Payam Haghighi

Toward Automatic Tolerancing of Mechanical Assemblies: First-Order GD&T Schema Development and Tolerance Allocation

Geometric and dimensional tolerances must be determined not only to ensure proper achievement of design function but also for manufacturability and assemblability of mechanical assemblies. We are investigating the degree to which it is possible to automate tolerance assignment on mechanical assemblies received only as STEP AP 203 (nominal) geometry files. In a previous paper, we reported on the preprocessing steps required: assembly feature recognition, pattern recognition, and extraction of both constraints and directions of control (DoC) for assembly. In this paper, we discuss first-order tolerance schema development, based purely on assemblability conditions. This includes selecting features to be toleranced, tolerance types, datums, and datum reference frames (DRFs), and tolerance value allocation. The approach described here is a combination of geometric analysis and heuristics. The assumption is that this initial geometric dimensioning and tolerancing (GD&T) specification will be sent to a stack analysis module and iterated upon until satisfactory results, such as desired acceptance rates, are reached. The paper also touches upon issues related to second-order schema development, one that takes intended design function into account.

Toward Automatic Tolerancing of Mechanical Assemblies: Assembly Analyses

Generating geometric dimensioning and tolerancing (GD&T) specifications for mechanical assemblies is a complex and tedious task, an expertise that few mechanical engineers possess. The task is often done by trial and error. While there are commercial systems to facilitate tolerance analysis, there is little support for tolerance synthesis. This paper presents a systematic approach toward collecting part and assembly characteristics in support of automating GD&T schema development and tolerance allocation for mechanical assemblies represented as neutral B-Rep. First, assembly characteristics are determined, then a tentative schema is determined and tolerances allocated. This is followed by adaptive iterations of analyses and refinement to achieve desired goals. This paper will present the preprocessing steps for assembly analysis needed for tolerance schema generation and allocation. Assembly analysis consists of four main tasks: assembly feature recognition (AFR), pattern detection, directions of control, and loop detection. This paper starts with identifying mating features in an assembly using the computer-aided design (CAD) file. Once the features are identified, patterns are determined among those features. Next, different directions of control for each part are identified and lastly, using all this information, all the possible loops existing in an assembly are searched.

Library of T-Maps for Dimensional and Geometric Tolerances

Tolerances are specified by a designer to allow reasonable freedom by a manufacturer for imperfections and inherent variability without compromising performance. T-Map is a hypothetical Euclidian point space model which describes all possible variations constrained by design or machining tolerances. The size and shape of T-Map reflects all variational possibilities for a target feature. According to ASME Y14.5M standard, tolerances are dimensional or geometric, and geometric tolerances are divided into different classes. T-maps have been created for all types of geometric tolerances with Primitive T-Map Elements. In this paper we have reviewed the basic ideas of T-maps, the reason it has been built, and the contribution it can have in design and manufacturing. It is then followed by a library of T-maps created for all Tolerance classes with brief description for each one.© 2014 ASME

Automatic Detection of Directions of Dimensional Control in Mechanical Parts

This research is part of a larger project which aims at developing a tool to help designers create effective GD&T schemas. The first step towards this goal is to determine the particular directions in which dimensions and tolerances need to be controlled. These directions we label here as “Directions of (Dimensional) Control” or DoC for short. Regardless of whether one uses chain dimensioning, reference dimensioning or geometric tolerancing, all size and basic dimensions of position line up in a finite number of directions or Directions of Control. This paper presents an approach for automatically identifying the directions of control from CAD models of mechanical parts. The only input to the system is the geometry of parts or assemblies in STEP file format. The analysis is done part by part for an assembly. First, planar and cylindrical features are recognized and their normal/axes extracted. The extracted features are then organized into groups of parallel normal or axes directions. Cylindrical features can belong to two or more Directions of Control, while planar features belong can only belong to one. Features in each DoC are then ordered based on perpendicular relative distances. Each ordered feature list forms a linear chain in which nodes represent features and links are attributed with relative distance to the nearest neighbors on each side. DoC chains are related to each other by relative orientation. Therefore, the chains are combined into a unified graph, using the junction nodes to contain the relative orientation between the chains. The extracted Directions of Control can be output in both textual and graphical form. Although the primary motivation for automatic DoC graph generation is computer assisted tolerancing and automatic tolerance analysis, the paper also discusses other applications in manufacturing.Copyright

