Two 8 mm metal rods spaced 6 cm apart are the basis. The Mendel printer has the rods closer together, so a Mendel carriage is not compatible with this repstrap. The change was done to make it easier to fix the makerbot extruder between the rods. The rods are fixed to wooden end plates using 2 mm aluminium strips.
Half round slots were made in the wood to make sure he rods stayed in the correct place.The half round slots were created by clamping two pieces of wood together and drilling an 8 mm hole though the middle.
A stepper motor complete with gears and a timing belt were salvaged from an old scanner. The scanner was working perfectly, but the latest drivers for it were for windows 98 and it did not work anymore with newer versions of windows!
Timing belts and the matching wheels for a particular diameter of a motor axle are hard to find, so using the complete set from the scanner was the best option.
The motor is fixed to the side of the X-axis.
The off-center position is not a problem as the Z-axis ball bearings which prevent the x-axis from twisting are quite far apart and the construction is very rigid.
Because the stepper motor is on the side, there is no need to have lots of additional idler wheels as in the Mendel design. The Mendel design is more symmetric, maybe prettier, but it looks like is got a bit overcomplicated at this point.
I was afraid the gears (15 and 75 teeth) would introduce play and the printing results would suffer, but the play is small and the gear ratio gives a 5 times higher resolution and a 5 times higher torque.
The timing belt is going to the other side of the x-axis to a idler wheel made of a small ball bearing with large washers on both ends.
The belt is connected to the carriage for the extruder. The carriage is made of aluminium profiles (10x10 mm square and 10x20 mm L shaped).
The carriage is only running on top of the horizontal rods in true Mendel fashion with ball bearing allowing for some errors in how parallel the rods are (check out the video here). I did not fit any ball bearings under the metal rods. Initially this was because it was not ready and I was so eager to try to get my printer to do something that I just postponed it, but later I started to think that this has some advantages as I mentioned the post about the extruder.
- when accidentally moving the Z-axis too low the extruder is not crashing into the build base, but lifts up automatically
- when the filament gets stuck the extruder is lifted and does not (directly) damage the z-axis or the extruder.
The x-axis is working fine with the carriage described. It may need to add extra ball bearing under the rods some day, but there are many other parts of the printer which should be upgraded first.
Another example of a printed object: the world famous whistle (thingiverse) which really works. The whistle is a digital design which was originally created last year by Zaggo and which within hours after posting on the website had physical implementations on different sides of the world. By now it has become a sort of standard print for new 3D printers.