Abstract

Reducing manufacturing cost is the main goal of machine tool precision allocation under the condition of meeting precision design requirements. The previous studies generally took “ensuring machining errors completely within the allowable range of design precision” as constraint. Due to the strict constraint, the cost reduction effect is limited. This paper proposes a new method of precision allocation based on precision reliability. The probability that the machining errors are within the design precision allowable range is taken as the measurement index of precision reliability, and the optimization model constraint is relaxed to “that the machining errors are within the allowable range of design precision with predefined precision reliability”, so as to obtain lower manufacturing cost under tolerable precision loss risk. The case study of a large-scale gear hobbing machine shows that this method can effectively reduce the manufacturing cost, and the precision allocation is more economical and reasonable.

Introduction

Due to the fierce competition in the machinery industry, higher requirements are put forward for the design of machine tools. The traditional empirical design method cannot meet the design requirements of balancing the precision, cost and reliability of machine tools. Optimization design is a kind of modern design method, which is to optimize the design parameters under various specified constraints to obtain the optimal value of one or more design indicators. It aims at improving product quality and reducing product cost.

The machine tool design mainly includes the structural design, electrical design, precision design, and thermal design. Precision design, also known as precision allocation, is to reasonably allocate the precision of each part of the machine tool according to the machining performance requirements of the machine tool. Due to the complex impact on product quality and cost, the precision allocation is a highly responsible task [1]. Precision allocation generally includes three steps: precision prediction, precision allocation optimization modeling and model solving.

Conclusions

Precision allocation is an important work in machine tool design, which is of great significance to ensure the machining precision, improve product quality and reduce manufacturing cost. The precision allocation method mainly includes the basic principle method, the method with the optimization objective of minimizing manufacturing cost, and the method with the optimization objective of minimizing quality loss. The precision allocation of motion parts is mostly based on the sensitivity (or contribution) of their errors to the manufacturing cost.

According to the modern mechanical precision design theory, the overall precision of machine tool transmission chain is a random variable with normal distribution. Taking the normal distribution random variables properties into consideration, the probability is very low with variable reaching the maximum. Obviously, it is too harsh for the overall precision to fully meet the requirements of design precision, and the precision allocation scheme based on this constraint is not economical and reasonable. In the design of precision allocation, it does not seek meeting 100 % design requirements. Based on the characteristics of normal distribution of the comprehensive machining precision, it takes the probability of the machining precision meeting the design requirements to reach a certain threshold as the constraint condition for precision allocation, which can ensure the machining precision of the CNC-LGHM and effectively reduce the manufacturing cost.