The repstrap printer does not have a frame of M8 threaded rods like the Mendel. Instead, it is built with a wooden plank (80x40x1.8 cm) as a basis. Two pieces of wood (about 30x8 cm) are fixed with screws to the front and the back edge of the basis. These pieces have two 8 mm holes for two metal rods 18 cm apart which form the Y-axis. The Y-axis has a carriage with the platform on which the printed models are built and enables the printer to move the object forwards and backwards.
All metal rods in this printer are 50 cm long. This is the only length I could find in the local hardware store. Mendel is using rods about 30 cm long and cutting the rod would leave me with useless left over pieces of 20 cm. So I decided to use the full 50 cm for X, Y and Z axis.
The carriage with the build platform is made of aluminium profiles and is using skate ball bearings. This was actually the first piece of the printer I made, and since my order of smaller, Mendel sized ball bearings was delayed, I started with ball bearing for inline skates with an external diameter of 22mm and an internal diameter of 8 mm. The ball bearings are not fixed with M8 bolts as this would increase the size and weight of the construction considerably; only M4 bolts and M4 threaded rod was used with M4 nuts inside the ball bearings. The external diameter of the M4 nuts happened to be pretty close to 8mm. This may not be the strongest construction, but it is working very well in this particular setup. In general it would be better to use small ball bearings.
The ball bearings are fitted in the same configuration as on the Z and X-axis ( 90+135+135 degrees), but only one ball bearing was put under each metal rod to pull down the carriage with a spring (actually a rubber band, this should be changed still, as rubber bands have a limited life expectancy). The bearing on the underside are necessary as tall printed objects seemed to have the power to lift the Y-carriage a bit from the metal rods.
The top of the carriage is a 6mm acrylic sheet of 20x15 cm. The sheet is resting on springs around the M4 threaded rods with M4 bolts on top to allow for precise leveling of the platform (in the picture the springs on the left are missing).
The Y-axis is moving with a stepper motor, gears and a timing belt which were salvaged from an old scanner. Just like the scanner used for the X-axis (see previous post) there was nothing wrong with the scanner, it just got too old to be interfaced to a modern PC; this scanner still had a SCSI interface on it!
I have opened several modern scanners, but it seems modern scanners do not always contain steppers but may use DC motors with some kind of optical measuring system to provide acurate positioning. It may be possible to use those too, but that will require more special hardware and software.
The complete construction of stepper motor and gears (the black plastic) was sawed off from a larger piece of plastic in the scanner and bolted under the Y-axis carriage with 3 M3 bolts. The bottom of the carriage looks like this:
The belt is fixed to the wooden support pieces on the front and back of the printer and the motor is pulling itself (with the carriage) along the belt. This has the disadvantage that the system in moving the motor which is some extra weight, but the belt is short and easy to attach to the support pieces.
As with the X-axis, play between the gears between the timing belt and the stepper motor is minimal and does not seem to cause big quality issues.
The 6mm acrylic base plate is NOT used to print on directly as it is too difficult to remove. A separate 3mm acrylic plate is clamped on the base and printing is done on that.
Many prints can be made on the top plate, but it can be seen already that tiny cracks are appearing on the surface caused by large local temperature differences during prints.
Because of the arrangement with the acrylic the actual usable build surface is only 15x15 cm. I would be quite easy to rearrange the carriage and have a build surface of 25x20 cm. However, without a heated build surface, objects that large can not be printed yet anyway.
The largest object printed so far was the A-Mazing box. The box has a maze on the side and opening can only be done when you find the correct route! Infomation about this model at thingiverse.
Comments and questions are welcome, the next post will be about the Z-axis completing the overall hardware setup for the 3D repstrap printer.