WEEK01: introduction, computer-aided designThis week we got a crash-course in CAD work. Neil used FreeCAD as an example to demonstrate parametric design, rendering, animation, and more. Our assignment is to model -- in as many ways we'd like -- a potential final project. One project idea I have mulled over for some time is constructing an open-source, easy-to-fabricate, sub-milligram analytical balance. These instruments are a critical tool for a wide variety of scientific research and education, but good ones (even on the used market) tend to cost in the thousands of dollars. Also, used scales are gross in a potentially dangerous way depending on their past lives. And despite the proliferation of open science instruments, it doesn't seem like anyone has yet tackled this key device.
- 3D printed scale mechanism design file, Fusion 360 format
- 3D printed polyamide tape clamp design file, Fusion 360 format
- Milled acetal flexure test coupon design file, Fusion 360 format
sketchesI started by sketching a potential finished balance in my notebook. I particularly like dot paper because it works for flat and isometric sketching: Translating the notes, because my handwriting isn't the best:
- The scale should have an easy-to-use serial port that connects via USB; no special hardware required beyond an FTDI driver. This port is used for scale configuration, datalogging, and remote control.
- An analytical balance probably isn't worthwhile if it can't resolve in the 0.1 mg range, with at least +/- 1.0 mg accuracy. Since a 0.1 g scale is part of the standard Fab Lab inventory, it's probably okay if the instrument tops out at <200 g.
- One of my favorite undergrad labs involved hanging a lump of copper from an analytical balance and suspending it in an open-top furnace. We were able to plot the copper oxidation rate and learned a great deal about things like passivation, diffusion, and oxide states. This scale needs an easily accessible hook for hanging stuff!
- A graphic display is tempting but should be considered a bonus feature; a ubiquitous 16x2 LCD is more than enough for most weighing tasks, and the serial port can handle the rest. The physical user interface should be dead-simple, probably just the display and a big tare bar.
- Analytical balances have draft shields. This is one place it could be fun to get weird on the design. Sick lambo doors on a lab instrument? Sounds stupid and awesome.
- A defining feature of an analytical balance, and what differentiates these instruments from common kitchen scales, is the use of a precision electromagnetic servomechanism. The weighing plate doesn't really move; instead, force is derived by measuring the power required to keep the scale balanced, using an electromagnet and an optical feedback device. Prototyping this part using a Fab Lab-accessible process is at the heart of this project.