David SherrerCase Study – Tolerance Analysis of a New Portable Modular Test Instrument21 May, 2000
Description Tooling of mechanical parts is expensive. Ensuring that the design works as intended the first time is critical for time to market and keeping within budget. Tolerance analysis of key interfaces is one tool for establishing proper part specifications for manufacturing. The tolerance analysis of a new modular test instrument developed at Agilent Technologies is presented. How the critical interfaces were identified along with the development of the tolerance model from the CAD data is presented. The results and key learnings from the analysis are also discussed.
What this Presentation Covers • Justification for doing the analysis • (Very) brief overview of the product (just enough for the discussion) • Process for discovering the critical interfaces • The development of the tolerance model • Results and key learnings from the analysis
What this Presentation Does Not Cover • Discussion of VSA and its capabilities • Comparison of VSA with other tolerance analysis packages • Comparison of the Monte Carlo method (which VSA uses) with other tolerance analysis methods
Justification for Doing the Analysis • Business is driving a new organizational model. Innovation and competition are changing the product lifecycle. • Stability in the schedule is just as important as reducing overall project schedules. • Tolerance analysis allows for constructive negotiations with suppliers over part specifications. Analysis allows for focusing on what is important. • High risk and expense to rework tooling. • Need to carry low inventory. Do not have the luxury of selective assembly on the production line.
Concept MR End of Life Product LifecycleBusiness Fundamental Drivers • Current • Future Number of Issues (create & solve) Shorten Shorten Minimize Minimize Minimize
New Portable Modular Test Instrument CAD Model Photo of Instrument
Process for identifying the Critical Mechanical Interfaces • Establish the requirements for the design. • Identify all the parts in the assembly. • Create an assembly flow diagram including any special fixtures or assembly steps. • Establish the interrelationships between the parts, i.e. compatibility, clearance, contact, and isolation. • Identify those interfaces that affect the key requirements of the design.
Critical Mechanical Interfaces • Connector on the plug-in module should align and engage with the connector on the backplane. • There must be adequate clearance between the plug-in module and the interior cavity of the extrusion. • Need to ensure that the lower arm of the plug-in module engages the slot in the extrusion. • The latch on the connector module must engage in the recess of the plug-in module**. • And many more … ** Critical interface discussed
Development of the Tolerance Model • First used the Unified Model of tolerance analysis with vector loops to determine the feasibility of the design and manufacturing processes being considered. Used common manufacturing tolerances for each part. • Validated the part tolerances with suppliers. • Created VSA model – Includes over 3000 lines of VSL code accounting for 379 tolerances and 66 measurements of features on the model (mainly concerned with the critical ones). Of those 66 measurements in the model, some are used to validate the integrity of the model • Tolerance model includes a “virtual” assembly to account for the form tolerances of the extrusion. • Performed 500,000 Monte Carlo simulations and 18,619 total High-Low-Median simulations. Total run time was 31.4 minutes on a Pentium III 650MHz PC.
Development of the Tolerance Model (Cont.) • Continued to refine the model based on measurements of the first prototype parts.
Results and Key Learnings from the Analysis • Analysis identified problems with how the parts were assembled in the CAD model (nominal dimensions). Noted misalignment of the connectors on the plug-in modules and those on the backplane pc board. • Analysis helped establish key part datums and assembly methods. • Analysis validated key critical interfaces and part specifications. • Monte Carlo method is easier to comprehend for design team and suppliers. • Analysis is still too tedious and time consuming. Results are very sensitive to the accuracy of the tolerance model. • Need methods for including effects of part deformation in the model.