4 days ago •
Uh oh. I'm in the competition. Guess I better get to work.
There's no chance of having the Decapede ready any time soon, so I bought some stepper drivers off eBay (Geeetech DRV8825) and started wiring them up on a breadboard and connecting them to the AUX pins on the RAMPS. Unfortunately, they did not come with thermal adhesive for the heat sinks.
The only thing the RAMPS needs in order to drive 2 extruders is an additional stepper driver. To run all 4, though, I will need to wire up some extra MOSFETs and voltage dividers for the extra heaters.
On the software side, I'm going to try merging the multiple extruder support from Marlin X2 into Bipolar Marlin. The RepRap X2 is a Prusa variant with dual extruders on independently moving X carriages. It may not look similar to the Theta printer, but from a programming standpoint, this is exactly what I need.
a month ago •
With all 4 extruders, the platter, and the Z axis, the machine requires 10 independently controlled motor axes. No current 3D printer controller is capable of driving more than 5 stepper motors at once. While researching options to overcome this limitation we discovered a project called Decapede , which aims to sell a 3D printer / CNC controller that can drive 10 motors (thus the name). The Decapede can also drive 8 heaters and read from 8 thermal sensors. The processor is an Arduino Mega 2560. More information can be found at http://printm3d.com/portfolio-item/decapede/ or http://reprap.org/wiki/Decapede.
We got in contact with the developers early on, who incorporated our feedback into the design and offered to let us test a prototype. Unfortunately, as time went on it became evident that the Decapede would not be read in time to complete the senior design project. Currently, several prototypes have been produced and various bugs have been discovered in the design. Once the issues are resolved, the developers plan to launch a crowdfunding campaign to fund production.
Our fallback plan was to use a standard 3D printer controller, the RepRap Arduino Mega Polulu Shield (RAMPS) version 1.4 (http://reprap.org/wiki/RAMPS). RAMPS can drive 5 motors and 3 heaters. This means that we would be unable to run all 4 extruders. By splicing the motor connections we were able to run two axes in parallel, thus allowing us to drive two extruders with the standard RAMPS controller.Power Supply
A standard ATX computer power supply was chosen for several reasons. First, they can provide the high power necessary to run the machine. Secondly, they are cheap and readily available. They also meet the 12 Volt requirement of our electronics. Modern switching power supplies are also very efficient and provide clean output.
In order to select the appropriate power supply we first had to determine the power requirements of the machine. The following chart shows a breakdown of each component and it’s peak wattage. We expect each component to draw less power under normal operation. Also, although we have a 240 Watt heated bed, it will only draw 120 Watts since it will only be operating at 12 Volts as opposed to 24.
Our selected power supply is a Thermaltake Toughpower 750W purchased from Newegg for $70. In order to convince the power supply to operate outside of a personal computer, we had to ground the PS_ON line. This was done by connecting a paperclip between the green wire and a black wire.Another important consideration is that not all power supplies are capable of delivering their maximum wattage on the 12 Volt line. We verified that our chosen power supply would deliver 720 Watts at 12 Volts.
a month ago •
The cross sectional view below shows the original design for the platter. It consists of two layers of circular plywood. The lower piece has a hole in the middle where a nut is glued in place to connect it to the shaft. The upper piece is connected to the lower piece by countersunk bolts around the perimeter.
In testing, the wooden platter proved to be inadequate. The top of the platter is required to be extremely level. Its height could not vary by more than 0.25mm across the entire surface (the height of a single plastic layer on a printed object). Unfortunately, the plywood was naturally warped far beyond the tolerance. This test video demonstrates the problem (http://youtu.be/nC2cthvXTew).
An attempt was made to level the wooden platter by soaking it in water and clamping it in place until dry. This was unsuccessful. We decided to construct a new platter out of machined aluminium, which would have no trouble meeting our flatness tolerance. It would also be better for the heated bed since aluminium is much more thermally conductive than wood.
A suitable plate of 7075 aluminium was acquired from the scrap pile in the machine shop. This picture shows the plate being machined on a CNC mill to a diameter of 304.8 mm and a thickness of 4.2 mm. A hole was drilled and tapped in the center for the M8x1.25 shaft.Heated Bed
The original design did not consider the need for a heated build platform. This is standard equipment on all FDM 3D printers. As an object is being printed, thermal contraction causes the bottom of the object to shrink while the top of the object is still hot. This causes the entire object to warp. In the worst cases, the object will completely peel off of the build surface before printing is completed.
The larger an object is, the more warping occurs. Also different plastics will warp more than others. Since the purpose of this printer is to build large object from multiple material, a heated build surface is essential.
Most 3D printers use an electric heating element and a temperature sensor under the build surface. Unfortunately, the spinning platter on this machine means the wires running to the heater would get wrapped around the shaft. The solution is simple. The electrical connections are passed through a slip ring to prevent the wires from twisting around the shaft. After extensive research, we found a circular 200W heater with a 240 mm diameter and a 12 connection slip ring.
The slip ring replaced the central bearing underneath the platter. A hollow coupler was used to connect the shaft to the slip ring. Holes were drilled in the side of the coupler for the wires to pass through. Since each connection on the slip ring is only rated for 2 Amps, multiple wires were used in parallel to carry the full current of the heater. The heater itself is designed to operate at 24 Volts. Unfortunately our power supply only provides 12 Volts. Although the heater is run at half power, it still gets sufficiently warm given enough time.