In 2.72 - Elements of Mechanical Design, my team was charged with designing, building, and testing a desktop lathe capable of cutting a 0.5" aluminum rod within a tolerance of +/- 0.002". To meet this functional requirement, we first had to create mathematical error models in MATLAB to parameterize how the geometry and material selection would affect the overall error. The use of applied mechanics provided an understanding of the performance of each individual part of the design and where tradeoffs could be made. From this, CAD models and FEA were used extensively to verify part stiffnesses, deflections, and stresses. Throughout the course of the class, we learned how mathematical models could be used to make informed design decisions and in turn how these design decisions affected manufacturability and ease of assembly. By the end of the semester, we were able to deliver a well-designed, assembled, and tested machine that performed within specifications. Our lathe survived the required drop test and performed the best out of 6 other undergraduate teams.
I served as one of the systems modeling specialist on my team and as the machining lead. My responsibility as a systems modeling specialist meant creating engineering calculators to drive design decisions such as bearing selection/layout, material selection/validation for components of the z-drive and x-drive, and pre-load. I was also heavily involved in the mechanical design of the x-drive and z-drive, which implemented the use of flexure bearings that allow for compliance in the degrees of freedom and high stiffness in the degrees of constraint. This practice of exact constraint design meant that we had some difficult components to machine, and I was also directly responsible for this aspect. Spending a significant amount of time in the machine shop meant that I was able to contribute a significant amount of knowledge that led to better design decisions.
Run-out testing performed on spindle
Perspective.. Which lathe came first?
The carriage has fixed linear bushings on the right side and a flexural mechanism with a potted linear bushing on the left side. The flexure provides lateral z rail compliance due to any misalignment from assembly or part tolerances. The dancing man, another flexural bearing, houses the leadscrew nut and attaches to the center of the carriage. It provides compliance to prevent binding and stiction from leadscrew misalignment.
You can see the breakdown of the Z-axis components. A noteworthy item here is the dancing man.. it looks very life-like!
Another flexural bearing that makes up the cross slide to provide X-axis tool movement. It has a total range of one half inch of travel.
The whole kit is actually quite small, but the composition of many parts is what made this worth every hour.
The competition required turning a blank round aluminum rod to 0.24" with a length of 1.00". The score was based on a formula that generated a point value based on machining time and final dimensions. Daniel Huertas is taking on the lead as the machinist here and has a supportive team feeding information on how best to proceed next.