Fab@Home:Model 1
From Fab @ Home
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Structure
Model 1 This is the first (basic) design of a fabber and a deposition tool. Its basic construction comprises a 3-axis Cartesian gantry positioning system driven by stepper motors attached to lead screws. Material deposition tools are modular, and the first tool we have designed is a syringe-based extrusion tool, which uses a linear stepper motor to control the syringe plunger position. The electronics and firmware of the current version of the system provide for up to 6 axes of bipolar stepper motor control at 24V, with 2 limit switches per axis of positioning plus an optional one limit switch for the remaining 3 axes. A microcontroller controls the positioning of the axes and is in bidirectional USB communications with the PC. An application running on the PC displays the real-time state of the machine numerically and graphically, and allows the user to manually position the axes, import and perform basic modifications to STL geometry data, apply specific material properties to each STL, and to generate and execute tool paths.
One concern in developing our design is that there is probably a minimum threshold of quality required in new technology kit for hobbyists, below which the excitement of the new technology will be masked by the malfunctions, maintenance problems, and poor aesthetics. Users must have a sense of what the technology is capable of before they can grasp how to modify it and apply it to their own purposes. Our first design has therefore focused more on ease of use, reliability, and aesthetics than on minimizing cost.
We have tried to assume a modest availability of technical tools and skills for the envisioned end user. The person interested in our kit would need to have a laptop or PC with a USB port, and basic assembly skills and tools, including soldering and crimping of cables and connectors, but as much as possible we have tried to design the kit such that it can be fully assembled with standard components and tools such as Allen keys, screwdrivers, scissors, pliers, and soldering iron. The main assembly tasks are snapping together the acrylic structure, inserting nuts and screws and threaded inserts, bolting together the positioning system components and mounting them to the structure, making cables to connect the microcontroller to the amplifier boards, and the motors to the amplifier boards, mounting the electronic boards to the chassis, and bundling and routing of cables. The user is expected to have some patience as well: completely assembling a kit from parts to operation requires roughly 12 hours of labor. Currently, the parts cost is estimated to be $2000. We hope that through increased volume, improvements by the user community and vendor sponsorship, these costs can be reduced significantly in the future.
Assembly
The structural components of our system are built from laser-cut acrylic sheet parts held together with snap-fit joinery and simple “T-nut” style screw/nut fastening, in which a square or hex nut is inserted into a slot in one part, and a screw threads in through a hole in a perpendicular part. The tight manufacturing tolerances achievable with laser cut acrylic enable us to produce good orthogonality and alignment in the machine base, increasing the ease of setup and reliability of the machine. Our first three units have been produced in-house with an Epilog Helix 35W laser engraver, cutting parts directly from SolidWorks drawing files, which are in turn generated from a SolidWorks 3D model of the complete system.
There are 33 acrylic sheet parts in the chassis, plus an additional 7 acrylic parts for the syringe tool. Currently, six sheets of 18” X 24” X 0.236” cast acrylic are used to produce the acrylic parts. The packing of parts into these sheets is not optimal – only about 50% of the sheet surface area is converted to parts. More efficient packing is feasible.
Cutting the parts for an entire machine requires three hours on our 35W laser cutter, and costs about $250 if done by a service provider. An additional benefit to having the structural parts laser cut, is that there is a large installed base of laser cutters/engravers in the sign making, and trophy and gift engraving industries. The custom nature of the work in these industries should make them amenable to kit builder’s approaching them to have parts made. In addition, most laser cutters used in these industries do not require specialized CAM software – they operate from an application printer driver, so almost any image editing or vector drawing program can be used to design or modify designs for laser cutting and engraving. Thus, the Fab@Home kit hacker does not need to invest in 3D CAD software to make or publish new hardware designs, and the bitmap engraving possible with these laser cutters allows them to customize the appearance of their machine with images and text. It is our hope that we can negotiate some reduced cost for laser cutting services for the Fab@Home community, at least for a standard design, in exchange for advertising on the Fab@Home website and a large volume of customers.
Positioning System
The linear motion components of the positioning system use off-the-shelf linear ball-bearing pillow blocks running on ½” diameter rails (McMaster-Carr, Inc.). The X and Y axes are in a gantry configuration with the deposition tool riding on the Y axis, which in turn rides on the X. The Z axis moves the build surface independently from the X and Y to minimize acceleration of parts as they are being fabricated. We have selected HSI Inc. linear stepper motors for our actuators because of their simple design, high resolution, and the semi-custom manufacturing focus of the company which permits specifying precisely the leadscrew and bearing journal dimensions required for our application, simplifying the overall design and assembly. For the X,Y, and Z axes we use NEMA size 14 bipolar motors with rotor-mounted lead screws. External polymer lead nuts are mounted to the axis carriages. In the case of the X axis, a timing belt and pulleys (Stock Drive Products, Inc.) are used to couple a slave leadscrew to the motor leadscrew to achieve symmetrical drive of the gantry. The force, maximum speed, and positioning resolution all depend upon the lead screw threading selected. We use the “B” series threading for the positioning axes, which allows for 15.8 μm travel per full step, a nominal top speed of 25 mm/s (1600 step/s), and a maximum thrust of 120 N.
Deposition Tool
The syringe tool structure (Figure 4) is also constructed of laser cut acrylic parts with snap fit joinery and T-nut fasteners. A linear stepper motor controls the position of the syringe piston. We employ as NEMA size 8 frame motor with a rotor mounted lead nut. The lead screw, which is not captive in the motor, has “7” series threading, 3.2 μm travel per full step, and the motor can achieve a top speed of 5.8 mm/s (1800 step/s), and a maximum thrust of 90 N. For the 10cc syringes we use, this amounts to a 1.1 cc/s maximum volume flow rate, and a maximum syringe pressure of 460 kPa (67 PSI).
