Problem Statement
The purpose of this lab was to explore fixture design, fixture design strategies, and modular fixturing and its components. Upon exploration into these three areas, it was necessary and beneficial to learn best practice and gain the experience appropriate to successful implementation of fixture design, strategies, and modular fixturing. Furthermore, it was also critical to develop a proficient and professional documentation scheme for presenting the data, results, and analysis of such exploration.
The benefit of exploring, learning, and adopting best practice for fixture design and all subsequent strategies and implementation, whether custom or modular, is in the value added to the manufacturing process. Fixturing is a necessary, important, and mission critical component to the manufacturing process. It is important to understand the relationship between the implementation of GD&T in design and how that relates to fixture design and strategies. That is, GD&T enhances the capabilities of fixturing in providing a framework for establishing robust and consistent fixturing strategies that can be used to ensure the integrity of design, the efficiency of manufacturing, and the reduction in waste, time, and cost.
Failure to employ GD&T within the design of fixtures and disregard for its import to the formulation of fixture strategies enhances exposure to error, redundancy, and introduces confusion to the fixturing process.
Additionally, this lab set out to analyze and identify the three machining operations that were required to produce the features indicated on the engineering drawing provided for the lab. From this analysis and determination, it was further determined the sort of fixture strategy necessary to achieve the machining of these features without interference to the fixturing components while maintaining workpiece rigidity and constraint.
Lab Setup
The procedure for this lab involved designing a fixture strategy using a part’s engineering drawing. To do this it was necessary to identify the part’s datums, then indicate their datum simulators on the drawing with symbolic representation. Along with the simulators, it was necessary to perform a force analysis to identify and indicate clamping and constraining forces necessary to constrain the workpiece during manufacturing. Once a strategy had been established in said manner, it was then necessary to identify and source modular fixturing components from Carr Lane documentation that were suitable for the specific strategy to be implemented. Once fixturing components were determined, it was then necessary to design the fixture with the use of SolidWorks CAD modeling, specifically, using assemblies within SolidWorks to create a fixturing setup for the part to be constrained.
The purpose for this procedure was to reinforce and solidify the practices common to the implementation of GD&T and fixture design and strategy. This was achieved through a practical hands on approach through the use and analysis engineering drawings to design a strategy. Then to use that strategy to inform the identification of fixturing components. And finally to design and model said components in a fixture setup suitable for workpiece holding.
There were several limitations with using both the software and the sourcing of fixturing components from the Carr Lane documentation. The limitations in the software ultimately stemmed from the inaccessibility of the entire Carr Lane component lineup. That is, the entire listing of Carr Lane fixturing components were not provided in the set of SolidWorks model files. Beyond this, SolidWorks was more than capable for designing with modular fixturing components within the assemblies function of the program.
As indicated in the Problem Statement section above, it was necessary to analyze the engineering drawing to determine the appropriate machining operations, and in conjunction with the force analysis, determine the appropriate fixture strategy and fixture components to resist machining forces and constrain the workpiece.
Data, Results, Analysis Theories
It should be noted that there are no mathematical relationships or equations pertaining to this lab. However, it is important to briefly discuss the significance of datums, datum features, and their simulators as it relates to fixture design and strategy. Given that datums are the theoretical planes, lines, points, or axes that represent the “perfect” referential basis for dimensioning and tolerance of component features, and that these datums are physically represented by specified datum features associated to functional, high tolerance, mating, or high valued component features, it is essential to establish all associated datum simulators as a) theoretically perfect geometric counterparts to the datum features, and b) practically as physical fixturing mechanisms or components that maintain and satisfy both the integrity and the parameters of datums and their features. To this effect, fixturing components are designed to be very high resolution, and thus are precision machined with low to no variability across all surfaces, edges, points or geometric shape.
Data
The following section includes the datum simulator and force analysis diagram, informational data regarding the three machining operations to be performed on the workpiece (including a feature callout diagram), modular fixture components bill of materials (BOM), the designated fixture strategy employed, and the finalized fixture designs produced in SolidWorks.
