Supersizing a Kossel Mini

Mini Kossel

     This time I'm upgrading the printer I started out with, a basic Mini Kossel kit, to a larger and taller frame. After replacing the original wheels with steel wheels last year, the aluminum towers finally wore down to the point that one of the carriages basically fell off the tower.

2020 Aluminum extrusion after running steel wheels for 14 months

     The obvious choice for replacing the damaged towers was OpenBuilds V-slot, same outer dimensions as the original extrusions allowing reuse of the printed frame parts. I also ended up swapping the steel wheels with the Delrin versions, don't want a repeat of what happened to the old towers.

Enlarged upper triangle

Enlarged lower triangle

     Since I was upping the size of the frame anyway, I cut the old towers down into new sides for the bottom triangle, the idea was to allow for mounting the power supply under the bed with the rest of the electronics, as well as allowing for upgrading the build plate in the future. Dimension wise, the side rails are now 30 cm long, allowing for anything up to a 25cm build plate. Reassembly was mostly the same as the original build process, just scaled up by a large margin.

Upgraded frame

     I did learn some new tricks to make assembly more precise, probably the most useful was using a spare section of extrusion to set the end-stop height to exactly below the upper triangle. All that's needed for this trick is a 3-inch C clamp, a spare or unused piece of the extrusion and the end-stop assembly.

The simple way to set endstop height

     Once the frame was rebuilt, the other parts that needed replacement were the rod-arms, the originals were both too short for the new size and one was cracked after a rather spectacular malfunction last fall. For the new rod-arms, the parts list is quite simple, 12 Traxxas 5347 joints (Amazon.ca), 6 12" lengths of 8-32 threaded rod, an 8-32 tap and AndrewBCN's assembly tool (Thingiverse, thing:701248). The 8-32 tap is optional, it just makes assembly easier by pre-cutting the first couple turns of the thread in the Traxxas joints.

Parts for new rod-arms with completed arm

     With the new rod-arms done, the next step was reinstalling the electronics and print bed. Since I'm not replacing the current print bed, some new mounts were needed, along with a new mounting bracket for the Re-ARM/Ramps boards. After creating a Fusion 360 mock-up of the bottom triangle, it was fairly easy to design new mounting brackets for the print bed, the Re-ARM was even simpler since the manufacture provides a cad file of the boards physical layout. The resulting STLs are here for download.

Re-ARM and electronics installed

Power supply mounting bracket

Mounting bracket installed on power supply

     As you can see, the power supply ended up under the bottom triangle, just wasn't space for the control boards otherwise. The brackets I've designed for mounting the power supply are in with the other STLs above, you'll need to mirror file with your slicer program to get both the left and right versions. Only other parts needed for mounting are some M4x20mm screws and the power supply, I'm using this one, but the brackets should work for any similar module.

Print bed installed with dust covers

     One of the recent improvements for deltas that's been trending on the net lately is adding protective covers to the corners over the lower pulleys and electronics bay. I've been meaning to add them for a while and came up with a simple way to make them out of some spare foam core.

Kossel on foam core for tracing

Kossel and Print bed outlines on foam core

     Yep, that's it, just stick the printer on the foam core and trace around the base with a marker. I also used an earlier version of the print bed supports to center it under the printer frame and then traced it out as well. After that it was just putting the printed parts in their approximate spots, trace the outlines and then cut out the section that's left. I did use a drill to put 3 holes in each for ventilation over the motors but that was basically it. last step was wrap the edges with electrical tape for safety and colour the top black for aesthetics.

Corner covers finished and installed

      Now, obviously the power supply is currently serving as the structural base for the entire frame, not the best idea for long term stability or noise. I found these tennis ball feet (thing:2158108) on Thingiverse, they're printable adaptors that let 3 standard tennis balls serve as vibration damping feet, so I used the Micro Kossel to print a set and installed them.

Tennis ball feet 2/3 installed

   And that was pretty much it, only things left to do were recalibration running a few test prints which turned out nicely.

Completed Mega Kossel

Re-ARM of a Mini Kossel

Panucatt Devices Re-ARM board Bottom View

     Last month, I covered some of the key upgrades on my mini Kossel, and most of them were things that improve the extrusion path. This time, I'm switching the controller board from an 8-bit Arduino Mega 2560 to a new 32-bit Smoothieware-based board called the Re-ARM for Ramps. 

New and Old: Re-ARM/Ramps (top right), Arduino Mega2560 (bottom left)

     Physically installing the Re-ARM was fairly simple, I just needed to remove the print bed and lift the Ramps assembly out from underneath. After that, a few minutes work with some pliers to gently separate the Mega and Ramps boards, then mount on the Re-ARM's headers with some gentle pressure until things are snug. Don't forget to set the Re-ARM's power mode before installing the Ramps, it's impossible to change the jumper with it in-place.

Re-ARM/Ramps with mounting bracket (on the left)

     After assembling the boards, I found that my Mega2560 mount was incompatible with the Re-ARM's footprint - mostly just a tolerances issue, the mounting bracket I was using is a bit sloppy, so I loaded the Re-ARM's digital reference into Fusion 360 and drew up the bracket above. It's designed to raise the Re-ARM to just the right height to allow the optional Ethernet adaptor to pass between the 2020 t-slot extrusions on the Kossel's base, and bolts down with a couple of #6 or M4 wood screws. The STL for it is here. 

Re-ARM/Ramps mostly installed

    Once the main boards where installed, it was time to add the graphics display. With the Arduino Mega, I'd used a Smart Controller display panel, but it wasn't compatible with the Re-ARM so I swapped it out for a Full Graphics Controller instead. The only real gotcha with using this display on the Re-ARM is that you need to splice part of the EXT2 cable to tap into one of the 5V rail pins since the main logic rails are operating at 3.3V.

Spliced line on EXT2 cable, it's the edge opposite the red reference line

5V pin for EXT2 splice

     And with that plugged in, I used the Micro Kossel to print some mounting brackets from Thingiverse for the display and then used some M3x20mm screws to install them.

Full Graphic Controller with Mounting brackets waiting for installation

Using zip-ties for cable management, helps keep things nice and tidy

Bracket fully installed using M4x0.8mm cap screw and M4 hex nut

     With that installed, all that's left is to remount the print-bed and deal with the software side of things. I'd originally used 3 M3x20 screws to hold the print-bed down, but these made it very vulnerable to warping and difficult to level after a reinstall, essentially requiring recalibration every time I needed to access the electronics. After running across thing: 1982435, I decided to make my own version with an extra slot for my heat shielding. Files are here.

Print-bed mounting blocks waiting for install

Test fitting heat shield

Finalizing exact positioning, note the M3x20mm screw and bolt for holding glass

     After sorting the heat shield, I still had a minor issue with the bed trying to float on top of the mounts, so I drafted up some extra wide M4 washers to hold things in place with some spare M4x20mm bolts I had left over from another project. You can see the floating issue in the next picture's lower left corner.

M4x20 bolts and custom washers waiting for final installation

Glass holder clip with M3 thumb wheel for holding the 195mm glass down

     As for holding the Buildtak/glass plate combo I use for an actual print surface onto the print-bed, my glass is an odd 195mm diameter, not the more standard 200mm or 220mm that you normally see, so the glass clips are customized for it. With that installed, it was largely just a matter of learning how to configure Smoothieware with the Mini Kossel's parameters, the official documentation, combined with the Re-ARM's setup guide were fantastic on this, then the usual calibration and it's done.

Retrofit complete, ready to print