Clarifications of a Datum Axis or Centerplane Specifying in Maximum Material Condition of Geometric Dimensioning and Tolerancing

Engineering and Engineering Technology students and professionals learning the processes and standards in computer-aided design (CAD) and computer-aided manufacturing (CAM) should learn and understand the methodology of geometric dimensioning and tolerancing (GD&T) to describe the intent and requirements for part and assembly geometries. Correct application of GD&T ensures that the part and assembly geometry defined on the drawing will have the desired form and fit (within limits) and function as intended. One learning difficulty in understanding GD&T is the concept of defining a datum axis or center plane using Maximum Material Condition (MMC). To overcome this difficulty, a new approach is presented that uses a modifier ○V (Virtual Condition) instead of ○M (MMC). A thorough rationalization of using ○V in datum axis specification is discussed. The paper also provides a convenient table on how to use this modifier

STEP-based Conceptual Framework for Measurement Planning Integration

AbstractMeasurement aims to check the product conformance or to control the manufacturing processes’ parameters. It needs to be planned in an integrated and interoperable manner with other manufacturing activities. Integration of measurement planning is based on the information provided by the design phase. This paper aims to assist the interoperability of the measurement plans through introducing the resource-independent measurement specifications (RIMS) concept. The paper presents a conceptual framework for representing a STEP-based measurement features from the coordinate metrology perspective. The proposed framework supports the direct formulation of the measurement process specifications in an operation-based manner and the realization the process control functionality of the measurement processes

Feature Cluster Algebra and Its Application for Geometric Tolerancing

abstract: The goal of this research project is to develop a DOF (degree of freedom) algebra for entity clusters to support tolerance specification, validation, and tolerance automation. This representation is required to capture the relation between geometric entities, metric constraints and tolerance specification. This research project is a part of an on-going project on creating a bi-level model of GD&T; (Geometric Dimensioning and Tolerancing). This thesis presents the systematic derivation of degree of freedoms of entity clusters corresponding to tolerance classes. The clusters can be datum reference frames (DRFs) or targets. A binary vector representation of degree of freedom and operations for combining them are proposed. An algebraic method is developed by using DOF representation. The ASME Y14.5.1 companion to the Geometric Dimensioning and Tolerancing (GD&T;) standard gives an exhaustive tabulation of active and invariant degrees of freedom (DOF) for Datum Reference Frames (DRF). This algebra is validated by checking it against all cases in the Y14.5.1 tabulation. This algebra allows the derivation of the general rules for tolerance specification and validation. A computer tool is implemented to support GD&T; specification and validation. The computer implementation outputs the geometric and tolerance information in the form of a CTF (Constraint-Tolerance-Feature) file which can be used for tolerance stack analysis.Dissertation/ThesisM.S. Mechanical Engineering 201

Automating GD&T Schema for Mechanical Assemblies

abstract: Parts are always manufactured with deviations from their nominal geometry due to many reasons such as inherent inaccuracies in the machine tools and environmental conditions. It is a designer job to devise a proper tolerance scheme to allow reasonable freedom to a manufacturer for imperfections without compromising performance. It takes years of experience and strong practical knowledge of the device function, manufacturing process and GD&T standards for a designer to create a good tolerance scheme. There is almost no theoretical resource to help designers in GD&T synthesis. As a result, designers often create inconsistent and incomplete tolerance schemes that lead to high assembly scrap rates. Auto-Tolerancing project was started in the Design Automation Lab (DAL) to investigate the degree to which tolerance synthesis can be automated. Tolerance synthesis includes tolerance schema generation (sans tolerance values) and tolerance value allocation. This thesis aims to address the tolerance schema generation. To develop an automated tolerance schema synthesis toolset, to-be-toleranced features need to be identified, required tolerance types should be determined, a scheme for computer representation of the GD&T information need to be developed, sequence of control should be identified, and a procedure for creating datum reference frames (DRFs) should be developed. The first three steps define the architecture of the tolerance schema generation module while the last two steps setup a base to create a proper tolerance scheme with the help of GD&T good practice rules obtained from experts. The GD&T scheme recommended by this module is used by the tolerance value allocation/analysis module to complete the process of automated tolerance synthesis. Various test cases are studied to verify the suitability of this module. The results show that software-generated schemas are proper enough to address the assemblability issues (first order tolerancing). Since this novel technology is at its initial stage of development, performing further researches and case studies will definitely help to improve the software for making more comprehensive tolerance schemas that cover design intent (second order tolerancing) and cost optimization (third order tolerancing).Dissertation/ThesisMasters Thesis Mechanical Engineering 201

Factors Relevant to Parallelism Inspection

Inspection of parallelism between two surfaces (planes) requires the establishment of a datum feature (surface), and determination of the envelope of points containing the inspected feature parallel to that datum feature. Verifying parallelism thus requires the establishment of the datum feature, and determining the separating distance between the two parallel planes. The parallelism determination can change with sampling (number of data points) and fitting of points. The datum feature establishment is very important to the inspection, and is also dependent on the number of points used to define it. In this thesis, the datum feature is least squares fit from the data collected using coordinate metrology. The inspected feature is then enveloped by minimum separation planes that contain the maximum deviation between the points. The separating distance between the enveloping planes is calculated, and termed the Parallelism Tolerance. Three levels each, of two sets of data representing the datum feature and inspected feature are collected, for 15 aluminum plates (3 parallelism geometries, 5 replicates). The independent factors are analyzed against the calculated parallelism values. Experimental analysis shows significant effect of sample size on the parallelism computed. As would be expected, the best parallelism values were obtained at the combination of the highest levels of sampling points for datum feature establishment and inspected feature verification

2003, “Computer Modeling of Geometric Variations in Mechanical Parts and Assemblies

This paper reports on part of a project related to the development of a computer model for GD&T (Geometric Dimensioning and Tolerancing