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When I wired up my Reprap and started printing, I noticed that all my prints were mirrored along the Y axis — I’d set it up with the end stop at the front and configured it so that the print bed moved forward when I clicked Y- in pronterface (printrun), and not back. (When the table moves back, the print head essentially moves forward along it.)

The end stops were already in place and I didn’t want to move them, but no combination of INVERT_Y_DIR and Y_HOME_DIR in Configuration.h would work. I even tried changing Y_ENDSTOP_INVERT, knowing full well that it wouldn’t help either.

So how do you flip an axis without moving the end stop or the home position? The answer is to change both INVERT_Y_DIR and Y_HOME_DIR in Configuration.h, and also swap the values for Y_MAX_PIN and  Y_MIN_PIN in pins.h for the specific motherboard you’re using.


This is the email (redacted) that went out to the complete list of club members about six days after the first meeting. (It was delayed primarily because I was awaiting comments from some of the advisers.) There was a technical glitch at that meeting that prevented a live demonstration, but we used that as an opportunity to teach a bit about how the printer operates. New terms or concepts were highlighted in bold type.

Greetings, parents and team members! (Parents, please ask the team members to read this email!)

I think we had a great first meeting. I know it was a lot to cover, and I’m sorry that I couldn’t do a live demonstration, but solving technical problems are all part of the process! I appreciated all of the great questions, and the interest you all showed. I’m confident that we can pull this off as a team if we stay focused and work hard.

A Couple of Build Options
There are two different 3D printer designs that I think we should decide between. The one I brought in is called a Prusa Mendel. It was designed in 2010 by Josef Prusa, who lives in the Czech Republic. It has a triangular frame. Please read a little about this design here and here

Another design, called the Printrbot, was developed last year by a man named Brook Drumm in California. Its frame looks more like an upside down T than a triangle. Several people (including Josef Prusa) have improved on this design in the meantime. Please read a little more about this design here

It’s not critical that you understand everything on those pages — just take a look at the pictures and review the text (particularly the Advantages and Disadvantages sections of the Mendel90 link, because these sections compare the two different models).

At our next meeting, I’d like you to share your thoughts about each design, so that we can decide which one to build as a team. Please come prepared!

About The Technical Problem
As you saw both in the video and at our first meeting, the printer has three axes of motion: X (the print head moves side to side), Y (the table moves front and back), and Z (the print head moves up and down). The motors that control the motion are not very smart — they just move in the direction the computer tells them to, and they’ll just keep going unless there’s something that tells them to stop. Some people accidentally damage their machines by telling the motors to move too far in one direction or another.

When a print starts, the machine moves the print nozzle to a “home” location. This is called “homing.” It knows where the home location is because there is a physical switch on each axis called an end stop. When I tell my machine to “home,” it moves the table forward until the Y axis end stop switch is hit, it moves the print head to the left until the X axis end stop switch is hit, and it moves the whole X axis (called the X Carriage) down until the Z axis end stop switch is hit.

The problem I had at the meeting turned out to be this: One of the wires leading to the Z end stop switch had worked loose on the drive over to the school. If I had told the printer to “home” in order to start a print, it would have moved the X carriage down, crashing into (and through) the glass table that the printer squirts plastic onto, because it would not have known when to stop!