Sun 18 Aug 2013
Posted by Lisa
Open Source Ecology’s HydraFabber, now in development, is a foldable, portable combination 3D printer, CNC circuit mill, and laser cutter. Its purposes will be to enable 2D and 3D rapid prototyping for open source machine design and to mill circuits needed for OSE’s modular machines in the Global Village Construction Set.
In coming weeks, OSE will take the HydraFabber to the finish line, realizing the dream of the 3-in-1 machine proposed by Leo Dearden and worked on by Nate Wettstein, John Kobmla Stager, and Tim Boyd.
At this point, the laser cutter design is ready for review—heads up, please, if you are a laser expert or have friends who are: we would like to have laser experts look at our design and calculations and offer feedback. We’ve already had much help from Tim Boyd, a laser expert and active contributor to our design sprints. Tim has produced the design for our laser-cutter component of the HydraFabber. The laser cutter enables 2D prototyping by cutting paper and cardboard patterns to be made in steel on the CNC Torch Table. In Part 1 of this post, Tim lays out the technical details and calculations for the laser cutter for review.
Dedicated Project Visitor (DPV) John Kobmla, meanwhile, is developing the CNC circuit mill part of the HydraFabber, and that work is nearing completion. In Part 2 of this post, we’ll take a detailed look at progress on the CNC circuit mill.
Part 1: Laser Cutter
Check out the conceptual diagram for the laser cutter and please pass this on to any of your laser expert friends. We are reviewing the current design and would like your feedback on (1) general design considerations, (2) verification of our calculations on laser cutting capacity, and (3) any comments on feasibility, especially if you have experience with lasers. Please send feedback to Tim Boyd.
After the calculations below are reviewed and after we receive feedback, we will purchase the parts and build the laser cutter.
The laser cutter design
The laser cutter uses a focused diode laser to cut paper and cardboard by thermal ablation, or the vaporization of the paper fibers. This process eliminates uncontrolled combustion.
We will scan the laser position in X dimension, and the paper will be panned in Y dimension. Ablation requires highly focused intensity; therefore, high laser power is required.
The necessary 975-nanometer wavelength, 10-watt fiber-coupled diode lasers are now commercially available at low cost. The laser power can be delivered to the X,Z scanning stage by a multimode fiber optic. The laser power is then focused onto the paper with a fiber-coupled optical assembly. The assembly focuses the power in a 200-micron spot at a distance of a few inches from focusing optics. This arrangement allows smoke and particulates to be vacuumed away from the optics.
A simple calculation regarding paper-cutting speed
Please review these calculations for our 10-watt laser cutter:
Ten watts of laser power focused in a 0.2 mm diameter spot results in an intensity of 30,000 watts per square cm or 300,000 times the intensity of sunlight.
Assuming the paper absorbs 1/2 of the incident power (or 5 joules per second) in a 0.2 mm diameter spot and assuming a paper thickness of roughly 0.1 mm, 5 joules/sec will act on a volume of roughly 3 millionths of a cubic centimeter.
Given a mass density of paper of 0.7 gram/cc, the mass is roughly 2 micrograms.
The specific heat of paper is 1.4 kilojoules per kilogram kelvin, or 1.4 joule per gram for 1 degree centigrade.
All of these factors result in a temperature rise of the volume of paper of roughly 1 million degrees centigrade per second.
Assuming that 1000 degrees is sufficient for vaporization, then a period of one millisecond is required for each spot. This suggests a cutting speed of 200mm per second.
Requirements for initial test
Before the laser module, fiber assembly, and focusing optics are procured, we must decide whether commercial power supplies for diode current and thermoelectric control should be procured or built by us and then tested by experienced personnel.
Also, laser safety is paramount. The 975 nm wavelength is the wavelength presently favored because high-power modules are most easily obtainable at this wavelength. However, these are eye-hazardous wavelengths, so the paper-cutting operation must be completely shielded by plastic that is totally opaque at these wavelengths. We should investigate dye-impregnated vinyl or other transparent plastic for its optical density at 975 nm. Fortunately such material is available for industrial application for the high power Nd:YAG wavelength of 1064 nm. However we must have a minimum of equipment to assess light leakage.
A big case, possibly one that folds up, will be needed to enclose the fiber and the X-Y axis. We are working on the first cut of a design for a frame that can be lowered down—a box of steel angle iron that will hold vinyl sheeting impregnated with dye that will filter out the light the laser emits, since that wavelength of light is dangerous to the eye. You can look through the vinyl and see the progress of the cut, but the vinyl will block the harmful 975 nm light, protecting the eyes from a burn or local blindness. It must be absolutely black at 975 nm to prevent light exposure to anyone who might be standing nearby. This is critical to prevent eye damage or even blindness and to meet OSHA requirements.
Vacuum pump considerations
The caustic of rays is the focus of optical power or radiant flux, or, more precisely, the geometrical curve of the envelope of rays where they focus concentrated light. In the illustration below, the caustic is the transparent object, red in color.
The caustic of rays will encompass whatever smoke and particulates are generated. Because smoke will interrupt the power from the lens, it is essential to maintain a small vacuum current to clear the region where work is taking place. An aquarium pump used in reverse might be sufficient to remove smoke from the enclosure, or a small circuit cooling fan might be used.
Bill of materials
We are ready to source the parts for the laser. The total cost for the 10-watt laser cutter portion is about $2000, as follows:
Part 2: The CNC Circuit Mill
The CNC circuit mill will enable OSE to produce the circuits needed for modules in various machines of the Global Village Construction Set. For the CNC circuit mill, John Kobmla has built upon the Ilan Moyer open source spindle from the Center for Bits and Atoms. John is currently working on the control software for running the CNC circuit mill, utilizing Fab Modules from MIT.
After a mere four weeks, the HydraFabber has a CNC mill spindle assembly. Now we move on to the milling bed and the mill controller—the Fab Modules. The CNC mill controller and bed are in the integration process and will be completed by Wed, Aug. 21st, 2013.
The journey has been interesting, to say the least, and fun, both for OSE and the HydraFabber Team. Our documentation shows how far we have come and the progress that has been made to bring us to the current achieved stage.
To see how the spindle goes together, view our spindle assembly animation:
And here is the spindle installed:
For more pictures, see John’s log.
Feedback is welcome. Special thanks to all our supporters and well wishers for all their encouragements and to those who have contributed to the various open source designs upon which the HydraFabber’s components are based. Our work stands on their shoulders to reach to realize new possibilities for open source rapid prototyping and circuit milling.