Portable Uploadable Problem Set 4
This is the kinematic coupling that I built and tested.
For design and build overview, please review Brainware and Hardware from week 2 respectively.
Testing
For this week, we were requried to test the following characteristics of our kinematic coupling:
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Repeatability
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Accuracy
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Stiffness
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Repeatability Test
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I decided to take this opportunity to perform another repeatability test, to compare my results to data from week 2's test(see Hardware 2). Previously, I measured a spread of 4.56mm after 10 trials, when the laser pointer was 6 feet away from the location of measurement. This corresponded to an angular variation of 0.14deg.
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This week, I clamped my coupling, with a pointer fixed to the top surface of the top half, on a bench vice. I set the location of measurement to be 15ft away from the pointer.
From the test, the horizontal spread was measured to be 22mm. With the pointer, 15ft away, the angular variation was 0.27deg. This is nearly double the variation estimated from week 2's experiment!
The vertical spread was measured to be 4mm. This corresponds to a vertical angular variation of 0.05deg.
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Accuracy Test:​
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In order to measure accuracy, I wanted to measure the distance between the centroids of the top surface of the top half and bottom surface of the bottom half of the couplings, respectively. I couldn't find a micrometer, hence I resorted to using a pair of vernier callipers, which were not ideal.
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The ​sample mean and standard deviations were 38.64mm and 0.02mm respectively.
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I plan of repeating this test with a suitable micrometer.
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Stiffness Test
With lack of access to suitable measurement instrumentation, I was not able to perform the stiffness test. However, a day ago I thought of two plausible ways of measuring this.
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Use an Instron to apply a known load and then measure the net displacement. I will need to wait for Makerworks to set up their newly acquired Instron first.
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Use dumbbells from the gym as loads and measure the vertical compression using a laser pointer mounted to the top of the dumbbell. This is more doable test but results will not be precise.
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Closing the Design Loop
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From Professor Slocum's Kinematic Coupling Spreadsheet, the net vector displacement (mostly from error along Z), was estimated to be 45nm. Clearly, the fabricated coupling did not meet this expectation. Although the dowel pins and the metal ball bearings were pressed into their respective pockets, they may not be resting on the same plane as each other given surface roughness and tolerance errors.
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The expected stiffness of the coupling is 30N/micron. The actual stiffness will be tested in the coming week and compared to this value.