Yearly Overhaul: Mega Kossel

Mega Kossel

       One year after building the Mega 2.0, some of the reused components started to wear out and cause issues, so this is an overview of what's been updated and replaced over the past couple of months, primarily the print-bed and its related wiring along with some worn out cables in a couple areas.

Mini Kossel Power Switch after overload surge

     The first issue that came up was the original power switch overheated and partially melted due to an improperly matched circuit breaker from an old refit allowing sustained over-current during the heat-up cycle at the start of a print, I think that's what happened anyway. Fortunately the fix was very simple, one of these IEC Socket with Switch and Fuse Holder units and a 5A glass fuse from the local electronics store, and some wire from an inexpensive extension cord covered the electrical side. Mounting the new plug was mostly a matter of designing a custom bracket for it to sit in and bolting that to the underside of the frame, then connecting everything to the power supply and that issue was fixed.

New power socket wired up and ready for mounting shell

Power Socket installed in printed housing for safety

      With the power input repaired, the second and more critical issue that came up was a mechanical failure of the original heat-bed power input cable where it was soldered to the Kapton heater disk. Re-soldering it worked temporarily but it broke a second time in the exact same spot and the second break ripped a hole in the copper layer of the circuitry, so it was time to retire the old thing and get a more solid MK3 bed variant to do the job.

New heat-bed stack components, from left to right:
spare aluminum, cotton insulation, 300mm MK3 bed, and Creality magnet kit

     Parts used in the new bed are basically the larger versions of the ones installed on the Sculptor and Ender 3. A 300mm MK3 aluminum bed is the electrical and structural core, with a sheet of cotton insulation to protect the electronics bay taped on the back. Upper surface is coated with the magnet sheet out of a Creality magnetic bed kit to mount the existing spring-steel sheet bed surface. I'm not using the Creality upper surface since they have a tendency to crack and breakdown after a fairly short usage lifetime.

New bed supports installed on Mega

     Of course, the change in bed shape means that a new set of bed supports are needed, some quick CAD work with the design files had the relevant parts drawn out and sent off to the Ender 3 and Sculptor for fabrication. Once that was done and bolted down, it was time to solder the electrical cables onto the new bed since it didn't come with the wires pre-installed, so here's some soldering in low-temperatures 101.

MK3 bed positioned on Ender 3 print surface for easy soldering.

     It was freezing cold out when this refit was done, workshop was just under 5°C average temperature, so the solder wasn't heating up correctly on the workbench or iron. Seems that solder needs about 20°C to work correctly, so I flipped the bed I was working on upside-down on my Ender 3's build-plate after cranking it up to 60°C. This managed to transmit enough heat into the parts being worked on to get things flowing correctly and it was fairly simple to finish installing after that.

Magnetic Print Bed Surface on a Budget

Mega Kossel with magnetic build plate

     When I rebuilt my Mini Kossel into the Mega Kossel last August, I installed upgraded rod-arms that allow for a maximum build diameter of 250mm (9.84 in), and I've been using a 190mm (7.5 in) piece of glass topped with Buildtak as my bed for the past 6 months, so I've really been using only 58% of my theoretical build diameter. I'd looked at upgrading the bed surface with the Buildtak Flexplate system in the past, but the only circular option currently available is a 300mm (12 in) set, far to large for my frame and 220mm heat-bed. Thus, I started looking at options for building my own custom version.

10-inch pizza lifter from the local dollar store

     Concept wise, the Flexplate system is fairly straightforward, an adhesive sheet with magnets in it that sticks to the printer's heat-bed or build plate, topped with a spring steel sheet with some Buildtak material on one side. For my version, I'm using some steel pizza lifters that I found at the local dollar store in the cooking supplies section of all things, $4 each and a bit of drilling to remove the riveted-on handle and I've got some fairly flat surface plates.

Adhesive backed magnets assortment from Lee Valley Tools

     For the magnetic part of things, I'm using some 6mm rare-earth magnets that I salvaged from an old toy building set that I had floating around, but Lee Valley sells some 1/4" ones that are a fairly close match. 

Bed mounting bracket with magnets installed

Other side of bed mount with magnets

I've designed a variation of the mounting brackets for my heat-bed that puts 36 of them in a ring around the entire bed, massive overkill when it comes to this type of magnet but having the bed surface start drifting loose is the last thing you want on a 3D printer of any type.

Corner brackets waiting for magnets

Brackets mounted on Mega Kossel

     After printing the brackets, I used some 5-minute epoxy to fix the magnets in place, I've found that it does an excellent job of holding parts to PLA prints under most conditions.

Magnetic bed fully installed on the Mega Kossel

     I've been using this bed configuration for the past 2 weeks already and it works quite well. This particular implementation is specific to the Mega Kossel, but if I was going to install something like this on an i3 style machine, I'd make custom brackets that put the magnets in a line along the front and back edges of the bed, then use a slightly oversize sheet of steel for the bed topping plate.