Stepper Driver Adjustment
The trim pots may need to be adjusted for stepper current. The ideal setting is above the point at which the stepper motors are too weak to complete a movement, and below the point where the driver enters thermal shutdown (a very temporary interruption of movement until the driver cools and resumes powering the stepper, which results in another thermal shutdown). The video above shows the three states - over-powered, under-powered, and properly adjusted. Begin a sample print at 100% speed and turn one axis clockwise until the stepper exhibits the under-powered behavior of missing steps. Then turn the pot counterclockwise until the stepper exhibits proper movement. If you reach the point of thermal shutdown before finding an ideal position, check for binding in the movement of that axis.
Rigidboard trimpots are adjusted CCW (counterclockwise) for MORE current and CW (clockwise) for less. NOTE that this is opposite the adjustments of standard Pololu A4988 boards.
Be careful when adjusting as several people have had the trimpots fall apart or fall off the board.
Use a nonmetal screwdriver (plastic or ceramic) to avoid short circuits.
Turn off the printer before connecting or disconnecting any cables (especially motor and extruder cables)
The mainboard has five A4988 stepper drivers onboard: one each for X,Y,Z, Extruder A, Extruder B. Each driver has a trimpot next to it for adjusting motor current. Full adjustment range is only about a half turn, and care should be taken not to drive the adjuster past the ends of it's range. A labeled picture of the board can be found here: http://rigidtalk.com/wiki/images/5/58/RBboardanalysis1.1.jpg
When the board is installed in the printer, the adjusters are accessible through the small round holes in the board cover, near each of the axis and extruder connectors. If a motor runs excessively hot it may need adjustment for less current, or if a motor tends to stall it may need adjustment for more current. Either condition can cause motors to skip or jitter/oscillate.
The stepper drivers can be adjusted without measuring voltages, by observing motor performance. Use a nonmetal screwdriver (plastic or ceramic) to avoid short circuits. Either run a print (preferably one that's been causing motor issues) or set up manual control using an LCD Controller, or Repetier Host, or Pronterface. CAREFULLY turn the pot fully CW (minimum current) then back CCW about 1/4 turn, or enough to make the motor run smoothly. From that point, if you are able to adjust while the motor is moving (either in a print or under manual control) turn the trimpot CW until the motor starts to stall or stutter, then turn CCW until steady performance resumes. Otherwise, alternate adjusting the pot slightly and then running the motor, with the same results in mind. The general idea is to find a low current setting that results in stalling or stuttering, and then boost the current a bit to get reliable motion without going too far CCW.
The numbers given below are for stock motors that are known to have come with Rigidbots. If you replace a motor, be certain to identify its rated current and determine an appropriate Vref setting according to the formula Vref=I*0.4. (Vref can be set to a lower value (at the expense of less torque) but should not be set higher.)
The Rigidboard has a "+" marked next to one terminal of each trimpot. Assuming the values given for R64 and R75 in the schematic are installed for all 5 voltage dividers on the board, that point is nominally at 0.84V (measured voltages on a random board varied from 0.92V to 0.73V for the 5 trimpots) and the other terminal on that side of the pot is at ground. The wiper's terminal is in the middle of the opposite side. Turning the adjuster CCW moves the wiper to "+" and CW moves it to ground. The voltage on the wiper (Vref) sets motor current according to I= Vref*2.5, so Vref at 0.8V --> 2A, Vref at 0.6V --> 1.5A, Vref at 0.4V --> 1A, etc.
It appears the Rigidbot uses 1.33A motors for the X axis and the extruders ("E"), and 1.66A motors for Y. In theory then, the maximum Vref settings should be 0.53V for X and E and 0.66V for Y. These settings should prevent the motors from getting too much current or overheating, but remember that the stepper drivers are on the Rigidboard, in a hot box, without a fan - they may still overheat at these settings, leading to thermal shutdowns and layer shifts/axis shifts in prints.
Some prints may be more prone to overheating the drivers than other prints, due to varying patterns of motion, especially if resonant vibrations and rattling occur. If missed steps or shifts occur, a lower Vref may help by limiting current to a greater extent. Reducing max acceleration settings for the affected axes can also help, as a low enough setting will eliminate resonance. Adding a fan to the electronics enclosure can further aid in keeping the drivers from overheating.
Both motor types have reportedly been used for Z, but since driver current is split between two Z motors, Vref could be set to give the full 2A capacity of the A4988 driver without overdriving either type. That probably isn't needed, since the Z axis doesn't require high speed or acceleration, and the driver would probably overheat without additional cooling. Somewhere in the range of Vref = 0.5-0.7V (yielding 1.25-1.75A) will most likely work fine, but your mileage and preferences may vary, particularly if your Z leadscrews, etc aren't well aligned/cleaned/lubed.**
[** A note regarding Z axis series connection: The stock arrangement is that the two Z motors are driven in parallel, meaning they each get 1/2 of the current limit set by trimpot adjustment. On the other hand when connected in series, each motor gets the full current limit. If you change to a series connection, as in this upgrade http://rigidtalk.com/wiki/index.php?title=Z-Axis_Stepper_Motor_-_Series_Connected_Splitter_Board , be sure to check and readjust the trimpot to give the correct current limit for a single motor.]
The trimpots can be adjusted with the board on the bench (with or without motor cables connected) or installed in the printer. Use a nonmetal screwdriver (plastic or ceramic) to avoid short circuits. The metal adjuster wheel of each trimpot is at wiper voltage, so it can be probed to measure Vref. Again, be very careful not to damage the adjuster, and be careful not to short it to anything else with the probe. Ground can be tapped at the X limit switch or at the left front pin of the LCD connector on the Rigidboard, but don't short it to surrounding pins, etc. AT YOUR OWN RISK you can use a metal screwdriver (yes, the one that you know you shouldn't be using) connected to a meter probe, to directly view the voltage as you adjust it. But don't blame the wiki if magic smoke comes out! Don't try that if you aren't fully comfortable with what you are doing, and keep in mind that a blown driver most likely means replacing your board.
A PDF of the schematic can be found here: http://www.rigidtalk.com/download/file.php?id=5 [Note that there appear to be some labeling discrepancies between the schematic and the board, e.g., the trimpot (R75) and voltage divider fixed resistor (R64) associated with E0/Extr A on the board are associated with E1/Extr B in the schematic. Based on measured voltages the fixed resistor for each trimpot is probably 49.9K, although the schematic shows 40K for 4 of them]
For more info on stepper drivers: http://www.pololu.com/product/1183 ; http://reprap.org/wiki/Pololu_stepper_driver_board