SCARA
Summary
The design of this robot is based heavily on the "HF06" robot designed by Martin Price (link). It was initially undertaken as a low-cost 3D positioning robot for use in touch-screen testing or button pressing/activation, but evolved into a 3D printer (with replacable tool head, so still capable of performing basic 3D positioner tasks). The X/Y resolution is lower than typical 3D printers, but is estimated to still be adequate (resolution is <1/4 the diameter of the extruded filament diameter).
Subsystems
Z-Axis Drive
The Z axis is driven by a 1.8 degree per step NEMA-23 motor, purchased at All Electronics (link). It uses an 8-wire interface, with 2 sets of coils wired in parallel or series for each phase). Wiring info is below:
- black to red = 3.4 ohms
- brown to blue = 3.5 ohms
- orange to green = 3.5 ohms
- yellow to white = 3.5 ohms
- black: 0 ohms to 1, 3.5 ohms to 3
- red: 0 ohms to 3, 3.5 ohms to 1
- brown: 0 ohms to 7, 3.5 ohms to 5
- blue: 0 ohms to 5, 3.5 ohms to 7
- white: 0 ohms to 4, 3.5 ohms to 2
- yellow: 0 ohms to 2, 3.5 ohms to 4
- orange: 0 ohms to 6, 3.5 ohms to 8
- green: 0 ohms to 8, 3.5 ohms to 6
The motor is coupled to a 1/4"-20 shaft which drives the SCARA X/Y positioner via an anti-backlash assembly comprised of two 1/4"-20 bolts held apart by a spring. The Z-axis assembly has 1.27 revolutions per mm, and using an 8-microstep controller gives 0.79375 um/step (1259.843 steps/mm), or 6.35 um/step (157.480 steps/mm) in full-step mode. The Z-axis print resolution will ultimately depend on machine tolerances and bed leveling, as it is clearly not limited by motor/mechanical resolution.
X/Y-Axis Drives
The X/Y axis are covered by the SCARA, and are driven by two 1.8 degree per step NEMA-17 motors, also purchased at All Electronics (link). Each motor is coupled to the 165mm arms via a 5:1 reduction pulley using GT2 2mm belts. Each motor will be driven by a 1/32 microstepping driver (DRV8825) for a total resolution of 0.01125 degrees/step, giving a worst-case linear step of 100um in X/Y (approximately 1/10th the resolution of a typical Cartesian bot, 1/10-1/3 that of a typical Delta, or about 1/2 to equal that of a DLP SLS printer).
Bill of Materials
| Part | Quantity | Unit Cost | Total Cost | Source |
| NEMA-17 1.8 deg. Motor | 2 | $6.50 | $13.00 | All Electronics |
| NEMA-23 1.8 deg. Motor | 1 | $8.50 | $8.50 | All Electronics |
| Shipping (All Electronics) | 1 | $7.00 | $7.00 | All Electronics |
| Motors | Total Cost | $28.50 | ||
| 1/4"-20 Threaded Rod 12" | 1 | $0.96 | $0.96 | Home Depot |
| 608ZZ Bearing | 4 | $0.305 | $1.22 | eBay |
| 8mm x 300mm Linear Rail | 2 | $6.69 | $13.38 | eBay |
| LM8UU Linear Bearing | 4 | $0.599 | $2.40 | eBay |
| Mechanicals | Total Cost | $17.96 | ||
| 12V 100W Power Supply | 1 | $11.94 | $11.94 | eBay |
| Stepper Driver (DRV8825) | 4 | $2.26 | $9.04 | eBay |
| Arduino Mega | 1 | $11.50 | $11.50 | eBay |
| RAMPS Board | 1 | $6.49 | $6.49 | eBay |
| Control Electronics | Total Cost | $38.97 | ||
| NEMA-17 1.8 deg. Hi-Torque Motor | 1 | $12.00 | $12.00 | eBay |
| MR105ZZ Bearing | 1 | $0.256 | $0.26 | eBay |
| Airtripper Metal Parts Kit | 1 | $12.79 | $12.79 | eBay |
| MK8 Drive Gear | 1 | $8.99 | $8.99 | eBay |
| E3D-v6 Bowden Extruder | 1 | $83.25 | $83.25 | E3D |
| Extruder | Total Cost | $117.29 | ||
| Total Unit Cost | $202.72 |