More Mini Kossel Updates

After nearly 4 months of supplying Mini Kossel kits we have had a lot of fantastic feedback from all the makers who have put the kits together. This is the third post on additions, changes and improvements (the first one is here, and the heated bed one here).

Simple second extruder fan mount

We have had a number of requests for a solution to allow a second, PWM controlled fan to blow on the part being printed, so I designed this modified J_head extruder mount:

This mounts on the end effector plate in the same way as the original one, with the exception that it is rotated by 60 degrees (one hole) to allow the z-probe to clear the second fan:

To minimise the air blowing on the heated bed from the existing, always-on, fan cooling the thermal break on the J-head, I added a strip of aluminium tape to the bottom of the always-on fan mount. The tape can withstand the hot-end temperature so I sealed all the way to the hot-end heater block insulation.

As the fan mount has to be rotated 60 degrees to allow the probe to clear the fan, one of the probe mount screws needs a nut adding:

The second fan is prepared with a male connector pin so it can be plugged and unplugged at the same point as the rest of the end effector wiring loom. In the long run the end effector wiring loom may move to a 2x5 pin plug.

The screws self tap into the plastic of the mount.

The fan wiring follows the same route as the remainder of the end effector/hotend wiring loom, with a plug lined up with the existing plug:

It connects to the RAMPS via screw terminal D9, between the extruder and heated bed (not connected in the picture below), allowing for PWM control when Motherboard number 33 is selected within Marlin configuration.h.

No firmware changes should be required from the standard setup and the cooling settings within Slic3r and Cura will turn the fan on after a specified number of layers.

The source files for the updated hot-end fan mount are on Github. and its also a "thing" on Youmagine!

New PSU for Heated Bed

After an issue with sourcing reliable and economical high current laptop power supplies, James Clutterbuck suggested the Dell DA-2; a 12V 18A power supply:

This beast is capable of providing enough power for the whole printer and heated bed so from now on complete printer kits ordered with heated beds will be supplied with just this power supply. Unfortunately although the DA2 is supplied with what appears to be a 2x4-way Molex Minifit plug, the housing doesn't mate correctly with the standard Molex 2x4 Minifit socket, and so needs to be changed (part numbers below).

Roland designed a new face plate that accommodates the Molex socket along with a power switch and the USB plug.

The power supply cable is wired to both sides of the RAMPS plug:

The assembled faceplate installed:

The documentation will be updated shortly. The source files are available on our Kossel Mini Repository on Github (USB-power-8way-V2). If you want to source the parts for the upgrade, along with the printed plate, you need:

Dell DA-2 Power supply (12V 18A)
8 way connectors: Molex 39-01-2080 and 39-01-2081
16AWG crimp pins for molex 39-00-0078 and 39-00-0082.
Standard case rocker switch such as this one.

Marlin improvements

The Think3dPrint3d Kossel fork of the Marlin firmware is available on github and comes pre-configured as a good starting point for Marlin on a Mini Kossel. I have recently implemented a couple of improvements. Firstly David Lapeš pointed out this feature request/ bug fix for the main version of Marlin which allows for negative position numbers to be correctly displayed on the PanelOne screen:

The second change is to add 4 menu items for filament management. Within the "Prepare" menu you can now choose to prime or retract the filament by a small amount (default 3mm) as well as either load or unload the filament completely (default 560mm).

The default values can be changed within configuration.h, line 399 onwards:

#define EASY_LOAD
#define BOWDEN_LENGTH 560
#define LCD_PURGE_LENGTH 3
#define LCD_RETRACT_LENGTH 3
#define LCD_PURGE_FEEDRATE 200
#define LCD_RETRACT_FEEDRATE 600
#define LCD_LOAD_FEEDRATE 500

#define LCD_UNLOAD_FEEDRATE 500

You can also set the feed rate for each action (in mm/minute)

This based on Lajos's changes to Marlin for the Tantilus printer, I changed his implementation to make it work with the delta firmware and simplify the options a bit.

You can upgrade to these changes by downloading the Think3dPrint3d version of Marlin and uploading it. Make sure you copy across any changes you have made to configuration.h such as the Z height and delta radius after calibration.

Sleeker cable management

Mark Burton has added "Go Faster" stripes to his Kossel mini in the form of these extrusion insets, which nicely hide away the endstop cables.

