Phrozen ARCO Z-lead screw flexing top cap set

I recently received my KickStarter “Phrozen ARCO” fdm printer. 

After going through all the calibrations and firmware updates, I started noticing Z-banding on all of my test  prints. 

Upon inspecting possible causes, I found the top of all 4 lead screws were fixed with a screw to a bearing. I was determined to design a fix for this, but when I removed the front left centre screw from the lead screw to get all the measurements,  the whole Z-screw dropped about 4 to 5mm. Creating vertical play of 4 to 5mm was not intended, so I looked for a way to fix the vertical movement.

After opening the printer bottom and inspecting the pulley assemblies, I discovered that the lead screws could be pushed quite a bit further into their respective belt pulleys. After securing all 4 lead screws again, the 4 to 5mm play was no longer present. So now I could start with designing the parts needed to give the tops of the 4 screws enough play to not cause Z-banding. 

Having Prusa XL printers in my workspace already gave me an idea on how to create that flexible top cap. The following design and testing was pretty straight forward and didn't take too long. Writing and documenting the necessary steps in this guide probably will take just as much time.

It is recommended to print the parts with either PETG or Nylon, since they require some flexibility. PLA can work, but because it is a stiffer material, it could possibly break after a while. 

You will need to print the 10mm m3 spacer 4 times!!

I have kept the tolerances tight, only 0.1mm between parts, so make sure your filament is well calibrated before printing the parts!

Here is a print example of Z-banding before and after this mod. 

So far it has improved significantly, that someone not looking for the Z-banding specifically, will not notice it. As a perfectionist I see it all and I want it all gone though, so I'll keep trying to find a way to eliminate it fully.

Steps to follow to replace the top bearing with a flexible top cap:

• Before turning of the printer and removing the power cable, it is a good idea to already have the gantry lowered to about the height of the plastic Z-alignment blocks.

• Remove the spool holder and the bowden coupling from the frame.

• Remove the 4 plastic top corner caps to get easier access to the top bearings.

• Before removing the top screws and bearings it is recommended to use the 2 plastic Z-alignment blocks to temporarily secure the gantry to the printer frame. This will ensure that the front and rear and left and right of the gantry assembly all stay aligned. Attach the 2 blocks with their transport screws to the frame and then the gantry with its transport screws to the alignment blocks.

• Remove the centre screw that is fixing the lead screw to the bearing. The lead screw will most likely drop a few mm, so don't be alarmed!

• Remove the 2 screws securing the bearing to the corner bracket.

• Repeat this for the remaining corners.

• Turn the printer upside down and remove the screws securing the bottom cover. The rubber feet do not need to be removed!

• When lifting the bottom cover, be careful with the wire attached to the main board cooling fan. Remove the red glue from the fan cable connector and remove the connector from the board. 

• You might need to cut a few zip ties to get the fan cable free, then put the bottom cover aside. 

• Inside each of the pulleys are 2 grub screws preventing the lead screws from turning inside the pulleys. Loosen those grub screws (red arrows) by 2 full turns. Loosen the centre screw completely (green arrow)

• Important step! Use the lead screw to bump out the top bearing, by turning the screw downwards until the bearing comes out of the top bracket.

• You can now tighten the centre screw fixing the lead screw into the pulley. You might need to guide the lead screw upward to fasten that centre screw until it can go no further. If all went well, all vertical play has now been removed from the lead screw assembly. If not repeat this step until the vertical play is gone. 

• Tighten the 2 grub screws again to prevent to lead screw from rotating inside the pulley. Check that everything is secured! Repeat this for the remaining pulley/screw assemblies.

• Attach the fan connector to the board and use some zip ties to bundle all the wires again and replace the bottom cover.

• Secure the bottom cover screws and turn the printer upright.

• You can add a small piece of heat shrink tube or electrical tape on the last 15mm of the lead screws. (Do not cover the screw holes!!) This is to minimise contact sound from the lead screw to the aluminium corner bracket. During fast print moves the lead screws vibrate quite a bit and one screw kept hitting the bracket, creating a very annoying rattle. The heat shrink tubes minimise these contact sounds. If you don't have heat shrink tube, a piece of electrical tape around the lead screw ends would act as a similar barrier.

• Now add the first lower part of the Z top cap to the corner bracket. It should snap fit into the bearing hole very nicely. Secure the lower half with the 2 screws that secured the bearing.

• snap fit the top spring part onto the lower part. The top parts are marked for their corner location!

• Check the hole diameter of the printed 10mm M3 bushings, so a M3 bolt can rotate freely inside and add it into the hole. Also make sure the bushing can rotate freely in the hole in the spring top cap!

• Secure the bushing by screwing in a 20mm m3 Allen bolt into the top of the lead screw. You might need to grip the lead screw tightly to secure this bolt. Repeat the process for the remaining corners. Make sure the bushing can still rotate in the top hole, the bolt is only there to keep the bushing in place, not to fix the lead screw to the spring!!

• Re-attach the 4 plastic corner caps.

• Re-attach the spool holder and the bowden coupling.

• Remove the top screws from the gantry alignment blocks. 

• Turn on the printer and raise the gantry until you can easily remove the screws securing the alignment blocks to the printer frame.

• It is wise to now do a new automatic bed levelling from the calibration menu. 

• After this everything is ready to start printing again.

.3mf file markings: 

• LF = left front

• RF = right front

• LR = left rear

• RR = right rear

Print settings:

• 0.2 layer height, 0.4 nozzle

• 4 perimeters

• 12 top and bottom layers to make it all solid

• No support necessary!!

Hardware needed: 

• 4x  M3x20mm allen bolts

• 4x  15mm length of 10mm diameter heat shrink tube or some electrical tape

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