Functional Tolerancing of a Gearbox

This paper proposes a scheme for the tolerance specification that uses the features’ function information and mating condition attributes in the assembly to derive an appropriate tolerance specification as per the design intents. The proposed mirror method provides a way to locate the critical components. It helps the user identify functional features and group them into clusters. Temporary DRF (Datum Reference Frame) is first generated for each cluster of features on critical components by selecting specific features as datum features. Other features (that are not datum features) present in the same cluster are then toleranced with respect to those datum features. The temporary DRFs as well as the tolerancing scheme are then copied (a mirror image of the critical component) to the other mating components (which are mating with the particular critical component). The final DRFs on the critical components are then decided by analyzing the temporary DRFs and the given functional requirements of each component. Appropriate geometric tolerance types and material conditions for toleranced features are generated following the standards and the industrial practices. INTRODUCTION Traditionally tolerances for manufactured parts are specified using symbolic schemes of GD&T as per ASME or ISO standards. Used appropriately, the symbols can give geometric definitions of components that represent design intents. GD&T describes the variational information related to the size, form, orientation, and location of part features. It has a designdimensioning philosophy that encourages designers to define a part based on how it functions in the final product. Tolerancing a component as per standards includes 1) datum feature selection, 2) dimensioning between datum features and toleranced features, 3) assignment of geometric tolerance type, value, material condition (for feature of size), and DRF in the feature control frame. Functional surfaces (edges or points) are normally chosen as datum features. The most important quality of a datum feature is that it must define the orientation and/or location of a part precisely in the assembly. This is particularly true for a primary datum feature. In general, the datum features need to be [MEA 98]: 1) functional (serving a purpose for the part’s operation); 2) representative of mating or sitting features and/or alignment edges (to assure that if they are inspected and accepted while oriented to, or located from datums constructed from those features, the controlled features will, indeed, mate and/or sit); 3) repeatable in manufacturing; 4) accessible during manufacturing and inspection operations. Conditions 1) and 2) are more important than conditions 3) and 4), because conditions 1) and 2) establish direct relationships between functions of the feature to the datum feature. We may add form or orientation tolerance to control datum feature so that they are repeatable in manufacturing and inspection. Some times, a functional feature may not be a good candidate for a datum feature if it has any accessibility problem during manufacturing, assembly and inspection. In that situation, we need to find out other feature that is accessible and whose orientation to the other functional features can be controlled by applying orientation tolerances. Functions and mating conditions must guide the datum selection process. Other practical needs should also be considered in selecting datum features. It should be noted that planar and cylindrical features are the most common features in any component. With sufficiently large surfaces, they are able to control the maximum number of DOFs (degree of freedom) of the orientation of any surface. They are easy to be simulated by surfaces of instruments used in manufacturing and inspection, such as working table of machine tool and inspection machine or surfaces of gages and fixers. Before introducing the proposed mirror method, it is important that we define the function and mating condition attributes of features that will guide clustering of features, selection of tolerance type, and selection of datum features. FUNCTION AND MATING CONDITION ATTRIBUTES OF FUNCTIONAL FEATURES In this paper, we differentiated mating condition attributes from function attributes because they serve differently in the tolerance selection process. Details of both attributes are provided in the following. Functional attributes serve some specific purposes in a part’s operation, such as seal, rotate, balance, gearing, fastening, press fit, clearance fit, and sliding etc. They should be used to guide the selection of tolerance types, tolerance values, and material conditions (Maximum Material Condition, Least Material Condition, Regardless of Feature Size MMC, LMC and RFS). Let us investigate some of the function attributes and examine how their requirements affect the tolerancing process. • Seal: arrests any leakage between two components. It requires tight form and size control (assuming no flexible sealing parts used). • Rotate: preserves the rotation between two components. The feature for “rotate”, such as cylindrical features that mate with bearings, requires tight location and form control. • Balance: preserves balance (symmetry) between two components. The feature for “balance”, such as a center hole or a boss to control the relative position of two components, doesn’t allow MMC referenced to it if it is chosen as a datum feature. • Gearing: preserves gear mesh between two components. Requires profile control of the gear mesh. • Fastening: connects two components. Requires projected tolerance zone consideration and MMC control for interchangeability. • Sliding: preserves sliding between two components (such as piston assembly). Requires form control on the contacting surfaces. • Press fit: does not allow any relative movement; The components after press fitted are considered as one single component. RFS is recommended. • Clearance fit: relative movement allowed. MMC is recommended. Mating condition attributes serve purposes in part’s location and orientation in the assembly. They can be categorized as: locate, sit, contact, align, etc. They can be used in guiding selection of datum features. • Locate: preserves the relative location of two components, such as a pin interface. The features that are used to locate, such as pin or pinholes, are normally short enough so that they will not control the orientation of component in the assembly but only location. They are not good candidates for primary datum features. However, as they control the position of component in the assembly, they are good candidates for secondary datum features. • Sit: preserves 3-points contact between two planes, controls 3 DOFs (Degrees Of Freedom): 2 rotations and 1 translation. If the two planes are large enough (for stability purpose), they can be best candidates for primary datum features. • Contact: preserves 1-point contact between two surfaces, controls 1 DOF: 1 translation. The surfaces are perfect candidate for tertiary datum feature. • Align: preserves 2-points contact between two surfaces, controls 2 DOFs: 1 rotation and 1 translation. The surfaces are perfect candidate for secondary datum features.