4 of the strips fit in each extrusion tower. The scad and stl files are on github.

 Thanks to everyone who has sent feedback and design improvements!

Kossel Mini - Heated Bed

We are finally ready to release the Heated bed kits for the Kossel Mini. It has taken a bit of time to confirm our preferred design and source the components in bulk. We have been running two test printers for almost two months now with this solution and will be writing to those who have already bought a kit shortly to offer them the upgrade first (as promised!).

Heated Bed

After investigating a number of options (PCB, Silicone heater, kapton heater on borosilicate glass, kapton heater on aluminium plate) we went for a Kapton heater on an aluminium plate.

The heater is rated at 10 A, giving an output of 120W at 12V, and supplied complete with a MF58104F3950 thermistor taped to the centre of the aluminium plate (thermistor table  on chirpy's blog) .  Wires are prepared for fitting to RAMPS. We have run these heaters continuously at 125ºC for 24 hours on our Kossel prototypes without any problems, and find that they heat up significantly faster than standard PCB heatbeds because of the higher power density (6kW/m2 vs 3kW/m2)

The aluminium plate acts as an excellent heat spreader for the Kapton heater and is easier to mount than borosilicate glass as it can be drilled and countersunk.

This is mounted in a "sandwich" with an aluminium-foil-tape covered cardboard insulator below (thanks to nophead for this design of heat insulator from the mendel 90).

This allows for various print surfaces to be clipped onto the aluminium plate, e.g; mirror glass with glue stick for PLA, Tufnol for nylon 618 or mirror glass with ABS juice or kapton for ABS. Best of all you can use multiple mirror plates plates to quickly swap on a new one while the other one cools.

In order to keep the electronics cool we have added a fan under the heated bed, complete with splitter cable:

And redesigned the bed mounts and z-probe retractor:

In order to power the bed we decided to add a second, 10A, laptop power supply:

And a matching second socket to the USB/Power plug plate. Because some customers have decided to use an ATX or similar power supply to power the printer and the heated bed, the second 10A power supply will be an optional extra in the kits.

The Mini Kossel Documentation has been updated to describe how to fit the heatbed either during initial build or as an upgrade. It's straight forward (mostly described by the pictures above).

Calibration

The T3P3 github has a slightly updated version of Marlin that we distributed with the kits so far, to include a thermistor table for the heated bed thermistor (number 11)

There are a few of things to consider when calibrating; the bed is now higher by ~10mm, and different bed (and nozzle) temperatures will lead to slightly different  Z = 0 positions due to thermal expansion. For example with a cold bed and cold hotend the Z-height was ~0.2mm lower that with a 80C bed and 225C nozzle.

The easiest way to handle this is to carry out the calibration steps (as described in my previous blog post) with the bed and hotend cold. Once the bed is level and there is no doming then you can measure the difference between Z = 0 on a cold bed and with the bed and nozzle at the centre print temperature for various materials. I measured my test printer with the heatbed between 60C and 110C and the hotend between 185C and 240C and there was <0.1mm of difference in Z height: all were ~0.2mm above the cold bed height.

I used the standard method of a piece of paper as a feeler gauge under the nozzle, stepping down by 0.1mm in Pronterface until I could feel the nozzle dragging. Be careful not to burn yourself on the hot bed! Once you have confirmed the difference (probably 0.2mm) then edit configuration.h :

#define MANUAL_Z_HOME_POS = 238.4 //cold = 238.6

and re-upload Marlin.

Z-probe retractor

The Z probe retract position will have slightly changed in X and Y and gone up in Z due to the new Z-Probe retractor

The documentation describes how to modify the Z-probe retract position in Marlin to compensate.

Extruder steps for different materials

Another thing to consider is that when using different materials, the extruder hobbed insert will bite into the filament more or less, changing the exact extruder steps per mm. With our standard mini extruder we use ~650 steps/mm for PLA. I found the following for different materials on a test printer:

ABS	660
PLA	650
Ninjaflex	700
Lawoo3d	650
T-glase	660
Nylon 618	660

The easiest way to handle these changes is to use:

M92 E650

in your start g-code and have different start g-codes for different materials in Slic3r.

Getting one!

We will be contacting those who have already bought a Mini Kossel kit from us first and offering a discounted upgrade kit, once these orders have shipped the upgrade will be available for general purchase.

All kits sold from now will have the option of a heated bed.