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.