The current syringe tool has been designed to allow EFD Inc. 10cc disposable syringe barrels to snap in and out, and for the piston to be quickly attached and released from the motor leadscrew for quick changing of materials. A metal nut fits tightly inside of disposable syringe pistons, and one end of the motor leadscrew has threading to match the nut. When firmly threaded into the nut, the leadscrew is prevented from rotating with the motor rotor, and hence the rotor motion is converted to linear motion. Manually unscrewing the leadscrew from the nut allows exchanging syringes regardless of how full without the need to move or remove the piston, facilitating fabrication of multiple-material objects, and conserving materials.
We have also developed a dual syringe tool (Figure 4) which allows two materials to be loaded simultaneously and independently deposited. As mentioned before, tools are bolted to the positioning system, and tools are intended to be modular. We have selected a syringe deposition tool as the first design because of the broad range of materials useable with such tools, and for the intuitiveness of operation.
Electronics and firmware
Personal computers today typically provide several USB connections, but no longer have RS-232 serial ports, though this is still the de facto standard serial communication method for robotics and microcontroller hobbyists. As a result, we have opted to support direct USB connection to our Fab@Home system, despite the additional development work and (internal) complexity that this entails. We chose to use a microcontroller with an on-chip USB 2.0 peripheral, the Philips LPC-2148 ARM7TDMI (Royal Philips Electronics N.V.). This is a very high performance, 60MHz, flash-memory microcontroller with a wealth of peripheral functions for future expansion, including ADC, DAC, PWM, counter/timers, real-time clock, high-speed GPIO, UARTs, SPI, I2C, not to mention the USB2.0 peripheral. In addition, it has 512kB of flash memory, and 40kB of RAM. The large program memory has enabled us to make a very easily understood and extended packet data protocol for communication between the PC application and the firmware. We use the large RAM space to buffer motion commands so that real-time motion does not depend on variations in communication bandwidth. With our current protocol, we can buffer roughly 670 path points for 6 axes of control. The microcontroller is powered by the USB, and thus can be communicated with even when the amplifier electronics are not powered. The high computational performance of the device enables the system to handle receiving and buffering path points, sending real-time status and position data, and controlling step and direction outputs for 6 axes at least 5kHz – we have not yet tested higher step rates. The microcontroller is available on a 1.5” X 2.5” board with header connectors for all pins and a USB connector for $39.95 in single quantity (LPC-H2148, Olimex Inc.).
Currently, we are using two 3-axis stepper motor amplifier boards (Figure 5) ($170 ea., Technological Arts, Inc.) to power the positioning system and syringe tool stepper motors. These boards have proven to be problematic, and we have had to upgrade some of the components on them to make them function reliably. These boards provide switch-mode current regulation for up to 3 bipolar stepper motors per board. The current regulation allows us to use 24VDC supply and 5V rated motors to get much higher acceleration (hence faster builds at finer resolutions), than would be possible at the nominal 5V. This is a design decision which increases cost significantly for the sake of making the technology more useable. A variety of other boards could be used.
The firmware for the LPC-2148 microcontroller was developed in C language, using the free GNU C ARM compiler, and Rowley CrossWorks for ARM development environment by Rowley Co. UK – available for 30 days unrestricted trial, and with a 99£ educational license, and a 499£ commercial license. CrossWorks is not essential - several freeware IDE’s exist which work with GNU C ARM. The firmware performs the following main functions:
- receiving and parsing of packetized commands from the PC via the USB
- buffering of motion path segments for fabrication paths
- immediate execution of jog motion commands
- configuration of limit switches (present/absent for each axis and direction)
- communicating axis position, limit switch state, and other system status to the PC via the USB
- controlling step and direction outputs for up to 6 axes at > 5kHz step frequency
As mentioned before, the microcontroller has additional resources available for future expansion, and the firmware has been designed with ease of expansion in mind.
Software
A PC application has been written which enables the user to control the machine, import, position, assign material properties to, and generate and execute manufacturing plans for geometry data which is imported in the form of STL files. In addition, the application has been designed around the concept that while the core application may eventually be improved by the user community, in the short term, the hardware configuration, the types of materials, and parameters for depositing materials would be far simpler and more interesting to explore and share. Thus the hardware configuration, material properties, and material deposition parameters are described in plain text parameter files. These files describe, for instance, the color and geometry data used to render the Fab@Home machine, the speeds and threading of the stepper motors, presence of limit switches, etc.
The PC application is targeted for the Microsoft Windows operating system. It is written in C++ using the Microsoft Visual Studio .NET development environment, Open GL for graphics rendering, and the Microsoft Foundation Class library for user interface components. The application has been designed with the aim of maximizing the intuitiveness of use (Figure 6). The user interface includes a 3D rendering of a Fab@Home machine which moves synchronously with the real-time position information sent back by the microcontroller. Dialog boxes allow importing and assigning material and tool properties to the part geometry, manual jogging of the axes via buttons and mouse scroll wheel, including the syringe tool motor, as well as a numerical view of the real-time position and status data from the microcontroller. The application also allows a rudimentary simulation of the fabrication process – the actual manufacturing plan is executed on a Fab@Home software emulator, and the motions are displayed in the GUI for quick checking of toolpaths.
