Announcements OpenRUGDMMC

Words from the project initiator

posted Dec 18, 2018, 1:20 PM by Tiberius Brastaviceanu

It will overcome some of the most important barriers to open source hardware production, resolving the absurd situation we find ourselves in, in which we cannot practically make even the most humble of the objects that form more complex hardware systems. Any mounting bracket made from metal, or object which is of larger size (such as a foot across, to the size of a vehicle chassis), or cannot have the poor tolerances and ridges that come with 3d printing, is a serious barrier.  When we encounter dozens of these, it scuttles prospective projects.

OpenRUGDMMAC will solve this.

I am currently trying to secure a room to work on it.  I have some $12k in funds saved up to give myself some time and money to work on it, but crowdfunding is a high priority.  

The immediate main stage, right now, is to produce a document describing the mark 1 prototype, and high level process algorithms in English or pseudo code.  I need to collaborate with others, in particular on the software for process planning, including a form of slicing at the right z heights based on model geometry, the generation and conversion of the machine agnostic path to a set of gcode programs, which have the deposition steps interleaved with the machining.  None of that is cutting edge.  However it would not be easy to produce from scratch.  The path planning for 3d rest milling is the hardest part.  There is extensive work done on this in the technological ecosystem which we can leverage to relatively rapidly get things set up.

In the very near term, to drum up collaborators and money, the production of a good set of seed documentation, including describing the process and the importance of it, which the long document is a start on (needing editing down and some animations more than anything), and to focus on what I would dub a mock up of the process, seems like a good approach.

For the mock up, I can draw a good test part in solidworks, then simply produce the machining gcode by slicing it manually, and using hsmworks to do the cam programming.  This produces the gcode required for all machining.  I am in the prices of trying to secure an auto tool change mill/router whose characteristics I know, having used it for some months previously.

The deposition step will require at least a modest apparatus.  However I may simply resort to spreading it on the layer like peanut butter, to keep everything very simple. The reality is that to do thing manually until it is more clear what works, is probably a good idea.  It is not clear how long it will take to set, if carbon dioxide may be used to speed setting, if the material can be dispensed from a syringe or piston or if some other means must be found, if vibration will be important, or whatever.

Supposing the layer height is a millimeter, and the build volume is five cm by five cm, a five cm block would take five hundred layers.  One thousand deposition steps, one for each material.  Clearly the layers must be rather thick, or it must be automated for more serious testing, then.  Also layer deposition time cannot be long. The layer height will often be much higher, if the material can set at such depth. The materials used in so called ceramic block casting can be applied as bulk solids.

The possibility of causing powder to bond together under ultrasonic ation and pressure is very interesting.  There is a document I found in which the authors were reporting successfully using 9000 pascals of pressure, 20 kilohertz at 1.8 microns amplitude vibrations in the direction parallel to the upper surface of the powder bed, to successfully bond 10 microns aluminum powder.  This is slightly surprising, as powder consolidation usually employs much higher powers and pressures, however they are probably tolerating lower densities of the resulting solid.  I wrote to the author.  They say they have more papers under review describing their experiments.

I have found a document describing the use of 1 percent by molarity of a transition metal powder of 325 mesh, combined with powdered graphite (mesh size I dont recall or was not listed) to grt the graphite to effectively bond together under pressure only.  Ultrasonication will probably enhance this.  The material had500 psi compressive strength, which should do for us.

Ultraso ic transducers in the 100 watt 20-40 khz tange are available through e.g. Amazon easily.  Higher powers should not be hard to obtain.  1.7 mhz is commonpy used in fog generators.  Some common machines I found use ten disks of a total of 25 watts each.  It remains a matter of hypothesis if higher frequencies would be helpful.  Also whether that is the input power or the output power, frankly. It appears practical to stack such transducers with channels for cooling water flow between them to achieve higher power intensity.  Unfortunately although promising, ultrasonic binding looks like it will take significant developme t effort, which may be unwise at this stage.  It may be more advisable to use binder and powder, such as low viscosity wax and a water soluble wax, instead.

Colloid makes a very important potential binder, silica or zirconia or even graphite colloids especislly..  it epuld be good in genersl for aluminum, plastics andgel cast ceramicsand useable for steel probably, with some precision loss.  However drying time may be an issue.  There are however many ways of causing colloids to gel, indeed the hard part is generally getting them to not gel.  We increase the colloidal concentration until it is easily destabilized by changes in ph or temperature, for instance, and then drop the temperature, or raise it upon deposition by ensuring the syringe containing the undeposited materisl was at a different tempaerature.  Or directing a straam of carbon dioxide at it to change the ph might work.  Gasses like ammonia are used industrially to do this during investment cadting coating operstions.  However they are slightly hazardous and harder to obtain.  Freezing works.  There are many options.  

The water in the colloidal system would dissolve any water soluble components in the support material.  There are colloidal systems based on alcohol instead of water, but it is probably more adviseable to use a system for the support material that is alcohol rather than water soluble instead, becausr yhe chemistry of water based colloids is much better known and manipulated.  A wax and an as yet unitentified solid, alcohol soluble material would be good.  Or soluble in hexane or some other solvent. Acetone is a bit harder to work with. They are all flammable. But we can roll with that for now. We need to peoduce test parts to bring in interest and money.  A common filler used in combination with wax in investment casting is cross linked polystryrene.  That might be a good choice.  The styrene monomer might do, too.  Really any random solid that is available in suitable form, low cost, recycleable, is the main thing, but I dont have many candidates right noe.  Perhaps a better approach is to start with a survey of easily available powders that are solvent soluble.  Hexamine is probe to sublimation at relatively low temeratures and used in fuel tablets, that might be kind of handy as it would be easy to remove even without sokvent.

Ulyimately I thi k I need to buy a bunch of different powders and get down to a lot of experimenting in the lab.  First, I need a lab. As of dec 7 I have signed a binding contract for a commercial bay that is a good start on a lab.  I have for some months is seed a storage unit so I have benches and may tools already.

However one of the major issues on the horizin is the sintering that occurs at high trmperature involved in casting steel and titanium etc.  Ultrasonic bonded solids can be expected to minimize this, as bondiing methods go, because thr small particles and small contact areas between particles that occur in colloid bonded powders are avoided.  The paricles are essentially already bonded with large contact areas, greatly reducing the ensuing shrinkage and distortion that will result from sintering.  Graphite is also one of the highest melting point materials we can easily work with, which implies relatively slow sintering rate.

Frequently in trying to design a promising apparatus, I encounter the problems the system is meant to solve.  Making custom parts is expensive and time consuming.

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