Results
The diagram of the datum simulator and force analysis provided the necessary framework for formulating a fixture strategy. That is, the ability to visually indicate on the diagram the placement of simulators relative to datum features and the force vectors relative to counter machining forces greatly enhanced the capacity to design an appropriate fixture setup for the workpiece. The positions of the simulators and the direction of the force vectors resulted in constrained workpiece with a net force and moment vector of zero.
The machining operations were identified and analyzed allowing for the consideration of fixture design elements to provide the appropriate amount of support at the features to be machined in order to eliminate any bending moments and thus deformation. The position of these features, the types of features, and the machining operations necessary to be performed to produce these features informed the selection of fixture components and their placement. The placement was decided such that the workpiece would be constrained in the XYZ direction through the use of serrated adjustable clamps, adjustable stops, and hook clamps. Furthermore, the position of fixture components was done such that maximum support of the workpiece was achieved, in addition to ensuring that fixturing would not interfere with machining operations.
As a result of the above paragraph, it was necessary to use many extension supports to achieve proper placement of fixture components. In conjunction with these supports, at least for the bore operations, relieved support cylinders were used to allow for debris extraction and proper clearance on tooling while maintaining integrity of the machined feature and the immediate surrounding region. For supporting the workpiece in the Z direction and in conjunction with datum A, support extensions were used with collared bushings, in addition to screw rest pads. For datum B and constraining the workpiece in the X direction, locating screws were used as edge locators. For datum C and constraining the workpiece in the Y direction, an adjustable stop was used. For clamping or counter forces and constraining the workpiece in the X and Y direction against datum B and C, serrated adjustable clamps were used and positioned such as to maintain the integrity of the 3 2 1 fixture scheme.
Analysis
It was found that challenges of fixture design in general are significant in trying to achieve a successful application of fixture strategies that constrain movement of the workpiece without over complexity and interfering with machining operations. The balance of needing a robust and reliable fixture setup that can achieve proper constraints without being over complicated and interfering with machining is a difficult task. This task is complicated by the necessity to have precision components that accurately replicate datum simulators in a practical and applicable way, so that the interface with machine is easy and repeatable. This is where a need for customization of fixture components can greatly enhance one’s ability to achieve a proper fixture setup and employ a successful fixture strategy given a specific workpiece and a machining operation. However, anything custom can be labor intensive, costing time and money. Another disadvantage of custom fixturing is its limitation for use because of the unique nature in which they are employed.
Regarding modular fixture design, especially considering the use of it for this lab, was both helpful and frustrating at times. Much of the frustration of modular fixturing came from the limitations of the modular system as a whole. That is, modular fixture is based on a preset grid system for which all subsequent fixture components are designed around. This forces a lot of unique geometry and limits the types and assortment of configurations you can create for a specific purpose. This isn’t all a negative aspect of the grid system, for it does make for a wide range of possible fixturing without the need for customized fixtures. In the case of this lab and the fixture setup that was designed, many other attempts with varying fixture components were made, but always ending up with one or two components that were misaligned or inadequately positioned for the intended function given the combination of fixture and placement up to that point.
Regarding considerations for fixture design for varying part quantities, it seems critical to incorporate quick clamping mechanisms for such an operation. It could also be valuable to design a fixture setup that allows for the work holding of multiple workpieces per batch. However, with increased batch size, it is important to consider exclusively the use of quick release clamping systems. Any other approach would be too time consuming and reduce productivity greatly.
Conclusion
To recap, this lab investigated the nature of fixture design and fixture strategy using an approach that employed datum simulation and force analysis on a given workpiece. Through the identification of datum simulator and forces, it was then determined what the most appropriate fixture strategy to employ would be, and in turn a fixture design was created through the use of modular fixture components. It was determined that modular fixturing is a economical and viable option for fixture design, though it does not come without some disadvantages and limitations. In the end, it is more feasible than customizing fixturing for every application needed, and provides a versatile alternative as such. I would definitely recommend expanding to a larger modular fixture kit were a large volume or an increased mix of workpieces desired to be machined.
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