Therefore, we first investigated the clamping quality factor of the transformable pin jigs to solve this problem. Since these two factors are determined by the loading position of the product on the transformable pin jigs, we proposed the method for optimal positioning of assembly parts to obtain the optimal fixture quality that minimizes the assembly errors. The proposed method is based on the iterative nearest point (ICP) algorithm and is improved to take into account the fixture quality factors of the transformable pin jigs.
INTRODUCTION
Background
A fixture quality is an important issue in manufacturing, as about 40% of sizing errors are caused by improper jig and fixture design (Nixon, 1971). The fixture quality refers to the degree of suppression of errors by the jig and fixture system during the manufacturing process. They could improve a product quality, whereas they had to redesign the jig and fixture system every time the product changed. In the past, the manufacturing companies produced the product for a long period (about 5 ~ 10 years), so the replacement cost of the jig and fixture system was not a big problem.
Motivation
Objective
LITERATURE SUERVEY
- Flexible fixture design
- Part stability
- Optimal fixture configuration
- Optimal part positioning
Kang has proposed evaluation criteria for part durability through the contact durability index (CSI) determined on the basis of the friction cone (Kang et al., 2003). Since external forces (e.g. clamping and machining forces) and internal forces (e.g. reactive forces) must be balanced in order for a part to remain stable during the manufacturing process, part stability is evaluated from the force balance. First, Qin estimated part stability with error due to part position and processing (Qin et al., 2006). Asante, on the other hand, analyzed part stability based on the relationship between external forces and deformation (Asante, 2010).
The stable condition means that the fixture limits the sliding and twisting of the part during the manufacturing process. Since the part is affected by the contact forces and the wrench, the movement and rotation of part will be minimized when those two forces and wrench are minimized. Alternatively, in the cutting process, the distribution of chip thickness can be used to evaluate the part stability.
In this work, the deformation of the part was modeled using the finite element method and was used on the objective function in the optimization process (Prabhaharan et al., 2006). This positioning is based on the analysis of the production process and the consideration of the part, including the material, geometry, desired dimensional tolerance and so on. Second, the initial placement to locate the part at the origin of the fixture system.
Finally, the maximization of the number of fasteners holding the part is performed using winding number theory.
TRANSFORMABLE PIN JIG SYSTEM
Transformable pin-jigs
Jig shape transformer
ASSEMBLY PART POSITIONING ON TRANSFORMABLE PIN-JIGS
Problem statement
The scratch occurs when the outside of the product is damaged by the sharp shape of the fixture. Because the special mold supports the product through surface contact that adapts to the shape of the product, holes often do not occur. However, because the point contact is made according to the shape of the product, the fastening quality is relatively lower than that of the special molds in the case of the transformable pin jigs.
The underlying contact pin is a pin that forms a contact on the bottom of the product. The boundary contact pin, on the other hand, is a pin that makes contact at the boundary of the product. Fixture quality is improved as the number of pins used to generate shapes increases, as it is beneficial to support the external force as the underlying contact pins increase in size and to hold the product as the boundary contact pins increase in size.
In an experiment to analyze the product deformations during the assembly process, we verified that the product deformation increases as the distance between the pin points and the connection points increases. Taking into account the components of the transformable pin jigs (the boundary contact pin and underlying contact pin) and the deformation factor of the part in the assembly process, to improve the clamping quality, increase the number of pins participating in the shape generation and the distance between the pins points and connection points to be reduced. Because this number of pins and spacing are determined according to the position of the mounting part, the transformable pin jigs require a mounting part to be placed in a position that improves clamping quality.
Therefore, in order to improve the quality of fastening, the objectives of i) reducing the distance between joint points and pin points and ii) increasing the number of pins used are defined as a point set registration problem.
Overview
Assembly part positioning on transformable pin-jigs
- Prerequisites
- Product data point set and pin point set
- Registration of product data point set and pin point set
Therefore, we used the point cloud that projected the CAD product in the x-y plane at the loading position as the product data point set, and the pin coordinates in the x-y plane are used as the pin point set. A set of product data points is obtained from a top view of the product projected onto the x-y plane. A product data point set consists of three different types of points which are junction, boundary and interior points.
