Kossel 220mm Hot Bed Spacers / Mounts by peaberry

Last crawled date: 3 years ago

These are spacers designed to mount a 220mm diameter hot bed to the Anycubic Kossel Linear (and other similar Kossel Delta printers). They're designed to fit a printer with 20x20x240mm extrusions (300mm triangle), using M4x10 socket headed screws and M4 square or T-nuts. The spacers are mounted 45mm from the end of the extrusion, and are shaped to fit the curvature of the bed at this position.
To provide clearance from the electronics, the spacers raise the hot bed 10mm above extrusion. The overall height is 13mm to be flush with the top surface of a 3mm thick hot bed.
There are two models: left and right handed, and 3x of each type need to be printed.
I've also added a PDF template for a heat shield, which is designed to sit between the electronics and hot bed. It has three rectangular feet which sit on the extrusion between each pair of spacers. For this I just used some 3mm hardboard covered in aluminium tape (as shown in one of the attached pictures).
After printing these, I also found these clips which work really well for mounting a 200mm glass bed to the heat plate: http://www.thingiverse.com/thing:1275136
Update 13/01/18: I've modified these to include a 3mm hole with a recess for an M3 nut (hotbed_spacer_02_right), and designed an additional clamp which fits on top of that (hotbed_clamp_02_right). I'm now using 3x of these with M3 countersunk screws and nuts to hold the bed down (i.e. only clamping the bed gently on 3x of the 6x spacers). My thinking is that this should be sufficient to hold the bed in place, without risk of distorting anything.
Update 25/01/18: I've added a new "tighter" version of the clamping piece, if you find the existing one a loose fit on your bed. The original fits perfectly and makes a tight fit on my own bed (which I just measured at 3.25mm thickness). However, the "tighter" version will be more suitable if your bed is less than 3mm thick.
It's a good idea to print these out of ABS rather than PLA so you can use higher bed temperatures for printing ABS, HIPS etc.
There were no instructions to install the heated bed. Using the Anycubic Trigorilla Ramps 1.4 compatible board, I connected a 12V 20A PSU to both the 11A and 5A power inputs, connected the bed to the HOTBED output (don't do this: read the warning below), and connected the heat bed thermistor (NTC 3950 100K) to the T1 sensor input. In the Marlin firmware, I configured TEMP_SENSOR_BED as type 11 (QWG-104F-3950 thermistor). The lookup table was missing from my older version of Marlin, so I added this table from the current Marlin development trunk.
WARNING: after running the heated bed successfully for quite a few weeks, the power MOSFET on my Trigorilla Ramps 1.4 board overheated (with a really nasty burning smell). On the Trigorilla board, it's an STB55NF06 MOSFET (B55NF06 GK162 V6 CHN 618) in DPAK surface mount packaging, and I guess it just doesn't have enough PCB copper area to dissipate the heat, resulting in thermal runaway. The plastic connector next to it also melted. Now I have to repair it, and I'm thinking of replacing with an external TO-220 packaged MOSFET with a decent heat sink. After this experience, I would not consider using the on-board MOSFET again...
I'm now using an external MOSFET board (HA210N06) instead of connecting direct to the Ramps board.