PrusaCalibrationNotesBraino

2015 January

This is in addition to my original build log http://reprap.org/wiki/PrusaBuildNotesBraino

The current task is to print Emmet's Gear Bearing.

http://www.thingiverse.com/thing:53451

With the "Perimeter width Test object" calibration block http://www.thingiverse.com/thing:264782 printed ok and seemed fine, emmets gear bearing is fused. The filament diameter and E-steps correction (Skeinforge) and extrusion multiplier (Slic3r) were tweaked until the Perimeter width Test object printed with the BOX width slightly LESS than 10mm and the SEAT opening was slightly greater than that. The 10mm box was 9.6 mm and the 10mm seat was 9.8 mm. However, there were gaps in the top solid layers. Ptining Emmet's gear bearing resulted in fused gears. So, back to the first principles. Re-check the machine calibration before adjusting the tolerances of the part itself.

Print Calibration Process

I will use Triffid Hunter's Calibration guide. http://reprap.org/wiki/Triffid_Hunter%27s_Calibration_Guide

Requirements

• A tool to precisely measure 100mm. A vernier caliper is ideal: Harbor freight digital caliper $4
• A tool to precisely measure something 0.5mm wide. A micrometer is ideal, but a vernier caliper will do: Harbor freight digital caliper $4
• Know your stepper motors' full steps per revolution value. steps = 360 / angle, so 1.8° = 200 steps, 0.9° = 400 steps, etc: 1200 pololus are 1.8 d = 200
• Know your stepper drivers' microsteps setting. Most Pololus are set to 16x. GEN3 driver 2.3 is fixed to 2 (half step). Some boards have 32x.: -- Pololus are 16x
• Know the number of teeth on your pulleys. Standard printed pulleys have 8 teeth. Most machined pulleys have 10 or 12 teeth since 8 tooth is technically too small for T5 belt: X axis 11 teeth; Y axis 11 teeth
• Know your belt pitch! XL and T5 belts /look/ the same, but the difference is important!: T5 belt
• Know the number of teeth on your extruder gears, or at least the gear ratio.: Large Gear: 43 teeth. Small Gear: 10 teeth. Ratio 43:10
• Remove all sources of backlash. Your parts won't be usable as calibration pieces if you have lash! : No lash that I can detect at this time,; belts are tight.
• Open the Online RepRap Calculator to find XY and E steps, layer heights, and acceleration. - must figure this out after I complete the above.

http://prusaprinters.org/calculator/

Steps

XY steps

...Using belts and pulleys, the XY steps-per-mm can be accurately calculated using your motor, pulley, and belt characteristics, and once set they shouldn't need to be calibrated further. Check. The basic formula is:

 steps_per_mm = (motor_steps_per_rev * driver_microstep) / (belt_pitch * pulley_number_of_teeth)
 steps_per_mm = (200 * 16) / (5 * 11) 
 steps_per_mm = (3200 ) / (55) 
 steps_per_mm = 58.181818

Z steps

...Using threaded rods for the Z axis. So to calculate how far the Z axis moves for each revolution of the motor, first you need to know how much rotation is being transmitted to the Z rods, and then use the "thread pitch" of the rod (distance-per-revolution) to determine the vertical motion.

The basic formula to calculate motion on a rotating rod is:

steps_per_mm = (motor_steps_per_rev * driver_microstep) / thread_pitch 
steps_per_mm = (200 * 16) / 1.25  [98861A080 M8 1,25 pitch threaded rod from McMaster Carr]
steps_per_mm = (3200) / 1.25 
steps_per_mm = 2560           // NEMA 17 with standard pitch M8 threaded rod:

E steps

There are an increasingly wide variety of motors and extruder setups to choose from. "Wade" extruders use a NEMA motor to drive a large reduction gear that turns a "hobbed bolt." Direct-drive extruders typically use a motor with a planetary gearbox to turn a drive gear, such as the popular MK7. Bowden setups can use either method to push the filament through a tube to the hot end. There are others, such as worm drives, but we won't get into those here. Calculate

For a typical Wade extruder, the hobbed bolt will be made from an M8 bolt, and its "effective diameter" will be around 7mm. The direct-drive MK7 gear is specified as having an effective diameter of 10.56mm. These are just starting points to get close to the correct value, and then you'll measure and calibrate to get the exact value later.

The standard formula is:

e_steps_per_mm = (motor_steps_per_rev * driver_microstep) * (big_gear_teeth / small_gear_teeth) / (hob_effective_diameter * pi)

Some typical examples:

// Classic Wade with a 39:11 gear ratio (200 * 16) * (39 / 11) / (7 * 3.14159) = 515.91048

// Gregstruder with a 51:11 gear ratio (200 * 16) * (51 / 11) / (7 * 3.14159) = 674.65217

// Gregstruder with a 43:10 gear ratio (200 * 16) * (43 / 10) / (7 * 3.14159) = 625.70681

// MK7 Direct Drive with 2engineers 50:1 planetary gear motor (48 * 16) * (50 / 1) / (10.56 * 3.14159) = 1157.49147