Tevo Tornado - belt-driven 2-nd Z-rod with minimal parts by AlMuz

Last crawled date: 3 years ago

WARNING
This mod reduces max print height by 4..8 mm
Single motor belt-driven design is better than two separate motors that will always have de-sync issues. That was the reason why I've picked Tornado over CR-10S (price the same, but AC heat-bed is a big win). I've decided to go with slightly simpler and cleaner design of the belt-driven second Z-rod for Tevo Tornado at the expense of slightly more complicated assembly:
Doesn't incorporate belt-tensioner (that used in other similar designs).
Both rods are suspended from the top by set-screws of pillow-block bearings, instead of being supported by the motor axle(s) and coupler(s) at the bottom. This approach completely eliminates Z bending and elephant foot artifacts on 3D prints. I can finally see distinctive second layer when printing (instead of nozzle messing the groves in the first layer).
Less parts, at the expense of slightly more complicated assembly process, but this also means somewhat faster assembly!
Additional hardware needed:
A set of 600 mm T8 lead screw rod with nut and 2x pillow-block bearings. Like this one: https://www.ebay.com/itm/T8-600mm-Stainless-Steel-Lead-Screw-Set-with-Mounted-Ball-Bearing-and-Shaft/173414745777
2x 40T GT2 pulleys - bigger diameter gives bigger precision of alignment between two rods. You can also 3D-print these and save some money.
784-2GT closed loop belt. Required tooth count depends on the diameter of pulleys, so if you get longer belt - you have to get/print bigger diameter pulley with more teeth.
4x M5 sliding T-nuts (like on a picture)
4x M5 20mm bolts
6x M3 12mm bolts
6x M3 nuts
12x M3 washers
Assembly Instructions
Part 1 - 3D-printed back-plate assembly
Pre-assemble the 3D-printed back-plate with the lead-screw nut on 90-deg mount using M3 bolts, nuts and washers. But don't tighten-up too much. You will need to slide the components later to align the 2-nd Z-rod vertically to the top-mounted pillow-block bearing.
To mount the 3D-printed back-plate assembly - you use the same nuts that hold the roller wheels in place. Instead of turning the nuts you will turn the bolts with allen key from the opposite side of the rollers assembly. However to get an access to the bolt of inner roller you have to remove the whole aluminum plate from the gantry - this is the most complicated part in the whole assembly process.
Part 2 - Replacing the original Z-rod on the motor side
After the 3D-printed back-plate assembly in place - remove the original top bearing assembly on the motor side.
Install the 1-st pillow block bearing using 3D-printed spacer. It should be aliened properly with the stepper motor axle and the original rod. If not aligned - you will need to print a different spacer of different height (scale the spacer part by Z and print it again).Tevo Tornado works OK without top-bearing in place. So just remove the pillow block and print another spacer (no need to reinstall original bearing assembly).
When 1-st block bearing installed - remove the original rod and insert the new 600 mm rod instead. Put the 40T pulley on top to measure the left-over height.
Remove the rod, cut it to the required length by hacksaw and install it back.
!!! NOTE !!! that properly installed Z-rod on the motor side should be suspended by set-screw of the pillow-block bearing on top. It must NOT be fully inserted into the coupler (when its end hits the coupler internals or motor axle). There should be like 1..2 mm gap inside, so the coupler will allow motor axle and the rod to float relatively to each other. Otherwise metal parts of the coupler, motor axle and the rod will rub each other, what may produce artifacts on printed parts due to Z-bending.
Don't install the 40T pulley on the motor-side yet, it is the last step of the assembly process.
Tighten up the set-screws and spacer bolts of the pillow-block bearing. Tighten-up the set screws of the shaft coupler to the motor at the bottom. Make sure that your printer still works with one axis installed on the motor side and 3D-printed back-plate assembly on the another side.
Part 3 - Installing 2-nd Z-rod with the belt and pulleys
Install the 2-nd pillow-block bearing in place using one more 3D-printed spacer, but don't tighten up the spacer bolts too much yet (you will slide it to adjust the belt tension).
Insert the original rod in it (the one you took off from the motor side). Fix the rod with a set-screws of the pillow-block bearing leaving enough length to install the 40T pulley on top.
Move the X-axis assembly to the very bottom (this will minimize the alignment errors).
Align the vertical position of the rod adjusting the 90-deg nut-mount on the 3D-printed back-plate assembly. But don't tighten up M3 screws yet (we still have to set correct distance between rods for the belt tension)!!!
Now you should have both Z rods installed on pillow-block bearings (suspended from the top by set-screws). Both pulleys should be removed. Its time to measure and set the correct distance between the rods to get good belt tension.
Take the closed loop 784T GT2 belt. Grab 40T pulleys (one in each hand) and hook the belt on them. Stretch the belt with pulleys. Use the distance between bores of the pulleys as an improvised ruler to measure the distance between the rods.
Shift the 2-nd Z pillow-block bearing (non-motor side) so that the distance between rods would be 3..5 mm longer than what an improvised stretched-belt ruler shows you.
Don't install the pulleys with the belt before shifting the pillow-block bearing - you will never get enough belt tension if the belt already installed.
When rod distance is set - tighten up the spacer bolts of the pillow block bearing.
Re-align the 2-nd Z-rod vertically again adjusting the 90-deg nut-mount on the 3D-printed back-plate assembly. Tighten up the M3 botls and nuts of the 3D-printed back-plate assembly.
Now its time to set the pulleys with belt on the Z-rods. To do that - hook the belt over the pulleys and stretch them apart (like you did for distance measurement between rods). Set one pulley on the motor-side rod by 4..5 mm (not fully), then stretch the belt with another pulley to the second Z-rod and set it the rod (also by 4...6 mm). So that horisontaly they are on the same level. You should feel the the belt has good enough tension (produces very low tone note as a guitar string). If you unable to hop the second pulley on the second Z - than the rods are too far away from each other (unscrew the 2nd Z-spacer and realign everything again).
Slide both pulleys simultaneously down until they rest on pillow-block bearings. Tighten-up the set screws on the motor-side only, leaving another side loose (you have to level the X-axis assembly first).
Use a square ruler to make sure that the alu extrusion of the X-axis assembly is perpendicular to both vertical alu extrusions of the Z-assembly. Rotate the 2-nd Z-rod to adjust the X-axis angle relatively to vertical extrusions. When set perfectly square - tighten-up the set-screws of the 40T puley on the 2-nd Z rod.
Level the bed and run a test print!