Hi Sha,

I’m sorry to hear about the difficulties.  There are several different issues here, and we need to work through them one at a time.

First off, I’d suggest putting away the paint, water, and paper until you can tell that the machine itself is operating correctly.  The robot is not yet configured correctly until the motion is clean, smooth, and without any “stuttering” type behavior of any sort. 

There are two different behaviors that might be described as “stuttering”: One, which we call “cogging” is where the motor tries to turn but cannot and jumps from one position to another. The other, which we call “stuttering” or “chattering” is where the motor is turning properly, but the carriage is undergoing stick and slip type vibration as it moves along the rod(s).  The latter can be annoying, but is not generally harmful (although it can cause the bushings to wear out prematurely), and is most likely caused by not having the two rods fully perpendicular.  However, what you are describing– where the motor loses position — is cogging, and is much more serious. 

Now, onto fixing the problem.

Unplug the bot (both USB and power), and turn the winches by hand— one at a time —to move the carriage to the home corner, and then to the opposite corner.  As you go, check the following:

(A) Is the spectra cord properly routed around the three ball bearings, riding in the groove?

(B) Are both rod ends fully square to the frame (up against their stops, without any gaps) at each end of travel?

(C) Is the motion clean, smooth, easy, and without binding of any kind?

If the answer to all three is a resounding “Yes, absolutely, positively!”, then your robot is mechanically set up correctly, and you likely need to adjust the motor current. (Skip down this text until you get to the part that says “If you need to adjust the motor current.”)

Otherwise, we need to work on the mechanical part.

Begin by turning the winches by hand to move the carriage to the center of the frame. Remove the rods from the carriage, by lifting up the rod end sliders and pulling the rods out, sort of like you did at the beginning of assembly step 28.  Leave the carriage attached to the robot by its cable guide, but set the carriage out of the way, for example on your table behind the frame. Re-install the rods.

Next, move the winches by hand, returning the two rods all the way left and back, as though you were moving the carriage back to the home corner.  Starting with the Y axis (motor 2 and its cord), check that the string is routed correctly over the entire path,as described in steps 15-18 of the assembly instructions. In particular, follow along with the numbered arrows on the frame, and make doubly sure that the cord is still laced into the grooves of the two ball bearings at arrow positions 3 and 6 on the WaterColorBot frame. Next, check that the rod ends are absolutely, positively square to the frame (without any gap, as shown in step 20). (If they were not square, then that would certainly cause the cogging that you had.)

Turn the Y winch by hand, all the way to the end of its travel and back.  The motion should be clean, smooth, easy, and without binding of any kind– much like an Etch-A-Sketch. (Your daughter should be able to turn it!)  If you have enough tension, the cord should will still be laced into the grooves of the ball bearings.  If you have too much tension, the motion may have too much friction, and won’t give that smooth motion that you need.  In practice, the amount of tension is pretty forgiving, up to those two limits.   Finally, after moving the rod to the end of its travel and back, check again that the rod ends are square to the frame.    

Then, repeat these steps for the other axis, and finally re-install the carriage.

First, be sure that you need to.  *DO NOT* adjust the motor current unless you’re sure that (A), (B), and (C) above are true. Then see our troubleshooting guide about how to adjust the motor current: