot be moved at all. The comparison is performed by considering the minimization of tardiness. While unrestricted rescheduling should generally give the best solution, moving jobs only forward can be more practical as in general production, material orders can be delayed but seldom advanced. This article points out that moving jobs only forward is not significantly worse than the unrestricted scheduling. When cases with small numbers of jobs and machines are analysed both policies give similar tardiness. Numerical experiments show that the differences between these two rescheduling policies are rather small in larger problems as well.

Block erection sequencing in shipbuilding with general lifting and joining times

This article studies the scheduling of block erection in shipbuilding. A mathematical model is formulated to minimize the makespan, i.e., the time between the start and finish of the erection schedule, when there are different lifting and block joining times. Structural stability and the no-skipped-blocks property are taken as constraints. The model is used to study the erection in three special cases. First, the case with a short lifting time is analyzed. Second, a solution is constructed for the case in which joining times are short and, third, the case in which the lifting and joining times are the same is analyzed. The article shows how these three cases can be solved efficiently, and, in addition, it gives insight into the erection scheduling such as the effects of the number of blocks in the horizontal and vertical directions on different joining times.

Block erection in the event of delays in shipbuilding: A scenario-based approach

Ships are constructed from individual steel structural units called blocks, which are typically erected into the ship one by one in a sequence. This sequence, the block erection schedule, is the main schedule in shipbuilding, and it is followed in other production phases. However, the problems in the production of the blocks can delay the arrival of the blocks in comparison to what was initially scheduled. To deal with these delays, this article studies five different planning methods: erecting blocks unfinished; the use of inventories; the use of rush jobs; sequence changes; and, if the delays cannot be dealt with, delaying the completion of the ship. In order to do that, a mathematical, scenario-based mixed-integer linear programming model is formed. The model is used with numerical results to show that the block structure, delay conditions, and the tardiness costs have significant effect on the optimal use of planning methods. Results show that inventories should be used when delays are probable, whereas blocks should be erected unfinished if the delays are very uncertain and there are high tardiness penalties. The uses of rush jobs and sequence changes depend on the structure of the ship.