Part Tolerance

Another challenge when determining tolerance is part rigidity. The inherent flexibility of a thin-section sealer makes it too flexible to be precisely measured. In this case, corporate knowledge, the engineer s experience, and machinability all play critical roles in the final decision. Engineers also refer to the industrial standard practice as a reliable reference. 

Orientation, Location and Run-Out, is another internationally recognized standard. Other references and specifications are available as well. 

In reverse engineering, dimensional tolerances are determined by variations in the sample measurements and accepted engineering practices. In accordance with the principle of truth in measurement, the resulting tolerances for the reverse engineered part should not exceed the minimum and maximum dimensions actually measured on the sampled OEM parts. Exceeding these limits requires justification and further substantiation. 

Statistics have been used for data analysis and reliability prediction in reverse engineering. The fundamentals of engineering statistics and its applications in data analysis of dimensional measurements and property evaluation will be discussed in Chapter 6. Its application to part reliability will be discussed in Chapter 7. 

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Concentricity Term to describe two circles or cylindrical shapes having a common center and common axis, such as the inside or outside diameters of a barrel or outside diameters of the surface and bearing surfaces of a screw. Deviation from concentricity is referred to as runout. Also refers to the relationship of all inside dimensions to all outside dimensions usually expressed in thousands of inch or millimeter FIM (full indicator movement). Deviation from concentricity is usually referred to as a runout. The concentricity should allow for the maximum part tolerance. The geometry of the part should help indicate the tolerance applied. 

Table 3.2 Examples of part tolerances for normal and precision classes...

Injection moulding leads to the best part tolerances but plastics differ because of their morphology, structure, rheology, etc. The geometry of a part and the complexity of the corresponding mould vary. Consequently, the tolerances of a part depend on ... 

Table 5.3, as an example, displays thermoplastic part tolerances for the normal and precision classes as defined by the NF-T58000 standard. 

Table 2-10. Parameters that Influence Part Tolerance.

An allowance is provided in the cavity for tight part tolerance requirements and plating. As plating highlights the mold surface, it is imperative that cavity polishing prior to plating be done precisely. Plating will not cover up a poorly polished surface. 

Tolerance interval n. The specified allowance of variation of a dimension (or other quantity) above and below the nominal or target values in a production part or product. For a process whose average level and random variation are in control, symmetrical tolerance limits should be at least six processes standard deviations apart in order to approach zero percent defective parts. Tolerances are better understood and getting much more attention now than a few decades ago when a New York molder was asked about tolerances on parts he was producing. His reply Hey, we got lotsa tolerance here We hire people no matter what color or nationality they are . 

In multicavity molds where close part tolerances are required, a balanced runner system is extremely important to achieve part-to-part uniformity. Fig. 7-3 depicts some multicavity runner systems. 

Figure 5 shows the typical thickness/penetration capabilities of various coating and surface treatments. As indicated in the figure, some surfaceengineering treatments penetrate into the surface and there is no intentional buildup on the surface. These are the surface-engineering processes described in Chapters 4 and 5. Other surface treatments coat or intentionally build up the surface. This is a selection factor. Can a part tolerate a buildup on the surface If not, the selection process is narrowed to the... 

Part number 2 cannot be moved directly into its final assembly position without first correcting for gripping errors, part-to-part tolerances or other errors that might cause interference between the parts. Instead, the piece is moved to a position close to its final assembly position, and is placed on the assembly table. The vision system takes a picture and returns the actual position of Part number 2, x. The relative moves required to assemble the piece are calculated. The translations are then executed to complete the assembly. The final assembly must be done in two steps a rotation to align the part properly and then one or more translations to mate it. The rotation must be done first, since rotation of the robot wrist will not necessarily rotate the part about its centroid. Rotating the part about a point other than the centroid will change the translational movements required for mating. 

There are a number of mathematical tools that are used in the world of manufacturing. Statistical Process Control is used to monitor and control the process parameters that are used to manufacture parts. Tolerance Analyses are used to analyze and predict fit and function of final assemblies. Pareto Analysis techniques are used to assess the contribution of various factors in problem situations. Design of Experiments techniques are used to quantify the variables in a given process or application. 

Infrared welding is much more forgiving of part tolerance stack up than radiant hot plate or laser welding it is similar to or even significantly better than vibration welding and ultrasonic welding. 

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