Update: Since writing this I have replaced the flex-coupler with a u-joint and the combination of the BZB and the u-joint has is providing an even better result
This upgrade is intended for a stock Tevo Tarantula with a z- rod that is found to wobble and where the user notices z-banding.
The Kit Tarantula implements a stepper motor at the top with a z-rod hanging from a flexible coupler. Before you go through the effort of printing and installing this upgrade, do the following verifications
- Ensure that the acrylic bracket on which the z-stepper motor mounts is exactly square to the frame. (as seen from the side) Use a small square to measure this. If it is not square ensure that the horizontal extrusion is flush against the vertical extrusions on both sides. If it is flush but still not square, use a shim on one side of mounting screws along the length of the bracket to shim it until it is square. Fasten the t-nuts. Verify again.
- Measure the distance of your z-rod from the vertical post. It should be the same at the top and at the bottom. This should be verified from the side view as well as from the back view. Adjust the motor position on the bracket and the lead nut mount on the X-frame until this is true.
- Move the z position in big steps by using the stepper motor. Position yourself so that you can see the flex couple turning. If the flex couple wobbles then do the following. Tie the X axis extrusion assembly to the top bar with a piece of thin rope. Undo the flex coupling grub screws. Screw the z lead screw manually downwards to remove it from the coupler. Remount the flex coupling to the stepper motor axle. The axle must go only as deep as the non flexible part of the coupler. Tighten at least one of the grub screws. Now align the lead screw with the coupler and screw it by hand to it is inserted into the coupler. The lead screw must go only as deep as the non flexible part of the coupler. Lock all the grub screws on the coupler. Check that coupler wobble has been resolved. Repeat step 2 above.
Remove the rope
Position the lead nut about halfway up the travel range (i.e. approx 100mm above the bed) Move the z-distance of the printing head by using the stepper motor while watching the bottom of the lead screw. If the lead screw centre line is projecting a small circle (wobble), read further. If not then you should be good.
If you had to make notable changes in any of steps steps 1 to 3 above, do a test print. If you still have notable z-banding and you have notable wobbling of the lead screw, then you may try this upgrade.
Note that this mod will not work if you support the bottom end of the z-rod with a bearing. In that case you will be better off with an oldham.
Extra Materials required
4 M4 x 12mm socket head screws
4 M4 washers (another four is optional)
3.9 mm drill bit (for those living in the imperial countries a 5/32" bit will do the trick)
3 mm drill bit (1/8" is good here)
I have printed mine using e-SUN PLA +. The Filament needs to be soft enough to be able to for the screws to cut their own thread into the material
Print the three stl's files provided. I have printed mine using a 0.2mm layer height
Remove all burr's and rough edges, unexpected protrusions, etc, from the prints using a hobby knife.
Starting with the largest print from BZB_1.stl, open up all three holes with the 3.9mm (5/32") drill bit. An M4 screw should now be able to turn freely in the holes without notable play.
Next, take the print from the BZB_2.stl and open up ONLY the holes on the two lower profile of the 4 sides, NOT on the two sides that has a miniature 'gable" look to it. Again, an M4 screw should be able to turn freely in the opened up holes.
Lastly, take the print from the BZB_3 file and open the 4 lead nut mounting holes with the 3mm drill (1/8").
Take the print from the BZB-3 and insert it into the BZB-2 with the two holes on the side facing the lower profile sides of BZB-2. Take an M4 x 12 Screw with a washer and screw it into the one side. The screw should self tap into the plastic. Don't go all the way. Repeat on the other side. Now adjust the screw to make sure that the BZB-3 will stay centre with notable gaps between the two parts . The BZB should be able to freely tilt about 10 degrees without hitting or rubbing against BZB-2. If there is space you can add a couple of optional washers between the two parts (in my case I still had a bit of and elephants foot problem, so the optional washers did not fit)
Take the assembled part and insert it into BZB-1. The bottom of the assembled part (BZB 2&3) should face away from the bottom of BZB-1. Align the holes and use the remaining screws and washers. If you have space add washers in between. Again, the BZB-2&3 assembly should be able to tilt freely in BZB-1 without rubbing against the sides (until it stops against BZB-1)
If you are happy with the above assembly then remove the lead screw and nut from the printer. Remember to use a rope to tie the X-axis extrusion assembly to the top bar. (or else it may crash into the bed)
Remove BZB-2&3 from BZB-1.
Take the original four M3 screws and nuts to remount the lead nut into the assembly. It must be inserted from the bottom and the screws from the recessed holes. Fasten the nuts. (M3 nuts against the bottom of the lead nut)
Undo the two M4 screws on the BZB 2&3 assembly by about 3 mm but without removing them from the assembly. Use a thin wire or toothpick and add lubricant to the threads just below the head. Don't let any lubricant get into the inner plastic part threads. Fasten the screws and adjust whilst checking for fee movement.
Insert the BZB-2&3 assembly into BZB-1 and assemble. Lubricate the outside parts of the threads without getting lubricant on the holes in the inner part. Check and adjust for free movement.
Thread the lead screw through the mounted lead nut and mount on the printer. Repeat steps 1 to 3 above.
The assembly is 5 mm longer and wider that the original bracket with the screw heads protruding as well. Ensure that all moving parts will still clear the assembly
Keep your fingers crossed and do a test print.
Let me know if you have any comments
Replacing the standard lead nut mount on a stock Tevo Tarantula
This is my first design using a 3-D design package(Design Spark Mechanical) It was designed with the intent to replace the standard bracket and assembly with this one, without having to do major readjustments.
The rationale behind the design is when you have a bent rod the angle at which the lead nut is positioned on the threads will change as the rod is being turned through 360 degrees. When the lead nut is fixed to the printing head platform, the forces on the platform will result in the lowering and lifting the head angle as it goes through the 360 degrees. In some cases it may also move the head most notably in the Y- direction as the beam is torqued on the slight flex provided by the rubber wheels. When the torque force is down the head is pushed closer to the previous layer and the new layer will be printed wider, and as the platform is torqued up the head is moved further from the previous layer the new layer will print thinner, resulting in the typical z-banding with peaks and valleys where the distance between peaks correlates with the amount the platform moves in the z-direction with one 360 degrees turn.
This design will mostly eliminate the torque forces on the platform as a result of the lead nut changing its horizontal angle, allowing mainly the z- motion to be transferred to the platform.
Ready for the lead nut to be mounted
From left to right BZB 1 2 &3 as well as the additional materials required
Opening up the holes for the lead nut to be mounted
Opening up the holes for the pivot screws in BZB-2
First functional prototype mounted