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Alignment and Assembly Principles
Published in Richard Leach, Stuart T. Smith, Basics of Precision Engineering, 2017
A major advantage in reducing the number of assembly steps and components is the reduction in the alignment effort. Although self-aligning features can be added, dowel pins or end stops for example, these may result in manufacturing tolerances being tightened to smaller numbers as each component needing to be assembled will require its own sub-error budgeting to meet the overall error budget. The effect of tolerances when multiple components are used in assemblies is called tolerance stack-up and often requires sophisticated analysis tools for this sub-error budgeting. Therefore, a monolithic component which combines several individual components may reduce manufacturing tolerances while maintaining component functionality and specifications. Of course, monolithic components have their disadvantages as well. In particular, the design freedom is limited by manufacturing methods (for instance it may be desirable to have internal cavities to lower the mass of the component) and machining errors (machine geometric errors and alignment errors when the component is handled for subsequent manufacturing steps).
Design for Quality
Published in David M. Anderson, Design for Manufacturability, 2020
A related problem is tolerance stack-up and worst-case tolerancing. Tolerance stack-up refers to the cumulative effect of all the tolerances in a “string” of dimensions (the combination of which affects the same overall dimension). Worst-case tolerancing refers to combining the “worst” of all the tolerances to analyze what the net effect will be. For instance, to do a worst-case analysis on the clearance between a shaft and a hole, one would consider the largest possible shaft in the smallest possible hole and the smallest possible shaft in the largest possible hole. This analysis will yield the extremes in clearance, which should conform to design requirements.
Building a prototype using off-the-shelf components
Published in Fuewen Frank Liou, Rapid Prototyping and Engineering Applications, 2019
Tolerance stack is a calculation which determines the maximum and minimum distance between two features on a part or in an assembly. Stacks help us ensure good product design and reduce product costs. Stacks can be linear or radial. Radial stacks often involve diametrical dimensions; think of it as linear but in a different direction. Basic stack steps are given as follows: Identify the problem: Identify the stack objective, list the conditions under which the stack is being calculated, and label the start point, end point, and direction of the stack. The conditions can include the temperature of the parts, whether the parts meet print specifications, whether the stack includes or excludes wear, the amount of deflection on the parts, and any unusual conditions present in the stack.Choose the desired answer: Write down the design goal before solving the problem. “What is the extreme maximum (or minimum) answer that would be acceptable and allow the product to function as intended?”Identify the stack path: A stack path is a series or chain of distances (part dimensions) from the start point of the stack to the end point of the stack. This chain of distances must consist only of known distances—which are dimensions on the drawing or a value calculated from dimensions on the drawing and must be the shortest possible chain of distances from the start point to the end point, and must be continuous—each distance must begin where the previous distance ends.Perform the math: Transfer the distances onto the stack form, add each column of numbers, check the subtotals, and evaluate the answer.
A study of tolerance allocation and stack-up analysis to improve the assembly precision of an injection mold
Published in Journal of the Chinese Institute of Engineers, 2023
Yuo-Tern Tsai, Kuan-Hong Lin, Chun-Sheng Chen
The general methods of tolerance designing and analysis can be referred to in the papers (Fischer 2011; Chase 1999). Tolerance stack-up analysis is to calculate the accumulated variations across a set of dimensions. It must select the schemes of the dimensions and the tolerances of the related parts and observe the tolerance variation of the assembly. Tsai and Wu (2013) investigated the method of tolerance re-distribution for a spindle to keep the assembly tolerance at a high-quality level. Focusing on group random assembly, Tsai, Chen, and Dai (2015) investigated the effect of grouping for components with uniform and normal distributions by the developed ‘grouped random assembly’ method. Lazzerini and Marcelloni (2000) proposed a genetic algorithm that generates and assesses the assembly plans.
Direct tolerance analysis of mechanical assemblies with normal and non-normal tolerances for predicting product quality
Published in International Journal of Computer Integrated Manufacturing, 2022
The conventional tolerance analysis methods generally use the worst case (WC) and root-sum squares (RSS) approaches for tolerance stack-up to estimate the tolerance accumulation on the assembly dimensions. Also, the conventional methods in the literature have been developed based on these main limitations that the assembly function is available in an explicit form and the dimensions are varied under the normality assumption. Therefore, to deal with these problems, a new tolerance analysis method was introduced based on the univariate DR method and the Pearson system. The proposed method can nonlinear tolerance analysis of the mechanical assembly without the need for existing the explicit assembly function. In addition, according to the proposed method, the dimensions can be varied under normal or non-normal distributions. The proposed approach was applied to tolerance analysis of some case studies and results were compared to improved-HLRF reliability index and MCS approaches. Results showed that the proposed approach can present more efficient results than the two other methods. The number of function evaluations in the proposed method to estimate the rejected product rate was much less than the other two methods. Unlike the conventional methods in the literature such as RSS and MCS, the proposed approach, is an accurate and efficient method to handle the tolerances that are distributed normally and non-normal without needing an explicit assembly function. It should be noted that the proposed method does not need any sensitivity analysis, so it is more efficient and practical in actual high-dimensional mechanical assemblies. The proposed algorithm can be easily automated to be used within CAD/CAM software for assembly quality control in industrial applications.
Integrated geometrical and dimensional tolerances stack-up analysis for the design of mechanical assemblies: an application on marine engineering
Published in Computer-Aided Design and Applications, 2018
Cristina Renzi, Alessandro Ceruti, Francesco Leali
The application of the tolerance stack up analysis in the early design phases is aimed to reduce redesign activities in later design stages. Hence, the main phases of the early design are integrated with steps for tolerance analysis.