Modular System Breakdown

Major subsystems of the OIT RepRap 

CARTESIAN ROBOT

Frame

The frame is made of 1/16 wall 2" X 2" mild steel square tubing tig welded into a cube. The z-axis is a scissor lift mounted to two parallel pieces of angle iron welded to the bottom of the frame. Holes and channels for the wiring harness were milled into the tubing prior to welding to provide hidden runs.

On the four vertical sections of the frame there are mounted two chrome rails on which the x/y axis carriage slides. These mount points were made with adjustment measures built in to take care of the inaccuracy of welding the frame together. These measures are in the form of vertical slots on the four uprights and horizontal slots on the mounting brackets that attach to the chrome rods. This allows for adjustment in the the vertical and horizontal directions so that a "true" alignment can be achieved without the time and expense of precision materials and fabrication being used on all parts of the frame assembly.

Design was done in AutoCAD, Inventor, and FeatureCAM with the parts being CNC'ed on a <insert CNC machine brand and model here> machine.

Z-Axis

The z-axis of the OIT RepRap is a custom designed and built scissor lift that moves the print surface up to the print head and then descends as each successive layer is printed out.

Y-Axis

The y-axis rides on top of the x-axis carriage assembly and consists of same style and size of rails. The truck assembly is the same but the spacing between the y-axis rails is much less being only wide enough for the Thermoplastic Extruder to sit between it.

X-Axis

The x-axis rides on 1.170" chrome rods that were scavenged off of some Universal Stereo-Plotters that the OIT Geomatics program had discarded. These rods saved the project a lot of money and time. The only modification that was made to these pieces was to cut them to the appropriate lengths and drill and tap the ends M6.

On the x-axis rails rides the x/y carriage assembly. This assembly consists of custom trucks made out of aluminum with 608 bearings in ABEC-3 that ride directly on the rails.

Electrical Box

The electrical box was a salvaged steel casement with rubber seals originally intended for an HVAC control system. Since we were on a very tight budget, $1,000 or less, we opted to use this box and simply spray painted it silver for a decent appearance. All electronics, computers, and power supplies are housed in this enclosure--subsystems described below.

Filament Feed

The plastic filament (3 mm nominal diameter) is fed down through the top of the machine from an un-powered reel. A tensioning screw ensures that there are no loose coils of filament clogging the system and causing problems.

ELECTRICAL

Computer

The computer running the RepRap Host Software (current v0.8.3) is placed in the Electrical Box and mounted outside of a standard computer case. This system consists of the mother board put on stand-offs inside the electrical box. The stand-offs are nylon and mount to a piece of MDF inside the Electrical Box.

Driver Boards

There are currently two versions of the electronics available on the official RepRap site: PIC 16F648A based and Arduino based. The OIT RepRap uses the Arduino based boards because of the better stepper driver capability of the boards. In total there are 6 boards that make up the system: 3 stepper driver boards, 1 extruder controller board, 1 power and communications board, and 1 Arduino controller board.

These boards connect two one another in a simple token ring topology which allows for easy extension of the system to support future upgrades. The design has been built and tested fully by others so we are not blazing any new trails here which cuts down on our build time.

Stepper Motors

Three, 200-step motors (Keling KL23H51-24-08B) drive the three axis. Coupling nuts attach these steppers to the ball screws used to drive the three axis. These ball screws were scavenged off of old robotics parts available at the school. They provide excellent response and precision without any associated extra costs due to their being "found" parts.

Wiring Harness

Cat 5e cabling is used for the entire wiring harness and is routed through the frame components to the sites needed. Extra wires are present but do not pose a problem owing to the casing on the cabling. Plug connectors are used at the terminus of each cable for easy attachment and detachment of components from the machine. This allows a very modular system design and facilitates expected future upgrades.