Kossel Mini - feedback and tweaks

After a manic month or so shipping Kossel Mini Kits we have been getting loads of feedback from those building the printer. A lot of the feedback was around the documentation which we have continued to adapt and develop, trying to ensure that if we get asked a question a couple of times we answer it in the documentation. Please keep this feedback coming - we really appreciate it!

Endstop trigger point adjustment

After re-reading minow's very helpful blog on in depth calibration we decided to make a minor change to the endstop trigger function. Previously the endstops triggered on the top of the X,Y and Z carriages:

These have been replaced with slimmer endstop mounts and a modified Delrin V-roller carriage adapter with M2.5 socket cap screw:

This means the endstop trigger point can be finely adjusted in order to level the bed very accurately.

The Delrin V-roller carriage adapter is modified from Haydn Huntley's design, while the slim line endstops are from Johann's original design. The modified files are available on our Github.

The changes can be retrofitted to existing Kossel Minis by simply drilling a hole for the M2.5 to self tap into and printing the thinner endstop mounts. It is fiddly to drill the holes in situ, even using a hand drill, so one Delrin V-roller can be removed to allow the carriage to be removed for easier access. We will shortly publish a new chapter of our Mini Kossel build manual showing this process in detail.

Endstop Trigger point calibration procedure

This process will be incorporated in the manual as soon as we are confident we have explored any downsides to this approach.

The endstops on the X Y and Z pillars should all trigger at the same distance from the bed. This procedure will show how to set these.

Start by using a ruler or measuring tape to set them approximately (within 1mm) at the same distance up the pillars.

Next fine tune each endstop trigger point in turn. This is an iterative process as each adjustment alters the others slightly. First adjust the Z height in the Marlin firmware to a few mm more than it needs to be and upload the firmware:

#define MANUAL_Z_HOME_POS 255

Then work through steps 1-5 below. Note you should do this with the extruder (and heated bed if you have one fitted) at the printing temperature.

1. Find the X axis endstop trigger point
1. Home the printer (G28)
2. Move the extruder to just in front of the X pillar (G1 X-76 Y-43 Z10 F7000). The tip should be approximately 5mm above the bed.
3. Lower the extruder using Pronterface until a piece of standard 80 gsm printer paper just drags on the nozzle.
4. Make a note of the Z height using M114:

SENDING:M114

X:-76.00Y:-43.00Z:5.10E:0.00

2. Adjust the Y axis endstop trigger point
1. Home the printer (G28)
2. Move the extruder to just in front of the Y pillar (G1 X76 Y-43 Z10 F7000). The tip should be approximately 5mm above the bed.
3. Lower the extruder using Pronterface until a piece of standard 80 gsm printer paper just drags on the nozzle.
4. Check the Z height using M114; the aim to get this to the same value as that recorded for the X axis.
5. Adjust the M2.5 screw on top of the Y carriage to raise or lower the trigger point (a 360 degree rotation of the M2.5 screw will give a 0.45mm change in trigger point)
6. Repeat steps a. to e. until the Z height checked in step d. is the same as that recorded for the X axis
3. Adjust the Z axis trigger point
1. Follow the same process as step 2, using (G1 X0 Y85 Z4 F7000) to place the extruder in front of the Z pillar
4. Work around steps 1-3 again because if large changes were required they will noticeably influence the set points of the other axes. Once they are within 0.1mm of each other, move to the next step
5. Finally we can set the correct “0” point for the centre of the bed.
1. Home the printer (G28)
2. Move the extruder to just above the center of the bed (G1 X0 Y0 Z10 F7000)
3. Lower the extruder using Pronterface until a piece of standard 80 gsm printer paper just drags on the nozzle.
4. Note the Z height using M114

SENDING:M114

X:0.00Y:0.00Z:5.20E:0.00

5. Subtract  this Z value from the one set for the manual Z home position at the beginning, change this in the Marlin firmware and re-upload the firmware. In this example it was 255-5.2 = 249.8

#define MANUAL_Z_HOME_POS 249.8

The endstop trigger points are now all within 0.1mm of each other and Z “0” is within 0.1mm of the top of the bed.

The next step is to calibrate the delta radius - Minow's blog outlines the process very well (see step 3).

New Extruder Mount

One of our customers, elmuchacho on thingiverse, has shared a number of designs as upgrades for the Mini Kossel. This is his extruder mount (his picture, then Think3dPrint3d's picture)

This extruder mount holds the extruder more firmly to the frame than the original Think3dprint3d zip tie version. It uses a nut threaded onto the PTFE tube rather than the pushfit connector (top picture).The new mount is shared by elmuchacho on thingiverse and tinkercad. He has also shared a version that uses the pushfit connector supplied with our kits.

To complement this I have reverted part of the extruder block to use the mounting screws and taken the opportunity to make a few other minor tweaks to improve printability - it's also available on the Think3dprint3d github.

Vibration damping

I have noticed a slight chatter in the Traxxas linkages. This is a common issue and the jury is still out on if it noticeably affects the print quality. That said, reducing this definitely makes the printer quieter. One solution is to use rubber bands:

   

These clips were also designed by elmuchacho: http://www.thingiverse.com/thing:371998

Dust is also using a similar idea on his Mini Kossel build (He is not building one of our kits but a similar printer).

Heatbed

We have confirmed the design for the heatbed (Kapton heater on Alu plate with insulation underneath). We are now just waiting for parts to arrive in bulk before we go ahead and offer them for sale. They will be offered at a discount to existing Mini Kossel customers and as an upgrade/stand-alone item as well. The details will follow in a separate blog post.

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!