Letters From the Fab Academy, Part 6

In this periodic series of “Letters,” Shawn Wallace, member of AS220, the Providence, RI community arts and technology space, shares his experiences with the Fab Academy, a distributed learning collaborative, built on the infrastructure of the Fab Lab network. — Gareth

Machine Design

By Shawn Wallace

One of the most exciting and challenging of the Fab Academy sessions this year was the unit on machine design during which we built a functional milling machine that was capable of replacing the $4,000 machine typically used in the lab. In the near future, it is entirely plausible that Fab Lab participants can easily assemble their own suite of tools for minimal cost.

Constructing the Mantis milling machine in the Barcelona Fab Lab

All of the labs participating in Fab Academy received a kit full of motors, bearings, machined rods, and electronics suitable for building a 3-axis CNC machine. Our goal — in addition to making a working machine — was to vet the parts list and procedures to see if they should be incorporated as a standard part of the Fab Lab inventory.

The engineering “project triangle” model is “fast, good, cheap: pick two.” You could say that the equivalent for DIY CNC machines is “stiff, square, cheap: pick two.” David Carr, one of the MIT grad students who helped teach the two-week session, tried to pick all three goals with his Mantis machine design. To achieve his goal of a sub-$100 milling machine, he needed to minimize the number of machined parts. The most important design decision was to make accurate holes for the parallel rods by stacking the parts and drilling them together, a traditional machine shop method called match drilling. The bearings and leadscrew nuts were then epoxied in place, resulting in a simple assembly that does not bind. You can read more about the design process at David’s Makeyourbot blog..

Assembly of the Mantis requires liberal amounts of 5-minute epoxy

The Fab Academy team worked together to assemble the body of the machine. All of the electronics were milled in the Lab from FR-1 copper circuit board stock and stuffed with surface mount components.

Noah Bedford took charge of the electronics system in the Providence Fab Lab

The Fab Academy Mantis kit was actually a combination of two projects, David’s Mantis and Ilan Moyer’s Fab in a Box project. The electronic control system for the machine was from the Fab in a Box, a personal fabrication toolset that can fit into a suitcase. The final working circuit boards are pictured below.

This project was the most challenging PCB fabrication the Fab Academy team had tackled to date

The motherboard (upper left) is an Atmega88 that talks to the various components over an RS-485-based network. RS-485 is a robust serial bus protocol that is well suited for integrating noisy nodes like the stepper and DC motors we are using. The three stepper motor control boards (bottom) are based on the Allegro A3967 integrated circuit, the same family of ICs used in the RepRap and MakerBot projects. The spindle motor is controlled by the H-bridge circuit (upper right) which is affixed to the Z-axis.

The networked infrastucture of the Fab in a Box is called Fabnet. The three axes and the spindle control are nodes on the network, each with an IP address and a virtual machine description that is interpreted by the controller. Files may be constructed in a Python-based command language or as RML files, and HPGL-based format compatible with the Roland milling machines in the Fab Lab.

Screenshot of the Fabnet virtual machine environment. Each part of the machine is individually addressable by IP address. New
axes or toolheads can be added by plugging in new nodes to the network.

Fab Net was created to drive another promising machine called the Multifab, which you can read more about on Ilan’s blog.

All of these milling machine designs came out of Neil Gershenfeld’s 2009 How to Make (Almost) Anything class that the Fab Academy students could also participate in via videoconference. Jonathan Ward built the first machine, followed by several other students who tweaked, improved, and pared the original design. It was interesting to witness how friendly competition can spark a micro-ecology of innovation.

Jonathan Ward works with Makeda and Noah at the Providence Fab Lab on another milling machine

The next (and last) installment in this series will showcase the students’ final projects, which will be presented at the Fab 6 conference in Amsterdam in August.

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