The Z-axis consists of:
- 2 freestanding vertical metal rods
- 2 separate stepper motors connected to threaded rods with a trapped nut in the x-axis ends
- 2 different sliding mechanisms connected to the x-axis ends
The repstrap does not have the Mendel threaded rod frame, but is built on a wooden basis. The vertical rods for the Z-axis are free standing, they are not supported at the top. This is another part which was thought to be temporary, but which turned out to be good enough and may be a pemanent solution. A 2 cm thick piece of hard wood is fixed to the basis and an 8 mm hole is drilled though the wood and halfway though the basis to give the rod a proper chance to stay put.
The 2 rods are connected to eachother with a thin strip of wood to make sure the distance between the rods is the same at the bottom and the top.
Two stepper motors were used for the z-axis when one would have been enough. It was easier to get an extra stepper motor than to get a set of pulleys and a timing belt to be able to use a single motor. So 2 motors were used (temporarily?); electrically the 2 motors are wired in series to the same stepper motor driver. This arrangement does not allow you to get the most power out of your motor (there is plenty of power available) but it makes sure the same current is running though the 2 motors and the the stepper motor driver will not get overloaded. The motors are connected to the threaded rods with a plastic tube which allows some movement to compensate for minor misallignment of the motor shaft and the threaded rod. Since steppermotors should not be loaded too much in the direction of the axle, a ball bearing is fixed in the wooden frame around the motors like this:
The motor is not resting on the basis, it is just very close, to avoid resonance.
The threaded rod is going though trapped nuts in the ends of the x-axis; the top of the threaded rod is not supported. The nuts are hot glued in some pieces of scrap plastic which are fixed with screws which have space to move horizontally. The hot glue is not very stong; this is on purpose. Already in several situations this has saved the printer form self destructing: when the z-axis is lowered too much the nuts will be separated from the plastic and will start to turn, thus stopping further damage.
The sliding mechanism around the vertical rods is close to Mendels, but the amount of ball bearings is less. One single ball bearing on the extreme left of the machine is the only bearing which is keeping the whole x assembly from moving to the right. The next 2 pictures show the arrangement on left (ball bearings in 135+90+135 configuration, allowing only up-down movement) and the right (180+180, allowing up-down and left-right movement).
The horizontal aluminium profile above is fixed with 2 vertical M3 bolts but it can move in holes which are about 5 mm wide so that the M4 bolt a the top can be used to adjust exactly how tight the ball bearings are gripping the metal rod.
The metal rods of the x-axis are going through the white plastic which is used on both sides to make a very rigid construction and to prevent the x-axis to be twisted.
As usual one printed object at the end of a post. The object is for testing only: a 10 cm long single layer wall thickness cylinder with a diameter of 4 cm. The object shows no bad effects of the free standing z-axis. There are some artefacts visible in the object which should not be there: everytime the printer is starting a new layer, too much plastic is deposited causing some blobs. The comb module in skeinforge will move the position of this point causing it to look like some kind of staircase. I hope something can be done in software to prevent this in the future: