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

Since I am unfamilliar with this process, I will copy the relevant instruction, and replace the formula with my printer's information. This will form my checklist to ensure I don't miss a step.

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: my 1200 pololus are 1.8° = 200 steps
• Know your stepper drivers' microsteps setting. Most Pololus are set to 16x.: -- my Pololus are 16x
• Know the number of teeth on your pulleys. 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

Notice that steps per mm are calulated separately for each of Extruder, Z axis, and XY axiis (X and Y have the same pullies)

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:

 xy_steps_per_mm = (motor_steps_per_rev * driver_microstep) / (belt_pitch * pulley_number_of_teeth)
 xy_steps_per_mm = (200 * 16) / (5 * 11) 
 xy_steps_per_mm = (3200 ) / (55) 
 xy_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:

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

E steps

... "Wade" extruders use a NEMA motor to drive a large reduction gear that turns a "hobbed bolt." [this is mine]

Calculate

....For a typical Wade extruder, the hobbed bolt will be made from an M8 bolt, and its "effective diameter" will be around 7mm. ... This is just a starting point to get close to the correct value, I'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)
e_steps_per_mm = (200 * 16) * (43 / 10) / (7 * pi)
e_steps_per_mm = 3200 * 4.3 / (21.9911)
e_steps_per_mm = 625.70629055

Some typical examples:

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

• • Note Error on Triffid hunter's page **