Automatic Detection and Extraction of Tolerance Stacks in Mechanical Assemblies

Tolerances are specified by a designer to allow reasonable freedom by a manufacturer for imperfections and inherent variability without compromising performance. It takes knowledge and experience to create a good tolerance scheme. It is a tedious process driven by the type of parts, their features and controls needed for each one of them. In this paper, we investigate the extent to which GDT we call them Direction of Control (DoC). Then we can create the GD&T schema, allocate tolerance values, and prepare it for tolerance evaluation. In this paper, we present an approach to automatically identify the dimensional loops based on assembly requirements. Assignment of tolerance values will be covered in future works as it is based on design function.Copyright

Preliminary Investigation on Generating an Explicit GD&T Scheme From a Process Plan

Geometric Dimensioning and Tolerancing (GDT converting plus/minus tolerances to appropriate geometric tolerances; and dealing with transient features — which are features that do not exist on the finished part used for GDT specs by the designer. We propose a new data structure, PCTF (process oriented constraint tolerance feature graph) to facilitate mapping between design and manufacturing tolerances.Copyright

Torsional Sensitivity and Resonant Frequency of an AFM With Parallel Sidewall Probes

The resonant frequencies and torsional sensitivities of an atomic force microscope (AFM) assembled cantilever probe which comprises a horizontal cantilever, two vertical extensions and two tips located at their free ends are studied. This probe makes the AFM capable of measuring, for instance, the outer/inner diameter, roundness and roughness of microstructures like micro-holes and micro nozzles which leads to a time-saving swift scanning process. In this work, the effects of the sample surface contact stiffness and the geometrical parameters such as the ratio of the vertical extension length to the horizontal cantilever length and the distance of the first vertical extension from the clamped end of the horizontal cantilever on torsional resonant frequencies and sensitivities are assessed. These geometrical effects are illustrated in some figures. The results show that the low-order vibration modes are more sensitive for low values of the contact stiffness but this situation is not valid for high values.Copyright

An Investigation on the Torsional Sensitivity and Resonant Frequency of an AFM With Sidewall and Top-Surface Probes

The resonant frequencies and torsional sensitivities of an atomic force microscope (AFM) with assembled cantilever probe (ACP) are studied. This ACP comprises a horizontal cantilever, a vertical extension and two tips located at the free ends of the cantilever and the extension which makes the AFM capable of simultaneous topography at top-surface and sidewalls of microstructures especially microgears which consequently leads to a time-saving swift scanning process. In this work, the effects of the sample surface contact stiffness and the geometrical parameters such as the ratio of the vertical extension length to the horizontal cantilever length and the distance of the vertical extension from clamped end of the horizontal cantilever on torsional resonant frequencies and sensitivities are assessed. These geometrical effects are illustrated in some figures. The results show that the low-order vibration modes are more sensitive for low values of the contact stiffness but the situation is reversed for high values.Copyright

An Investigation on the Flexural Sensitivity and Resonant Frequency of an AFM With Sidewall and Top-Surface Probes

The resonant frequencies and flexural sensitivities of an atomic force microscope (AFM) assembled cantilever probe which comprises a horizontal cantilever, a vertical extension and two tips located at the free ends of the cantilever and the extension are studied. This probe makes the AFM capable of simultaneous topography at top-surface and sidewalls of microstructures especially microgears which leads to a time-saving swift scanning process. In this work, the effects of the sample surface contact stiffness and the geometrical parameters such as the ratio of the vertical extension length to the horizontal cantilever length and the distance of the vertical extension from clamped end of the horizontal cantilever on the resonant frequencies and flexural sensitivities are assessed. These geometrical effects are illustrated in some figures. The results show that the low-order vibration modes are more sensitive for low values of the contact stiffness but the situation is reversed for high values.Copyright