Large RepStrap

I am currently in the process of building a 2 foot by 2 foot by 1 foot machine. Here are some things to think about in your design, and some of the solutions I have found in my quest for answers.

First: Accuracy of movement must be better than the thickness of the application stream or it will not adhere to what was put down in the last layer. If you are applying a .5 mm extrusion, your movement accuracy needs to be about +/-0.15 mm or better to get a usable product. Many RepRaps are extruding at .3 mm and finding it difficult to maintain enough accuracy at 8" x 6" x 4".

Second: Your table must be extremely flat and consistent to be able to print that .5 mm extrusion stream on it at any point and get it to adhere. I am planning to use a stationary piece of 1/4" thick glass.

Third: The cost of materials for large prints is going to be astronomical. I am designing a head system that uses chopped abs rather than filament. It is approximately one tenth the material cost and readily available in large quantities in a variety of materials and colors. I am designing my head system to have a melting pot and pump that moves the melted material through a heated silicone tube to a light weight head with a solenoid valve and extruder tip. The material hose will be encased by another silicone hose of larger diameter that will have a recirculating supply of hot oil to maintain the temperature of the melted material. This way, when the machine is turned off, there is a way to reheat the material in the tube since it will become solid when cool. The silicone tube is good to about 30 PSI at around 500 degrees fahrenheit. I do not believe more than 15 to 20 PSI will be necessary. With this system, the addition of additional heads is not a difficult or expensive undertaking.

Fourth: Inertia is the largest enemy for RepRap printers. If you watch them run, they beat themselves to death in making the print head bounce all over the place very quickly. If you look at a typical inkjet printer, it goes across, then back, across, then back, turning the flow of ink on and off in minute segments very quickly and accurately. The RepRap systems don't work well in this form because the filament flow does not stop quickly and leaves strings from one end location to the next start location. I believe that the use of my head design will eliminate this. This will then allow greater accuracy and longevity of the carriage, along with faster print speeds.

Five: Using dual rods with bushings or ball bearings to move the head on requires a design that allows for miss-alignment of the two parallel rods without binding or losing accuracy of positioning. It is impossible to maintain perfect alignment over large distances. I have decided that I am going to only use one rod in the x direction and one rod in the y direction. These rods will be at a right angle to each other and will be at different heights so one will pass over the other. My head carriage will be a flat plate with two linear bearings mounted down the middle of the plate on the bottom side at each end, and another set mounted down the middle of the plate at a right angle to the first set on the top of the plate. Each bar will create movement of the head by sliding the plate along the other bar as it moves sideways. In this way it only takes two shafts to move the head and also to constrain it. Each shaft will have a gear attached to each end that will roll on a fine toothed gear track, each shaft being driven directly by a stepper motor attatched to one end outside of the gear. The four gear tracks will make a square which will be raised up and down by all-thread driven by individual stepper motors each mounted at the four corners of the square frame (somewhat similar to the shape of the original RepRap).This allows for self leveling through software and a sensor mounted on the head that can accurately detect the distance to the surface of the print (There really only needs to be three of these, but three looks a little awkward).

Six: The vertical travel of the head will be by raising the entire square track system vertically. It will need to be constrained accurately in order to maintain print resolution accuracy. If there are four post of hollow large diameter tubes at each of the four corners that are all perpendicular to the glass table top with a good degree of accuracy, then if the corners of the frame follow those tubes with linear bearings, the possibility of binding is quite likely. I propose to run a bearing against one side and spring load another bearing against the opposite side of each post. The posts will work in pairs diagonally across the square frame. In this way, one diagonal constrains movement one way (+x+y/-x-y) and the other pair constrains the movement the other way (+x-y/-x+y). This eliminates any possibility of binding.

Seven: The Arduino boards are the top choice for control electronics at present, but in the near future there will be a new board being put forth by an individual working out of the Seattle Metrix Create Space that will include on the board all of the components of the Arduino boards and also all the components of the stepper motor drivers with nice motor attachment connectors. I believe the board will come with everything needed to run four separate stepper motors. Remember that one stepper controller can operate more than one motor if they are wired in parallel. Since my head design will not be in need of a stepper motor to feed the material but meerly a dc or ac motor, there should be enough to run all stepper motors independantly with the exception of the y axis where the four lifting points will all be in parallel and will be running together (leaving a free motor controllers).

I have put a lot of work into this design, so if anyone uses it to build a RepStrap machine, I would love to hear about your success or failure and pictures would be wonderful. Please contact me at MrMaint@gmail.com, This design is copyrighted, so if you would like to use it for commercial interest, please contact me at the above email address. Thank you, Alex Bruski

This sounds interesting Alex. I'm interested in building a ultra high speed large area printer at some point. You should post some photos for us to see your creation.