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.

Kossel Mini and More

Our Mini Kossel is a version of the excellent Kossel delta robot printer from Johann Rocholl, the designer of the original Rostock delta. It's very quick and easy to put together and relatively economical to source parts for. We have designed some additional parts and incorporated a number of variations from around the community. The changes we have made are described below and all the additional source files are available on Github (look in the T3P3 additions directory)

The Mini Kossel can be bought as a kit or assembled from www.think3dprint3d.com  There are 5 colours in stock or we will print you a set in any custom colour we can source and print. All our parts are printed in ABS on our Mendel90 Lasercut production printers which continue to churn out excellent-quality parts every day after almost a year in service.

Linear Rails v Rollers

The initial Kossel design used linear rails: 

Picture (c) hiwin.com

These are great but add significantly to the cost. Using rollers on the aluminium extrusions themselves has been suggested in a number of places:

http://www.electronhacks.com/2013/12/kossel-mini-3d-printer-vertical-movement-tutorial/

http://www.thingiverse.com/thing:308369

and this comparison here: http://www.builda3dprinter.eu/rails-wheels/.

These convinced us to try the roller based option. As we are using Mitsumi aluminium extrusions we got Delrin rollers precision machined:

After many hundred of hours printing the bearings and extrusions are not showing visible wear. The only lubrication we have used is a spray of light oil containing PTFE (for example GT85). It is very simple to tighten the adjusting capscrew to adjust the pre-load and take up any slack if they do wear.

Johann is looking into using recirculating Delrin balls directly on the extrusion as well - well worth following up as and when time permits. Delrin balls are quite pricey, though, but it would be awesome if Airsoft BB pellets turned out to work well. The best carriage for these turned out to be Haydn Huntley's.

RepRapPro mini extruder

This extruder has been proven over many thousands of hours, both on RepRapPro's Mendel and Huxley printers and on our Mendel90 Lasercut multi extruder machine.

We modified this slightly to use easily obtainable pushfit bowden fittings and made a our own version of the quick and simple zip tie mount to fit to the extrusion:

Power and electronics mountings

To keep the kit as simple and user friendly as possible we designed a USB and power plug plate:

That provides a neat interface with the RAMPS in the base of the printer. The non-heated bed version uses a single 5 amp laptop style power supply. We are investigating options for the heated bed version but one being considered is an extension of this plate with another plug for a heated bed power supply. The RAMPS is mounted on another simple plate:

The picture also shows some simple tabs to keep a round glass mirror plate fixed securely using M3 penny washers.

Filament management

A reel holder and filament guide complete the kit:

Build Manual

We have written a comprehensive Kossel Mini Assembly Manual for the kit which we hope will become a useful resource for all. We would really welcome feedback on the manual and suggestions for additional information to add. 

Prints

One of the Mini Kossel beta testers did these prints of the EggO egg "thrones" by mageli which are awesome. 

PanelOne LCD

Following on from the case design tutorial in SCAD here is more information of the PanelOne, a simple LCD controller for RAMPS. It is optimised for those who want to rout/etch a single sided PCB with only through-hole components. It also uses widely-available 10-way IDC ribbon cables.

The more fiddly elements of the SD card socket and logic level converter are left to an optional, and generally available, daughter board such as this one from adafruit:

image from adafruit.com

or this one:

image from hobbytonics.co.uk

The circuit board itself is designed in KiCAD as a single sided board, and was originally designed for Sumpod who commissioned us to design a board that could be routed on a PCB mill and fitted the dimensions of their printer enclosure.

In keeping with the simplicity of the design there is no adaptor board required at the RAMPS end as the pinouts are arranged to match with the AUX2 and AUX3 headers.

The complete KiCad files for the PanelOne are available on github.

Coming Soon

Watch this space... we are working on adding a heated bed and an interesting twist on multiple extruders!