Base contact, on the other hand, can be made at all points in the product's set of data points. The number of points is determined as shown in Table 4-1. PRODUCT DATA POINT SET CLASSIFICATIONS. Type Description Number of points. a) the product image and (b) the set of product data points converted from the image.
The iterative closest point (ICP) algorithm was used to align product data point set and pin point set. If the centroids of product data point set and pin point set are defined as ̅ and 𝑝̅ with constant weight, the point deviations from the centroid are given by ⃗𝑖′ and 𝑝⃗𝑗′. Typically the junction point set has higher weight than others as the number of junction points is least.
Applying a weight creates the center of gravity of the product data point set closer to the point where the weight is high.
CASE STUDY
Overview
Assembly part positioning
- Toy product data pointset
- Toy model
- Car Dootrim panel
The result of the proposed assembly part positioning method, as shown in the Figure 5-2, is the most suitable for this requirement of junction point alignment as the initial position (coincidence of centroids of product point set and pin point set) and the classical ICP. Furthermore, the number of active pins at the optimal position increased from the proposed method. The number of active boundary contact pins was increased to 7 from 0, but the number of underlying contact pins was reduced to 1 from 3. There was a trade-off between the number of boundary and underlying contact pins as the product area was limited.
Nevertheless, it could be considered to be improved as the total number of active legs was increased. a) Initial position, (b) classical ICP and (c) ICP with proposed objective function (data point set for toy products). The only 5 x 5 pin jigs needed to hold this toy model, the number of pins was set to 5 for each X axis and Y axis. The total number of both active legs (boundary and underlying contact legs) was the same at 10, but the number of limit contact legs was increased to 3 from 2.
It can be more effective at holding the product during the assembly process because the boundary contact pins are more suitable for holding a product than the underlying contact pins. The specifications and number of points were presented in Table 5-5, and each pin in the transformable pin jigs was used considering its size. a) general view, (b) top view and (c) product data point set (car door trim). The total number of both active pins was equal to 45, but the number of boundary contact pins was increased because according to the proposed method, the boundary contact pin was more effective in holding the product.
The proposed assembly part positioning method moves the product downwards, as a result more limit contacts were possible at the top edge of the product.
Deformation analysis
- Setup
- Result
This experiment was designed to prove the coupling effect of point alignment with part deformation. As a result, the optimal position was determined to be [-5.87mm, 11.35mm, 2.65°] which are the translation along the x-axis and y-axis, and the rotation around the z-axis, respectively. The result, as shown in Table 5-7~8, the stress and deformation were reduced at the first joint point, while there was no great difference at the other joint points.
According to this result, the proposed assembly part positioning method could improve the attachment quality by reducing the stress and deformation of the assembly part. The reason for this phenomenon was that the proposed method focused on minimizing total fitting error. Therefore, if the objective function is redesigned for each joint point having the same misfit, then the deformation and stress at the joint point will be close to equally distributed.
This redesigned method does not guarantee that total misalignment is minimized, but it does guarantee nearly uniform assembly quality between connection points. Average (105 N/m2) Standard deviation (104 N/m2) At initial position At optimal position At initial position At optimal position.
CONCLUSION AND FURTHER WORKS
Conclusion
The result shows that the optimization reduces one of the connection points while maintaining the deformation of the remaining connection points. To summarize, the deformation factors in the transformable pin jigs were defined as the number of active pins and the alignment of join point and pin point. The assembly part location based on these two deformation factors was presented as point set registration problem.
To solve this problem, the iterative nearest point algorithm was used and an improved objective function for transformable pin-jigs was proposed. The deformation of the product evaluated by finite element analysis was minimized at the optimal load position calculated by the proposed component positioning method. Therefore, we found that the proposed component positioning method for transformable pins could reduce product deformation and prevent assembly errors.
Further works
Paper presented at the 대한기계학회 IT융합부 문춘계학술대회. Principle and simulation of fixture configuration design for sheet metal assembly with laser welding, part 2: optimal configuration design with genetic algorithm. Principle and simulation of fixture configuration design for sheet metal assembly with laser welding, part 1: finite element modeling and a prediction and correction method.
Paper presented at 대한기계학회 춘추학술대회. Systematic modeling of workpiece-fixture geometry preselection and compatibility for workpiece machining error prediction. Optimization of equipment layout for flexible aerospace parts based on intelligent self-reconfigurable installation system.
